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Rita TONINI

Personale tecnico amministrativo
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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Pubblicazioni

2018 - Transmission electron microscopy study of helium implanted silicon [Capitolo/Saggio]
Frabboni, Stefano; Corni, Federico; Tonini, Rita; Nobili, Carlo Emanuele; Ottaviani, Giampiero
abstract

The structural evolution of helium-related extended defects in silicon, formed after intermediate dose helium implantation and annealing, has been investigated. It is found that the highest helium concentration (annealing at 300°C) is associated with clusters of bubbles arranged in a platelet-like morphology. At 500°c the helium concentration markedly decreases and clusters of cavities formed by a central, large cavity surrounded by small cavities (planetarylike structures) are detected. Thermal treatment at 900°c accomplishes complete helium effusion from the sample leaving behind empty cavities i.e. voids.

2014 - Nanovoid Formation and Dynamics in He+-Implanted Nanocrystalline Silicon [Articolo su rivista]
Lorenzi, Bruno; Frabboni, Stefano; Gazzadi, gian carlo; Tonini, Rita; Ottaviani, Giampiero; Narducci, Dario
abstract

Helium implantation in single crystal silicon is known to lead, after a proper thermal treatment, to the formation of voids with diameters ranging between 10 nm and 30 nm. Formation of voids is governed by the coalescence of vacancies created by implantation, initially trapping helium atoms. At high temperatures (), helium leaves the nanobubbles and outdiffuses, while the now empty voids grow in size and eventually change their shape to form tetrakaidecahedra (Wulff construction). In this communication, we report how He+ implantation in heavily boron-doped nanocrystalline silicon shows a completely different dynamics. Annealing at leads to the formation of large voids, located around grain boundaries, along with a large number of nanovoids with an average diameter of 2-4 nm and an estimated density of distributed throughout the grains. Annealing at higher temperature (up to ) also induces a decrease of the void size with a change in their density, finally accounting to . The high temperature annealing also causes vacancy evaporation down to a depth of 80-100 nm from the outer surface. The possibility of obtaining a stable, uniform distribution of nanometer-sized voids is of major relevance as a novel tool for phonon and electron engineering in thermoelectric materials.

2014 - Paradoxical Enhancement of the Power Factor of Polycrystalline Silicon as a Result of the Formation of Nanovoids [Articolo su rivista]
Lorenzi, B.; Narducci, D.; Tonini, Rita; Frabboni, Stefano; Gazzadi, G. C.; Ottaviani, Giampiero; Neophytou, N.; Zianni, X.
abstract

Hole-containing silicon has been regarded as a viable candidate thermoelectric material because of its low thermal conductivity. However, because voids are efficient scattering centers not just for phonons but also for charge carriers, achievable power factors (PFs) are normally too low for its most common form, i.e. porous silicon, to be of practical interest. In this communication we report that high PFs can, indeed, be achieved with nanoporous structures obtained from highly doped silicon. High PFs, up to a huge 22 mW K-2 m(-1) (more than six times higher than values for the bulk material), were observed for heavily boron-doped nanocrystalline silicon films in which nanovoids (NVs) were generated by He+ ion implantation. In contrast with single-crystalline silicon in which He+ implantation leads to large voids, in polycrystalline films implantation followed by annealing at 1000A degrees C results in homogeneous distribution of NVs with final diameters of approximately 2 nm and densities of the order of 10(19) cm(-3) with average spacing of 10 nm. Study of its morphology revealed silicon nanograins 50 nm in diameter coated with 5-nm precipitates of SiB (x) . We recently reported that PFs up to 15 mW K-2 m(-1) could be achieved for silicon-boron nanocomposites (without NVs) because of a simultaneous increase of electrical conductivity and Seebeck coefficient. In that case, the high Seebeck coefficient was achieved as a result of potential barriers on the grain boundaries, and high electrical conductivity was achieved as a result of extremely high levels of doping. The additional increase in the PF observed in the presence of NVs (which also include SiB (x) precipitates) might have several possible explanations; these are currently under investigation. Experimental results are reported which might clarify the reason for this paradoxical effect of NVs on silicon PF.

2012 - Using evidence from nanocavities to assess the vibrational properties of external surfaces [Articolo su rivista]
G. F., Cerofolini; CORNI, Federico; FRABBONI, Stefano; OTTAVIANI, Giampiero; E., Romano; TONINI, Rita; D., Narducci
abstract

Internal surfaces of nanocavities are an exceptionally useful laboratory wherein one can spotlight the factors ruling the intricate interplay between morphology and chemistry at silicon surfaces. At the same time, they offer unparalleled opportunities to validate the assignment of vibrational signals of silicon-terminating species under almost ideal experimental conditions. In the case of hydrogen, evidence will be provided of the detailed evolution of H-related species at surfaces depending on their orientation. Also, preliminary results concerning nitrogen at and around nanocavity surfaces will be reported.

2010 - A Tool for the Spectroscopic Investigation of Hydrogen-Silicon Interaction [Articolo su rivista]
E., Romano; G. F., Cerofolini; D., Narducci; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita
abstract

The preparation of an array of nanocavities (NCs) in silicon via thermal treatments of high-fluence heliumimplantedsilicon is a well established process. The NCs have oered a powerful tool for the preparation of welldefined and ordered internal silicon surface, enabling the experimental investigation of its free energy. Here,instead, we interpret the NCs as an ideal nanolaboratory for the study of the adsorption and desorption of H2on silicon. We will present the determination of the internal pressure through the equilibrium abundances ofmono-hydride dimers and dihydrides obtained with infrared spectroscopy.

2010 - Adsorption equilibria and kinetics of H2 at nearly ideal (2 x 1) Si(1 0 0) inner surfaces [Articolo su rivista]
G. F., Cerofolini; E., Romano; D., Narducci; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita
abstract

Silicon nanocavities can be terminated with hydrogen by wet chemical etching. Their infrared spectra can to a large extent be interpreted in terms of silicon monohydrides on H(7 x 7) Si(1 1 1), H(1 x 1) Si(1 1 1) and H(2 x 1) Si(1 0 0), and of silicondihydrides on H(1 x 1) Si(1 0 0). The time evolution under isothermal conditions (600 °C) of the (1 0 0) faces admits a description in terms of transformation from H(1 x 1) Si(1 0 0) into (2 x 1) Si(1 0 0) with simultaneous H2 adsorption onto (2 x 1) Si(1 0 0) neat dimers. In so doing the inner H2 pressure decreases by about one order of magnitude from the initial value of 3000 Torr. The unique properties of nanocavities allow their use as nanoreactors; this has led to the determination of the sticking coecient for H2 adsorption in conditions of pressure, temperature and adventitious contamination otherwise not achievable.

2009 - Evidence for H-2 at high pressure in the silicon nanocavities after dipping in HF solution [Articolo su rivista]
Romano, E.; Cerofolini, G. F.; Narducci, D.; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita
abstract

The immersion in HF solutions of silicon containing nanocavities (produced by the annealing at high temperature, 950 degrees C, of silicon implanted with helium at high fluence, 2 X 10(16) cm(-2)) results in the injection of hydrogen in an infrared-mute state (most likely HA into the nanocavities. The pressure achieved in the cavities is sufficiently high to stabilize the hydrogen coverage of the inner surfaces at: temperatures exceeding by 200 degrees C the one of complete desorption from the outer surface.

2009 - Hydrogen injection and retention in nanocavities of single-crystalline silicon [Articolo su rivista]
G. F., Cerofolini; E., Romano; D., Narducci; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita
abstract

The control of the chemical state of the inner surfaces of nanocavities (NCs) produced by the annealing of helium-implanted silicon has influence on lifetime control, gettering and wafer bonding. In this work it is demonstrated that the etching in HFaq of (1 0 0) silicon containing a buried array of NCs produces a giant injection of hydrogen with the consequent passivation of the inner surfaces, mainly via the formation of silicon monohydride at (1 1 1) faces and monohydride dimers at 2 × 1 reconstructed (1 0 0) faces. These terminations are very stable and survive heat treatments at 700 °C.

2009 - Nanocavities in silicon: An infrared investigation of internal surface reconstruction after hydrogen implantation [Articolo su rivista]
E., Romano; D., Narducci; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita; G. F., Cerofolini
abstract

The preparation of tetrakaidecahedron-shaped nanocavities in silicon via thermal treatments of high-fluence helium-implanted silicon is a well established process. When the mean distance between Such cavities is on the length scale of the exciton diameter, they are expected to modulate the silicon band structure. This property, however, can hardly be exploited due to the large number of dangling bonds remaining on the cavity inner surface at the end of the process. An easy way to reduce their amount is to passivate them with hydrogen. To investigate the interaction of hydrogen with (i) bulk silicon and (ii) the inner surface of the nanocavities, hydrogen was implanted in silicon on preformed nanocavity arrays and the evolution of the Si-H complexes after isochronal annealings in the temperature range 150-80 degrees C was sensed by infrared spectroscopy in multiple internal reflection geometry. In contrast with previous findings, we will provide evidence that there is no measurable redistribution from bulk defects to inner surface during the thermal treatments. Thus, within the limits of experimental sensitivity. surface passivation was proved to occur upon implantation. Annealing eliminates first the most reactive species (SiH3 and SiH2 above 500 and 550 degrees C, respectively): at higher temperatures only signals related to the remaining H passivation of the ideal reconstructed H (1 x 1)-Si(111). H (7 x 7)-Si(111), and H (2 x 1)-Si(100) Surfaces are observed.

2007 - Structural properties of reactively sputtered W-Si-N thin films [Articolo su rivista]
Vomiero, A; Marchi, Eb; Quaranta, A; DELLA MEA, G; Brusa, Rs; Mariotto, G; Felisari, L; Frabboni, Stefano; Tonini, Rita; Ottaviani, G; Mattei, G; Scandurra, A; Puglisi, O.
abstract

Tungsten-silicon-nitrogen, W-Si-N, ternary thin films have been reactively sputter deposited from W5Si3 and WSi2 targets using several nitrogen partial pressures. The films have been thermal annealed in the 600-1000 degrees C temperature range and a wide region of the W-Si-N ternary phase diagram has been explored by changing the N-2/Ar ratio during the deposition. Multitechnique approach was adopted for the analysis of the samples. Composition has been determined via ion beam analysis; chemical states were investigated using x-ray photoelectron spectroscopy (XPS); crystalline structure was studied using transmission electron microscopy (TEM) and x-ray diffraction (XRD) and surface morphology by scanning electron microscope. The films deposited in pure argon atmosphere are tungsten rich and approach the target contents as N2/Ar ratio is varied during deposition. Tungsten enrichment in the films is caused by resputtering of silicon which can be inhibited by the formation of silicon nitride, allowing films with Si/W ratio closer to the target compositions. The higher capability to form nitrides with silicon than with tungsten favors enhancement of nitrogen. content in samples deposited from the silicon rich target (WSi2). The samples with excess nitrogen content have shown losses of this element after thermal treatment. XPS measurements show a break of W-N bonds caused by thermal instability of tungsten nitrides. TEM and XRD revealed the segregation of tungsten in form of metallic or silicide nanoclusters in samples with low nitrogen content (W58Si21N21 and W24Si42N34). High amounts of nitrogen were revealed to be highly effective in inhibiting metallic cluster coalescence. Measurements of electrical resistivity of as deposited films were performed using four point probe technique. They were found to lie in the range between 0.4 and 79 m Omega cm depending on sample composition. (c) 2007 American Institute of Physics.

2006 - X-ray reflectivity study of hydrogen implanted silicon [Articolo su rivista]
P., Dubcek; B., Pivac; S., Bernstorff; Corni, Federico; Tonini, Rita; Ottaviani, Giampiero
abstract

The X-ray reflectivity (XRR) technique was used to study monocrystalline silicon samples implanted with H-2 ions at an energy of 31 keV and to the dose of 2 x 1016 hydrogen atoms/cm(2). All samples were subsequently isochronally annealed in vacuum at different temperatures in the range from 100 to 900 degrees C. Although the hydrogen depth distribution was expected to be smooth initially, fringes in the XRR spectra were observed already in the implanted but not annealed sample, revealing the presence of a well-defined film-like structure. Annealing enhances the film top to bottom interface correlation due to structural relaxation, resulting in the appearance of fringes in the larger angular range, already at low annealing temperatures. The thickness of the film decreases slowly up to 350 degrees C where substantial changes in the roughness are observed, probably due to the onset of larger clusters formation. Further annealing at higher temperatures restores the high correlation of the film interfaces, while the thickness decreases with the temperature more rapidly. (c) 2006 Elsevier B.V. All rights reserved.

2005 - Vibrational spectroscopy study of Ar+-ion irradiated Si-rich oxide films grown by plasma-enhanced chemical vapor deposition [Articolo su rivista]
G., Mariotto; G., Das; A., Quaranta; G., DELLA MEA; Corni, Federico; Tonini, Rita
abstract

SiOx thin films with different stoichiometry degree were obtained by plasma-enhanced chemical vapor deposition on crystalline silicon substrates from SiH4 and N2O gas mixtures. Two twin sets of samples were irradiated by 380 keV Ar+ ions at a fluence of 5×1016 ions/cm2 at room temperature and at 500 °C, respectively, and then annealed in vacuum at different temperatures, between 500 and 1100 °C. A set of unirradiated samples has been annealed in the same conditions in order to discriminate the contribution of ion irradiation and of thermal treatments to the changes of the film microstructure. The structural modification of the oxide network and the growth of Si nanoclusters have been studied by vibrational spectroscopy techniques. Fourier transform infrared absorption spectra evidenced that ion irradiation induces a hydrogen loss of about 50%, and that postirradiation thermal treatments lead to the recovery of the irradiation defects and to the out diffusion of the residual hydrogen. After heating at 800 °C, irradiated and unirradiated samples exhibit substantially the same structure from the point of view of infrared-absorption spectra. In the meanwhile, the Si-O-Si stretching peak blue shifts, but never reaches the wavenumber value of pure silica owing to the presence of nitrogen into the network. Raman spectra of as-irradiated films reveal the presence of an amorphous silicon phase within the damaged layer of the oxide matrix. Raman spectra of irradiated samples undergoing thermal treatments at high temperature indicate a rearrangement of the film microstructure with the progressive clustering of the amorphous silicon phase. However, no clear spectroscopic evidence is gained about the crystallization of silicon nanoclusters, even after annealing at the highest temperature. In fact, the Raman scattering from silicon nanocrystals is partially hidden by the Raman peak of the c-Si substrate.

2004 - Copper–titanium thin film interaction [Articolo su rivista]
L., Castoldi; G., Visalli; S., Morin; P., Ferrari; S., Alberici; Ottaviani, Giampiero; Corni, Federico; Tonini, Rita; Nobili, Carlo Emanuele; M., Bersani
abstract

Interaction between 5 mum thick copper and 50 nm thin titanium films was investigated as a function of annealing temperature and time using MeV He-4(+) Rutherford backscattering, X-ray diffraction and dynamic Secondary Ion Mass Spectrometry. Samples were made by depositing 10 nm of titanium on a PECVD silicon oxynitride, followed by 50 nm of titanium nitride and 50 nm of titanium in the said order. In the same system 100 nm of copper were subsequently sputtered; finally 5 mum of copper were grown by electroplating. This complex structure was chosen in order to investigate the possibility of using copper interconnects also in power devices. To investigate the composition and growth of Ti-Cu compound on the buried interface, it was necessary to develop a special procedure. The results of the investigation show the formation of a laterally non-uniform layer of TiCu4, which is presumably preceded by the formation of CuTi. The growth of the compound is kinetically controlled by means of a diffusion coefficient having 1.7 eV activation energy and a 5 x 10(-2) cm(2)/s pre-exponential factor. The formation of a titanium-copper compound ensures a reliable and low resistance electrical contact especially at the vias. The Ti/TiN/Ti acts efficiently as a sacrificial and inert diffusion barrier. No copper was detected on the silicon oxynitride surface even after a 20-min 500 degreesC heat treatment.

2004 - GISAXS Study of Hydrogen Implanted Silicon [Articolo su rivista]
B., Pivac; P., Dub; Ek, ; S., Bernstorff; Corni, Federico; Tonini, Rita
abstract

The grazing incidence small angle X-ray scattering (GISAXS) technique was used to study monocrystalline silicon samples implanted with H-2 ions at an energy of 31 keV and to the dose of 10(16) ions/cm(2). Samples were annealed isochronally at different temperatures. Although the H depth distribution was expected to be smooth initially, nanosized features, like agglomerates of defects, have been detected. Annealing destroys this feature due to the relaxation of defects, i.e. redistribution of vacancies and hydrogen. Above 300degreesC a well defined film with highly correlated borders is formed on the edge of the layer rich in defects, whose thickness is slowly decreasing from 25 to 22 nm with increasing annealing temperature. Moreover, defects as well as hydrogen are migrating towards the surface with increasing annealing temperature, as indicated by the increase in surface roughness.

2004 - Nanovoid formationin helium-implanted single-crystal silicon studied by in situ techniques [Articolo su rivista]
Frabboni, Stefano; Corni, Federico; Nobili, Carlo Emanuele; Tonini, Rita; Ottaviani, Giampiero
abstract

In situ transmission electron microscopy and thermal desorption spectrometry have been employed to observe the evolution of vacancy-type extended defects and the corresponding helium state in helium implanted single crystal silicon during thermal ramping annealing. The structural evolution starting with the formation of a platelike cluster of high pressurized helium bubbles and ending in an empty nanovoid is performed conserving tha total volume of vacancy-type extended defects forming each cluster. Structural adjstements occur by surface diffusion inside each cluster following a migration and coalescence mechanism in presence of high pressure helium for 350°C /T/570°C. A conservative Ostwald ripening is observed for 570/T/700°C in presence of helium desorption.

2004 - Phase formations in Co-Silicon system [Articolo su rivista]
Corni, Federico; Tonini, Rita; Ottaviani, Giampiero; S., Alberici; D., Erbetta; T., Marangon
abstract

Interactions between thin cobalt, 40 nm, and a thick - 250 nm -silicon film were investigated by in situ sheet resistance measurements. Poly and amorphous silicon films, both deposited by CVD, were used. MeV He-4(+) Rutherford Backscattering and X-ray diffraction techniques were used to infer the kinds and thicknesses of the compounds formed. Co2Si and CoSi form, on both silicon substrates, in the same way and in substantial consistency with the data reported in literature. The formation of CoSi2 seems to be nucleation controlled on polysilicon, while on amorphous the formation is controlled by diffusion and no nucleation barrier has been detected. The amorphous silicon remains amorphous during the CoSi2 formation, thus suggesting that the model which attributes the absence of a nucleation barrier to silicon crystallization does not hold. (C) 2004 Elsevier B.V. All rights reserved.

2004 - Transmission Electron Microscopy study of Helium Implanted Silicon [Relazione in Atti di Convegno]
Frabboni, S.; Corni, F.; Tonini, R.; Nobili, C.; Ottaviani, G.
abstract

The structural evolution of helium related extended defects in silicon, formed after intermediate dose helium implantation and annealing has been investigated. It is found that the highest helium concentration (annealing at 300ºC) is associated to clusters of bubbles arranged in a platelet-like morphology. At 500ºC the helium concentration markedly decreases and clusters of cavities formed by a central, large cavity surrounded by small cavities (planetary like structures) are detected. The thermal treatment at 900ºC accomplishes the complete helium effusion from the sample leaving behind empty cavities i.e. voids.

2004 - Transmission electron microscopy study of blisters in high-temperature annealed He and H co-implanted single-crystal silicon [Articolo su rivista]
Frabboni, Stefano; Gc, Gazzadi; L., Felisari; Tonini, Rita; Corni, Federico; Ottaviani, Giampiero
abstract

Transmission electron microscopy has allowed us to draw the three-dimensional structure of blisters formed after high-temperature annealing of He-H co-implanted silicon by combining the unique capability of site-selective cross sectioning of the focused ion beam with conventional plan view images. It has been shown that blisters are formed by crystalline lamellae strongly bended, plastically deformed, and suspended over buried empty cavities. The volume of the protruding blister surface is almost equal to the buried empty volumes, thus suggesting a mechanism for blister formation based on H and He precipitation and migration of silicon atoms toward the surface.

2003 - Early stages of bubble formation in helium-implated (100) silicon [Articolo su rivista]
B., Pivac; O., Milat; P., Dubcek; S., Bernstorff; Corni, Federico; Nobili, Carlo Emanuele; Tonini, Rita
abstract

Early stages of bubble formation in Czochralski grown, (100)-oriented silicon are investigated by implanting 2 x 10(16) He+ cm(-2) at 20 keV and treating the samples in the temperature range 100-450 degreesC. Elastic recoil detection is used to measure the helium content and depth distribution, while the gas release is studied by thermal desorption spectrometry. To evidence the radiation damage evolution and the defect behavior, the results from Rutherford backscattering spectrometry in channeling conditions, cross sectional transmission electron microscopy, and grazing incidence small angle X-ray scattering are critically analyzed and compared. The study leads to the identification of the steps followed by the defects during annealing and suggests that critical condition for the production of stable bubbles is the presence at low temperature of agglomerates composed of more than four vacancies decorated by helium. This kind of defect offers helium the room necessary to transform into gas phase and confer thermal stability to the bubble embryos.

2003 - GISAXS study of structural relaxation in amorphous silicon [Articolo su rivista]
Dubcek, P; Pivac, B; Bernstorff, S; Tonini, Rita; Corni, Federico; Ottaviani, Giampiero
abstract

The structural changes induced in single crystal silicon implanted with silicon ions above the amorphisation threshold were studied by Grazing Incidence Small Angle X-ray Scattering technique. Silicon samples were implanted with silicon ions at 30 keV to the dose of 5 x 10(15) atoms/cm(2). A well-defined layer of amorphous silicon, thick about 40 nm was formed below the surface. As implanted samples were subsequently relaxed by thermal annealing at 350 degreesC. The analysis have shown that the amorphous layer exhibits a granular structure that develops with annealing. A model will be presented for the film structure changes obtained by data evaluation based on the distorted wave Born approximation. (C) 2002 Elsevier Science B.V. All rights reserved.

2003 - Grazing incidence small-angle X-ray scattering study of defects in deuterium implanted monocrystalline silicon [Articolo su rivista]
P., Dubcek; B., Pivac; S., Bernstorff; Corni, Federico; Tonini, Rita; Ottaviani, Giampiero
abstract

Grazing incidence small-angle X-ray scattering was used to study monocrystalline silicon samples implanted with deuterium ions at an energy of 24 keV and to the dose of 2 x 10(16) ions/cm(2). Samples were annealed isochronally at different temperatures in the range from 393 to 973 K. Due to the relaxation of the defects structures, i.e. redistribution of vacancies and deuterium, strong particle like contribution is observed in addition to the rough surface scattering, already at 393 K annealing. During the annealing, this particles (agglomerations of vacancies) are gradually dissolved till 623 K annealing temperature. Another agglomeration mechanism takes over at about 773 K when a different type of particle growth is observed, and these are dissolved again at about 973 K. The sizes of detected particles are in 2-3 nm range. Also, the interference type of scattering from a film of about 30 nm thickness (the top layer, mostly unaffected by implantation) is observed. This film is gradually getting thinner with the increasing annealing temperature, due to the redistribution of the defects and the structure relaxation.

2002 - DLTS and EPR study of defects in H implanted silicon [Articolo su rivista]
V., Miksic; B., Pivac; B., Rakvin; H., Zorc; Corni, Federico; Tonini, Rita; Ottaviani, Giampiero
abstract

Single crystal CZ Si samples were implanted with hydrogen ions to the dose of 2E16 He ions/cm(2) at room temperature and subsequently annealed in vacuum in the temperature interval from 100 to 900 degreesC. The aim of the experiment was to determine the conditions for bubble formation within the solid film, which may have important technological application. Defects produced in such samples were studied by deep-level transient spectroscopy (DLTS) and electron paramagnetic resonance (EPR) spectroscopy. It is shown that high dose hydrogen implantation produces vacancy-related and silicon selfinterstitial clusters. The latter are thought to be responsible for the formation of the weak displacement field. The annealing at higher temperatures creates multivacancy-related clusters responsible for the strong displacement field formation. (C) 2002 Elsevier Science B.V. All rights reserved.

2002 - Damage evolution in helium-hydrogen co-implanted (100) silicon [Articolo su rivista]
Tonini, Rita; Corni, Federico; Nobili, Carlo Emanuele; Ottaviani, Giampiero; Cazzaniga, F; Queirolo, G.
abstract

The modifications induced in single-crystal silicon by helium and hydrogen complantation have been investigated as a function of isochronal heat treatments in the 100-900 degreesC temperature range. 30 keV helium ions were implanted into (100) p-type silicon wafer held at RT with a dose Of 1x10(16) cm(-2); Successively 1x10(16) cm(-2) hydrogen ions were implanted at 24 keV. Various analytical techniques have been used to characterize the in-depth samples: 2 MeV He-4(+) backscattering in channeling conditions (RBS-channeling) to detect damage distribution; elastic recoil detection (ERD) technique with 8 MeV N-15(++) and 2 MeV He-4(+) to measure the helium and hydrogen concentration; cross sectional transmission electron microscopy (XTEM) analysis to characterize the distribution of the defects. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to analyze the surface morphology. According to the backscattering-channeling data two kinds of damage are present: one, damage A, located in a narrow region at a depth corresponding to helium and hydrogen mean projected range and another one, damage B, located from 270 nm to the sample surface. Damage A increases up to 400 degreesC and at higher temperatures tends to disappear. Damage B reaches a maximum after 500 degreesC and stays constant at higher temperatures. Above 500 degreesC the sample surface exhibits blistering and exfoliation phenomena. The area fraction covered by the blisters is proportional to the dechanneling signal caused by damage B. Above 700 degreesC helium and hydrogen are completely desorbed. The presence of blisters and of the related dechanneling signal indicate that silicon is plastically deformed.

2002 - In-situ time-resolved reflectivity: a technique useful to investigate solid-state transformations [Articolo su rivista]
Corni, Federico; Tonini, Rita; Pontiroli, A; Pavia, G; Queirolo, G; Zonca, R.
abstract

Solid-state transformations occurring in thin films can be studied by directing a laser beam on to the sample surface and measuring the reflected intensity by means of a photodiode. The technique, called Time-Resolved Reflectivity, is an in-situ technique very fast and simple. Under proper conditions, it allows the entire evolution of a transformation to be followed while it occurs during a thermal treatment, allowing the kinetics of the process to be studied. In this work we present a method and some criteria to extract physical information on layered samples interesting for the microelectronic industry. The surface reflectivity depends on the refractive indices (wavelength dependent) of the materials employed and on the thicknesses of the layers. The results extracted from the raw reflectivity data (obtained with a conventional He-Ne 632.8 nm laser) by means of few simple assumptions, are discussed and compared to those obtained with more conventional, but in most cases, destructive and ex-situ techniques. (C) 2002 Elsevier Science B.V. All rights reserved.

2002 - Reply to comments on 'EPR study of He-implanted Si' by P. Pivac, B. Rakvin, R. Tonini, F. Corni, G. Ottaizani, Published in Mater. Sci. Eng. B73 (2000) 60-63 - Written by M. Kakazey, M. Vlasova, and J.G. Gonzalez-Rodriguez - Reply to discussion [Articolo su rivista]
Pivac, B; Rakvin, B; Tonini, Rita; Corni, Federico; Ottaviani, Giampiero
abstract

Reply to comments on 'EPR study of He-implanted Si' by P. Pivac, B. Rakvin, R. Tonini, F. Corni, G. Ottaizani, Published in Mater. Sci. Eng. B73 (2000) 60-63 - Written by M. Kakazey, M. Vlasova, and J.G. Gonzalez-Rodriguez - Reply to discussion

2002 - Some aspects of blistering and exfoliation of helium-hydrogen coimplanted (100) silicon [Articolo su rivista]
Corni, Federico; Tonini, Rita
abstract

A Si(He,H) system, produced by ion implantation and suitable for SOI fabrication, is studied during and after the thermal treatment which induces surface blistering and exfoliation. The attention is focused on the influence of storage time between the implantation stage and the annealing stage, since it is expected that even at low temperatures the phenomena taking part in the blistering and exfoliation process are active. Three different storage environments are selected: (1) liquid nitrogen, (2) room temperature and (3) 100 °C in air. The main result is that long time sample storage has the effect of decrease the exfoliation efficiency. Moreover, optimum conditions for exfoliation are met at storage time around 24 h at 100 °C, where the maximum fraction (1) of the sample surface (about 16%) results exfoliated and (2) of He atoms (about 80%) are desorbed. The discussion evidences that He desorbtion during annealing and exfoliation efficiency are strictly correlated. Moreover, a phenomenological model explaining the observed storage effect is sketched, deduced from the present knowledge of the Si(H), Si(He) and Si(He,H) systems.

2001 - Helium/deuterium co-implanted silicon – a thermal desorption spectrometry investigation [Articolo su rivista]
Corni, Federico; Nobili, Carlo Emanuele; Tonini, Rita; Ottaviani, Giampiero; Tonelli, Massimo
abstract

Thermal desorption spectrometry has been applied to investigate the blistering and exfoliation phenomena which occur at the surface of a p-type (100) silicon wafer coimplanted with helium and deuterium. During the heat treatments in linear temperature ramp, an explosive emission of both gases occurs. The phenomenon is kinetically controlled with an effective activation energy of 1.3±0.2 eV. In addition, the desorption spectra present a second contribution, attributed to deuterium emission from buried cavities. Also in this case, the process is kinetically controlled with an effective activation energy of 1.9±0.3 eV. Thermal desorption spectrometry is a suitable technique to have information about various phenomena which occur during blistering and exfoliation.

2000 - EPR study of He-implanted Si [Articolo su rivista]
Pivac, B; Rakvin, B; Tonini, Rita; Corni, Federico; Ottaviani, Giampiero
abstract

Electron paramagnetic resonance has been used to study the influence of thermal treatments on defect evolution in helium-implanted Czochralski single-crystal silicon. It is shown that the thermal treatment induces helium migration and capturing by vacancy clusters that transform into pressurized bubbles. Such transformation produces a strain field, which in turn affects the dangling bond's lineshape in its vicinity. It is shown that the strain field causes asymmetry of dangling bond lineshape that is proportional to the strain field. This selects the electron paramagnetic resonance as a convenient technique For the monitoring of the early phases of bubble formation. (C) 2000 Elsevier Science S.A. All rights reserved.

2000 - Electron paramagnetic resonance study of S2 defects in Hydrogen implanted Silicon [Articolo su rivista]
B., Rakvin; B., Pivac; Tonini, Rita; Corni, Federico; Ottaviani, Giampiero
abstract

Electron paramagnetic resonance spectra of S2 paramagnetic center in Float zone silicon (FZ-Si) and Czochralski silicon (CZ-Si) produced by H2 ion-implantation and by subsequent annealing have been studied. At room temperature the spectrum exhibits an isotropic line g 2X0066 0X0002, which, then broadens and transforms in a complex spec- trum at 120 K. This complex spectrum can be decomposed into isotropic and anisotropic component. The relative concentration of anisotropic component is larger in CZ-Si and smaller in FZ-Si than the respective isotropic compo- nent. The g tensor evaluated from the anisotropic component at 120 K shows that the spectrum originated from the center with trigonal symmetry. The involvement of hydrogen and oxygen atoms in the center has been discussed. It is suggested that the S2 center is developed in the multi-vacancy defect containing oxygen and hydrogen atoms. From reversible thermal eect of the linewidth narrowing the motional properties of the dangling bond in the S2 center is described by the activation energy of 0.03 eV. Ó 2000 Elsevier Science B.V. All rights reserved.

2000 - Evolution of vacancy-like defects in He-implanted (100) Si studied by thermal desorption spectrometry [Articolo su rivista]
Corni, Federico; G., Calzolari; Gambetta, Francesca; Nobili, Carlo Emanuele; Tonini, Rita; Zapparoli, Mauro
abstract

hermal desorption spectrometry (TDS) has been applied to investigate the thermal evolution of vacancy-like defects in helium-implanted (100) silicon samples with a dose of 2x1016 cm-2 at 20 keV. The measured spectra present features which can be interpreted as fingerprints of the modifications occurring in the sample. The defects that are recognized which affect the desorption are: thermally unstable helium-vacancy complexes, pressurized gas bubbles organized in planar structures (cracks) and thermally stable cavities. The attribution is supported by the results obtained by complementary techniques, such elastic recoil detection, channeling Rutherford backscattering spectrometry, cross sectional transmission electron microscopy and positron annihilation spectroscopy which have been employed on isothermally pre-annealed samples in the range 100-800oC

2000 - Formation of vacancy clusters and cavities in He-implanted silicon studied by slow-positron annihilation spectroscopy [Articolo su rivista]
R. S., Brusa; G. P., Karwasz; N., Tiengo; A., Zecca; Corni, Federico; Tonini, Rita; Ottaviani, Giampiero
abstract

The depth profile of open volume defects has been measured in Si implanted with He at an energy of 20 keV, by means of a slow-positron beam and the Doppler broadening technique. The evolution of defect distributions has been studied as a function of isochronal annealing in two series of samples implanted at the fluence of 5×1015 and 2×1016He cm-2. A fitting procedure has been applied to the experimental data to extract a positron parameter characterizing each open volume defect. The defects have been identified by comparing this parameter with recent theoretical calculations. In as-implanted samples the major part of vacancies and divacancies produced by implantation is passivated by the presence of He. The mean depth of defects as seen by the positron annihilation technique is about five times less than the helium projected range. During the successive isochronal annealing the number of positron traps decreases, then increases and finally, at the highest annealing temperatures, disappears only in the samples implanted at the lowest fluence. A minimum of open volume defects is reached at the annealing temperature of 250°C in both series. The increase of open volume defects at temperatures higher than 250°C is due to the appearance of vacancy clusters of increasing size, with a mean depth distribution that moves towards the He projected range. The appearance of vacancy clusters is strictly related to the out diffusion of He. In the samples implanted at 5×1015cm-2 the vacancy clusters are mainly four vacancy agglomerates stabilized by He related defects. They disappear starting from an annealing temperature of 700°C. In the samples implanted at 2×1016cm-2 and annealed at 850–900°C the vacancy clusters disappear and only a distribution of cavities centered around the He projected range remains. The role of vacancies in the formation of He clusters, which evolve in bubble and then in cavities, is discussed.

2000 - GISAXS study of defects in He implanted silicon [Articolo su rivista]
P., Dubcek; O., Milat; B., Pivac; S., Bernstorff; H., Amenitsch; Tonini, Rita; Corni, Federico; Ottaviani, Giampiero
abstract

The modifications induced in single-crystal silicon by implanted helium have been investigated by grazing incidence small angle X-ray scattering technique. The samples prepared by implanting 2×1016 cm−2 helium ions at 20 keV in silicon wafers held at 77 K were thermally treated for 2 h in the 100–800°C temperature range. It is shown that implantation produced a film with slightly lower density than that of undamaged single crystal silicon. Further thermal treatment caused shrinking of the film leaving well ordered subsurface layer and damaged film below. Upon annealing at 600°C this structure apparently disappear, to be formed again upon annealing at 800°C.

2000 - Hydrogen and helium bubbles in silicon [Articolo su rivista]
G. F., Cerofolini; Corni, Federico; Frabboni, Stefano; Nobili, Carlo Emanuele; Ottaviani, Giampiero; Tonini, Rita
abstract

Hydrogen is a quite common impurity in semiconductor-silicon technology: it is unintentionally but unavoidably added to the silicon after crystal growth during wafer processing, and continues to be present during wet oxidation, film depositions, etching and annealing steps. The effects of hydrogen in single crystal silicon at low concentration have been the subject of many papers, books and conference proceedings. Much less considered is the case of hydrogen at massive concentration. One Anal effect of heavy hydrogen loading is the formation of cavities and bubbles, with size up to 100 nm. Cavities and bubbles are also observed after helium loading by high-fluence ion implantation. This article reviews the basic mechanisms responsible for the formation and growth of such structures in single-crystalline silicon. In particular, starting from the loading (ion implantation) and having in mind the formation of the cavities, this paper will cover: the effects of substrate temperature, the interaction of vacancies and self-interstitials with the impurity, the mechanisms of gas segregation inside the cavities, the pressure which arises because of the segregation and the subsequent displacement field in the crystal, the stability against heat treatments of the gas in the cavities and of the cavities themselves. The understanding of the physical processes should lead to gain more insight in the processes of cleavage of the Si-Si bond and vacancy agglomeration which can induce not only the formation of cavities and bubbles, but also planar cutting or explosion. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

2000 - Large angle convergent beam electron diffraction strain measurements in high dose helium implanted silicon [Articolo su rivista]
Gambetta, Francesca; Frabboni, Stefano; Tonini, Rita; Corni, Federico
abstract

Large angle convergent beam electron diffraction (LACBED) technique has been applied to <110> TEM cross section of silicon samples implanted with 2x1016 cm-2 He ions at an energy of 20 KeV in order to evaluate the stress/strain field in the implanted layer. The stress/strain field depends on the mismatch between the lattice of the silicon substrate and the one of the defective layer which contains clusters of point defects and small He bubbles. Lattice mismatch causes changes in the spacing and inclination of diffraction planes producing a shift and a rotation of diffraction intensity lines (Bragg contours (BC)) visible in LACBED patterns. In particular, when the electron beam is parallel to the rotation axis, the BC simply rotate. This is the case of mismatches observed with (333) BC when the electron beam is incident on the sample along a direction close to <110> zone axis. Measurements show a peak of the stress/strain field (intensity 1.5 GPa) at a depth close to the helium projected range, where small bubbles occasionally form aggregate oriented along <011> direction.

2000 - Thermal desorption spectra from cavities in helium-implanted silicon [Articolo su rivista]
Cerofolini, Gf; Calzolari, G; Corni, Federico; Frabboni, Stefano; Nobili, Carlo Emanuele; Ottaviani, Giampiero; Tonini, Rita
abstract

Thermal desorption spectra at constant ramp rate have been determined after helium implantation into bare silicon prepared for a large set of experimental conditions. The spectra can phenomenologically be classified as composed by two peaks: the alpha peak, centered on a temperature of 750-800 degrees C with a shoulder extending to lower temperature (down to 550 degrees C), and the beta peak, centered on a lower temperature depending on the implantation-annealing conditions. The alpha peak is attributed to the emission from cavities, while the beta peak is attributed to the emission from vacancylike defects. A detailed theory describing helium effusion from stable cavities as controlled by the interatomic helium-helium potential is proposed and found to reproduce accurately most of the alpha peaks. The postimplantation of hydrogen into samples displaying a pure beta emission results in an alpha peak which can be described by the same model as above provided that the cavities are unstable and shrink during desorption in such a way as to maintain constant the concentration of contained helium.

2000 - Ultradense gas bubbles in Hydrogen- or Helium-implanted (or co-implanted) Silicon [Articolo su rivista]
Cerofolini, G. F.; Calzolari, G.; Corni, Federico; Nobili, Carlo Emanuele; Ottaviani, Giampiero; Tonini, Rita
abstract

Ultradense gas bubbles in Hydrogen- or Helium-implanted (or co-implanted) Silicon

1999 - Helium-implanted silicon: A study of bubble precursors [Articolo su rivista]
Corni, Federico; Calzolari, G; Frabboni, Stefano; Nobili, Carlo Emanuele; Ottaviani, Giampiero; Tonini, Rita; Cerofolini, Gf; Leone, D; Servidori, M; Brusa, Rs; Karwasz, Gp; Tiengo, N; Zecca, A.
abstract

The interaction of helium atoms with the radiation damage imparted to (100) silicon single crystal by He+ implantation at 5 x 10(15) cm(-2), 20 keV, and liquid-nitrogen temperature is investigated by means of various complementary techniques during and after thermal treatments. Thermal programmed desorption was used to study the dissociation kinetics of helium from the defects and to plan suitable heat treatments for the other techniques. The helium profiles were determined by 8 MeV N-15(2+) elastic recoil detection, quantitative data on damage were obtained by channeling Rutherford backscattering spectrometry, double crystal x-ray diffraction, and positron annihilation spectroscopy. Isothermal treatments at 250 degrees C produce first helium redistribution and trapping in vacancy-like defects, rather than helium desorption from traps. The process is thermally activated with an effective activation energy, dispersed in a band from 1.1 to about 1.7 eV. For higher temperature treatments (2 h at 500 degrees C) the traps are almost emptied and at 700 degrees C all vacancy-like defects are annealed out. No bubbles or voids are observed by transmission electron microscopy, either in the as-implanted or in annealed samples. (C) 1999 American Institute of Physics. [S0021-8979(99)00903-2].

1999 - Pre-cavities defect distribution in He implanted silicon studied by slow positron beam [Articolo su rivista]
Brusa, Rs; Karwasz, Gp; Zecca, A; Corni, Federico; Tonini, Rita; Ottaviani, Giampiero
abstract

Several techniques were applied to study distributions of point defects created after He implantation in Si at an energy of 20 keV. The evolution of the defect distributions as a function of isocronal thermal anneling was studied in the 150-900 degrees C temperature range. In particular Doppler-broadening measurements with a slow positron beam were performed to gain information on open volume defects precursors of the cavities that are formed in He implanted silicon after thermal treatment. Profiles of displaced Si atoms, He, and vacancies are presented for the meaningful thermal treatments, and discussed.

1999 - Vacancy-gettering in silicon: Cavities and helium-implantation [Articolo su rivista]
Corni, Federico; Tonini, Rita; Frabboni, Stefano; Nobili, Carlo Emanuele; Calzolari, G; Masetti, S; Tamarozzi, P; Pavia, G; Cerofolini, Gf
abstract

Cavities in single crystalline silicon are shown to act as gettering centers for the vacancies produced during an ion implantation process: the effect of the gettering is observed as an increase of their total volume. Helium implantation at 20 keV with a dose of 2 x 10(16) cm(-2) plus a heat treatment at 850 degrees C for 3 hours produces the cavities; vacancies are supplied by a subsequent 0.8 x 10(16) cm(-2) helium implant. In this case the vacancy-gettering efficiency, i.e. average increase of vacancies involved in stable voids per helium ion, is found to be 0.53 vacancies ion(-1); about 4 times more compared to the case when the same 0.8 x 10(16) cm(-2) implant is performed on a virgin crystal. A further effect is the peculiar helium behavior which acts as a stabilizer of the vacancies produced during the implantation. The differential vacancy-gettering efficiency is found to increase with the helium dose: about 0.13 vacancies ion(-1) at 0.8 x 10(16) cm(-2), 0.4 vacancies ion(-1) at 2 x 10(16) cm(-2), and 3.8 vacancies ion(-1) at 2.8 x 10(16) cm(-2) .

1998 - Electron paramagnetic resonance evidence for reversible transformation of thermal donor into shallow donor-type center in hydrogen-implanted silicon [Articolo su rivista]
Rakvin, B; Pivac, B; Tonini, Rita; Corni, Federico; Ottaviani, Giampiero
abstract

Electron paramagnetic resonance spectrum of the proton-related thermal donor (TD) assigned as NL8 paramagnetic center has been detected at 110 K after heat treatment of the hydrogen-implanted Czochralski-Si at 773 K. The effect of temperature on reversible transformations of the anisotropic spectrum of NL8 center into the isotropic singlet line was studied in the temperature region from 110 to 240 K. The analysis of the singlet provides an evidence that this signal originates from the proton- related shallow donor type at g = 1.9987. The changes in the linewidth have been used to evaluate the parameters [1/tau = 0.66 x 10(12) exp(-Delta E/kT); Delta E = 169 meV] for thermally activated electron emission to the conduction band from the second donor state of the NL8 center. These results represent direct experimental evidence of reversible transformation of the TD+ charged center into the shallow donor-type center. (C) 1998 American Institute of Physics. [S0003-6951(98)01548-4].

1998 - Growth kinetics of a displacement field in hydrogen implanted single crystalline silicon [Articolo su rivista]
D., Bisero; Corni, Federico; Frabboni, Stefano; Tonini, Rita; Ottaviani, Giampiero; R., Balboni
abstract

The growth of a displacement field in single crystal silicon resulting from high dose hydrogen implantation and subsequent heat treatments has been investigated by MeV He-4(+) Rutherford backscattering in channeling conditions, double crystal x-ray diffraction, and transmission electron microscopy. The results obtained in samples annealed for various times in the temperature range 220-350 degrees C have been explained in terms of a kinetic model which assumes the formation of clusters of hydrogen molecules. The growth of the displacement field is thermally activated with an activation energy of 0.50+/-0.05 eV, suggesting that the limiting process could be the release of hydrogen atoms bounded to defects created by ion implantation.

1998 - High-dose helium-implanted single-crystal silicon: Annealing behavior [Articolo su rivista]
Tonini, Rita; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Cerofolini, G. F.
abstract

The modifications induced in single-crystal silicon by implanted helium have been investigated by ion beam techniques. The damage has been detected by 2 MeV 4He+ backscattering in channeling conditions and the helium in-depth distribution by 7 and 8 MeV 15N++ elastic recoil scattering. The samples prepared by implanting 2×1016 cm−2 helium ions at 20 keV in silicon wafers held either at 77 K (LNT sample) or at 300 K (RT sample) have been heat treated for 2 h in the 100–800 °C temperature range. In the as-implanted LNT sample the damage maximum is at 130±20 nm and shifts in-depth to 180±10 nm after annealing at 200 °C, in the as-implanted RT sample, the damage maximum is already located at 180±10 nm. In the 250–500 °C temperature range, the LNT and RT samples follow the same annealing path with only slight differences in the temperature values; in both cases, the dechanneling signal increases and reaches a maximum value of nonregistered silicon atoms of 2.2–2.5×1022 at/cm3. In the same temperature range, the helium signal becomes narrower, builds up in a region centered on 220±20 nm and no appreciable loss of helium can be detected. The growth of the damage is consistent with the creation of cracks and a etherogenous distribution of bubbles filled with high pressure helium which stress the lattice; for the channeling Rutherford backscattering technique, their action is similar to silicon interstitials. At temperatures above 500 °C, helium is released from the samples; this process is associated with a decrease of the damage and the formation and increase in size of voids. At 900 °C empty voids with a diameter around 20 nm are found.

1998 - The effect of biaxial stress on the solid phase epitaxial crystallization of GexSi((1-x)) films [Articolo su rivista]
Corni, Federico; Frabboni, Stefano; Tonini, Rita; Leone, D; de Boer, W; Gasparotto, A.
abstract

Two fully-strained, Si rich, GexSi((1-x)) alloys deposited on (100) Si by CVD were amorphized including part of the substrate. and recrystallized at different temperatures in the range 500-620 degrees C. The amorphous-crystal interfaces remain flat during crystallization, and the films grow coherently with the substrate producing crystalline, defect-free, strained heteroepitaxial layers. The kinetics of the process, compared with literature data of the corresponding relaxed alloys, are affected by the presence of the in-plane stress at the interface, resulting in a decrease of the velocity, in quantitative agreement with the predictions of the activation strain model in the case of biaxial stress.

1997 - Effects of static disorder on LACBED patterns of single crystal silicon implanted with hydrogen [Relazione in Atti di Convegno]
Frabboni, Stefano; Gambetta, F; Tonini, Rita; Balboni, R; Armigliato, A.
abstract

The LACBED technique has been applied to &lt;110&gt; TEM cross-sections of Si sample implanted with H to study the effect of the static disorder on the Rocking Curves (RC) of high angle diffracted beams in presence of platelets and cavities. This effect has been revealed by comparing the RC originating from the perfect crystal and from the implanted layer, respectively. The results are consistent with an atomic root mean square displacement of the order of 10(-2) nm which compares quite well with x-ray measurements performed in the Bragg-symmetric geometry.

1997 - Evolution of defect profiles in He-implanted silicon studied by slow positrons [Relazione in Atti di Convegno]
Brusa, Rs; Karwasz, Gp; Tiengo, N; Zecca, A; Corni, Federico; Nobili, Carlo Emanuele; Ottaviani, Giampiero; Tonini, Rita
abstract

He implanted in silicon forms clusters and then out-diffuses from the surface of the crystal when the implanted silicon samples are annealed from 200 to 700 degrees C, Evaporated He leaves empty voids in the Si crystal. The dimension of the voids and the dynamics of the void formation are strictly related to the He dose. A variable-energy positron beam has been used for monitoring the formation and the evolution of the void distribution in p-type (100) Si (1.7-2.4 Omega cm), created by implantation at 77 K of He ions ( 5x10(15) ions/cm(2)) at 20 keV. The samples were treated at 250 degrees C for different annealing times. Thermal desorption (TD) measurements of He are also presented.

1997 - Helium in silicon: Thermal-desorption investigation of bubble precursors [Articolo su rivista]
Corni, Federico; Nobili, Carlo Emanuele; Ottaviani, Giampiero; Tonini, Rita; Calzolari, G; Cerofolini, Gf; Queirolo, G.
abstract

Thermal desorption measurements are performed on (100)-oriented p-type Si wafers implanted with He ions at 20 keV. The doses have been selected in order to produce crystal damage avoiding the formation of detectable bubbles. The He effusion kinetics, studied both in isothermal and in constant heating rate conditions, exhibit effective activation energy heterogeneity indicating the presence of various kinds of traps, precursor of the bubbles. The energy distribution results peaked at about 1.1 eV with an exponential decay towards higher energies and a width of about 0.2 eV. A semiquantitative model, based on the present knowledge about the Si:He system, is proposed, that accounts for He filled nanoblisters formation through interstitial He clustering and precipitation. The observed energy heterogeneity is ascribed to variations of the He solution energy from these cavities due to He-He and He-wall interactions.

1997 - Infrared light emission due to radiation damage in crystalline silicon [Articolo su rivista]
Bisero, D; Corni, Federico; Ottaviani, Giampiero; Tonini, Rita; Pavesi, L.
abstract

We have observed a set of broad luminescence bands between 1.07 and 0.85 eV, in He-implanted Si annealed in vacuum. These emissions are very similar to those of H-implanted and annealed Si, demonstrated by different groups in last years, in which H was believed to play a fundamental role. A comparison between the photoluminescence of He-implanted Si and of H-implanted Si, has allowed to conclude that the infrared photoluminescence of the Si:H system does not depend on H presence, but must be completely ascribed to the damage produced by the bombardment of Si with light ions. (C) 1997 Published by Elsevier Science Ltd. All rights reserved.

1997 - Infrared study of Si-rich silicon oxide films deposited by plasma-enhanced chemical vapor deposition [Articolo su rivista]
Sassella, A; Borghesi, A; Corni, Federico; Monelli, A; Ottaviani, Giampiero; Tonini, Rita; Pivac, B; Bacchetta, M; Zanotti, L.
abstract

A single chamber system for plasma-enhanced chemical vapor deposition was employed to deposit different films of SiOx:N,H with 0.85 less than or equal to x less than or equal to 1.91, which are studied here by Fourier transform infrared transmission spectroscopy. The sample composition was determined by Rutherford backscattering spectrometry, nuclear reaction, and elastic recoil detection analysis. Moreover, physical properties such as thickness uniformity, deposition rate, density, wet and dry etch rates, and stress are determined. A quantitative study of Si-OH, N-H, and Si-H bonds was performed and interpreted on the basis of the random bonding model; in addition, the presence of NH2, Si-O-Si, H2SiO2, and Si-N groups was detected. The effect of sample annealing at 600 and 900 degrees C was studied and two species of Si-H bonds were identified, one more stable and the other one easily releasable. A reordering effect of annealing was also detected as-a reduction of the amorphous network stress and as the increase of the bond angle in the Si-O-Si groups up to the value typical of thermal SiO2.

1997 - Photoluminescence characterization of SiGe QW grown by MBE [Articolo su rivista]
De Padova, P; Perfetti, P; Felici, R; Priori, S; Quaresima, C; Pizzoferrato, R; Casalboni, M; Prosposito, P; Corni, Federico; Tonini, Rita; Grilli, A; Raco, A.
abstract

Photoluminescence (PL) emission spectra have been carried out on a partially relaxed Si/Si1-xGex/Si quantum well (QW) grown by solid-source MBE at a nominal composition of x = 17%, The results are discussed in terms of the film morphology, as derived from structural techniques (SEM, RHEED, RES and XRD): especially as regards the presence of a high density of dots-like 3D structures. PL spectra show an intense brood band, attributed to excitonic recombinations at interstitial plateletes in the dots, with smaller X-NP and X-TO excitonic lines due to phonon-resolved interband transitions originated from the strained epitaxyal layer grown before the dot formation.

1997 - Positron annihilation studies of silicon-rich SiO2 produced by high dose ion implantation [Articolo su rivista]
G., Ghislotti; B., Nielsen; P., Asoka Kumar; K. G., Lynn; L. F., Di Mauro; Corni, Federico; Tonini, Rita
abstract

PAS is used to study Si-rich SiO2 samples prepared by implantation of Si (160 keV) ions at doses in the range 3e16-3e17 and consequent thermal annealing at high temperature (up to 1100 oC)....

1997 - Radiation enhanced transport of hydrogen in SiO2 [Articolo su rivista]
Corni, Federico; Monelli, A; Ottaviani, Giampiero; Tonini, Rita; Queirolo, G; Zanotti, L.
abstract

2.2 MeV He-4(+) and 7 MeV N-15(2+) ion beams have been used to investigate, by in situ measurements, the hydrogen desorption processes in SiO2 thin films deposited by plasma enhanced chemical vapor deposition (PECVD). An effective cross-section of 3 x 10(-16) cm(2) for He and 17 x 10(-16) cm(2) for N has been measured for the ion-thin-film interaction phenomena, The structure crystalline silicon/thermally grown SiO2/PECVD film has also been investigated for the hydrogen radiation-enhanced diffusion in thermally grown SiO2. The data are consistent with a diffusion process with a diffusion coefficient of (5.0 +/- 0.5) x 10(-26) cm(2)/ion from a constant source at a concentration C-0 = (2.5 +/- 0.2) x 10(21) at/cm(3).

1997 - Visible light emission from silicon implanted and annealed SiO2 layers [Articolo su rivista]
Ghislotti, G; Nielsen, B; Asokakumar, P; Lynn, Kg; Dimauro, Lf; Bottani, Ce; Corni, Federico; Tonini, Rita; Ottaviani, Giampiero
abstract

Silicon implanted and annealed SiO2 layers are studied usin g photoluminescence (PL) and positron annihilation spectroscopy (PAS). Two PL emission bands are observed. a band centered at 560 nm is present in as-implanted samples and it is still observed after 1000 degrees C annealing. The emission time is fast. A second band centered at 780 nm is detected after 1000 degrees C annealing. The intensity of the 780 nm band further increased when hydrogen annealing was performed. The emission time is long (1 mu s to 0.2 ms). PAS results show that defects produced by implantation anneal at 600 degrees C. Based on the annealing behavior and on the emission times, the origin of the two bands is discussed.

1996 - Bandgap widening in quantum sieves [Articolo su rivista]
Cerofolini, Gf; Meda, L; Bisero, D; Corni, Federico; Ottaviani, Giampiero; Tonini, Rita
abstract

Dense distributions of separate insulating regions in silicon may produce Anderson localization and hence a kind of confinement. This weak confinement is expected to be responsible for visible photoluminescence with different features from that observed in porous or nanocrystalline silicon. Dense distributions of separate insulating regions may be produced by ion implantation of a suitable species followed by an appropriate heat treatment. The visible photoluminescence observed in ion-implanted Si:H, Si:O and Si:He is explained in terms of exciton weak confinement.

1996 - Hydrogen determination in Si-rich oxide thin films [Articolo su rivista]
Monelli, A; Corni, Federico; Tonini, Rita; Ferrari, C; Ottaviani, Giampiero; Zanotti, L; Queirolo, G.
abstract

Quantitative in-depth distribution of the elements contained in silicon-rich oxide thin films deposited on single-crystal silicon by low temperature plasma-assisted deposition has been performed by a combination of various MeV ion beam techniques. The quantity of oxygen and nitrogen has been measured by nuclear reactions, the silicon content has been determined by Rutherford backscattering, and elastic recoil detection was used for hydrogen. All the samples contain not only Si and O, but also N and H, which are residuals from the reactions involved in the deposition process. We did find that the MeV beam used in the nuclear techniques can induce a process of hydrogen desorption, which causes the measured H content to be a function of the He dose received by the sample. This phenomenon, not previously reported, must be taken into account to give the comet H content. The study of the kinetics of the He-induced hydrogen desorption has been used to correct the experimental data and to determine the original hydrogen content. The correction factor is in most of the cases close to 2 and outside any experimental error. Moreover the studies of the kinetics give information on the kinds of hydrogen complexes contained in the films. The results suggest that, on the basis of the strength of the binding energies, hydrogen is present in at least two different configurations, weakly and strongly bonded. In the first configuration hydrogen is easily desorbed either under the action of the ion beam or of the heat treatment at 600 degrees C, in the second, hydrogen is lost only after treatment at 900 degrees C.

1996 - Silicon interstitials generation during the exposure of silicon to hydrogen plasma [Articolo su rivista]
Tonini, Rita; Monelli, A; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Queirolo, G.
abstract

Interstitials in silicon can be produced by hydrogen ions through chemical action and physical impact. To establish the relative importance of the two processes, wafers of single crystal silicon kept at a temperature of 250 degrees C were exposed to a hydrogen plasma, with and without an accelerating voltage (500 V). As gettering centers interstitial-type dislocation loops were made, before H exposure, by implanting 1.9 x 10(15) at. cm(-2) 110 keV P atoms and annealing at 1000 degrees C for 30 s. The analysis was performed using MeV ion channeling, elastic recoil detection analysis and cross-section transmission electron microscopy. The results suggest that H is captured by the defects: in the case of unbiased plasma, H goes in the region of the extrinsic defects, while in the case of biased plasma, H is trapped near the surface where the defects are produced by the energetic H ions themselves. In both cases the data are consistent with the formation of hydrogen nanoblisters with the subsequent breaking of silicon bonds and injection of silicon interstitials. The migration of interstitials occurs at a temperature as low as 250 degrees C.

1996 - Stress and interface morphology contributions in the crystallization kinetics of a GexSi1-x thin layer on (100)Si [Articolo su rivista]
Corni, Federico; Frabboni, Stefano; Tonini, Rita; Ottaviani, Giampiero; Queirolo, G.
abstract

A study of the solid phase epitaxial growth from amorphous phase of a strained GexSi1-x thin layer made by Ge-74(+) ion implantation of (100) Si is presented. Ge peak concentration is 6.4 at. %. Principally employed techniques are time-resolved reflectivity for crystallization rate measurements and cross sectional transmission electron microscopy (TEM) for interface morphology imaging. The kinetics is heavily affected by the mismatch stress induced by the Ge; the experimental data cannot however be explained considering only the average stress along the amorphous-crystal interface and another mechanism is required. Cross sectional TEM observations of partially crystallized samples reveal the tendency of the interface to roughen with an evolution reflected in the crystallization activation energy Such roughness is assumed to affect the kinetics by locally enhancing the interface stress.

1995 - Hydrogen precipitation in highly oversaturated single-crystalline silicon [Articolo su rivista]
Cerofolini, Gf; Balboni, R; Bisero, D; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita; Brusa, Rs; Zecca, A; Ceschini, M; Giebel, G; Pavesi, L.
abstract

A review of high concentration Hydrogen in Silicon is presented.

1995 - Visible photoluminescence from He‐implanted silicon [Articolo su rivista]
D., Bisero; Corni, Federico; Nobili, Carlo Emanuele; Tonini, Rita; Ottaviani, Giampiero; C., Mazzoleni; L., Pavesi
abstract

Visible photoluminescence has been observed at cryogenic temperatures from crystalline Si bombarded with He and exposed to H either as plasma or gas in the 250-450 degrees C temperature range. The experimental results are consistent with the formation of Si nanoparticles produced by He segregation, which is responsible for exciton localization, and H passivation of the nonradiative recombination centers.

1994 - Recrystallization of strainedlayers with various Gegradients and in the presence of impurities [Articolo su rivista]
Corni, Federico; Tonini, Rita; Balboni, R; Vescan, L.
abstract

The recrystallization of amorphous GexSi1-x thin films on Si with uniform and with increasing Ge content is studied. The amorphous-crystal interface roughness is modified by the presence of C and O impurities before entering the GexSi1-x layer. The crystallization kinetics are heavily affected by the resulting interface morphology. A minimum velocity is found in the region of the impurities where the interface starts to roughen; the strain effect induced by the Ge within the GexSi1-x is overcome and the crystallization proceeds according to kinetics very similar to those for pure Si.

1994 - Visible luminescence from silicon by hydrogen implantation and annealing treatments [Articolo su rivista]
Pavesi, L; Giebel, G; Tonini, Rita; Corni, Federico; Nobili, Carlo Emanuele; Ottaviani, Giampiero
abstract

Luminescence at an energy higher than the Si band-gap energy has been observed following H implantation and annealing treatments of Si samples. This phenomenon is discussed considering the damage caused by the H implantation and its evolution with thermal treatments. No definitive answer on the origin of the luminescence is given but various possible models are proposed.

1993 - Giant radiation damage produced by the impact of heavy molecular ions onto silicon single crystal [Articolo su rivista]
Cerofolini, Gf; Bertoni, S; Meda, L; Balboni, R; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita; Para, Af
abstract

The giant radiation damage imparted to single-crystalline silicon by the impact of Re2(CO)2+ at 140 keV is studied by conventional and high-resolution transmission electron microscopy. Evidence for damage produced by the correlated motion of the atoms initially forming the molecular ion is presented.

1992 - A fast technique for the quantitative analysis of channeling RBS spectra [Articolo su rivista]
Cerofolini, Gf; Corni, Federico; Ottaviani, Giampiero; Tonini, Rita
abstract

A straightforward method for the analysis of channeling RBS spectra is developed and validated. This method needs a minimum of computational complexity and does not require the a priori knowledge of the location of the scattering centres and of their dechanneling cross section. The method is applied to channeling RBS spectra obtained along the [100] direction in a hydrogen-implanted (100) silicon crystal to verify that the evaluated displaced-atom depth distribution is independent of the probe energy.

1992 - Crystallization kinetics of boron- and germanium-implanted 〈100〉 Si: a balance between doping and strain effects [Articolo su rivista]
Corni, Federico; Tonini, Rita; Ottaviani, Giampiero; Servidori, M; Priolo, F.
abstract

Electrical and strain arguments were used to explain the differences in growth rate observed for silicon solid phase epitaxy in the presence of electrically active dopants and neutral impurities. In the case of dopants, strain effects seem to be absent or at least to play a negligible role, while for neutral impurities strain seems to be very important. The purpose of this work was to study the role of dopants in the presence of intentionally strained silicon films. The studies were performed by comparing the growth rates measured in samples implanted with boron and germanium, and the growth rates obtained in samples implanted with only germanium or only boron. With respect to intrinsic silicon, boron produces a rate enhancement, germanium a decrease and boron-germanium a slight increase. The results are interpreted by taking into account the strain and by assuming that strain and electrical effects are independent.

1992 - HYDROGEN-RELATED COMPLEXES AS THE STRESSING SPECIES IN HIGH-FLUENCE, HYDROGEN-IMPLANTED, SINGLE-CRYSTAL SILICON [Articolo su rivista]
Cerofolini, Gf; Meda, L; Balboni, R; Corni, Federico; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita; Anderle, M; Canteri, R.
abstract

This work is devoted to the characterization of the Si:H system obtained by high-fluence, low-energy, hydrogen implantation into single-crystal silicon. The implanted hydrogen profile and the ones resulting after thermal annealing in the range 100-800-degrees-C are detected by secondary-ion mass spectrometry and elastic-recoil detection analysis. The displacement field in the crystal, measured by channeling Rutherford-backscattering spectrometry, is found to depend on the direct radiation damage, the extended defects formed after ion implantation (revealed by transmission electron microscopy), and the implanted species. The contribution to the displacement field due to hydrogen-related defects has a characteristic reverse annealing in the range 100-400-degrees-C, essentially due to their formation kinetics.

1992 - Processing high-quality silicon for microstrip detectors [Articolo su rivista]
Nava, Filippo; Ottaviani, Giampiero; Tonini, Rita; Frabboni, Stefano; A., Alberigi Quaranta; P., Cantoni; P. L., Frabetti; L., Stagni; G., Queirolo; P. F., Manfredi
abstract

An investigation has been made into the behaviour of high-purity silicon (HP-Si) during the fabrication of microstrip detectors. The resistivity of the silicon used is 3 k-OMEGA cm. The investigation is centred on standard bipolar processes based on ion implantation. It is found that, comparing the processes used, the best diode characteristics are achieved when a heat treatment at 600-degrees-C is used after the ion-implantation step, whereas the worst results from an implantation and a 900-degrees-C heat treatment. Thus it is shown that if integration of the electronic circuitry and the detector on a single chip is required, then the high-temperature heat treatments must be done before the ion-implantation step needed for detector fabrication.

1992 - SEARCH FOR NUCLEAR-REACTIONS PRODUCED BY THE IMPACT OF HEAVY MOLECULAR-IONS ONTO LID [Articolo su rivista]
Cerofolini, Gf; Corni, Federico; Ottaviani, Giampiero; Tonini, Rita
abstract

Preliminary evidence for light element fusions induced by the impact of heavy molecular ions, with mass around 430 a.m.u. (obtained by fragmentation of Re2(CO)10) and energy 140 keV, onto LiD target is presented. The experimental arrangement excludes the impact of fast light particles due to fragmentation inside the acceleration column and supports the idea of a <<new>> underlying physics.

1991 - EVIDENCE FOR MOLECULAR-HYDROGEN IN SINGLE-CRYSTAL SILICON [Articolo su rivista]
Meda, L; Cerofolini, Gf; Ottaviani, Giampiero; Tonini, Rita; Corni, Federico; Balboni, R; Anderle, M; Canteri, R; Dierckx, R.
abstract

Rutherford backscattering in channeling in combination with elastic recoil detection analysis has been used to study the formation and destruction of hydrogen supermolecular complexes (H2)n formed in single crystal silicon after hydrogen implantation and annealing in the range 150-800-degrees-C. Secondary ion mass spectroscopy has been used to confirm this interpretation by giving direct evidence for H-2. The supermolecular configuration can be interpreted as the nucleus for the formation of bubbles.

1991 - INFLUENCE OF IMPLANT DOSE AND TARGET TEMPERATURE ON CRYSTAL QUALITY AND JUNCTION DEPTH OF BORON-DOPED SILICON LAYERS [Articolo su rivista]
Fabbri, R; Servidori, M; Solmi, S; Frabboni, Stefano; Ottaviani, Giampiero; Tonini, Rita; Canteri, R.
abstract

30 keV boron ions are implanted at doses of 2 x 10(14) and 2 x 10(15) cm-2 in <100> silicon wafers kept at room or liquid-nitrogen temperatures. The samples are analyzed by double-crystal X-ray diffraction, transmission electron microscopy and secondary ion-mass spectrometry before and after furnace annealing at 800-degrees-C. The low-dose implant does not amorphize the substrate at any of the temperatures, and residual defects together with a remarkably enhanced boron diffusion are observed after annealing. The high-dose implant amorphizes the substrate only at low temperature. In this case, unlike the room-temperature implant, the absence of any residual defect, the incorporation of the dopant in substitutional position and a negligible profile braodening of boron are obtained after annealing. In principle, this process proves itself a promising step for the fabrication of p+/n shallow junctions with good electrical characteristics.