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Rita MAGRI
Professore Associato
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica
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Pubblicazioni
2024
- Structures and electronic states of nickel-rich oxides for lithium ion batteries
[Articolo su rivista]
Yousuf, Saleem; Mridha, Md Maruf; Magri, Rita
abstract
A new superstructure of layered pristine LiNiO2 (LNO) was obtained by first replicating 16 times the optimized structure of the 12-atom primitive cell of the R3m space group and then relaxing both cell parameters and internal positions. The structural optimization is accompanied by a very significant lowering of the internal energy. The R3m space group is the space group observed experimentally by XRD. In contrast to the structure commonly proposed in the literature, this new crystal structure, which still belongs to the R3m space group, shows size and charge disproportionation of the NiO6 octahedra. The charge disproportionation results in a change in the Ni oxidation state from Ni3+ calculated in the 12-atom primitive unit cell, to Ni4+ and almost Ni2+. This is the first time that such charge disproportionation has been calculated for the R3m crystal structure of LNO. The Ni-O bond length distribution of the new structure agrees well with experimental values. Our results show that the choice of the simulation unit cell is important for determining the energetics of this class of oxide material, proposed for cathodes in lithium ion batteries (LIBs). We used this new structure as a template for the study of the structural and electronic changes induced by delithiation and by Mn for Ni cation substitution, originating the solid solutions LiNiyMn(1-y)O2 (LNMO). Our results, surprisingly, agree well with existing experiments and explain observed trends better than previous studies.Correct atomistic models of LNO and LNMO (N = Nickel, M = Manganese) oxide materials are critical for a correct prediction of the charging and discharging behavior of Li-ion batteries with NMC cathodes.
2023
- A first-principles study of self-healing binders for next-generation Si-based lithium-ion batteries
[Articolo su rivista]
Maji, R.; Salvador, M. A.; Ruini, A.; Magri, R.; Degoli, E.
abstract
Silicon anodes typically suffer from poor intrinsic conductivity and dramatic volume change during charge/discharge cycles, which hinders their commercialization in high energy density lithium-ion batteries (LiBs). This issue can be alleviated by embedding particles of the active material in an adhesive matrix, such as a polymer binder, that can accommodate large volume changes during lithiation and delithiation. Several research efforts have aimed at enhancing the adhesive, elastic, electrical, and ionic properties of binders for use in silicon anodes. Therefore, stable silicon/polymer interfaces are crucial for the performance of high capacity silicon-based LiBs. In this research, we focused on the definition of the mechanisms that determine the adhesion properties of a couple of recently proposed self-healing polymers, on Si-surfaces. The structural and electronic properties as well as the energetics of boronic acid-doped polyaniline and polyvinyl alcohol monomers absorbed on Si (110) and Si (111) surfaces have been investigated through first-principles calculations based on the density functional theory. We showed that the coabsorption of these two monomers increases the absorption energy and in general improves the adhesion properties of both polymers on both Si-surfaces, especially on the Si (111) facet.
2023
- Measuring thermal conductivity of nanostructures with the 3ω method: the need for finite element modeling
[Articolo su rivista]
Peri, L.; Prete, D.; Demontis, V.; Degoli, E.; Ruini, A.; Magri, R.; Rossella, F.
abstract
Conventional techniques of measuring thermal transport properties may be unreliable or unwieldy when applied to nanostructures. However, a simple, all-electrical technique is available for all samples featuring high-aspect-ratio: the 3? method. Nonetheless, its usual formulation relies on simple analytical results which may break down in real experimental conditions. In this work we clarify these limits and quantify them via adimensional numbers and present a more accurate, numerical solution to the 3? problem based on the Finite Element Method (FEM). Finally, we present a comparison of the two methods on experimental datasets from InAsSb nanostructures with different thermal transport properties, to stress the crucial need of a FEM counterpart to 3? measurements in nanostructures with low thermal conductivity.
2023
- Structural and Dynamic Characterization of Li-Ionic Liquid Electrolyte Solutions for Application in Li-Ion Batteries: A Molecular Dynamics Approach
[Articolo su rivista]
Salvador, Ma; Maji, R; Rossella, F; Degoli, E; Ruini, A; Magri, R
abstract
Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li-PF6, Li-BF4, and Li-TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation-anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation-anion combination and of different fractions of lithium in the solutions to assess their relative advantage for Li-ion battery applications, by comparing the solvation and structural properties of the systems. This is the most-comprehensive work so far to consider pyrrolidinium-based ILs with different anions and different amounts of Li: from it, we can systematically determine the role of each constituent and its concentration on the structural and dynamic properties of the electrolyte solutions.
2022
- Interaction of Hydrogen with Cu-Modified Cerium Oxide Surfaces
[Articolo su rivista]
Vikatakavi, Avinash; Benedetti, Stefania; Righi, Giulia; Magri, Rita; D'Addato, Sergio; Luches, Paola; Selloni, Annabella
abstract
2022
- Investigation of the structural and electronic differences between silver and copper doped ceria using the density functional theory
[Articolo su rivista]
Righi, G.; Benedetti, S.; Magri, R.
abstract
Using the density functional theory (DFT) we have investigated how Ag and Cu atoms, substitutional to Ce, arrange themselves within Ceria (CeO2) and their effect on the ceria lattice, the oxidation states of the metal atoms, and the oxygen vacancy formation energies. Noble metal doped ceria has been proposed in substitution of platinum for a number of catalytic reactions. We have considered single noble metal atoms substituting Ce atoms in the (111) CeO2 surface unit cell, and investigated the thermodynamic stability of few configurations of one, two, and four Ag or Cu atoms in the unit cell. We have found that the noble metal atoms prefer to be located in the cation layer closer to the surface. An interesting result is that Cu causes a strong lattice distortion contrary to Ag. Ag, instead, causes a stronger reduction (oxygen loss) than Cu, since the electrons released in the oxygen vacancy formation are transferred mainly to Ag atoms, whose oxidation state tends towards +1, their preferred one, as found experimentally. In the Cu doped ceria, instead, part of the electrons reduces cerium atoms from +4 to +3 since Cu tends to stay in the +2 oxidation state, its preferred one. When we increase the concentration of Ag and Cu, substituting four Ce atoms, the noble metal atoms prefer to sit closer to each other, forming structural motifs resembling those typical of the Ag2O and CuO oxides.
2021
- Enhanced thermoelectric properties in Sb/Ge core/shell nanowires through vacancy modulation
[Articolo su rivista]
Bhuyan, P. D.; Gajjar, P. N.; Magri, R.; Gupta, S. K.
abstract
In the present work, we have modified the physical and electronic structure of Sb/Ge core/shell nanowires via vacancy creation and doping with foreign atoms with the aim to improve their thermoelectric energy conversion efficiency. Sb/Ge-NWs having a diameter of 1.5 Å show metallicity with 2Go quantum conductance. The stability of the nanowires is assessed through the calculation of their formation energy. The formation of one vacancy at either the Sb- and Ge-site modifies substantially the electronic properties. From the comparison of the thermoelectric properties of the nanowires with and without the vacancy, we have found that the figure of merit for the Sb/Ge NW with one Sb vacancy increases of 0.18 compared to the pristine NW. The NW doping with different transition metals: Fe, Co, Ni and Cu have been found to also enhance the conversion efficiency. Thus, our calculations show that the thermoelectric performance of metal–semiconductor core–shell NWs can be in principle improved as much as 80% by vacancy formation and doping.
2020
- Increasing Optical Efficiency in the Telecommunication Bands of Strain-Engineered Ga (As,Bi) Alloys
[Articolo su rivista]
Tisbi, E.; Placidi, E.; Magri, R.; Prosposito, P.; Francini, R.; Zaganelli, A.; Cecchi, S.; Zallo, E.; Calarco, R.; Luna, E.; Honolka, J.; Vondracek, M.; Colonna, S.; Arciprete, F.
abstract
The search for semiconducting materials with improved optical properties relies on the possibility to manipulate the semiconductors band structure by using quantum confinement, strain effects, and by the addition of diluted amounts of impurity elements such as Bi. In this study, we explore the possibility to engineer the structural and physical properties of the Ga(As,Bi) alloy by employing different stress conditions in its epitaxial growth. Films with variable concentration of Bi are grown by molecular beam epitaxy on bare GaAs(001) crystals and on partially relaxed (In,Ga)As double buffer layers acting as stressors aiming to control the Bi incorporation into the alloy and improving the optical properties in terms of efficiency. A combination of several structural and electronic characterization techniques and dedicated density-functional-theory calculations allows us a systematic comparison between the samples grown under compressive and tensile strain. We demonstrate the possibility to grow Ga(As,Bi) under different strain conditions without affecting its crystal quality. The different strain conditions strongly impact the Bi incorporation in the GaAs matrix and the luminescence properties of the sample. We find (i) a striking improvement of the photoluminescence with a strongly increased radiative efficiency when Ga(As,Bi) is grown under tensile strain and (ii) an interesting higher redshift with respect to Ga(As,Bi) grown compressively on GaAs. These two effects allow us to reach the important photoluminescence emission at 1.3 µm with a Bi concentration as low as 4.9% compared to 7.5% needed for samples grown directly on GaAs. This is a significant achievement for the application of the Ga(As,Bi) material in optoelectronic devices.
2020
- Methane Activation on Metal-Doped (111) and (100) Ceria Surfaces with Charge-Compensating Oxygen Vacancies
[Articolo su rivista]
Righi, G.; Magri, R.; Selloni, A.
abstract
Reducing the temperature for methane activation is an important objective that would benefit many technological applications. In this work, we explore the possibility to achieve this goal using single-atom catalysts (SACs) on ceria surfaces. We focus on Ag SACs, which have recently been suggested to be promising catalysts for both H2 and CH4 oxidation. Using first-principles calculations, we investigate methane activation on CeO2(111) and CeO2(100), two frequently exposed surfaces on ceria nanoparticles. The presence of Ag is found to reduce the activation energy for methane dissociation on both surfaces. On Ag-doped CeO2(111), the formation of methanol via the Mars-van Krevelen mechanism has a slightly lower energy barrier than the dissociation to CH3 + H, suggesting that methanol is a likely product of methane activation on this surface. A novel aspect of this work is the focus on stable surface structures where each Ag dopant substituting Ce forms a complex with a charge-compensating surface oxygen vacancy. These complexes are found to play a critical role and accounting for their presence is essential for a proper description of the surface reactivity.
2020
- Reduced cerium configurations in CeO2/Ag inverse catalysis
[Articolo su rivista]
Righi, G.; Anderlini, L.; Magri, R.
abstract
Inverse catalysts consisting of thin layers of a reducible oxide supported on noble metal surfaces have recently attracted much attention and shown to be competitive with the conventional catalysts, where metal nanoparticles are instead deposited on metal-oxide supports, in terms of their catalytic activity. We present here a computational study based on the density functional theory of the reduction geometries induced on ceria layers by a Ag support and by the creation of oxygen vacancies. We find that electrons are transferred from the Ag support to the ceria layers. These electrons localize at some Cerium sites to form 2D long range ordered patterns of Ce3+ and Ce4+ cations. Depending on their location the oxygen vacancies contribute and modify these patterns. The oxidation state of the cerium atoms affects the catalytic activity of the oxides.
2020
- Surface Reactivity of Ag-Modified Ceria to Hydrogen: A Combined Experimental and Theoretical Investigation
[Articolo su rivista]
Benedetti, S.; Righi, G.; Luches, P.; D'Addato, S.; Magri, R.; Selloni, A.
abstract
We investigate the mechanism of H2 activation on Ag-modified cerium oxide surfaces, of interest for different catalytic applications. The study is performed on thin epitaxial cerium oxide films, investigated by X-ray photoemission spectroscopy to assess the changes of both the Ag oxidation state and the concentration of Ce3+ ions, O vacancies, and hydroxyl groups on the surface during thermal reduction cycles in vacuum and under hydrogen exposure. The results are interpreted using density functional theory calculations to model pristine and Ag-modified ceria surfaces. Although the reactivity of ceria toward H2 oxidation improves when a fraction of Ce cations is substituted with Ag, the concentration of reduced Ce3+ ions in Ag-modified ceria is found to be lower than in pure ceria under the same conditions. This behavior is observed even though the number of surface oxygen vacancies caused by the thermal treatment under hydrogen exposure is larger for the Ag-modified surface. These results are explained in terms of a change of the oxidation state of the surface Ag, which is able to acquire some of the extra surface electrons created by the oxygen vacancies and the adsorbed hydrogen atoms. Our findings provide new insights into the reactivity of Ag-modified ceria, which has been proposed as a promising alternative to platinum electrodes in electrochemical devices.
2020
- Surface reducibility, reactivity, and stability induced by noble metal modifications on the γ -Fe2O3maghemite (001) surfaces
[Articolo su rivista]
Righi, G.; Magri, R.
abstract
In these last years large research efforts have been devoted to the synthesis and investigation of reducible metal oxide surfaces modified with metal atoms and nanoparticles for improving their performance in a number of advanced applications. Among reducible metal oxides, iron oxides have the advantage to be made up from two of the most common elements on Earth. In this paper we analyze the structural, electronic, and magnetic consequences of the insertion of isolated noble metal atoms (Cu, Ag, Au) on the γ-Fe2O3 (001) surface. We have considered many different configurations for the single atoms: adsorbed, substitutional to iron atoms, or to oxygen atoms, and, using first principles calculations, we have studied how the presence of the noble metal atoms on the surface influences the surface stability, its reducibility, and, therefore, its catalytic activity, and how these properties depend on the kind of noble metal atom and its position. Our results show that noble metal atoms adsorbed on the surface facilitate the adsorption of CO molecules, and, among them, Cu atoms are those that bind best to the surface also providing the strongest adsorption energy for the CO molecule. At ambient temperature and pressure the noble metal atoms prefer to substitute the iron atoms than to just adsorb on the surface, but for Ag atoms the adsorption and substitutional energies are very close. The surfaces with Ag in place of Fe are the most reducible and reactive for exchange of oxygen atoms. Finally, Au is the best noble metal for oxygen substitution. Our results provide useful insights for the researchers designing and synthesizing new noble metal - iron oxides nanostructures for applications in biology, medicine, catalysis, and chemical analysis.
2019
- Attractive interactions between like-oriented surface steps from an
ab initio
perspective: Role of the elastic and electrostatic contributions
[Articolo su rivista]
Righi, Giulia; Franchini, Anna; Magri, Rita
abstract
In this work, we show that by using the density functional approach, in which the electronic degrees of freedom are separated by the ionic ones, it is possible to individuate and separately study the elastic and electrostatic step interactions, traditionally introduced in the literature as the only two kinds of step interaction expected at T=0. We have applied the method to the technologically important GaAs(001) surface and found some unexpected results for the relatively short step distances accessible to the ab initio approaches, contradicting those of the continuum models so far employed for the study of the elastic step interactions: (i) the sign of the step interaction depends on the step termination and is due to the electrostatic interaction; (ii) the elastic interaction does not contribute to the step interaction, contrary to the common belief of a strong elastic repulsive interaction between like-oriented steps. We show that this is due to the electron behavior. When considering only ion displacements and pointlike steps as in the continuum theories, we recover the classical results and repulsive step elastic interactions; (iii) the experimentally observed Ab step termination shows a weakly attractive step interaction whereby attractive step interactions between like-oriented steps on an unstrained surface are believed not to exist. The proposed method of separating elastic and electrostatic interactions for further analysis of their dependence on the configurational degrees of freedom can be extended to other defective situations.
2019
- Band folding, strain, confinement, and surface relaxation effects on the electronic structure of GaAs and GaP: from bulk to nanowires
[Articolo su rivista]
dos Santos, Cláudia Lange; Piquini, Paulo; Magri, Rita
abstract
It is common to find materials that show strikingly different properties between its bulk and nanometric forms. In this paper we show how to link the electronic structures of two III-V systems, one a direct gap material, GaAs, and the other an indirect gap material, GaP, from their bulks right down to the shape of thin nanowires. The understanding of how these changes occur represents a scientific and technological challenge and is relevant for the design and prediction of novel nanostructured materials. GaAs and GaP bulk and nanowire systems are studied in the zinc-blende and wurtzite structures both free of strain and subjected to biaxial strains perpendicular to the [111]/[0001] direction (the kind of strain that the materials are subjected to when grown one on top of the other). We provide an interpretation of the band structure of nanowires, grown along the [111] (zinc-blende structure) and the [0001] (wurtzite structure) directions, in terms of the bulk band structures of the corresponding binary compounds. The procedure reveals the origin of the valence and conduction valleys relevant to determine the nature (direct or indirect) of the band gaps and the kind (direct and pseudodirect) of the valence to conduction transitions. Thus, by calculating only the bulk bands it is possible to describe the behavior of the nanowire bands even for very thin nanowires. The effects on the band structures due to biaxial strain are analogously analyzed, providing for bulk GaP the first results in literature. The role of confinement, and surface relaxation, in determining the nanowire electronic structure of thin nanowires are analyzed separately revealing that the change in the nature of the band gap is due mainly to surface relaxation effects, not confinement. We show that the change of the gap (indirect/direct) from the bulk to the 1D systems is mainly due to the competition between the energies of bulk conduction valleys which are differently influenced by confinement and strain. This effect is shown also by other low dimensional materials like the 2D materials extending only few atomic layers in one dimension. The competing valleys are already present in the bulk band structure. While the main effect of confinement is to open all gaps, it is not necessarily the main cause of the direct/indirect change in the nature of the electronic gap as instead is usually claimed in the literature. Our study can be used to understand and engineer the structure of many nanostructures systems by just better analysing the behavior of the bulk bands.
2019
- H2 Dissociation on Noble Metal Single Atom Catalysts Adsorbed on and Doped into CeO2 (111)
[Articolo su rivista]
Righi, Giulia; Magri, Rita; Selloni, Annabella
abstract
We used density functional theory (DFT) calculations to investigate the dissociation of H-2 on an Ag single atom catalyst adsorbed on the pristine CeO2 (111) surface (Ag/CeO2), or substituting a surface Ce atom on the reduced (Ag:CeO2-x) and partially hydrogenated (Ag:H-CeO2) surfaces. The initial state of the H-2 dissociation reaction in the different investigated models corresponds to distinct oxidation states, +1, +2, or +3, of the Ag atom, thus allowing us to examine the influence of the charge transfers between the noble metal, the oxide, and the hydrogen atoms on the reaction pathway and activation energy. In all investigated models, the computed barrier of H-2 dissociation is lowered by about 0.6 eV in comparison to that on metal-free CeO2. On Ag/CeO2 and Ag:CeO2-x, also the energy of H-2 dissociative adsorption is smaller than that on metal-free ceria. These results suggest that CeO2 modified with dispersed Ag atoms is a promising anode material for proton exchange membrane fuel cells. Further comparison of our results for Ag to analogous calculations for Cu and Au single atom catalysts reveals trends in the computed barriers that can be related to the change of the metal oxidation state in the reaction.
2019
- Reduction Properties of (001) Maghemite Surfaces
[Relazione in Atti di Convegno]
Righi, G.; Magri, R.
abstract
Using Density Functional Theory (DFT) calculations we have studied the reduction properties of the (001) maghemite (y - Fe 2 O 3 ) surface, which is a promising material for catalysis applications. We have calculated the formation energies of oxygen vacancies on the surface correlating them with the oxygen atom positions. We have found that the oxygen vacancy formation energy is lower for the oxygen atoms not bonded to the tetrahedral irons in the layer below the surface. Also, the presence of an iron vacancy in the third layer below the surface lowers the oxygen vacancy formation energy. An high oxygen vacancy formation energy correlates with a change in the oxidation state of some iron atoms.
2019
- Reduction and Oxidation of Maghemite (001) Surfaces: The Role of Iron Vacancies
[Articolo su rivista]
Righi, G.; Magri, R.
abstract
The knowledge of surface reduction and oxidation energetics in reducible oxides is essential for the design of improved catalysts for oxidation reactions. This is particularly true for iron oxides, a very attractive material system, because of the availability and biocompatibility of its constituents. In this work, by means of the density functional theory, we have thoroughly studied the ?-Fe2O3(001) maghemite surfaces, taking full account of iron vacancies beyond a mean field approach. Despite the structural similarity with the more studied magnetite Fe3O4 surfaces from which maghemite differs only for the presence of iron vacancies in the octahedral sites and for the absence of reduced Fe2+ cations, the surface properties are quite different. Our investigation shows that the presence of Fe vacancies is responsible for an increase in the surface reducibility. Also, it favors surface oxidation. The main reason is that the Fe vacancies cause a decrease of electronic charge of the surface oxygen atoms, which then become more reactive. We have considered different surface terminations and found that the reduced surfaces are more stable than the simple bulk-truncated ones. The reduction leads to a new reconstruction of the surface, which is the most stable surface termination among those investigated. We have examined the charge transfers and the modifications in the electronic structure caused by the surface reduction.
2018
- Metal-support interaction in catalysis: The influence of the morphology of a nano-oxide domain on catalytic activity
[Articolo su rivista]
Najafishirtari, Sharif; Guglieri, Clara; Marras, Sergio; Scarpellini, Alice; Brescia, Rosaria; Prato, Mirko; Righi, Giulia; Franchini, Anna; Magri, Rita; Manna, Liberato; Colombo, Massimo
abstract
Using wet chemistry synthesis methods we prepared nanodumbbell structures as a model oxide supported metal catalyst. In this peculiar configuration, a single metallic domain (M) is connected to a single metal oxide (MOx) one. The size, composition and morphology of each domain can be carefully controlled, allowing us to investigate the effects resulting from a hollow morphology of the MOx domains, while all other material’s properties were kept constant. We chose the CO oxidation as a model oxidation reaction and increasing the population of nanocrystals (NCs) with hollow oxide domains resulted in a decrease in catalytic activity. Despite the manipulation of oxide morphology affected the surface charge of the Au domain, the bulk oxide reducibility and the crystallinity of the nanosized oxide support, the rate limiting step of CO oxidation was not affected. The same apparent activation energy was indeed measured independently from the population of NCs with hollow oxide domains. The difference in catalytic performance was thus ascribed to a different number of interfacial active sites when the morphology evolved from full to hollow.
2018
- Si and Ge based metallic core/shell nanowires for nano-electronic device applications
[Articolo su rivista]
Bhuyan, Prabal Dev; Kumar, Ashok; Sonvane, Yogesh; Gajjar, P. N.; Magri, Rita; Gupta, SANJEEV KUMAR
abstract
One dimensional heterostructure nanowires (NWs) have attracted a large attention due to the possibility of easily tuning their energy gap, a useful property for application to next generation electronic devices. In this work, we propose new core/shell NW systems where Ge and Si shells are built around very thin As and Sb cores. The modification in the electronic properties arises due to the induced compressive strain experienced by the metal core region which is attributed to the lattice-mismatch with the shell region. As/Ge and As/Si nanowires undergo a semiconducting-to-metal transition on increasing the diameter of the shell. The current-voltage (I-V) characteristics of the nanowires show a negative differential conductance (NDC) effect for small diameters that could lead to their application in atomic scale device(s) for fast switching. In addition, an ohmic behavior and upto 300% increment of the current value is achieved on just doubling the shell region. The resistivity of nanowires decreases with the increase in diameter. These characteristics make these NWs suitable candidates for application as electron connectors in nanoelectronic devices.
2017
- Strain-engineered arrays of InAs quantum dots on GaAs(001): Epitaxial growth and modeling
[Articolo su rivista]
Latini, V.; Placidi, E.; Magri, R.; Tisbi, E.; Patella, F.; Arciprete, F.
abstract
Working under critical conditions for dot nucleation in a Molecular Beam Epitaxy chamber, we were able to drive the formation of InAs dot chains to precise locations in multilayered samples grown on a rippled GaAs(001) surface. We discussed the role of the elastic field and the surface curvature in determining the dot arrangement at each stacked layer, proving a new mechanism of self-organization of the dots. In particular, we succeeded in controlling the interplay between elastic and curvature effects and we showed how a selection process is achievable in the chain formation. The role of the stress field was also studied by means of Finite Element Method simulations, and we gained a valuable understanding of the interlayer dot correlations for dot arrays with variable cap thicknesses. We proved the existence of an anisotropy in the cap formation of isolated dots, which appeared to be directly related to our peculiar growth geometry and experimental set-up.
2016
- Erratum: Step energy and step interactions on the reconstructed GaAs(001) surface (Physical Review B - Condensed Matter and Materials Physics (2014) 90 (115314) DOI: 10.1103/PhysRevB.90.115314)
[Articolo su rivista]
Magri, Rita; Gupta, SANJEEV KUMAR; Rosini, Marcello
abstract
We report here the corrected values for the step energies and the step interactions calculated in our paper for a large set of different step configurations. We have found that one parameter, i.e., the smearing parameter governing the band occupation around the Fermi level, was not converged enough to predict the step properties with sufficient accuracy. Since the surfaces are metallic a smearing function had to be used for the state occupation around the Fermi level. We used the smearing function proposed by Marzari and Vanderbilt. We found that the parameter entering this expression is a particularly sensitive one. The previous paper used a commonly chosen value of 0.02 Ry. However, we found that a much smaller value for this parameter was necessary to obtain well converged values for the surface energies. The convergence of the smearing parameter has been carefully checked. Figure presented.
2016
- Stress-determined nucleation sites above GaAs-capped arrays of InAs quantum dots
[Articolo su rivista]
Latini, V.; Placidi, E.; Arciprete, F.; Tisbi, E.; Patella, F.; Magri, Rita
abstract
We studied the stress field at the surface of GaAs capping layers of variable thicknesses burying
InAs quantum dot arrays using the Finite Element method to solve numerically the equations of the
elastic field. The aim is to determine the stress-determined favorable sites for dot nucleation. We
show that: (i) depending on the cap thickness, dot distances, and array orientation, sudden transitions
in the stress-strain fields occur, leading from a vertical alignment of the dots to an antialigned
correlation. We find that just few determined positions are favorable for dot nucleation and
exclude some other sites previously indicated as favorable in the literature; (ii) the critical thicknesses
at which the switch between the vertical alignment and the anti-aligned positions occurs
depend on the distance between the dots in a square array and on the ratio between the two different
distances if the arrays are rectangular; (iii) the transitions occur within a few nanometer range of
the capping layer thickness, and the elastic field undergoes large changes in its properties before
and after the transition. This behavior has been revealed by a very accurate fit of the tangential
stress field using appropriate fit functions. The fit and parameter functions allow to easily reproduce
the stress field in different contexts and are useful in growth simulation models. The results suggest
that by properly engineering the capping layer thicknesses in the layers of a stack, it is possible
to obtain different three-dimensional quantum dot lattices starting from an initial fixed dot array.
Our results are in agreement with the available experimental data.
2014
- Manipulating surface diffusion and elastic interactions to obtain quantum dot multilayer arrangements over different length scales
[Articolo su rivista]
E., Placidi; F., Arciprete; V., Latini; S., Latini; Magri, Rita; M., Scuderi; G., Nicotra; F., Patella
abstract
An innovative multilayer growth of InAs quantum dots on GaAs(100) is demonstrated to lead to self-aggregation of correlated quantum dot chains over mesoscopic distances. The fundamental idea is that at critical growth conditions is possible to drive the dot nucleation only at precise locations corresponding to the local minima of the Indium chemical potential. Differently from the known dot multilayers, where nucleation of new dots on top of the buried ones is driven by the surface strain originating from the dots below, here the spatial correlations and nucleation of additional dots are mostly dictated by a self-engineering of the surface occurring during the growth, close to the critical conditions for dot formation under the fixed oblique direction of the incoming As flux, that drives the In surface diffusion.
2014
- Role of as in the anisotropic positioning of self-assembled InAs quantum dots
[Relazione in Atti di Convegno]
Arciprete, F.; Placidi, E.; Magri, R.; Fanfoni, M.; Balzarotti, A.; Patella, F.
abstract
Progress in tailoring the size, shape and positioning of Quantum Dots on the substrate is crucial for their potential applications in new optoelectronic devices for nano-photonics as well as in quantum information and computation. Using Molecular Beam Epitaxy in pulsed deposition mode we demonstrate that the nucleation of InAs Quantum Dots can be selectively guided on the GaAs(001) surface by a suitable choice of the kinetic parameters for the growth of both the GaAs buffer layer and the InAs Quantum Dots. By developing a two-species rate-equation kinetic model we show that the positioning of the Quantum Dots on only one side of mounds of the GaAs buffer can be traced back to the very small As flux gradient between the two mound slopes (ΔFA/F A ≈1-5%) caused by the proper tilting of the incoming As flux. Such gradient originates, at the relatively high growth-temperature, a net cation flow from one slope of the mound to the other that is responsible for the selective growth. © 2013 Materials Research Society.
2014
- Selective growth of InAs quantum dots on GaAs driven by as kinetics
[Articolo su rivista]
Magri, Rita; E., Placidi; F., Arciprete; F., Patella
abstract
n this paper we examine experimentally and theoretically how the As flux direction and intensity influences the recently observed selective growth of InAs quantum dots on the rippled surface of GaAs, where the dots line up only against well determined slopes of the mounded GaAs surface facing the impinging Arsenic flux. We have previously shown that the observed phenomenon is exclusively related to the As constituent, challenging the widespread belief that As plays only a minor role in the dot formation. The selective growth is obtained by Molecular Beam Epitaxy at high growth temperature and under a high As/In flux ratio. To interpret the experimental results, we use a newly developed kinetic model incorporating new features: (i) anions are explicitly considered; (ii) cations and anions follow a different kinetics; (iii) the dot surface is distinguished by the dot bulk and atoms are transferred back and forth between the two phases depending on the surface interaction with the environment. We find that even a very small difference in the As flux intensity impinging over two substrate regions produces a cation current flow from one region to the other, so that the dots form only on the region exposed to the largest flux.
2014
- Step energy and step interactions on the reconstructed GaAs(001) surface
[Articolo su rivista]
Magri, Rita; Sanjeev K., Gupta; Rosini, Marcello
abstract
Using ab initio total energy calculations we have studied the relation between the step atomic configuration and its properties (step energy, donor/acceptor behavior, and step interaction) on a beta(2)(2 x 4) reconstructed GaAs (001) surface. The results have been tested against the widely used elastic dipole model for the step energy and step interaction considered valid for stress-free surfaces. We have found that acceptor-behaving steps have an attractive interaction and donor-behaving steps have a repulsive interaction in contrast with the elastic dipole model which predicts always a repulsive interaction between like-oriented steps. To account for the attractive interaction we consider the electrostatic dipole interaction having the L-2 scaling with the step distance L and therefore compatible with the standard elastic model. Using a model charge distribution with localized point charges at the step based on the electron counting model we show that the electrostatic step interaction can indeed be generally attractive and of the same order of magnitude of the negative elastic dipole interaction. Our results show however that the usually employed dipole model is unable to account for the repulsive/attractive step interaction between donorlike/acceptorlike steps. Therefore, the ab initio results suggest an important electronic contribution to the step interaction, at least at the short step distances accessible to the first-principles study. Our results explain qualitatively many experimental observations and provide an explanation to the step bunching phenomenon on GaAs(001) induced by doping or by critical growth conditions as due to the stabilization of attractively interacting step structures. These ideas would lead to the development of a bottom-up surface step engineering.
2013
- Kinetically driven selective growth of InAs quantum dots on GaAs
[Articolo su rivista]
Fabrizio, Arciprete; Ernesto, Placidi; Magri, Rita; Davide Del, Gaudio; Fulvia, Patella
abstract
We show that, by changing and tuning the direction of the As flux on a rippled substrate, at temperatures higher than 530 degrees C and high As/In flux ratio, a selective growth of InAs dots can be obtained on GaAs. This is an undisclosed effect related to the Arsenic flux in the molecular beam epitaxial growth of InAs quantum dots (QDs) on GaAs(001). This effect cannot be explained by a shadowing effect, due to the gentle slopes of the mounds (1-3 degrees), and reveals instead that As plays a fundamental role at these growth conditions. We have developed a kinetic model, which takes into account the coupling between cations and anions, and found that the very small surface gradient in the anion flux, due to the oblique evaporation on the mounded surface, is responsible for a massive drain of cations toward the surface anion-rich areas, thus generating the selective growth of QDs.
2013
- The Unexpected Role of Arsenic in Driving the Selective Growth of InAs Quantum Dots on GaAs
[Articolo su rivista]
F., Arciprete; E., Placidi; Magri, Rita; M., Fanfoni; A., Balzarotti; F., Patella
abstract
Here we show a new effect due to the arsenic flux in the molecular beam epitaxy growth of InAs quantum dots on GaAs(001) at temperatures higher than 500 degrees C and high As/In flux ratio. We show that, by changing and tuning the direction of the As flux on a rippled substrate, a selective growth can be obtained where the dots form only on some appropriately orientated slopes of a sequence of mounds elongated along the ((1) over bar 10) surface direction. Since the relative As flux intensity difference over the two opposite mound slopes is very small (2-5%), the observed large effect cannot be explained simply as a pure shadowing effect and reveals instead that As, whose contribution to the modeling of growth has often been ignored or underestimated, probably for a lack of knowledge, plays a fundamental role at these growth conditions. To explain our experiment, we have developed a kinetic model that explicitly takes into account the coupling between cations (In) and anions (As) and found that the very small surface gradient in the anion flux, due to the oblique evaporation on the mounded surface, is responsible for a massive drain of cations toward the surface anion-rich areas, thus generating the selective growth of quantum dots. We expect a comparable behavior for the anions of other III-V and II-VI compound semiconductors.
2012
- InAs Epitaxy on GaAs(001): A Model Case of Strain-Driven Self-Assembling of Qunatum Dots
[Capitolo/Saggio]
E., Placidi; F., Arciprete; Magri, Rita; Rosini, Marcello; A., Vinattieri; L., Cavigli; M., Gurioli; E., Giovine; L., Persichetti; M., Fanfoni; F., Patella; A., Balzarotti
abstract
We review basic topics of self-aggregation process of InAs quantum dots on the GaAs(001) surface with reference to our recent experimental and theoretical studies. Atomic-force and scanneling-tunneling microscopy, and reflection high-energy electron diffraction measurements are presented for discussing issues such as formation and composition of the wetting layer, evolution of the 2D to 3D transition, size distribution and equilibrium shape of the islands. Single-dot emission is demonstrated by micro-photoluminescence spectra of samples where quantum dots were confined on selected nanoscale areas of the surface by molecular-beam epitaxial growth on lithographed substrates. Theoretical ab-initio studies of the In diffusion on the wetting layer, and simulations with the finite element method of the elastic energy relaxation of the island-substrate system are also discussed.
2011
- Role of surface structural motifs on the stability and reflectance anisotropy spectra of Sb-rich GaSb(001) reconstructions
[Articolo su rivista]
C., Hogan; Magri, Rita; R., Del Sole
abstract
The structure of the technologically important-but still mostly unknown-GaSb(001)-c(2 x 6) surface reconstruction is investigated by means of ab initio simulations of reflectance anisotropy spectroscopy (RAS) and total energy calculations. A large number of reconstruction models for the GaSb(001) surface in the Sb-rich coverage regime are considered. The influence of each single surface structural motif on the RAS spectra is studied in detail, as well as their role in the surface stability with regard to application of the electron counting rule (ECR). We interpret the features of the RAS data measured for this reconstruction and suggest a new model for the c(2 x 6) phase. In this model a few Sb atoms in the second layer are randomly substituted by Ga, forming surface antisite defects. When used to fulfill the ECR, this "doping" effect considerably lowers the total energy of the long chain c(2 x 6) reconstruction model, making it competitive with the more stable short-chain (4 x 3) reconstructions. Formation of the surface antisites occurs spontaneously in the presence of dynamical negative charge fluctuations and is favored by the excellent matching between GaSb(001) and metallic Sb and by the natural softness of the Ga-Sb bonds. Calculations of the reflectance anisotropy spectra confirm that this structure is a major component of a largely disordered surface, where motifs of the stable (4 x 3) reconstructions are also present.
2010
- Effects of wetting layer structure on surface phase stability and on indium surface diffusion
[Relazione in Atti di Convegno]
Rosini, Marcello; P., Kratzer; Magri, Rita
abstract
We study the effects of surface reconstruction and step formation on the surface phase stability, of an InAs wetting layer on GaAs(001). In particular we focus our attention on the α2 and β2 (2 × 4) surface reconstructions. The two investigated reconstructions have been shown to be formed at an high In coverage, at the onset of the 2D→3D transition. The analysis of the connection between the step stability and the strain distribution around the step edges leads to the conclusion that the favoured step geometries are those minimising the strain. Finally, In diffusion on the flat reconstructed wetting layers has been investigated.We find: (i) the elements of the surface reconstructions favouring In diffusion; (ii) that In diffusion on these surfaces is strongly anisotropic, favoring the [-110] direction; (iii) that the As surface dimers introduce additional adsorption sites with high barriers for In escape.
2010
- In adsorption and diffusion on in-rich (2×4) reconstructed InGaAs surfaces on GaAs(001)
[Relazione in Atti di Convegno]
Rosini, Marcello; Righi, Maria Clelia; Kratzer, Peter; Magri, Rita
abstract
We investigate the potential energy surface (PES) and the adsorption properties of an In adatom on InAs (2×4) reconstructed wetting layers (WLs) deposited on a GaAs substrate. The results are then used to derive the diffusion properties of a single In adatom on the WLs. We find that: (i) the adsorbate diffusion is highly anisotropic; (ii) the adsorption sites within the As dimers have to be taken into account since they strongly affect diffusion; (iii) the most stable adsorption sites are the ones within the dimers and those located besides the in-dimers.
2010
- Optical properties of GaSb(001)--c(2x6): the role of surface antisite defects
[Articolo su rivista]
C., Hogan; Magri, Rita; R., Del Sole
abstract
We consider the formation of surface antisite defects on a previously proposed model for the GaSb(001)-c(2 x 6) surface. Based on ab initio total energy calculations, we show how these defects stabilize the otherwise metallic surface and how their formation is driven by the excess charge associated with the Sb-rich surface conditions. The surface-sensitive optical technique of reflectance anisotropy spectroscopy is shown to be crucial for detecting the defects, and computation of spectra yields a good agreement with experiment when defects are included in the surface reconstruction.
2010
- Spontaneous Formation of Surface Antisite Defects in the Stabilization of the Sb-Rich GaSb(001) Surface
[Articolo su rivista]
C., Hogan; Magri, Rita; R., Del Sole
abstract
This Letter solves the long-standing puzzle [Phys. Rev. Lett. 79, 693 (1997)] of why GaSb(001) apparently violates the electron counting rule (ECR) in forming a reconstruction featuring long Sb-dimer chains, rather than the c(4 x 4) reconstruction found in all other arsenide and antimonide III-V compounds in the V-rich regime. We find that an alternative strategy, that in fact satisfies the ECR, is followed by the Sb-rich GaSb(001) surface, whereby long Sb-dimer chains are stabilized by randomly distributed subsurface Ga antisite defects. The excess of surface Sb drives the defect formation that in turn stabilizes the surface in a metastable phase. The transition to the c(4 x 4) reconstruction, where the ECR is instead satisfied through missing dimers, is therefore inhibited. Our conclusions are supported by ab initio simulations of experimental reflectance anisotropy spectra.
2010
- Structural stability of clean GaAs nanowires grown along the [111] direction
[Relazione in Atti di Convegno]
Magri, Rita; Rosini, Marcello; F., Casetta
abstract
Using a first-principles approach we have calculated the formation energies of small diameter GaAs nanowires (NWs) with both zinc-blende and wurtzite structure grown along the [111] direction. The section of the wires is hexagonal and the side facets are oriented either {11-20} and {10-10} in the case of the wurtzite structure, and {110} and {112} for the zinc-blende structure. The formation energy of the nanowires as a function of their radius is then interpreted in terms of a model in which the energy contributions from the bulks, the flat surfaces and the ridges are taken explicitly into account. We find that the nanowire stability is mainly explained by the competition between the bulk energy, favoring the zincblende structure and the surface energies favoring the wurtzite structure. We find also that the directly calculated formation energies of some small diameter wurtzite NWs can be reproduced by our model taking into account only the bulk and flat surface contributions. That is, the ridges do not contribute substantially to the nanowire formation energy. Inspection of the ridge structure and band structure reveals that this good agreement occurs when the NWs are semiconducting and the ridges do not add more dangling bonds to the surface with respect to those provided by the sidewalls. Within our model we find the critical diameter for the wurtzite-zinc-blende transition at 6.3 nm.
2010
- Structure and optical properties of the Sb-stabilized GaSb(001) surface
[Relazione in Atti di Convegno]
C., Hogan; Magri, Rita; R., Del Sole
abstract
We suggest a model for the c(2 x 6) phase of the Sbstabilized GaSb(001) surface, whereby Sb atoms in the second layer are partially substituted by Ga. This ‘doping’ effect enables the surface to fulfil the electron counting rule whilst maintaining a reconstruction based on long dimer chains.Total energy calculations verify that the suggested reconstruction is relatively stable, and calculations of the reflectance anisotropy spectra confirm that it comprises a major component of the largely disordered surface
2010
- Surface Effects on the Atomic and Electronic Structure of Unpassivated GaAs Nanowires
[Articolo su rivista]
Rosini, Marcello; Magri, Rita
abstract
On the basis of accurate ab initio calculations, we propose a model for predicting the stability of III−V nanowires (NW) having different side walls and ridge configurations. The model allows us to obtain the NW formation energies by performing calculations only on relatively “small” systems, small diameter NWs and flat surfaces, to extract the contributions to the stability of each structural motif. Despite the idea illustrated here for the case of hexagonally shaped GaAs NWs grown along the [111]/[0001] direction, the method can also be applied generally to other differently shaped and oriented III−V NWs. The model shows that NW surfaces (side walls plus ridges) mainly determine the NW stability, so the changes to the surface structure (e.g., induced by defects or different growth conditions) would modify the final NW structure in a remarkable way. We find that wurtzite and zinc blende nanowires have similar energies over a wide diameter range, thus explaining the observed polytypism. Furthermore, new more stable ridge reconstructions are proposed for zinc blende nanowires. The surface-related structural motifs also have clear fingerprints on the NW electronic structure. We find that the more stable nanowires are all semiconducting. The band gaps are ruled by surface states and do not follow the trend mandated by the quantum confinement effect. Small diameter wurtzite nanowires have an indirect band gap, but for some of them, an indirect to direct transition can be foreseen to occur at larger diameters. Surface states have a larger impact on the zinc blende NW band gaps than on the wurtzite NW ones. Zinc blende nanowire band gaps reduce significantly with increasing nanowire radius, reaching the bulk value at a diameter of about 30 Å. The surface structure and the high surface related DOS below the conduction band are going to affect the nanowire dopant incorporation and efficiency when doping is carried out during the NW growth.
2010
- Surface compositional profiles of self-assembled InAs/GaAs quantum rings
[Relazione in Atti di Convegno]
Magri, Rita; S., Heun; G., Biasiol; A., Locatelli; T. O., Mentes; L., Sorba
abstract
The surface composition profiles of self-assembled InAs/GaAs quantum rings (QR) are studied both experimentally and theoretically. By using X-ray Photoemission Electron Microscopy (XPEEM) we obtain a 2D composition mapping of unburied rings, which can be directly related to the QR topography measured by Atomic Force Microscopy (AFM). Top-surface composition mapping allows us to obtain information on structures which cannot be directly accessed with cross-sectional studies since overgrowing the QRs with a thick GaAs film alters both their morphology and composition. The 2D surface maps reveal a non-uniform distribution across the rings with an In richer InGaAs alloy in the central hole regions. Elastic energy calculations via a Valence Force Field (VFF) approach show that, for a given shape of the rings and a fixed total number of Ga and In atoms, an In enrichment of the alloy in the central hole region, together with an In enrichment of the surface layers, leads to a lowering of the total strain energy.
2009
- Ab-initio calculations of luminescence and optical gain properties in silicon nanostructures
[Articolo su rivista]
Degoli, Elena; Guerra, Roberto; Iori, Federico; Magri, Rita; Marri, Ivan; O., Pulci; Bisi, Olmes; Ossicini, Stefano
abstract
Density-functional and many body perturbation theory calculations have been carried out in order to study the optical properties both in the ground and excited state configurations, of silicon nanocrystals in different conditions of surface passivation. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular, we show that only the presence of a surface Si–O–Si bridge bond induces an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible PL experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO2 matrix, the optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. For each system considered, optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc.
2009
- In adatom diffusion on InxGa1-xAs/GaAs(001): effects of strain, reconstruction and composition
[Articolo su rivista]
Rosini, Marcello; Kratzer, P; Magri, Rita
abstract
By using density functional theory (DFT) calculations of the potential energy surface in conjunction with the analytical solution of the master equation for the time evolution of the adatom site distribution, we study the diffusion properties of an isolated In adatom on InxGa1-xAs wetting layers (WL) deposited on the GaAs(001). The WL reconstructions considered in this study are, listed in the order of increasing In coverage: c(4 x 4), (1 x 3), (2 x 3), alpha(2)(2 x 4) and beta(2)(2 x 4). We analyze the dependence of the diffusion properties on WL reconstruction, composition and strain, and find that: (i) diffusion on the (2 x N) reconstructions is strongly anisotropic, owing to the presence of the low barrier potential in-dimer trench, favoring the diffusion along the [(1) over bar 10] direction over that along the [110] direction; (ii) In diffusion at a WL coverage theta = 2/3 monolayers (ML; with composition x = 2/3) is faster than on clean GaAs(001) c(4 x 4), and decreases at theta = 1.75 ML (x = 1; e. g. InAs/GaAs(001)); (iii) diffusion and nucleation on the (2 x 4) WL is affected by the presence of adsorption sites for indium inside the As dimers; (iv) the approximation used for the exchange-correlation potential within DFT has an important effect on the description of the diffusion properties.
2009
- Indium surface diffusion on InAs (2x4) reconstructed wetting layers on GaAs(001)
[Articolo su rivista]
Rosini, Marcello; Righi, Maria Clelia; P., Kratzer; Magri, Rita
abstract
In this paper we present a study of In surface diffusion on InAs wetting layers deposited on the (001) surface of GaAs. The alpha2(2×4) and beta2(2×4) reconstructions stabilized by a high In concentration are considered. The low symmetry of the alpha2(2×4) reconstruction allowed us to understand the effect of the wetting-layer symmetry on the adsorbate diffusion. We find that (i) the diffusion coefficient value is larger for In motion on the alpha2 reconstruction than on the beta2 reconstruction. This is due to the presence on beta2 of an additional As dimer that rises locally the potential energy surface and offer an additional site to which the In adatom can bind strongly. (ii) The In adsorption sites located within the As dimers have to be taken into account properly for these specific reconstructions, since they greatly affect the value of the diffusion coefficient. This is in contrast to what happens for the other reconstructions reported in the literature. (iii) The adsorbate diffusion is highly anisotropic with the [[overline 1]10] direction favored over the [110] direction, due to the presence of low-potential channels along [[overline 1]10]. (iv) The anisotropy is slightly smaller on the alpha2 reconstruction than on the beta2 reconstruction because on the beta2 there is an additional diffusion channel along the [[overline 1]10] direction.
2009
- Optical properties of silicon nanocrystallites in SiO2 matrix:Crystalline vs. amorphous case
[Articolo su rivista]
Guerra, Roberto; Marri, Ivan; Magri, Rita; L., Martin Samos; O., Pulci; Degoli, Elena; Ossicini, Stefano
abstract
Within a first-principles framework we show the dependenceof the optical properties of silicon nanocrystallites embeddedin a silica matrix on the crystalline vs. amorphous order ofthe system. Moreover we calculate how many-body effectsmodify the electronic and optical properties of the embeddedsilicon nanodots. A discussion about the different roles playedby dimensionality, interface properties, disorder and excitoniceffects on the electronic and optical properties of the confinednanostructures is presented.
2009
- Silicon nanocrystallites in a SiO2 matrix: Role of disorder and size
[Articolo su rivista]
Guerra, Roberto; Marri, Ivan; Magri, Rita; L., Martin Samos; O., Pulci; Degoli, Elena; Ossicini, Stefano
abstract
We compare, through first-principles pseudopotential calculations, the structural, electronic, and optical properties of different size silicon nanoclusters embedded in a SiO2 crystalline or amorphous matrix with that of freestanding, hydrogenated, and hydroxided silicon nanoclusters of corresponding size and shape. We findthat the largest effect on the optoelectronic behavior is due to the amorphization of the embedded nanocluster. In that, the amorphization reduces the fundamental gap while increasing the absorption strength in the visible range. Increasing the nanocluster size does not change substantially this picture but only leads to the reduction in the absorption threshold, following the quantum confinement rule. Finally, through the calculation of the optical absorption spectra both in an independent-particle and a many-body approach, we show that the effect of local fields is crucial for describing properly the optical behavior of the crystalline case while it is of minor importance for amorphous systems.
2009
- Surface compositional mapping of self-assembled InAs/GaAs quantum rings
[Articolo su rivista]
Biasiol, G; Magri, Rita; Heun, S; Locatelli, A; Mentes, To; Sorba, L.
abstract
The composition profile of self-assembled InAs/GaAs quantum rings is investigated both experimentally and theoretically. Our study is aimed at obtaining information on unburied rings, which cannot be directly accessed in cross-sectional analysis. Two-dimensional surface chemical maps obtained by X-ray photoemission electron microscopy reveal a non-uniform composition profile with a double structure composed of an In-rich core, corresponding to the central hole of the ring, surrounded by a rim with a stronger In-Ga intermixing. These results are substantiated by an atomistic model which, for a given shape, identifies the composition distribution that minimizes the elastic energy of the system, The good agreement between experiment and theory allows us to identify the minimization of strain energy as the main driving force for the formation of quantum rings.
2008
- Adsorption of Indium on an InAs wetting layer deposited on the GaAs(001) surface
[Articolo su rivista]
Rosini, Marcello; Magri, Rita; AND KRATZER, P.
abstract
n this work, we perform a first-principles study of the adsorption properties of an In adatom deposited on a 1.75 ML( monolayer) InAs, forming a wetting layer on GaAs(001) with the alpha2(2×4) or beta2(2×4) reconstruction. Thus, we are addressing here the important case of the wetting layer at a high In coverage, just before the occurrence of the two-dimensional to three-dimensional transition. The structural properties of these reconstructions have been studied: we determine the equilibrium geometry of the surfaces and their stability for various growth conditions. We have then carried out a detailed study of the potential energy surface (PES) for an In adsorbate, finding the minima and the saddle points. The main characteristics of the PES and the bonding configurations of the In adatom on the surface are analyzed by comparing them with analogous studies reported in the literature, trying to extract the effects due to (i) the compressive strain to which the InAs adlayer is subjected, (ii) the particular surface reconstruction, and (iii) the wetting layer composition. We found that, in general, stable adsorption sites are located at (i) locations beside the As in-dimers, (ii) positions bridging two As in-dimers, (iii) between two adjacent ad-dimers (only in beta2), and (iv) locations bridging two As ad-dimers. We find also other shallower adsorption sites which are more reconstruction specific due to the lower symmetry of the alpha2 reconstruction compared to the beta2 reconstruction. We point out that the alpha2 reconstruction has the lowest symmetry among the reconstructions studied so far for this system.
2008
- Electronic structure of self-assembled InAs/InP quantum dots: Comparison with self-assembled InAs/GaAs quantum dots
[Articolo su rivista]
M., Gong; K., Duan; C. F., Li; Magri, Rita; G. A., Narvaez; L., He
abstract
We investigate the electronic structure of the InAs/InP quantum dots using an atomistic pseudopotential method and compare it to that of the InAs/GaAs quantum dots (QDs). We show that even though the InAs/InP and InAs/GaAs dots have the same dot material, their electronic structures differ significantly in certain aspects, especially for holes: (i) The hole levels have a much larger energy spacing in the InAs/InP dots than in the InAs/GaAs dots of corresponding size. (ii) Furthermore, in contrast with the InAs/GaAs dots, where the sizable hole p, d intrashell level splitting smashes the energy level shell structure, the InAs/InP QDs have a well defined energy level shell structure with small p, d level splitting, for holes. (iii) The fundamental exciton energies of the InAs/InP dots are calculated to be around 0.8 eV (~1.55 µm), about 200 meV lower than those of typical InAs/GaAs QDs, mainly due to the smaller lattice mismatch in the InAs/InP dots. (iv) The widths of the exciton P shell and D shell are much narrower in the InAs/InP dots than in the InAs/GaAs dots. (v) The InAs/GaAs and InAs/InP dots have a reversed light polarization anisotropy along the [100] and [1[overline 1]0] directions.
2008
- First-Principles Study of Silicon Nanocrystals: Structural and Electronic Properties, Absorption, Emission, and Doping.
[Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes; Degoli, Elena; Marri, Ivan; F., Iori; E., Luppi; Magri, Rita; R., Poli; G., Cantele; D., Ninno; F., Trani; M., Marsili; O., Pulci; V., Olevano; M., Gatti; K., GAAL NAGY; A., Incze; G., Onida
abstract
Total energy calculations within the Density Functional Theory have been carried out in order to investigate the structural, electronic, and optical properties of un-doped and doped silicon nanostructures of different size and different surface terminations. In particular the effects induced by the creation of an electron-hole pair on the properties of hydrogenated silicon nanoclusters as a function of dimension are discussed in detail showing the strong interplay between the structural andoptical properties of the system. The distortion induced on the structure by an electronic excitation of the cluster is analyzed and considered in the evaluation of the Stokes shift between absorption and emission energies. Besides we show how many-body effects crucially modify the absorption and emission spectra of the silicon nanocrystals. Starting from the hydrogenated clusters, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si-O-Si bridge bond originates significative excitonic luminescence features in the near-visible range. Concerning the doping, we consider B and P single- and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced,favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals showing the possibility ofan impurity based engineering of the absorption and luminescence properties of Si nanocrystals.
2008
- Pseudopotential calculations of band gaps and band edges of short-period (InAs)n (GaSb)m superlattices with different substrates, layer orientations, and interfacial bonds
[Articolo su rivista]
Piquini, P; Zunger, A; Magri, Rita
abstract
The band edges and band gaps of (InAs)n/(GaSb)m (n,m=1,20) superlattices have been theoretically studied through the plane-wave empirical pseudopotential method for different situations: (i) different substrates, GaSb and InAs; (ii) different point group symmetries, C2v and D2d; and (iii) different growth directions, (001) and (110). We find that (a) the band gaps for the (001) C2v superlattices on a GaSb substrate exhibit a nonmonotonic behavior as a function of the GaSb barrier thickness when the number of (InAs)n layers exceed n=5; (b) substrate effects: compared with the GaSb substrate, the different strain field generated by the InAs substrate leads to a larger variation of the band gaps for the (001) C2v superlattices as a function of the InAs well thickness; (c) effect of the type of interfacial bonds: the In-Sb bonds at the interfaces of the (001) D2d superlattices partially pin the band edge states, reducing the influence of the confinement effects on electrons and holes, and lowering the band gaps as compared to the (001) C2v case. The valence band maximum of the (001) D2d superlattices with Ga-As bonds at the interfaces are shifted down, increasing the band gaps as compared to the (001) C2v case; (d) effect of layer orientation: the presence of In-Sb bonds at both interfaces of the (110) superlattices pin the band edge states and reduces the band gaps, as compared to the (001) C2v case. An anticrossing between the electron and hole levels in the (110) superlattices, for thin GaSb and thick InAs layers, leads to an increase of the band gaps, as a function of the InAs thickness; (e) superlattices vs random alloys: the comparison between the band edges and band gaps of the superlattices on a GaSb substrate and those for random alloys, lattice matched to a GaSb substrate, as a function of the In composition, shows that the random alloys present almost always higher band gaps and give a clear indication of the effect of superlattice's ordering and period on the behavior of the band gaps and band edges. Inclusion of interfacial interdiffusion, using the approach of Magri and Zunger [Phys. Rev. B 65, 165302 (2002)], is shown to significantly increase the band gaps relative to the predictions for abrupt superlattices, bringing the results closer to experiment. It is noteworthy that k·p model fit instead measured gaps corresponding to interdiffused interfaces using a chemically abrupt model.
2008
- THEORETICAL STUDIES OF ABSORPTION, EMISSION AND GAIN IN SILICON NANOSTRUCTURES
[Capitolo/Saggio]
Degoli, Elena; Guerra, Roberto; Iori, Federico; Magri, Rita; Marri, Ivan; Ossicini, Stefano
abstract
Density-functional and many body perturbation theory calculations have been carried out in order to study the structural, electronic, and opti- cal properties both in ground and excited state configuration, of silicon nanocrystals in different conditions of surface passivation and doping. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidences that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the phys- ical properties of these systems. In particular we show that only the presence of a surface Si-O-Si bridge bond induce an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible photoluminescence (PL) experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO2 matrix, the electronic and optical properties are discussed in detail. The strong in- terplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. Finally, concerning doping, we will show that, thanks to electronic transitions between donor and acceptor states present at the band edges and considering also the effect of quantum confinement it is possible to engineer the absorp- tion and emission spectra of Si nanocrystals. For each considered system optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc.
2007
- Codoping goes Nano: Structural and Optical Properties of Boron and Phosphorus Codoped Silicon Nanocrystals
[Relazione in Atti di Convegno]
Magri, Rita; Iori, Federico; Degoli, Elena; O., Pulci; Ossicini, Stefano
abstract
Doping control at the nanoscale can be used to modify optical and electronic properties thus inducing interesting effects that cannot be observed in pure systems. By using Density Functional Theory, Silicon Nanocrystals (Si-nc) of different size (diameter ranging from 1.1 nm to 1.8 nm) have been studied localizing impurities at different substitutional sites and calculating the impurity formation energies. Starting from hydrogen terminated silicon Si-nc, we found that codoping is always energetically favored with respect to a single B- or P-doping and that the two impurities tend to occupy nearest neighbor sites near the surface. The formation energy depends on the distance between the two impurities. The codoped Si-nc present bandedge states localized on the impurities which are responsible for a red-shift of the absorption threshold with respect to that of pure undoped Si nanocrystals. Concerning the emission spectra, we find a Stokes shift of the photoluminescence to a lower energy with respect to the absorption edge due to the nanocrystal (nc) structural relaxation after the creation of the electron-hole pair. We have calculated the absorption and emission spectra going beyond a single-particle approach showing the important role played by the many-body effects. The presence of electronic quasi-direct optical transitions between the donor and acceptor states within the band-gap makes it possible to engineer the optical properties of Si-nc.
2007
- Doping in silicon nanocrystals
[Articolo su rivista]
Ossicini, Stefano; Degoli, Elena; F., Iori; O., Pulci; G., Cantele; Magri, Rita; Bisi, Olmes; F., Trani; D., Ninno
abstract
The absorption and, for the first time, the emission spectra of doped silicon nanocrystals have been calculated within a first-principles framework including geometry optimization. Starting from hydrogenated silicon nanocrystals, simultaneous n- and p-type doping with boron and phosphorous impurities have been considered. We found that the B–P co-doping results to be easier than simple B- or P-doping and that the two impurities tend to occupy nearest neighbours sites inside the nanocrystal itself. The co-doped nanocrystals bandstructure presents band edge states that are localized on the impurities and are responsible of the red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in fair agreement with the experimental outcome. The emission spectra show a Stokes shift with respect to the absorption due to the structural relaxation after the creation of the electron–hole pair. Moreover, the absorption and emission spectra have been calculated for a small co-doped nanocrystal beyond the single particle approach by introducing the selfenergycorrection and solving the Bethe–Salpeter equation scheme. Our procedure shows the important role played by the many-bodyeffects.
2007
- Doping in silicon nanostructures
[Articolo su rivista]
F., Iori; Ossicini, Stefano; Degoli, Elena; E., Luppi; R., Poli; Magri, Rita; G., Cantele; F., Trani; AND D., Ninno
abstract
We report on an ab initio study of the structural, electronic and optical properties of boron and phosphorous doped silicon nanocrystals. The scaling with the Si-nanocrystal size is investigated for both the neutral formation energies (FE) and the impurity activation energies. Both these energies scale with the nanocrystal inverse radius. The optical properties reveal the existence of new absorption peaks in the low energy region related to the presence of the impurity. The effects of B and P co-doping show that the formation energies are always smaller than those of the corresponding single-doped cases due to both carriers compensation and minor structural distortion. Moreover in the case of co-doping the electronic and optical properties show a strong reduction of the band gap with respect to the pure silicon nanocrystals that makes possible to engineer the photoluminescence properties of silicon nanocrystals.
2007
- Engineering silicon nanocrystals: Theoretical study of the effect of codoping with boron and phosphorus
[Articolo su rivista]
Iori, F.; Degoli, E.; Magri, R.; Marri, I.; Cantele, G.; Ninno, D.; Trani, F.; Pulci, O.; Ossicini, S.
abstract
We show that the optical and electronic properties of nanocrystalline silicon can be efficiently tuned using impurity doping. In particular, we give evidence, by means of ab-initio calculations, that by properly controlling the doping with either one or two atomic species, a significant modification of both the absorption and the emission of light can be achieved. We have considered impurities, either boron or phosphorous (doping) or both (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupy nearest neighbor sites near the surface. In addition, such systems present band-edge states localized on the impurities giving rise to a red-shift of the absorption thresholds with respect to that of undoped nanocrystals. Our detailed theoretical analysis shows that the creation of an ele...
2007
- Excitons in Silicon Nanocrystallites: the Nature of Luminescence
[Articolo su rivista]
E., Luppi; F., Iori; Magri, Rita; O., Pulci; Degoli, Elena; Ossicini, Stefano; V., Olevano
abstract
The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter have been calculated within a first-principles framework. Our calculations include geometry optimization and the many-body effects induced by the creation of an electron-hole pair. Starting from hydrogenated silicon clusters of different sizes, different Si/O bondings at the cluster surface have been considered. We found that the presence of a Si-O-Si bridge bond causes significant excitonic luminescence features in the visible range that are in fair agreement with experiment
2007
- Role of surface passivation and doping in silicon nanocrystals
[Articolo su rivista]
Magri, Rita; Degoli, Elena; Iori, Federico; Luppi, Eleonora; Pulci, O.; Ossicini, Stefano; Cantele, G; Trani, F; Ninno, D.
abstract
The absorption and the emission spectra of undoped and doped silicon nanocrystals of different size and surface terminations have been calculated within a first-principles framework. The effects induced by the creation of an electron-hole pair on the atomic structure and on the optical spectra of hydrogenated silicon nanoclusters as a function of dimension are discussed showing the strong interplay between the structural and optical properties of the system. Starting from the hydrogenated clusters, (i) different Si/O bonding at the cluster surface and (ii) different doping configurations have been considered. We have found that the presence of a Si-O-Si bridge bond at the nanocrystal surface gives rise to significant excitonic luminescence features in the near-visible range that are in fair agreement with photoluminescence (PL) measurements on oxidized and SiO_{2} embedded nanocrystals. The study of the structural, electronic and optical properties of simultaneously n- and p-type doped hydrogenated silicon nanocrystals with boron and phosphorous impurities have shown that B-P co-doping is energetically favorable with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbors sites. The co-doped nanocrystals present band edge states localized on the impurities that are responsible of a red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in agreement with the experiment.
2006
- Doping in silicon nanocrystals: An ab initio study of the structural, electronic and optical properties
[Articolo su rivista]
Iori, Federico; Degoli, Elena; Luppi, Eleonora; Magri, Rita; Marri, Ivan; G., Cantele; D., Ninno; F., Trani; Ossicini, Stefano
abstract
There are experimental evidences that doping control at the nanoscale can significantly modify the optical properties with respect to the pure systems. This is the case of silicon nanocrystals (Si-nc), for which it has been shown that the photoluminescence (PL) peak can be tuned also below the bulk Si band gap by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. In this work, we report on an ab initio study of impurity states in Si-nc. We consider B and P substitutional impurities for Si-nc with a diameter up to 2.2 nm. Formation energies (FEs), electronic, optical and structural properties have been determined as a function of the cluster dimension. For both B-doped and P-doped Si-nc the FE increases on decreasing the dimension, showing that the substitutional doping gets progressively more difficult for the smaller nanocrystals. Moreover, subsurface impurity positions result to be the most stable ones. The codoping reduces the FE strongly favoring this process with respect to the simple n-doping or p-doping. Such an effect can be attributed to charge compensation between the donor and the acceptor atoms. Moreover, smaller structural deformations, with respect to n-doped and p-doped cases, localized only around the impurity sites are observed. The band gap and the optical threshold are largely reduced with respect to the undoped Si-nc showing the possibility of an impurity-based engineering of the Si-nc PL properties. (c) 2006 Elsevier B.V. All rights reserved.
2006
- Study of arsenic for antimony exchange at the Sb-stabilized GaSb(0 0 1) surface
[Articolo su rivista]
Righi, Maria Clelia; Magri, Rita; Bertoni, Carlo Maria
abstract
In this paper we present a first-principle study on the energetics of a single As2 molecule on GaSb(0 0 1) reconstructed surface. In order to shed light into the mechanisms of anion exchange at the Sb-rich GaSb(0 0 1) surface, we studied firstly As2 adsorption and then As for Sb exchange. We identify a surface region where both the processes are energetically favored. The results of this twofold analysis can be combined to derive possible reaction paths for the anion exchange process. © 2005 Elsevier B.V. All rights reserved.
2006
- The structural, electronic and optical properties of Si nanoclusters: effects of size, doping and surface passivation
[Capitolo/Saggio]
Degoli, Elena; E., Luppi; Magri, Rita; F., Iori; G., Cantele; Ossicini, Stefano
abstract
AB-initio routines
2006
- Understanding doping in silicon nanostructures
[Articolo su rivista]
Ossicini, Stefano; F., Iori; Degoli, Elena; E., Luppi; Magri, Rita; R., Poli; G., Cantele; F., Trani; D., Ninno
abstract
The effects of both single doping and simultaneous codoping on the structural, electronic, and optical properties of Si nanocrystals are calculated by the first-principles method. We show that the amount of the nanocrystal relaxation around the impurity is directly related to the impurity valence. Moreover, both the neutral impurity formation energies and the impurity activation energies scale with the reciprocal radius. Interestingly, no significant variation of the activation energy on the impurity species is found, and the cluster relaxation gives a minor contribution to it. The role of the impurity position within the nanocrystal has also been elucidated showing that the subsurface positions are the most stable ones. We show that, if the carriers in the Si nanocrystals are perfectly compensated by simultaneous doping with the n- and p-type impurities, the nanocrystals undergo a minor structural distortion around the impurities. The formation energies are always smaller than that for the corresponding single-doped cases. Moreover, in the case of codoping, the bandgap is strongly reduced with respect to the gap of the pure crystals showing the possibility of an impurity-based engineering of the photoluminescence properties of the Si nanocrystals.
2005
- Ab-initio Calculations Of The Electronic Properties of Hydrogenated and Oxidized Silicon Nanocrystrals: Ground and Excited States
[Relazione in Atti di Convegno]
Ossicini, Stefano; Bisi, Olmes; Cantele, G; Degoli, Elena; DEL SOLE, R; Gatti, M; Incze, A; Iori, Federico; Luppi, Eleonora; Magri, Rita; Ninno, D; Onida, G; Pulci, O.
abstract
The electronic and optical properties of hydrogenated silicon nanocrystals (H-Sinc) have been investigated through ab-initio techniques (Pseudopotential approach in ground and excited state electronic configurations, TDDFT, GW) as a function of size and symmetry. The presence of an electron-hole pair in the nanocrystals causes a strong deformation of the structures with respect to the ground-state configuration, and this is more evident for smaller systems and at the surface of the H-Si-nc. Also the nature of the distorsion changes: for small clusters it is strongly localized, while as the size increases the distortion is spread out over the entire structure. The structural modifications are immediately reflected into the electronic structure. Actually, we have found the expected decrease of the energy gap on increasing the nanocrystal dimension for the ground state and, for the excited state configuration, a reduction of the energy gap the more significant the smaller is the nanocrystal. For the excited nanoparticles the HOMO and LUMO become strongly localised in correspondence of the distortion, giving rise to defect-like states which reduce the gap; these effects are stronger in smaller clusters. Thus, we can deduce that the absorption of resonant radiation by the nanocrystal in its ground state configuration induces a transition between the HOMO and LUMO levels, which for all these nanocrystals is optically allowed. Such a transition is followed by a cluster relaxation in the excited state configuration giving rise to distorted geometries and to new LUMO and HOMO, whose energy difference is smaller than that in the ground-state geometry. It is between these two last states that emission occurs; the Stokes Shift between absorption and emission changes as a function of the dimension. Thus we have substituted some H with O both double bonded or in a bridge configuration with respect to Si. The substitution of H with O as passivating agent results in a different cluster geometry and in a reduction of the energy band gap depending on the type of O-Si bond. Moreover also the optical properties strongly depend on the different O-Si bond type. The results provide a consistent interpretation of the photoluminescence redshift observed in oxidized samples and of recent outcomes on Si single quantum dot photoluminescence bandwidth. Three fundamental aspects come out: first, there is a strong interplay between structural and electronic properties, mostly when excited configurations are concerned. Second, a consistent explanation of emission processes can be carried out only if such excited configurations are accounted for. Third, optical gaps cannot be calculated simply as the HOMO-LUMO energy separation without introducing errors as larger as smaller is the nanocrystal.
2005
- Ab-initio Calculations Of The Electronic Properties of Silicon Nanocrystals: Absorption, Emission, Stokes Shift
[Relazione in Atti di Convegno]
Degoli, Elena; G., Cantele; E., Luppi; Magri, Rita; Ossicini, Stefano; D., Ninno; Bisi, Olmes; G., Onida; M., Gatti; A., Incze; O., Pulci; R., DEL SOLE
abstract
The structural, optical and electronic properties of silicon nanocrystals are investigated as a function of the dimension as well as the surface passivation. Both the ground- and an excited-state configuration are studied using ab-initio calculations. Atom relaxation under excitation is taken into account and related with the experimentally observed Stokes shift.
2005
- Electronic excitations in solids: Density functional and Green's function theory
[Articolo su rivista]
Pulci, O; Marsili, M; Luppi, Eleonora; Hogan, C; Garbuio, V; Sottile, F; Magri, Rita; Del Sole, R.
abstract
Theoretical approaches for ab initio studies of the electronic and optical properties of matter are here reviewed. Examples within Density Functional Theory, Many-Body perturbation Theory and Time Dependent Density Functional Theory are presented and discussed, pointing out advantages and drawbacks of the different schemes.
2005
- Electronic, structural and optical properties of hydrogenated silicon nanocrystals: the role of the excited states
[Relazione in Atti di Convegno]
G., Cantele; Degoli, Elena; Luppi, Eleonora; Magri, Rita; D., Ninno; Bisi, Olmes; Ossicini, Stefano; G., Iadonisi
abstract
In this paper we report on a first-principle calculation of the electronic and structural properties of hydrogenated silicon nanocrystals both in the ground- and in an excited-state configuration. The presence of an electron-hole pair created under excitation is taken into account and its effects on both the electronic spectrum and the cluster geometry are pointed out. The interpretation of the results is done within a four-level model, which also allows the explanation of the experimentally observed Stokes shift. Size-related aspects are also analysed and discussed.
2005
- Evolution of the band-gap and band-edge energies of the lattice-matched GaInAsSb/GaSb and GaInAsSb/InAs alloys as a function of composition
[Articolo su rivista]
Magri, Rita; Zunger, A; Kroemer, H.
abstract
Using atomistic pseudopotential calculations we predict the evolution of the valence-band maximum energy E-upsilon(x,y) and conduction-band minimum energy E-c(x,y) for a compositionally graded quaternary Ga1-yInyAsxSb1-x alloy lattice matched to GaSb or InAs as a function of (x,y) or, equivalently, as a function of distance from the substrate. We find upward-concave bowing for both E-c and E-upsilon, in contradiction with simple interpolative models. A transition from staggered (type II) to broken-gap (type III) lineup relative to GaSb is predicted to occur at x=0.81 and y=0.92 on a GaSb substrate, and at x=0.59 and y=0.62 on an InAs substrate. In the latter case, the quaternary alloy has a minimum gap at x=0.85 and y=0.87.
2005
- First-principles study of Sb-stabilized GaSb(001) surface reconstructions
[Articolo su rivista]
Righi, Maria Clelia; Magri, Rita; Bertoni, Carlo Maria
abstract
We report results of ab initio total-energy and electronic- structure calculations for the Sb-stabilized GaSb(001) surface. We consider different reconstruction models proposed in the literature on the basis of experimental observations in typical GaSb growth conditions and present the T=0 surface stability diagram. We found that dimer-based (4 x 3) reconstructions are favored over widely proposed (1 x 3)/c(2 x 6) models that have a similar structure, but do not satisfy the electron counting rule. We discuss also the stability of the beta 2(2 x 4) reconstruction for comparison with the As-based GaAs(001) and InAs(001) surfaces that present this phase in a wide range of surface preparation conditions. We predict the surface band structure of the GaSb(001) stable configurations and identify the nature of the surface bands. All the (4 x 3) reconstructions turned out to be semiconducting with an energy gap slightly smaller than the GaSb bulk value.
2005
- First-principles study of n- and p-doped silicon nanoclusters
[Articolo su rivista]
G., Cantele; Degoli, Elena; Luppi, Eleonora; Magri, Rita; D., Ninno; G., Iadonisi; Ossicini, Stefano
abstract
We report on an ab initio study of the structural and electronic properties of B- and P-doped Si nanoclusters. The neutral impurities formation energies are calculated. We show that they are higher in smaller nanoclusters and that this is not related to the structural relaxation around the impurity. Their dependence on the impurity position within the nanocluster is also discussed. Finally, we have calculated the B and P activation energies showing the existence of a nearly linear scaling with the nanocluster inverse radius. Interestingly, no significant variation of the activation energy on the impurity species is found and the cluster relaxation gives a minor contribution to it.
2005
- Formation energies of silicon nanocrystals: role of dimension and passivation
[Relazione in Atti di Convegno]
Degoli, Elena; Ossicini, Stefano; G., Cantele; Luppi, Eleonora; Magri, Rita; D., Ninno; Bisi, Olmes
abstract
The structural properties of small silicon nanoclusters as a function of dimension and surface passivation are studied from ab initio technique. The formation energies are calculated and the relative stability of the considered clusters is predicted and discussed.
2005
- P and B single- and co-doped silicon nanocrystals: Formation and activation energies, electronic and optical properties
[Relazione in Atti di Convegno]
Ossicini, S.; Iori, F.; Degoli, E.; Luppi, E.; Magri, R.; Cantele, G.; Trani, F.; Ninno, D.
abstract
We report on an ab initio study of the structural and electronic properties of B and P doped Si nanocrystals (Si-nc). The formation energies (FE) scale with the radius, the activation energies with the inverse radius. The effects of B and P co-doping show that the FE are always smaller than that for the corresponding single-doped cases and that is possible to engineer the photoluminescence properties of Si-nc.
2005
- Simultaneously B- and P-doped silicon nanoclusters: Formation energies and electronic properties
[Articolo su rivista]
Ossicini, Stefano; Degoli, Elena; Iori, F; Luppi, E; Magri, Rita; Cantele, G; Trani, F; Ninno, D.
abstract
The effects of B and P codoping on the impurity formation energies and electronic properties of Si nanocrystals (Si-nc) are calculated by a first-principles method. We show that, if carriers in the Si-nc are perfectly compensated by simultaneous doping with n- and p-type impurities, the Si-nc undergo a minor structural distortion around the impurities and that the formation energies are always smaller than those for the corresponding single-doped cases. The band gap of the codoped Si-nc is strongly reduced with respect to the gap of the pure ones showing the possibility of an impurity based engineering of the photoluminescence properties of Si-nc.
2005
- The electronic and optical properties of silicon nanoclusters: absorption and emission
[Articolo su rivista]
Luppi, Eleonora; Degoli, Elena; G., Cantele; Ossicini, Stefano; Magri, Rita; D., Ninno; Bisi, Olmes; O., Pulci; G., Onida; M., Gatti; A., Incze; R., Del Sole
abstract
The electronic and optical properties of silicon nanocrystals passivated with hydrogen and oxygen have been investigated both in the ground- and in an excited-state configuration, through different ab-initio techniques. The presence of an electron-hole pair leads to a strong interplay between the structural and optical properties of the system. The structural distortion of the nanocrystals induced by an electronic excitation is analysed together with the role of the symmetry constraint during the relaxation. The structural distortion can account for the experimentally observed Stokes Shift. Size-related aspects are also analysed and discussed.
2004
- Ab-initio structural and electronic properties of hydrogenated silicon nanoclusters in their ground and excited state
[Articolo su rivista]
Degoli, Elena; G., Cantele; E., Luppi; Magri, Rita; D., Ninno; Bisi, Olmes; Ossicini, Stefano
abstract
Electronic and structural properties of small hydrogenated silicon nanoclusters as a function of dimension are calculated from ab initio technique. The effects induced by the creation of an electron-hole pair are discussed in detail, showing the strong interplay between the structural and optical properties of the system. The distortion induced on the structure after an electronic excitation of the cluster is analyzed together with the role of the symmetry constraint during the relaxation. We point out how the overall effect is that of significantly changing the electronic spectrum if no symmetry constraint is imposed to the system. Such distortion can account for the Stokes shift and provides a possible structural model to be linked to the four-level scheme invoked in the literature to explain recent results for the optical gain in silicon nanoclusters. Finally, formation energies for clusters with increasing dimension are calculated and their relative stability discussed.
2004
- Formation energies of silicon nanocrystals: role of dimension and passivation
[Abstract in Atti di Convegno]
Ossicini, Stefano; Degoli, Elena; Cantele, G; Luppi, Eleonora; Magri, Rita; Ninno, D; Bisi, Olmes
abstract
The structural porperties of small silicon nanoclusters are investigated.
2004
- Giant birefringence in zinc-blende-based artificial semiconductors
[Articolo su rivista]
Jancu, Jm; Vasanelli, A; Magri, Rita; Voisin, P.
abstract
We use extended-basis empirical tight-binding calculations and examine the anisotropy of the refractive index in ultrashort-period superlattices of materials sharing no common atom. We find that a strong birefringence can be engineered in these articial semiconductors, allowing phase matching for frequency difference generation. The prominent role of epitaxial constraint and bond-length alternation is evidenced.
2004
- Structural, electronic and optical properties of silicon nanoclusters: the role of the size and surface passivation
[Relazione in Atti di Convegno]
Cantele, G; Ossicini, Stefano; Degoli, Elena; Luppi, Eleonora; Magri, Rita; Ninno, D; Bisi, Olmes
abstract
Thes structural, optical and electronic properties of silicon nanocrystals are investigated.
2003
- Electronic and optical properties of silicon nanocrystals: structural effects
[Articolo su rivista]
Degoli, Elena; Ossicini, Stefano; M., Luppi; E., Luppi; Magri, Rita; G., Cantele; D., Ninno; N., Iadonisi
abstract
The aim of this work is to investigate the structural, electronic and optical properties of hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations. Recent results about ab-initio calculations of Stokes shift as a function of the cluster dimension and of optical gain will be presented here. A structural model that can be linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO2 matrix will be suggested too.
2003
- Experimental and theoretical joint study on the electronic and structural properties of silicon nanocrystals embedded in SiO2: active role of the interface region
[Relazione in Atti di Convegno]
N., Daldosso; M., Luppi; G., Dalba; L., Pavesi; F., Rocca; F., Priolo; G., Franzò; F., Iacona; Degoli, Elena; Magri, Rita; Ossicini, Stefano
abstract
The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 has been studied by x-ray absorption spectroscopy (XAS) and by ab-initio total energycalculations. Si-nc have been formed by PECVD deposition of SiOx with different Si content (from 35 to 42 at.%) and thermal annealing at high temperature (1250 °C). The comparisonbetween total electron yield (TEY) and photoluminescence yield (PLY) spectra has allowed the identification of a modified region of SiO2 (about 1 nm thick) surrounding the Si-nc, which participates to the light emission of Si-nc. Total energy calculations, within the density functional theory, clearly show that Si-nc are surrounded by a cap-shell of stressed SiO2 with a thickness of about 1 nm. The optoelectronic properties show the appearance of localized states not only in the Si-nc core region but also in the modified SiO2 region
2003
- Gain theory and models in silicon nanostructures
[Relazione in Atti di Convegno]
Ossicini, Stefano; C., Arcangeli; Bisi, Olmes; Degoli, Elena; M., Luppi; Magri, Rita; L., DAL NEGRO; L., Pavesi
abstract
The main goal in the information technology is to have the possibility of integrating low-dimensional structures showing appropriate optoelectronic properties with the well established and highly advanced silicon microelectronics present technology. Therefore, after the initial impulse given by the work of Canham on visible luminescence from porous Si, nanostructured Si has received extensive attention both from experimental and theoretical point of view during the last ten years. This activity is mainly centered on the possibility of getting relevant optoelectronic properties from nanocrystalline Si, which in the bulk crystalline form is an indirect band gap semiconductor, with very inefficient light emission in the infrared. Although some controversial interpretations of the visible light emission from low-dimensional Si structures still exist, it is generally accepted that the quantum confinement, caused by the restricted size, and the surface passivation are essential for this phenomenon.Here we will review our activity in the field of the theoretical determination of the structural, electronic and optical properties of Si nanocrystals (Si-nc). The present work aims at answer a very important question related to the origin of the enhanced photoluminescence in Si-nc embedded in SiO2. In fact, optical gain has been recently observed in ion implanted Si-nc and in Si-nc formed by plasma enhanced chemical vapour deposition and annealing treatments. We propose, here, an analysis of the experimental findings based on an effective rate equation model for a four level system; moreover looking at our theoretical results for the optical properties of Si-nc we search for structural model that can be linked to the four level scheme. As final outcome, due to the results for the optoelectronic properties of Si-nc in different interface bond configurations, we demonstrate that in order to account for the striking photoluminescence properties of Si-nc it is necessary to take carefully into account not only the role of quantum confinement, but also the role of the interface region surrounding the Si-nc.
2003
- Predicting interband transition energies for InAs/GaSb superlattices using the empirical pseudopotential method
[Articolo su rivista]
Magri, Rita; Zunger, A.
abstract
Recent measurements surprisingly show that the lowest valence-to-conduction confined transitions in narrow (InAs)(8)/(GaSb)(n) and (InAs)(6)/(GaSb)(n) superlattices increase in energy as the barrier thickness n increases. We show that in addition to the mesoscopic geometric quantities (well and barrier sizes), an atomic-scale description of interdiffused interfaces is needed to correctly reproduce the observed spectroscopic trend. The interdiffused interface is modeled via diffusion equations. We compare our atomistic empirical pseudopotential calculation in which only the bulk binary data are fit to experiment, with contemporary methods in which agreement with experiment is forced using ideally abrupt interfaces.
2003
- Role of interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO2
[Articolo su rivista]
N., Daldosso; M., Luppi; Ossicini, Stefano; Degoli, Elena; Magri, Rita; G., Dalba; P., Fornasini; R. GRISENTI, F. ROCCA; L., Pavesi; S. BONINELLI, F. PRIOLO; C., Spinella; F., Iacona
abstract
Light emitting silicon nanocrystals embedded in SiO2 have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered TEM analysis and by ab-initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding SiO2 is not sharp: an intermediate region of amorphous nature and of variable composition links the crystalline Si with the amorphous stoichiometric SiO2. This region plays an active role in the light emission process.
2003
- Surface and confinement effects on the optical and structural properties of silicon nanocrystals
[Relazione in Atti di Convegno]
Ossicini, Stefano; Magri, Rita; Degoli, Elena; Luppi, Marcello; Luppi, Eleonora
abstract
In this work we investigate, by first-principles calculations, the structural, electronic and optical properties of: (1) oxygenated silicon-based nanoclusters of different sizes in regime of multiple oxidation at the surface, and (2) hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations within density functional theory.In the first case we have varied systematically the number of Si=O bonds at the cluster surface and found a nonlinear reduction of the energy gap with the Si=O bond number. A saturation limit is reached, which allows us to provide a consistent interpretation of the photoluminescence (PL) redshift observed in oxidized porous silicon samples. Our results help also to explain some very recent findings on the single silicon quantum dot photoluminescence bandwidth.
2003
- Theory of optical properties of 6.1 Å III-V superlattices: The role of the interfaces
[Articolo su rivista]
Magri, Rita; A., Zunger
abstract
Interfacial interdiffusion in quantum wells and superlattices could alter the interfacial strain, band alignment, and even the atomic symmetry at the interface, thus potentially changing the electronic and optical properties. We consider-the InAs/GaSb system describing the interdiffused interfaces via a simple kinetic model of molecular beam epitaxy growth. The predicted atomic positions after interdiffusion are then used in a pseudopotential theory to describe the electronic and optical consequences of interdiffusion. We determine (i) the effects of different interfacial bonding compositions on the electronic and optical properties; (ii) the segregation profiles at the normal and inverted interfaces; and (iii) the effect of structural disorder on band gaps. The application of our method to the InAs/GaSb superlattices allows us to explain numerous observed results and trends.
2003
- Theory of optical properties of segregated InAs/GaSb superlattices
[Articolo su rivista]
Magri, Rita; A., Zunger
abstract
The authors study the effects of interfacial atomic segregation on the electronic and optical properties of InAs/GaSb superlattices. They describe their atomistic empirical pseudo-potential method and test its performance against the available experimental data. They show its ability to predict the band structure dependence on the detailed atomic configuration, and thus to properly account for the effects of interfacial atomic segregation and structural disorder. They also show how their method avoids the approximations underlying the pseudopotential method of Dente and Tilton, which gives different results. The application of the proposed method to the InAs/GaSb superlattices allows the explanation of some observed experimental results, such as: the bandgap difference between (InAs)(8)/(GaSb)(8) superlattices with almost pure InSb-like or GaAs-like interfaces; the large blue shift of the bandgap when the growth temperature of the superlattice increases; and the blue shift of the bandgap of (InAs)8/(GaSb)(n) superlattices with increasing GaSb period n. They present a detailed comparison of their predicted blue shift with that obtained by other theories.
2003
- Will silicon be the photonics material of the third millennium?
[Relazione in Atti di Convegno]
L., Pavesi; L., Dal Negro; N., Daldosso; Z., Gaburro; M., Cazzanelli; F., Iacona; G., Franzo; D., Pacifici; F., Priolo; Ossicini, Stefano; Luppi, Marcello; Degoli, Elena; Magri, Rita
abstract
CMOS circuitry dominates the current semiconductor market due to the astonishing power of silicon electronic integration technology. In contrast to the dominance of silicon in electronics, photonics utilises a diversity of materials for emitting, guiding, modulating and detecting light. In the last ten years a big research effort was aimed to render Si an optical active material so that it can be turned from an electronic material to a photonic material. For some the future of Si-based photonic lays in 'hybrid' solutions, for others the utilisation of more photonic functions by silicon itself. During the last two years many breakthroughs in the field have appeared. In this paper we will review what we believe the most important: optical gain in silicon nanostructures.
2002
- Density functional calculations of the structural, electronic and optical properties of semiconductor nanostructures
[Capitolo/Saggio]
Ossicini, Stefano; Degoli, Elena; Luppi, M.; Magri, Rita
abstract
Series in Micro and Nanoengineering
2002
- Effects of interfacial atomic segregation and intermixing on the electronic properties of InAs/GaSb superlattices
[Articolo su rivista]
Magri, Rita; Zunger, A.
abstract
Abrupt InAs/GaSb superlattices have In-Sb and Ga-As interfacial chemical bonds that are not present in the constituent materials InAs and GaSb. We study the effect of interfacial atomic mixing on the electronic structure of such superlattices, including electron and hole energies and wave function localization, interband transition energies, and dipole matrix elements. We combine an empirical pseudopotential method for describing the electronic structure with two different structural models of interfacial disorder. First, we use the "single-layer disorder" model and change in a continous way the composition of the interfacial bonds. Second, we study interfacial atomic segregation using a layer-by-layer kinetic model of molecular beam epitaxy growth, fit to the observed scanning tunneling microscopy segregation profiles. The growth model provides a detailed structural model of segregated InAs/GaSb superlattices with atomic resolution. The application of the empirical pseudopotential ...
2002
- Gain theory and models in silicon nanostructures
[Monografia/Trattato scientifico]
Ossicini, Stefano; Arcangeli, C; Bisi, O.; Degoli, Elena; Luppi, M.; Magri, Rita; DAL NEGRO, L.; Pavesi, L.
abstract
The main goal in the information technology is to have the possibility of integrating low-dimensional structures showing appropriate optoelectronic properties with the well established and highly advanced silicon microelectronics present technology. Therefore, after the initial impulse given by the work of Canham on visible luminescence from porous Si, nanostructured Si has received extensive attention both from experimental and theoretical point of view during the last ten years. This activity is mainly centered on the possibility of getting relevant optoelectronic properties from nanocrystalline Si, which in the bulk crystalline form is an indirect band gap semiconductor, with very inefficient light emission in the infrared. Although some controversial interpretations of the visible light emission from low-dimensional Si structures still exist, it is generally accepted that the quantum confinement, caused by the restricted size, and the surface passivation are essential for this phenomenon.Here we will review our activity in the field of the theoretical determination of the structural, electronic and optical properties of Si nanocrystals (Si-nc). The present work aims at answer a very important question related to the origin of the enhanced photoluminescence in Si-nc embedded in SiO2. In fact, optical gain has been recently observed in ion implanted Si-nc and in Si-nc formed by plasma enhanced chemical vapour deposition and annealing treatments. We propose, here, an analysis of the experimental findings based on an effective rate equation model for a four level system; moreover looking at our theoretical results for the optical properties of Si-nc we search for structural model that can be linked to the four level scheme. As final outcome, due to the results for the optoelectronic properties of Si-nc in different interface bond configurations, we demonstrate that in order to account for the striking photoluminescence properties of Si-nc it is necessary to take carefully into account not only the role of quantum confinement, but also the role of the interface region surrounding the Si-nc.
2002
- Segregation effects on the optical properties of (InAs)/(GaSb) superlattices
[Articolo su rivista]
Magri, Rita; A., Zunger
abstract
We combine a kinetic model of MBE growth with the empirical pseudopotential band structure method to study the effects of interfacial disorder and segregation on the optical properties of InAs/GaSb superlattices. We fit the layer-by-layer growth model to the observed STM segregated profiles, extracting surface-to-subsurface atomic exchange energies, These are then used to obtain a detailed simulated model of segregated InAs/GaSb superlattices with atomic resolution. The application of the pseudopotential calculations to Such structures reveals remarkable electronic consequences of segregation, including a blue shift of band gap with increasing sample growth temperature. (C) 2002 Elsevier Science B.V. All rights reserved.
2002
- Si nanostructures embedded in SiO[sub 2]: electronic and optical properties
[Relazione in Atti di Convegno]
Ossicini, Stefano; Degoli, Elena; Luppi, Marcello; Magri, Rita
abstract
We present ab initio results for the structural, electronic and optical properties of silicon nanostructures confined by silicon dioxide. We investigate the role of the dimension, symmetry and bonding situations at the interfaces. In particular we consider Si/SiO2 superlattices and Si nanocrystals embedded in SiO2 matrix. In the case of Si/SiO2 superlattices the presence of oxygen defects at the interface and the dimensionality are the key points in order to explain the experimental outcomes concerning photoluminescence. For Si nanocrystals embedded in SiO2 we show, in agreement with experimental results, the close interplay between chemical and structural effects on the electronic and optical properties.
2001
- Effects of interfacial atomic segregation on optical properties of InAs/GaSb superlattices
[Articolo su rivista]
Magri, Rita; Zunger, Alex
abstract
Rapid Communications - Largely because of the lack of detailed microscopic information on the interfacial morphology, most electronic structure calculations on superlattices and quantum wells assume abrupt interfaces. Cross-sectional scanning tunneling microscopy (STM) measurements have now resolved atomic features of segregated interfaces. We fit a layer-by-layer growth model to the observed STM profiles, extracting surface-to-subsurface atomic exchange energies. These are then used to obtain a detailed simulated model of segregated InAs/GaSb superlattices with atomic resolution. Applying pseudopotential calculations to such structures reveals remarkable electronic consequences of segregation, including a blueshift of interband transitions, lowering of polarization anisotropy, and reduction of the amplitude of heavy-hole wave functions at the inverted interface.
2001
- IN PLANE ANISOTROPY OF THE OPTICAL PROPERTIES OF (In0.5Ga0.5As)n/(InP)n SUPERLATTICES.
[Articolo su rivista]
Magri, Rita; Ossicini, Stefano
abstract
ABSTRACT In this paper we study the electronic and optical properties of (In0.5Ga0.5As)n/(InP)n superlattices, where the Ga0.5In0.5As alloy is described both through the virtual crystal approximation (VCA) and through an appropriate ordered ternary structure. By first-principles calculations of the dielectric tensor elements we address the issue of the giant polarization anisotropy of the optical absorption experimentally observed in these superlattices. The magnitude of the anisotropy depends on the splitting between the hole states at the valence band top which is due to the lowering of the overall symmetry to the C2v point group and it is greatly influenced by strain not only at the interfaces but also in the bulk alloy.
2001
- The electronic and optical properties of InGaAs/InP and InAlAs/InP superlattices
[Articolo su rivista]
C., Ghidoni; Magri, Rita; Ossicini, Stefano
abstract
We study using first-principle calculations the electronic and optical properties of In0.5Ga0.5As/InP and In0.5Al0.5As/InP superlattices, where the InGaAs and InAlAs alloys are described through an appropriate ordered ternary structure. The calculated electronic properties show that the substitution of Ga with Al originate an opening of the band gap from the infrared to the near visible and a transformation of the band alignment from type I to type II. Through the analysis of the optical properties we discuss successfully the giant polarization anisotropy observed in these systems. (C) 2001 Elsevier Science B.V. All rights reserved.
2000
- Anticrossing and coupling of light-hole and heavy-hole states in (001) GaAs/AlxGa1-xAs heterostructures
[Articolo su rivista]
Magri, Rita; A., Zunger
abstract
Heterostructures sharing a common atom such as AlAs/GaAs/AlAs have a D-2d point-group symmetry which allows the bulk-forbidden coupling between odd-parity light-hole states (e.g., lh1) and even-parity heavy-hole states (e.g., hh2). Continuum models, such as the commonly implemented (standard model) k.p theory miss the correct D-2d symmetry and thus produce zero coupling at the zone center. We have used the atomistic empirical pseudopotential theory to study the lh1-hh2 coupling in (001) superlattices and quantum wells of GaAs/AlxGa1-xAs. By varying the Al concentration x of the barrier we scan a range of valence-band barrier heights DeltaE(v)(x). We find the following: (i) The lh1 and hh2 states anticross at rather large quantum wells width or superlattice periods 60 <n(c)< 70 monolayers. (ii) The coupling matrix elements V-lh1.hh2(k parallel to =0) are small (0.02-0.07 meV) and reach a maximum value at a valence-band barrier height DeltaE(v)approximate to 100 meV, which corresponds to an Al composition x(Al) = 0.2 in the barrier. (iii) The coupling matrix elements obtained from our atomistic theory are at least an order of magnitude smaller than those calculated by the phenomenological model of Ivchenko et al. [Phys. Rev. B 54, 5852 (1996)]. (iv) The dependence of V-lh1,V-hh2 on the barrier height DeltaE(v)(x) is more complicated than that suggested by the recent model of Cortez et al., [J. Vac. Sci. Technol. B 18, 2232 (2000)], in which V-lh1.hh2 is proportional to the product of DeltaE(v)(x) times the amplitudes of the lh1 and hh2 envelopes at the interfaces. Thus, atomistic information is needed to establish the actual scaling.
2000
- Anticrossing semiconducting band gap in nominally semimetallic InAs/GaSb superlattices
[Articolo su rivista]
Magri, Rita; L. W., Wang; Alex, Zunger; I., Vurgaftman; AND J. R., Meyer
abstract
While (InAs)n/(GaSb)n (001) superlattices are semiconducting for nnc the InAs electron level eInAs is below the GaSb hole level hGaSb, so the system is converted to a nominal semimetal. At nonzero in-plane wave vectors (k|| [not equal] 0), however, the wave functions eInAs and hGaSb have the same symmetry, so they anticross. This opens up a "hybridization gap" at some k||=k*||. Using a pseudopotential plane-wave approach as well as a (pseudopotential fit) eight-band k·p approach, we predict the hybridization gap and its properties such as wave-function localization and out-of-plane dispersion. We find that recent model calculations underestimate this gap severely.
1998
- Ab-initio investigation of the polarization anisotropy of the optical absorption in (InGa)As-InP superlattices
[Articolo su rivista]
Magri, Rita; Ossicini, Stefano
abstract
In this paper we present a theory which accounts for both the intensity and the shape of the experimentally observed giant polarization anisotropy of the optical absorption in (InGa)As-InP superlattices. By first-principles self-consistent calculations of the superlattice crystal potential and of the dielectric tensor elements we derive a selection rule'' by which the radiation polarization along the 110 and -110 directions choose only one optical transition of the doublet corresponding to transitions to the conduction band from the two upper crystal field split hole states which are degenerate in common atom superlattices. However, due to the similar intensity of the oscillator strength of these transitions, it turns out that the magnitude of the anisotropy depends only on the magnitude of the energy gap between the split states.
1998
- ROLE OF SYMMETRY REDUCTION IN THE POLARIZATION DEPENDENCE OF THE OPTICAL ABSORPTION IN NON-COMMON ATOM SUPERLATTICES
[Articolo su rivista]
Magri, Rita; Ossicini, Stefano
abstract
Role of symmetry
1996
- Electronic properties of Sb deposited on GaAs(110) in the submonolayer coverage regime
[Articolo su rivista]
Magri, Rita
abstract
We present theoretical calculations of the electronic structure of a Sb/GaAs(110) interface in a submonolayer deposition regime. We have studied two different structural models of the Sb islands on GaAs(110) corresponding to a coverage theta =0.625. A self-consistent first-principles pseudopotential method using density-functional theory within the local-density approximation has been used to calculate the minimum-energy atomic configuration of the Sb islands and of the GaAs substrate. In particular, we have determined the equilibrium geometry and the electronic properties of the Sb terrace edges. The terrace terminated with a Sb atom bonded to an As atom of the substrate is energetically favored with respect to the edge having the Sb atom bonded to a Ga substrate atom. In both cases there is a tendency of the Sb atom on the edge to move towards a site at which a threefold coordination can be reestablished by forming a bond with another atom of the substrate. This is achieved by red...
1996
- Structural and electronic properties of Sb islands on GaAs (110)
[Articolo su rivista]
Magri, Rita; Manghi, Franca; CALANDRA BUONAURA, Carlo
abstract
We present the results of theoretical calculations of the electronic structure of the Sb/GaAs(110) interface in the submonolayer coverage regime performed in the full ab initio self-consistent pseudopotential scheme. Different structural models for the edges of the extended Sb islands have been considered and their equilibrium geometry has been determined by total energy minimization. The single particle band structure shows interface states arising mainly from the incomplete bonding of the Sb adatoms at the terrace edge to the substrate, which fall within the optical gap. Distinct features are associated to different island terminations. The interface turns out to be metallic in all the considered cases with a partially occupied peak at the Fermi level. We have also studied the effect of including explicitly the on-site Hubbard electron-electron correlation in the calculation of the quasiparticle spectrum, obtaining the observed semiconducting interface when the Coulomb interaction parameter U is larger than 3 eV. The interface states within the optical band gap can be present also at higher coverages when some disorder exists, evidentiating a general mechanism for the Fermi level pinning at this interface.
1994
- Electron electron correlation for Sb on GaAs(110)
[Relazione in Atti di Convegno]
Magri, Rita; Manghi, Franca; CALANDRA BUONAURA, Carlo
abstract
We have studied the effect of including the on-site Hubbard electron-electron correlation in the calculation of the quasiparticle spectrum, obtaining the observed semiconducting interface when the Coulombinteraction parameter U is larger than 3 eV. The interface states within the optical band gap can bepresent also at higher coverages when some disorder exists, evidentiating a general mechanism forthe Fermi level pinning at this interface.
1994
- ORDERING IN BXC1-X COMPOUNDS WITH THE GRAPHITE STRUCTURE
[Articolo su rivista]
MAGRI, Rita
abstract
We present a theoretical study of the structural stability of ordered configurations of BxC1-x solid solutions with the graphite structure. The calculations were carried out self-consistently using norm-conserving pseudopotentials in the local-density approximation. The cohesive energies of a number of ordered structures at different compositions x have been subjected to a cluster expansion, which enables us to predict the cohesive energy of the random alloy at the corresponding compositions and to gain insight on the tendency of the system towards ordering. We find that, at T = 0 K, some ordered configurations have a cohesive energy higher than the corresponding random distribution at the same compositions. The inspection of the cluster interactions leads moreover to the conclusion that segregation of boron in graphite is highly discouraged and long-range order is likely to occur.
1992
- DIAMOND-LIKE ORDER IN ZINCBLENDE COMPOUNDS
[Articolo su rivista]
Laks, Db; Magri, Rita; Zunger, A.
abstract
We show that the energy of substitutional randomization of atoms in zinc-blende compounds is surprisingly small. This suggests the existence of a new class of defects in these materials ("random aggregates"), which consist of regions of less than or similar to 10 atoms where the sites of the diamond lattice are randomly occupied by A or B atoms in place of the ordered AB crystal occupancy. The structural and electronic properties of these defects are outlined.
1991
- Electronic structure and density of states of the random Al0.5Ga0.5As, Ga0.5As0.5P, and Ga0.5In0.5As semiconductor alloys
[Articolo su rivista]
Magri, Rita; S. FROYEN AND A., Zunger
abstract
The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical bowing of the direct band gaps are calculated for three perfectly random semiconductor alloys within the first-principles pseudopotential method using the concept of ``special quasirandom structures'' (SQS's). The SQS's are periodic structures with moderately large unit cells whose sites are occupied by atoms in a way designed to reproduce the structural features of the infinite, perfectly random substitutional alloys. In avoiding averaging over atoms (as in the virtual-crystal approximation) or over atomic environments (as in the site-coherent-potential approximation), this approach is capable of revealing the multisite nature of chemical disorder, as well as atomic-relaxation effects. We show how the existence of different local environments about chemically identical sites leads to splittings and fine structures in the density of states, and how atomic relaxations are induced by such n...
1991
- Predicting structural energies of atomic lattices
[Articolo su rivista]
MAGRI, Rita; J. E., BERNARD; A., ZUNGER
abstract
The complexity of current ab initio quantum-mechanical calculations of the total energy of given distributions of atoms on a periodic lattice often limits explorations to just a few configurations. We show how such a small number of calculations can be used instead to compute the interaction energies of a generalized Ising model, which then readily provides predicted energies of many more interesting configurations. This is illustrated for AlAs/GaAs systems.
1991
- Real-space description of semiconducting band gaps in substitutional systems
[Articolo su rivista]
MAGRI, Rita; ZUNGER, A.
abstract
The goal of ``band-gap engineering'' in substitutional lattices is to identify atomic configurations that would give rise to a desired value of the band gap. Yet, current theoretical approaches to the problems, based largely on compilations of band structures for various latice configurations, have not yielded simple rules relating structural motifs to band gaps. We show that the band gap of substitutional AlAs/GaAs lattices can be usefully expanded in terms of a hierarchy of contributions from real-space ``atomic figures'' (pairs, triplets, quadruplets) detemined from first-principles band-structure calculations. Pair figures (up to fourth neighbors) and three-body figures are dominant. In analogy with similar cluster expansions of the total energy, this permits a systematic search among all lattice configurations for those having ``special'' band gaps. This approach enables the design of substitutional systems with certain band-gap properties by assembling atomic figures. As an il...
1991
- Thermodynamic instability of ordered (001) GaAlAs in bulk form
[Articolo su rivista]
Magri, Rita; A., Zunger
abstract
Recent tight-binding calculations of bulk electronic total energies by Koiller, Davidovich, and Falicov (KDF) [Phys. Rev. B 41, 3670 (1990)] indicated the tendency for Al1-xGaxAs alloys to form ordered structures. The stablest structure they predicted was the monolayer (AlAs)1(GaAs)1[001] superlattice, which was recently observed in homogeneous vapor-phase growth. In light of these results we have examined the possibility that bulk energetics can explain this ordering. We have subjected KDF's tight-binding total-energy calculations and, separately, our own first-principles pseudopotential total-energy calculations to a statistical-mechanics analysis of order-disorder transitions. We find that bulk thermodynamics is inconsistent with the observed ordering; hence, explanations must be sought elsewhere (e.g., surface thermodynamics or kinetic effects).
1990
- Characterization of Bioacceptable Carbon Materials
[Articolo su rivista]
Magri, Rita; C., Mariani; Ottaviani, Giampiero
abstract
Bioacceptable carbon films deposited with a new low temperature technique have been investigated using Raman and X-ray photoelectron spectroscopy. Our results reveal a substantial similarity between the low temperature deposited films and the well-known bioacceptable low temperature isotropic (LTI) pyrocarbon. The Raman spectra, as well as the larger density and oxygen content in the bulk, lead to the speculation of the presence of the common tetrahedral bonds in these materials.
1990
- Electronic band structure of the (GaAs)1/(InAs)1 (111) superlattice
[Articolo su rivista]
Magri, Rita
abstract
We present a first-principles calculation of the electronic band structure of (GaAs)1/(InAs)1 (111) superlattice, performed by using ab initio norm-conserving pseudopotentials. The folding in the smaller Brillouin zone and the new symmetry properties of the electronic states have been thoroughly investigated by a comparison with the band structure of the corresponding disordered alloy treated within the virtual-crystal approximation. The interactions between the folded states giving rise to energy splittings and electronic charge localization on given layers have been analyzed at the high-symmetry points. The effects of the new ordering potential, introduced by differentiating the two cations in the virtual crystal, and of the strain potential—arising when the superlattice is subjected to a lateral strain on the (111) planes—are separately investigated. We find a strong charge localization in the first two conduction states at the Gamma point and a considerable minimum-band-gap redu...
1990
- Ground state structures and the random state energy of the Madelung lattice
[Articolo su rivista]
Magri, Rita; S. H., Wei; A., Zunger
abstract
Rapid Communications - We consider the classic Madelung problem of a lattice with N sites labeled i, each occupied by either an A or a B atom, and bearing a point charge Qi that depends on the environment of i. We find that, out of the 2N possible lattice configurations of this binary A1-xBx fcc alloy, the lowest-energy ``ground-state structures'' are the A3B-, A2B2- and AB3-ordered superlattices with ordering vector (1,0,1/2). On the other hand, for the pseudobinary A1-xBxC zinc-blende alloy, the ground state corresponds to phase separation into AC+BC. Contrary to the accepted view, the Madelung energy of the random binary alloy is found to be nonvanishing.
1989
- Search for stable configuration of (GaAs)1(InAs)1 (111) superlattice
[Articolo su rivista]
Magri, Rita; CALANDRA BUONAURA, Carlo
abstract
The epitaxial formation energy of a (GaAs)1 (InAs)1 (III) superlattice grown on an InPsubstrate is calculated by using a self-consistent pseudopotential approach within the densityfunctional theory. Atomic configurations are considered which have two distinct bonddistances and correspond to different relaxations along the [111] direction. The minimumenergy is found assigning to the superlattice the same bond distances obtained by minimizingthe total energy of the two binary systems under the epitaxial constraint. For all thegeometries we find that the superlattice is unstable with respect to disproportionation intothe constituents•
1989
- Structural stability and valence charge density in (GaAs)1(InAs)1 (111) superlattice
[Articolo su rivista]
Magri, Rita; CALANDRA BUONAURA, Carlo
abstract
We present a theoretical study of the structural stability and the ground-state electronic distribution of a (GaAs)1/(InAs)1 superlattice grown along the [111] direction. The calculations were carried out self-consistently using norm-conserving pseudopotentials in the local-density approxima- tion. We find that the unrelaxed superlattice with a single anion-cation bond length is unstable with respect to disproportionation into the binary systems. By allowing for relaxation through plane displacements along the [111] directions under the constraints of having only two different bond lengths, the energy may be significantly lowered, but the superlattice remains unstable. The effects of deformation, chemical exchange, and relaxation on the valence charge distribution are investigated and the peculiar characteristics of the chemical bond in the superlattice are pointed out.
1989
- Surface effects in the electronic properties of YBa2Cu3O7
[Articolo su rivista]
CALANDRA BUONAURA, Carlo; Goldoni, Guido; Manghi, Franca; Magri, Rita
abstract
The electronic structure of basal plane surfaces of the high-T, superconducting material YBa,Cu,O, has been determined using a tint-binding Hamilto~~n derived from a first-p~nciple calculation of the bulk phase. The surface local density of states has been obtained for four different surface configurations, which differ for the yttrium and oxygen content. The results show that the presence and state of the surface have a strong influence on the distribution of single particle states. The nature of the surface induced modifications is detailed and the relevance of the results to the understanding of experimental data is shortly discussed.