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Dipartimento di Scienze e Metodi dell'Ingegneria
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Dipartimento di Scienze e Metodi dell'Ingegneria

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2023 - Molecular Bridges Link Monolayers of Hexagonal Boron Nitride during Dielectric Breakdown [Articolo su rivista]
Ranjan, A; O'Shea, Sj; Padovani, A; Su, T; La Torraca, P; Ang, Ys; Munde, Ms; Zhang, Ch; Zhang, Xx; Bosman, M; Raghavan, N; Pey, Kl

We use conduction atomic force microscopy (CAFM) to examine the soft breakdown of monocrystalline hexagonal boron nitride (h-BN) and relate the observations to the defect generation and dielectric degradation in the h-BN by charge transport simulations and density functional theory (DFT) calculations. A modified CAFM approach is adopted, whereby 500 x 500 nm2 to 3 x 3 mu m2 sized metal/h-BN/ metal capacitors are fabricated on 7 to 19 nm-thick h-BN crystal flakes and the CAFM tip is placed on top of the capacitor as an electrical probe. Current-voltage (I-V) sweeps and time-dependent dielectric breakdown measurements indicate that defects are generated gradually over time, leading to a progressive increase in current prior to dielectric breakdown. Typical leakage currents are around 0.3 A/cm2 at a 10 MV/cm applied field. DFT calculations indicate that many types of defects could be generated and contribute to the leakage current. However, three defects created from adjacent boron and nitrogen monovacancies exhibit the lowest formation energy. These three defects form molecular bridges between two adjacent h-BN layers, which in turn "electrically shorts" the two layers at the defect location. Electrical shorting between layers is manifested in charge transport simulations, which show that the I-V data can only be correctly modeled by incorporating a decrease in effective electrical thickness of the h-BN as well as the usual increase in trap density, which, alone, cannot explain the experimental data. An alternative breakdown mechanism, namely, the physical removal of h-BN layers under soft breakdown, appears unlikely given the h-BN is mechanically confined by the electrodes and no change in AFM topography is observed after breakdown. High-resolution transmission electron microscope micrographs of the breakdown location show a highly localized (<1 nm) breakdown path extending between the two electrodes, with the h-BN layers fractured and disrupted, but not removed.

2022 - Dielectric breakdown in HfO2 dielectrics: Using multiscale modeling to identify the critical physical processes involved in oxide degradation [Articolo su rivista]
Strand, Jack; La Torraca, Paolo; Padovani, Andrea; Larcher, Luca; Shluger, Alexander L.

2022 - The electrons' journey in thick metal oxides [Articolo su rivista]
Caruso, F.; La Torraca, P.; Larcher, L.; Tallarida, G.; Spiga, S.

Originally introduced in electronic manufacturing to replace the SiO2 insulating layer, metal oxides are now extensively used in a multitude of electronic devices. Understanding charge transport mechanisms in metal oxides is of paramount importance for device optimization; however, a detailed and self-consistent discussion of electron conduction at all applied electric fields is lacking in the literature. In this work, we investigated the conduction mechanisms in three model systems, Al2O3, HfO2, and Al-doped HfO2 metal-insulator-metal capacitors, determining the path that the electrons travel within the metal oxide. Traps properties are extracted from experimental current-voltage characteristics using the Ginestra® simulation software. Furthermore, the analysis allowed to visualize the location of traps most involved in the conduction and the dominant transport mechanisms at each applied electric field. Despite the different oxide properties, a similar trend was recognized at low electric fields, the electron transport through the oxide is negligible, and the dominant contribution to the measured current is ascribed to the charge/discharge of traps located near the metal/oxide interfaces, leading to displacement currents. At high electric fields, the transport of electrons occurs through the defect rich oxides in the two following ways: if a large density of traps is energetically located near the electrodes Fermi level (as in HfO2), the electrons tunnel from trap to trap until they reach the anode; otherwise, when traps are closer to the conduction band (as in Al2O3 and AlHfO), the electrons tunnel from the cathode into one trap and then into the oxide conduction band, interacting only with traps near the cathode.

2021 - Circuit model for thermoviscous propagation in annular waveguides [Articolo su rivista]
Ricci, Y.; La Torraca, P.; Larcher, L.

This paper presents a circuit model of the thermoviscous acoustic wave propagation in waveguides with annular cross section. The model, validated against finite element method simulations of the input acoustic impedance, captures the annular waveguide behavior with good accuracy within a frequency bandwidth consistent with the lumped-element approximation. The cascading of multiple circuit models easily allows extending the bandwidth while preserving the same accuracy. The circuit model was derived from the low reduced frequency (LRF) wave propagation model in rectangular layers, representing a valid approximation of the complex LRF solution in annular waveguides. The simplified analytical description allows for the formulation of a compact T-network model comprised of standard circuit elements. This circuit model can be implemented in circuit simulators to accelerate both the analysis and engineering of devices having elements with annular cross section, such as micro-electro-mechanical systems devices or microphones.

2021 - Design and Fabrication of a Pillar-based Piezoelectric Microphone exploiting 3D-Printing Technology [Articolo su rivista]
Ricci, Y.; Sorrentino, A.; La Torraca, P.; Cattani, L.; Cotogno, M.; Cantarella, G.; Orazi, L.; Castagnetti, D.; Lugli, P.; Larcher, L.

This letter presents a 3-D-printed piezoelectric microphone with enhanced voltage sensitivity. The sensitivity is improved by a combination of a single-pillar mechanical design and a specific polyvinylidene fluoride (PVDF)-film electrode patterning. The moving part of the mechanical structure and the chassis are 3D-printed as a single unit and trimmed by laser cutting, allowing for a simple fabrication of the device. The measured sensitivity of 1 mV/Pa (±6 dB) in the bandwidth 500–2500 Hz agrees with simulations, showing an improvement over similar pillar-based piezoelectric sensor solutions. The sensitivity performance is shown to be comparable to existing microphones with different technologies. The microphone is also characterized by excellent linearity within the measurable range. 3D-printing technique can thus be adopted for the manufacturing of low cost and highly customizable microphone sensors.

2021 - Extraction of Defects Properties in Dielectric Materials from I-V Curve Hysteresis [Articolo su rivista]
Torraca, P. L.; Caruso, F.; Padovani, A.; Spiga, S.; Tallarida, G.; Larcher, L.

Atomic defects in high-k materials affect the performance, reliability, variability, and scaling potential of electronic devices. Their characterization is thus of paramount importance, and methods exploiting electrical measurements are highly demanded. In this work we present a novel method for extracting the defect properties from I-V curve hysteresis measured at low electric field in thick metal-insulator-metal (MIM) stacks. The I-V curve hysteresis allows detecting the defects located near the electrode-insulator interfaces and aligned with the stack Fermi level, and extracting their properties. The defects are profiled cross-correlating the information provided by the low-field current hysteresis and the high-field steady-state current. This technique can be applied to MIM stacks fabricated in Back-End-of-Line for capacitors, embedded memories and thin film transistors.

2020 - Corrigendum to “Linearization of thermoacoustic loudspeakers by adaptive predistortion” [Sens. Actuators A: Phys. 297 (2019) 111551] (Sensors and Actuators: A. Physical (2019) 297, (S0924424719307903), (10.1016/j.sna.2019.111551)) [Articolo su rivista]
La Torraca, P.; Ricci, Y.; Bobinger, M.; Pavan, P.; Larcher, L.

The authors would like to add the following text as acknowledgement The authors would like to thank ASK industries S.P.A. for financial support and technical assistance. This work is supported by the Italian Ministry of Economic Development (MISE)'s FUND FOR THE SUSTAINABLE GROWTH (F.C.S) under grant agreement (CUP) B48I15000130008, project VASM (“Vehicle Active Sound Management”). The authors acknowledge the support of the Tiziano Nili (Project Leader) and Luca Cattani (Team Leader) The authors would like to apologise for any inconvenience caused.

2019 - Linearization of thermoacoustic loudspeakers by adaptive predistortion [Articolo su rivista]
La Torraca, P.; Ricci, Y.; Bobinger, M.; Pavan, P.; Larcher, L.

In this work we present a novel driving technique for thermoacoustic (TA) loudspeakers. The proposed technique allows linearizing the pressure response of TA loudspeakers while reducing the average power dissipation on the device, and thus its working temperature. This is achieved exploiting an adaptive predistortion algorithm, implemented through digital signal processing. The controlled TA loudspeakers show exceptionally low values of total harmonic distortion and intermodulation distortion in their pressure response, exceeding the performance of previously proposed techniques, and a significantly reduced working temperature.

2019 - Multiscale modeling for application-oriented optimization of resistive random-access memory [Articolo su rivista]
La Torraca, P.; Puglisi, F. M.; Padovani, A.; Larcher, L.

Memristor-based neuromorphic systems have been proposed as a promising alternative to von Neumann computing architectures, which are currently challenged by the ever-increasing computational power required by modern artificial intelligence (AI) algorithms. The design and optimization of memristive devices for specific AI applications is thus of paramount importance, but still extremely complex, as many dierent physical mechanisms and their interactions have to be accounted for, which are, in many cases, not fully understood. The high complexity of the physical mechanisms involved and their partial comprehension are currently hampering the development of memristive devices and preventing their optimization. In this work, we tackle the application-oriented optimization of Resistive Random-Access Memory (RRAM) devices using a multiscale modeling platform. The considered platform includes all the involved physical mechanisms (i.e., charge transport and trapping, and ion generation, diusion, and recombination) and accounts for the 3D electric and temperature field in the device. Thanks to its multiscale nature, the modeling platform allows RRAM devices to be simulated and the microscopic physical mechanisms involved to be investigated, the device performance to be connected to the material's microscopic properties and geometries, the device electrical characteristics to be predicted, the effect of the forming conditions (i.e., temperature, compliance current, and voltage stress) on the device's performance and variability to be evaluated, the analog resistance switching to be optimized, and the device's reliability and failure causes to be investigated. The discussion of the presented simulation results provides useful insights for supporting the application-oriented optimization of RRAM technology according to specific AI applications, for the implementation of either non-volatile memories, deep neural networks, or spiking neural networks.

2018 - High Efficiency Thermoacoustic Loudspeaker Made with a Silica Aerogel Substrate [Articolo su rivista]
La Torraca, P.; Bobinger, M.; Pavan, P.; Becherer, M.; Zhao, S.; Koebel, M.; Cattani, L.; Lugli, P.; Larcher, L.

The extremely low thermal effusivity of the silica aerogel is exploited to develop a high efficiency thermoacoustic (TA) loudspeaker with solid substrate. The deposition of the electrically conductive, low heat capacity active film on the silica aerogel surface is achieved with both the spray coating of silver nanowires and the sputter coating of gold films. The uniform spray coating of the hydrophobic silica aerogel is enabled by a low pressure plasma treatment, which however impairs its robustness. The spray-coated samples prove to be fragile when subjected to elevated temperatures and thus not suitable for TA applications. Sputter coating, not requiring any treatment of the aerogel surface, allows the fabrication of working TA loudspeaker samples with a 100 nm gold active film. The electroacoustic response of the gold-sputtered silica aerogel TA loudspeaker is characterized at different input power levels. The experimental results are compared with those present in literature, showing an improved efficiency with respect to the other TA loudspeakers with solid substrate.

2018 - On the frequency response of nanostructured thermoacoustic loudspeakers [Articolo su rivista]
La Torraca, P.; Bobinger, M.; Servadio, M.; Pavan, P.; Becherer, M.; Lugli, P.; Larcher, L.

In this work, we investigate the thermal and acoustic frequency responses of nanostructured thermoacoustic loudspeakers. An opposite frequency dependence of thermal and acoustic responses was found independently of the device substrate (Kapton and glass) and the nanometric active film (silver nanowires and nm-thick metal films). The experimental results are interpreted with the support of a comprehensive electro-thermo-acoustic model, allowing for the separation of the purely thermal effects from the proper thermoacoustic (TA) transduction. The thermal interactions causing the reported opposite trends are understood, providing useful insights for the further development of the TA loudspeaker technology.

2018 - Solution-Processing of Copper Nanowires for Transparent Heaters and Thermo-Acoustic Loudspeakers [Articolo su rivista]
Bobinger, Marco; La Torraca, Paolo; Mock, Josef; Becherer, Markus; Cattani, Luca; Angeli, Diego; Larcher, Luca; Lugli, Paolo

In this study, we present a copper nanowires (CuNWs) based spray deposition process for the fabrication of transparent heaters and thermo-acoustic loudspeakers. We developed a scalable and solution-based synthesis process for CuNWs, which allows to fabricate spray deposited transparent electrodes that show performances comparable to indium tin oxide based TEs, at much lower material and deposition costs. Without any post-processing, the CuNWs films exhibit a sheet resistance as low as 12.6 Ω/□ at a high transparency of 77%. CuNW-based transparent heaters and thermo-acoustic loudspeakers are accurately characterized and modeled in both the thermal and the acoustic domain, showing performances aligned with the state-of-the art.

2017 - Characterization and modelling of transparent heaters based on solution-processed copper nanowires [Relazione in Atti di Convegno]
Bobinger, Marco; Mock, Josef; Becherer, Markus; Torraca, Paolo La; Angeli, Diego; Larcher, Luca; Lugli, Paolo

In this study, we present an environmentally friendly and solution-based synthesis for copper nanowires (CuNWs) at a moderate process temperature. Transparent electrodes (TEs) are fabricated by spray-deposition and evaluated in terms of their electro-optical performance. Using ImageJ, the CuNW diameters are determined in an automated and reproducible way. Without any post-processing, the films show a sheet resistance as low as 12.6 Ohm/sq at a high transparency of 77 %. Further, CuNW-based transparent heaters are characterized and accurately modelled using the Crank-Nicolson finite method that accounts for the heat losses and the resistance-temperature dependence of the films.

2017 - Infrared, transient thermal, and electrical properties of silver nanowire thin films for transparent heaters and energy-efficient coatings [Articolo su rivista]
Bobinger, Marco; Angeli, Diego; Colasanti, Simone; La Torraca, Paolo; Larcher, Luca; Lugli, Paolo

n this study, we investigate the infrared and electrical propertiesas well as the thermal response of transparent silvernanowire (AgNW) based thin-film heaters, when subjected toJoule heating. Controlling the number of layers and hence thedeposition time, our spray-coating technique allows to modulatethe thermal and electrical properties of the thin films in a precisemanner. In addition, this technique enables the fabrication ofhomogeneous and large-area heaters, which, in terms of theirelectro-optical properties, nicely compare to the performances ofstate-of-the-art AgNW transparent electrodes. The thermal response and the electrical properties are accurately reproducedby a purposely developed physical model, which shows that thetemperature dependence of the AgNW film resistance is loweredby a factor of 2 compared to bulk silver, independently of thenumber of deposited layers. Compared to uncoated glass,the emissivity decreases by 58% at a coverage rate of 58%. At thesame time, the AgNW film can sustain a transparency as high as81.3%. Therefore, AgNW-based thin films can be used as a low-emissivity coating, for e.g., energy-efficient window glazingapplications. Finally, we accurately determine the fragmentationtemperature of AgNWs, which sets the ultimate limitation of usefor heating applications.

2017 - Physical modeling and characterization of thermo-acoustic loudspeakers made of silver nano-wire films [Articolo su rivista]
La Torraca, P.; Bobinger, M.; Pavan, Paolo; Seeber, B.; Lugli, P.; Larcher, L.

Recent developments of ultra-low heat capacity nanostructured materials revived the interest in the thermo-acoustic (TA) loudspeaker technology, which shows important advantages compared to the classical dynamic loudspeakers as they feature a lower cost and weight, flexibility, conformability to the surface of various shapes, and transparency. The development of the TA loudspeaker technology requires accurate physical models connecting the material properties to the thermal and acoustic speaker's performance. We present here a combined theoretical and experimental analysis of TA loudspeakers, where the electro-thermal and the thermo-acoustic transductions are handled separately, thus allowing an in-depth description of both the pressure and temperature dynamics. The electro-thermal transduction is analyzed by accounting for all the heat flow processes taking place between the TA loudspeaker and the surrounding environment, with focus on their frequency dependence. The thermo-acoustic conversion is studied by solving the coupled thermo-acoustic equations, derived from the Navier-Stokes equations, and by exploiting the Huygens-Fresnel principle to decompose the TA loudspeaker surface into a dense set of TA point sources. A general formulation of the 3D pressure field is derived summing up the TA point source contributions via a Rayleigh integral. The model is validated against temperature and sound pressure level measured on the TA loudspeaker sample made of a Silver Nanowire random network deposited on a polyimide substrate. A good agreement is found between measurements and simulations, demonstrating that the model is capable of connecting material properties to the thermo-acoustic performance of the device, thus providing a valuable tool for the design and optimization of TA loudspeakers.

2017 - Tailoring the Aqueous Synthesis and Deposition of Copper Nanowires for Transparent Electrodes and Heaters [Articolo su rivista]
Bobinger, Marco; Mock, Josef; LA TORRACA, Paolo; Becherer, Markus; Lugli, Paolo; Larcher, Luca

Due to the high abundance of copper on the earth and its high intrinsic electrical conductivity, copper nanowires (CuNWs) represent a promising material for transparent electrodes. In this work, an environmentally friendly and scalable synthesis that requires a low process temperature is studied. The optimum temperature is found at 79 °C, which results in nanowires with the lowest diameters. The as-synthesized solution is sprayed to transparent conducting films, which are in turn subjected to various post-treatments such as thermal sintering or washing with propionic acid to enhance their electro-optical performance. Following both the optimum protocol for the synthesis and post-treatment, a sheet resistance of 10.3 Ω â »â 1at a transparency of 83.4% is achieved. Moreover, the CuNW-films are tested as transparent heaters and show a homogeneous heat distribution. For the electrical properties of the films, a temperature dependence of resistance that is lowered around 28% compared to the one for bulk copper is found.

2016 - Characterization and modeling of the thermal and electrical properties of transparent silver nanowire thin-film heaters [Relazione in Atti di Convegno]
Bobinger, Marco; Colasanti, Simone; Lugli, Paolo; Angeli, Diego; La Torraca, Paolo; Larcher, Luca

In this study, we present the characterization and the modeling of transparent silver nanowire thin-film heaters in terms of their transient thermal response when subjected to Joule heating and their electrical properties. The electrical properties, which showed a conductance-temperature dependence that is reduced down to a factor of 2 compared to the value for bulk silver, could be modeled accurately by simulation results. In addition, our transparent electrode deposition technique, i.e. spray-coating, allowed for an excellent reproducibility and provided homogeneous and large films that compare to state-of-the-art silver nanowire transparent electrode performance.