Publications 2021

Script list Publications

(1) N-Type Ge/Si Antennas for THz Sensing
C.A. Chavarin, E. Hardt, S. Gruessing, O. Skibitzki, I. Costina, D. Spirito, W. Seifert, W.M. Klesse, C.L. Manganelli, C. You, J. Flesch, J. Piehler, M. Missori, L. Baldassarre, B. Witzigmann, G. Capellini
Optics Express 29(5), 7680 (2021)
(DFG-ESSENCE)
Ge-on-Si plasmonics holds the promise for compact and low-cost solutions in the manipulation of THz radiation. We discuss here the plasmonic properties of doped Ge bow-tie antennas made with a low-point cost CMOS mainstream technology. These antennas display resonances between 500 and 700 GHz, probed by THz Time Domain Spectroscopy. We show surface functionalization of the antennas with a thin layer of α-lipoic acid, that red-shifts the antenna resonances by about 20 GHz. Moreover, we show that also antennas covered with a protective silicon nitride cap layer exhibit a comparable red-shift when covered with the biolayer. This suggests that the electromagnetic fields at the hotspot extend well beyond the cap layer, enabling the possibility to use the antennas with an improved protection of the plasmonic material in conjunction with microfluidics.

(2) Perspectives on Electrically Pumped Ge/SiGe QW Emitters at THz Frequencies
C. Ciano, M. Montanari, L. Persichetti, D. Stark, G. Scalari, J. Faist, L. Di Gaspare, G. Capellini, C. Corley, T. Grange, S. Birner, M. Virgilio, L. Baldassarre, M. Ortolani, M. De Seta
Proc. 45th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz 2020), (2021)
DOI: 10.1109/IRMMW-THz46771.2020.9370671, (FLASH)

(3) Liquid-Phase Exfoliated Gallium Selenide for Light-Driven Thin-Film Transistors
N. Curreli, M. Serri, M.I. Zappia, D. Spirito, G. Bianca, J. Buha, L. Najafi, Z. Sofer, R. Krahne, V. Pellegrini, F. Bonaccorso
Advanced Electronic Materials 7(3), 2001080 (2021)
DOI: 10.1002/aelm.202001080
Gallium selenide (GaSe), a layered semiconductor of Group‐III monochalcogenides, has been recognized by the scientific community in recent years as an appealing material in the fields of photonics and (opto)electronics. Thanks to its pseudodirect bandgap and its thickness‐dependent (opto)electronic properties, GaSe has emerged as a promising candidate for the implementation of thin‐film transistors (TFTs) and photodetectors with fast response and high sensitivity. Solution processing of 2D materials provides low‐cost inks that allow the design and realization of printed electronic devices, enabling this technology to move from the laboratory to the industry. In this work, a solution‐processed GaSe‐based light‐driven transistor is presented. Liquid phase exfoliation (LPE) is used to exfoliate bulk GaSe in isopropanol, formulating a functional ink that is subsequently deposited by spray coating onto Si/SiO2 substrates. The GaSe phototransistor exhibits a p‐channel behavior with a high on/off ratio (≈103) that is gate‐voltage dependent. Moreover, the device response also depends on the illumination with a maximum responsivity of 13 A W–1 to UV–visible light and a fast response time of 35 ms. This study demonstrates that liquid phase exfoliated GaSe is a promising candidate for the design and realization of next‐generation (opto)electronic devices.

(4) Mechanical Switching of Orientation-Related Photoluminescence in Deep-Blue 2D Layered Perovskite Ensembles
B. Dhanabalan, A. Castelli, L. Ceseracciu, D. Spirito, F. Di Stasio, L. Manna, R. Krahne, M.P. Arciniegas
Nanoscale 13(7), 3948 (2021)
The synergy between the organic component of two-dimensional (2D) metal halide layered perovskites and flexible polymers offers an unexplored window to tune their optical properties at low mechanical stress. Thus, there is a significant interest in exploiting their PL anisotropy by controlling their orientation and elucidating their interactions. Here, we apply this principle to platelet structures of micrometre lateral size that are synthesized in situ into free-standing polymer films. We study the photoluminescence of the resulting films under cyclic mechanical stress and observe an enhancement in the emission intensity up to ∼2.5 times along with a switch in the emission profile when stretching the films from 0% to 70% elongation. All the films recovered their initial emission intensity when releasing the stress throughout ca. 15 mechanical cycles. We hypothesize a combined contribution from reduced reabsorption, changes on in-plane and out-of-plane dipole moments that stem from different orientation of the platelets inside the film, and relative sliding of platelets within oriented stacks while stretching the films. Our results reveal how low-mechanical stress affects 2D layered perovskite aggregation and orientation, an open pathway toward the design of strain-controlled emission.

(5) Three-Dimensional Interfacing of Cells with Hierarchical Silicon Nano/Microstructures for Midinfrared Interrogation of In Situ Captured Proteins
J. Flesch, M. Bettenhausen, M. Kazmierczak, W.M. Klesse, O. Skibitzki, O.E. Psathaki, R. Kurre, G. Capellini, S. Guha, T. Schroeder, B. Witzigmann, C. You, J. Piehler
ACS Applied Materials & Interfaces 13(7), 8049 (2021)
DOI: 10.1021/acsami.0c22421, (DFG-ESSENCE)
Label-free optical detection of biomolecules is currently limited by a lack of specificity rather than sensitivity. To exploit the much more characteristic refractive index dispersion in the mid-infrared (IR) regime, we have engineered three-dimensional IR-resonant silicon micropillar arrays (Si-MPAs) for protein sensing. By exploiting the unique hierarchical nano- and microstructured design of these Si-MPAs attained by CMOS-compatible silicon-based microfabrication processes, we achieved an optimized interrogation of surface protein binding. Based on spatially resolved surface functionalization, we demonstrate controlled three-dimensional interfacing of mammalian cells with Si-MPAs. Spatially controlled surface functionalization for site-specific protein immobilization enabled efficient targeting of soluble and membrane proteins into sensing hotspots directly from cells cultured on Si-MPAs. Protein binding to Si-MPA hotspots at submonolayer level was unambiguously detected by conventional Fourier transform IR spectroscopy. The compatibility with cost-effective CMOS-based microfabrication techniques readily allows integration of this novel IR transducer into fully fledged bioanalytical microdevices for selective and sensitive protein sensing.

(6) The Formation of a Sn Monolayer on Ge(100) Studied at the Atomic Scale
E.V.S. Hofmann, E. Scalise, F. Montalenti, T.J.Z. Stock, S.R. Schofield, G. Capellini, L. Miglio, N.J. Curson, W.M. Klesse
Applied Surface Science 561, 149961 (2021)
The growth of multi-layer germanium-tin (GeSn) quantum wells offers an intriguing pathway towards the integration of lasers in a CMOS platform. An important step in growing high quality quantum well interfaces is the formation of an initial wetting layer. However, key atomic-scale details of this process have not previously been discussed. We use scanning tunneling microscopy combined with density functional theory to study the deposition of Sn on Ge(1 0 0) at room temperature over a coverage range of 0.01 to 1.24 monolayers. We demonstrate the formation of a sub-2% Ge content GeSn wetting layer from three atomic-scale characteristic ad-dimer structural components, and show that small quantities of Sn incorporate into the Ge surface forming two atomic configurations. The ratio of the ad-dimer structures changes with increasing Sn coverage, indicating a change in growth kinetics. At sub-monolayer coverage, the least densely packing ad-dimer structure is most abundant. As the layer closes, forming a two-dimensional wetting layer, the more densely packing ad-dimer structure become dominant. These results demonstrate the capability to form an atomically smooth wetting layer at room temperature, and provide critical atomic-scale insights for the optimization of growth processes of GeSn multi-quantum-wells to meet the quality requirements of optical GeSn-based devices.

(7) Oxides based Resistive Switching Memories
S. Kalem, S. B. Tekin, Z. E. Kaya, E. Jagaluier, R. Roelofs, S. Yildirim, O. Yavuzcetin, Ch. Wenger
Proc. SPIE Oxide-based Materials and Devices XII (SPIE OPTO 2021), 11687, 116871L (2021)
DOI: 10.1117/12.2585681, (Panache)

(8) Fast Scatterometric Measurement of Periodic Surface Structures in Plasma-Etching Processes
W.M. Klesse, A. Rathsfeld, C. Groß, E. Malguth, O. Skibitzki, L. Zealouk
Measurement 170, 108721 (2021)
DOI: 10.1016/j.measurement.2020.108721
To satisfy the continuous demand of ever smaller feature sizes, plasma etching technologies in microelectronics enable the fabrication of device structures in the nanometer range. To control these processes, real-time access to the structure’s dimensions is needed. We develop a special method of optical critical dimension metrology and evaluate the feasibility of reconstructing the etched dimensions from experimental reflectivity spectra of the surface, taken about every second. For a periodic 2D model structure etched into a silicon, we develop and test a fast algorithm.
To reduce the computing time, we generate a library of spectra before the etching. We demonstrate that, by replacing the numerically simulated spectra in the reconstruction algorithm by spectra interpolated from the library, it is possible to compute the geometry parameters in times less than a second. Finally, to also reduce memory size and computing time for the library, we reduce the scanning of the parameter values to a sparse grid.

(9) Phase Transitions in Low-Dimensional Layered Double Perovskites: The Role of the Organic Moieties
B. Martin-Garcia, D. Spirito, G. Biffi, S. Artyukhin, F. Bonaccorso, R. Krahne
The Journal of Physical Chemistry Letters 12, 280 (2021)
DOI: 10.1021/acs.jpclett.0c03275
Halide double perovskites are an interesting alternative to Pb-containing counterparts as active materials in optoelectronic devices. Low-dimensional double perovskites are fabricated by introducing large organic cations, resulting in organic/inorganic architectures with one or more inorganic octahedra layers separated by organic cations. Here, we synthesized layered double perovskites based on 3D Cs2AgBiBr6, consisting of double (2L) or single (1L) inorganic octahedra layers, using ammonium cations of different sizes and chemical structures. Temperature-dependent Raman spectroscopy revealed phase transition signatures in both inorganic lattice and organic moieties by detecting variations in their vibrational modes. Changes in the conformational arrangement of the organic cations to an ordered state coincided with a phase transition in the 1L systems with the shortest ammonium moieties. Significant changes of photoluminescence intensity observed around the transition temperature suggest that optical properties may be affected by the octahedral tilts emerging at the phase transition.

(10) Characterization and Separation of Live and Dead Yeast Cells using CMOS-Based DEP Microfluidics
H. Matbaechi Ettehad, Ch. Wenger
Micromachines 12(3), 270 (2021)
DOI: 10.3390/mi12030270, (BioBic)
This study aims at developing a miniaturized CMOS integrated silicon-based microfluidic system, compatible with a standard CMOS process, to enable the characterization, and separation of live and dead yeast cells (as model bio-particle organisms) in a cell mixture using the DEP technique. DEP offers excellent benefits in terms of cost, operational power, and especially easy electrode integration with the CMOS architecture, and requiring label-free sample preparation. This can increase the likeliness of using DEP in practical settings. In this work the DEP force was generated using an interdigitated electrode arrays (IDEs) placed on the bottom of a CMOS-based silicon microfluidic channel. This system was primarily used for the immobilization of yeast cells using DEP. This study validated the system for cell separation applications based on the distinct responses of live and dead cells and their surrounding media. The findings confirmed the device’s capability for efficient, rapid and selective cell separation. The viability of this CMOS embedded microfluidic for dielectrophoretic cell manipulation applications and compatibility of the dielectrophoretic structure with CMOS production line and electronics, enabling its future commercially mass production.

(11) Optimized Programming Algorithms for Multilevel RRAM in Hardware Neural Networks
V. Milo, F. Anzalone, C. Zambelli, E. Perez, M.K. Mahadevaiah, O.G. Ossorio, P. Olivo, Ch. Wenger, D. Ielmini
Proc. International Reliability Physics Symposium (IRPS 2021), (2021)
DOI: 10.1109/IRPS46558.2021.9405119, (Total Resilience)

(12) Optimized Programming Algorithms for Multilevel RRAM in Hardware Neural Networks
V. Milo, F. Anzalone, C. Zambelli, E. Perez, M.K. Mahadevaiah, O.G. Ossorio, P. Olivo, Ch. Wenger, D. Ielmini
Proc. International Reliability Physics Symposium (IRPS 2021), (2021)
DOI: 10.1109/IRPS46558.2021.9405119, (NeuroMem)

(13) Optimized Programming Algorithms for Multilevel RRAM in Hardware Neural Networks
V. Milo, F. Anzalone, C. Zambelli, E. Perez, M.K. Mahadevaiah, O.G. Ossorio, P. Olivo, Ch. Wenger, D. Ielmini
Proc. International Reliability Physics Symposium (IRPS 2021), (2021)
DOI: 10.1109/IRPS46558.2021.9405119, (KI-PRO)

(14) THz Intersubband Absorption in N-Type Si1−xGex Parabolic Quantum Wells
M. Montanari, C. Ciano, L. Persichetti, C. Corley, L. Baldassarre, M. Ortolani, L. Di Gaspare, G. Capellini, D. Stark, G. Scalari, M. Virgilio, M. DeSeta
Applied Physics Letters 118(16), 163106 (2021)
DOI: 10.1063/5.0048344, (FLASH)
High-quality n-type continuously graded Ge-rich Si1-xGex parabolic quantum wells with different doping levels were grown by using ultrahigh-vacuum chemical vapor deposition on Si(001) substrates. A thorough structural characterization study highlights an ideal parabolic compositional profile. THz intersubband absorption has been investigated in modulation-doped samples and samples directly doped in the wells. The comparison of experimental absorption data and theoretical calculations allowed us to quantify the impact of electron correlation effects on the absorption resonances in the different doping conditions and for electron sheet densities in the (1/6) x 1011 cm-2 range. A single optical resonance is present in modulation doped samples. Its peak energy and line shape are independent of temperature-induced variations of the electron distribution in the subbands up to 300 K, in agreement with the generalized Kohn theorem. This achievement represents a relevant step forward for the development of CMOS compatible optoelectronic devices in the THz spectral range, where thermal charge fluctuations play a key role.

(15) Performance Assessment of Amorphous HfO2-based RRAM Devices for Neuromorphic Applications
O.G. Ossorio, G. Vinuesa, H. Garcia, B. Sahelices, S. Duenas, H. Castan, E. Perez, M.K. Mahadevaiah, Ch. Wenger
ECS Transactions 102(2), 29 (2021)
(Total Resilience)
The use of thin layers of amorphous hafnium oxide has been shown to be suitable for the manufacture of Resistive Random-Access memories (RRAM). These memories are of great interest because of their simple structure and non-volatile character. They are particularly appealing as they are good candidates for substituting flash memories. In this work, the performance of the MIM structure that takes part of a 4 kbit memory array based on 1-transistor-1-resistance (1T1R) cells was studied in terms of control of intermediate states and cycle durability. DC and small signal experiments were carried out in order to fully characterize the devices, which presented excellent multilevel capabilities and resistive-switching behavior.

(16) Performance Assessment of Amorphous HfO2-based RRAM Devices for Neuromorphic Applications
O.G. Ossorio, G. Vinuesa, H. Garcia, B. Sahelices, S. Duenas, H. Castan, E. Perez, M.K. Mahadevaiah, Ch. Wenger
ECS Transactions 102(2), 29 (2021)
(KI-PRO)
The use of thin layers of amorphous hafnium oxide has been shown to be suitable for the manufacture of Resistive Random-Access memories (RRAM). These memories are of great interest because of their simple structure and non-volatile character. They are particularly appealing as they are good candidates for substituting flash memories. In this work, the performance of the MIM structure that takes part of a 4 kbit memory array based on 1-transistor-1-resistance (1T1R) cells was studied in terms of control of intermediate states and cycle durability. DC and small signal experiments were carried out in order to fully characterize the devices, which presented excellent multilevel capabilities and resistive-switching behavior.

(17) Performance Assessment of Amorphous HfO2-based RRAM Devices for Neuromorphic Applications
O.G. Ossorio, G. Vinuesa, H. Garcia, B. Sahelices, S. Duenas, H. Castan, E. Perez, M.K. Mahadevaiah, Ch. Wenger
ECS Transactions 102(2), 29 (2021)
(NeuroMem)
The use of thin layers of amorphous hafnium oxide has been shown to be suitable for the manufacture of Resistive Random-Access memories (RRAM). These memories are of great interest because of their simple structure and non-volatile character. They are particularly appealing as they are good candidates for substituting flash memories. In this work, the performance of the MIM structure that takes part of a 4 kbit memory array based on 1-transistor-1-resistance (1T1R) cells was studied in terms of control of intermediate states and cycle durability. DC and small signal experiments were carried out in order to fully characterize the devices, which presented excellent multilevel capabilities and resistive-switching behavior.

(18) A Versatile, Voltage-Pulse Based Read and Programming Circuit for Multi-Level RRAM Cells
S. Pechmann, T. Mai, M. Völkel, M.K. Mahadevaiah, E. Perez, E. Perez-Bosch Quesada, M. Reichenbach, Ch. Wenger, A. Hagelauer
Electronics (MDPI) 10(5), 530 (2021)
(KI-PRO)
In this work, we present an integrated read and programming circuit for Resistive Random Access Memory (RRAM) cells. Since there are a lot of different RRAM technologies in research and the process variations of this new memory technology often spread over a wide range of electrical properties, the proposed circuit focuses on versatility in order to be adaptable to different cell properties. The circuit is suitable for both read and programming operations based on voltage pulses of flexible length and height. The implemented read method is based on evaluating the voltage drop over a measurement resistor and can distinguish up to eight different states, which are coded in binary, thereby realizing a digitization of the analog memory value. The circuit was fabricated in the 130 nm CMOS process line of IHP. The simulations were done using a physics-based, multi-level RRAM model. The measurement results prove the functionality of the read circuit and the programming system and demonstrate that the read system can distinguish up to eight different states with an overall resistance ratio of 7.9.

(19) Optimization of Multi-Level Operation in RRAM Arrays for In-Memory Computing
E. Perez, A.J. Perez-Avila, R. Romero-Zaliz, M.K. Mahadevaiah, E. Perez-Bosch Quesada, J.B. Roldan, F. Jimenez-Molinos, Ch. Wenger
Electronics (MDPI) 10(9), 1084 (2021)
DOI: 10.1016/j.mee.2019.05.004, (Total Resilience)
Accomplishing multi-level programming in resistive random access memory (RRAM) arrays with truly discrete and linearly spaced conductive levels is crucial in order to implement synaptic weights in hardware-based neuromorphic systems. In this paper, we implemented this feature on 4-kbit 1T1R RRAM arrays by tuning the programming parameters of the multi-level incremental step pulse with verify algorithm (M-ISPVA). The optimized set of parameters was assessed by comparing its results with a non-optimized one. The optimized set of parameters proved to be an effective way to define non-overlapped conductive levels due to the strong reduction of the device-to-device variability as well as of the cycle-to-cycle variability, assessed by inter-levels switching tests and during 1k reset-set cycles. In order to evaluate this improvement in real scenarios the experimental characteristics of the RRAM devices were captured by means of a behavioral model, which was used to simulate two different neuromorphic systems: an 8x8 vector-matrix-multiplication (VMM) accelerator and a 4-layer feedforward neural network for MNIST database recognition. The results clearly showed that the optimization of the programming parameters improved both the precision of VMM results as well as the recognition accuracy of the neural network in about 6 % compared with the use of non-optimized parameters.

(20) Optimization of Multi-Level Operation in RRAM Arrays for In-Memory Computing
E. Perez, A.J. Perez-Avila, R. Romero-Zaliz, M.K. Mahadevaiah, E. Perez-Bosch Quesada, J.B. Roldan, F. Jimenez-Molinos, Ch. Wenger
Electronics (MDPI) 10(9), 1084 (2021)
DOI: 10.1016/j.mee.2019.05.004, (Neutronics)
Accomplishing multi-level programming in resistive random access memory (RRAM) arrays with truly discrete and linearly spaced conductive levels is crucial in order to implement synaptic weights in hardware-based neuromorphic systems. In this paper, we implemented this feature on 4-kbit 1T1R RRAM arrays by tuning the programming parameters of the multi-level incremental step pulse with verify algorithm (M-ISPVA). The optimized set of parameters was assessed by comparing its results with a non-optimized one. The optimized set of parameters proved to be an effective way to define non-overlapped conductive levels due to the strong reduction of the device-to-device variability as well as of the cycle-to-cycle variability, assessed by inter-levels switching tests and during 1k reset-set cycles. In order to evaluate this improvement in real scenarios the experimental characteristics of the RRAM devices were captured by means of a behavioral model, which was used to simulate two different neuromorphic systems: an 8x8 vector-matrix-multiplication (VMM) accelerator and a 4-layer feedforward neural network for MNIST database recognition. The results clearly showed that the optimization of the programming parameters improved both the precision of VMM results as well as the recognition accuracy of the neural network in about 6 % compared with the use of non-optimized parameters.

(21) Advanced Temperature Dependent Statistical Analysis of Forming Voltage Distributions for Three Different HfO2-Based RRAM Technologies
E. Perez, D. Maldonado, C. Acal, J.E. Ruiz-Castro, A.M. Aguilera, F. Jimenez-Molinos, J.B. Roldan, Ch. Wenger
Solid State Electronics 176, 107961 (2021)
DOI: 10.1016/j.sse.2021.107961, (Total Resilience)
In this work, voltage distributions of forming operations are analyzed by using an advanced statistical approach based on phase-type distributions (PHD). The experimental data were collected from batches of 128 HfO2-based RRAM devices integrated in 4-kbit arrays. Three di erent switching oxides, namely, polycrystalline HfO2, amorphous HfO2, and Al-doped HfO2, were tested in the temperature range from -40 to 150 oC. The variability of forming voltages has been usually studied by using the Weibull distribution (WD). However, the performance of the PHD analysis demonstrated its ability to better model this crucial operation. The capacity of the PHD to reproduce the experimental data has been validated by means of the Kolmogorov-Smirnov test, while the WD failed in many of the cases studied. In addition, PHD allows to extract information about intermediate probabilistic states that occur in the forming process and the transition probabilities between them; in this manner, we can deepen on the conductive lament formation physics. In particular, the number of intermediate states can be related to the device variability.

(22) Optimization of Multi-Level Operation in RRAM Arrays for In-Memory Computing
E. Perez, A.J. Perez-Avila, R. Romero-Zaliz, M.K. Mahadevaiah, E. Perez-Bosch Quesada, J.B. Roldan, F. Jimenez-Molinos, Ch. Wenger
Electronics (MDPI) 10(9), 1084 (2021)
DOI: 10.1016/j.mee.2019.05.004, (KI-PRO)
Accomplishing multi-level programming in resistive random access memory (RRAM) arrays with truly discrete and linearly spaced conductive levels is crucial in order to implement synaptic weights in hardware-based neuromorphic systems. In this paper, we implemented this feature on 4-kbit 1T1R RRAM arrays by tuning the programming parameters of the multi-level incremental step pulse with verify algorithm (M-ISPVA). The optimized set of parameters was assessed by comparing its results with a non-optimized one. The optimized set of parameters proved to be an effective way to define non-overlapped conductive levels due to the strong reduction of the device-to-device variability as well as of the cycle-to-cycle variability, assessed by inter-levels switching tests and during 1k reset-set cycles. In order to evaluate this improvement in real scenarios the experimental characteristics of the RRAM devices were captured by means of a behavioral model, which was used to simulate two different neuromorphic systems: an 8x8 vector-matrix-multiplication (VMM) accelerator and a 4-layer feedforward neural network for MNIST database recognition. The results clearly showed that the optimization of the programming parameters improved both the precision of VMM results as well as the recognition accuracy of the neural network in about 6 % compared with the use of non-optimized parameters.

(23) Advanced Temperature Dependent Statistical Analysis of Forming Voltage Distributions for Three Different HfO2-Based RRAM Technologies
E. Perez, D. Maldonado, C. Acal, J.E. Ruiz-Castro, A.M. Aguilera, F. Jimenez-Molinos, J.B. Roldan, Ch. Wenger
Solid State Electronics 176, 107961 (2021)
DOI: 10.1016/j.sse.2021.107961, (NeuroMem)
In this work, voltage distributions of forming operations are analyzed by using an advanced statistical approach based on phase-type distributions (PHD). The experimental data were collected from batches of 128 HfO2-based RRAM devices integrated in 4-kbit arrays. Three di erent switching oxides, namely, polycrystalline HfO2, amorphous HfO2, and Al-doped HfO2, were tested in the temperature range from -40 to 150 oC. The variability of forming voltages has been usually studied by using the Weibull distribution (WD). However, the performance of the PHD analysis demonstrated its ability to better model this crucial operation. The capacity of the PHD to reproduce the experimental data has been validated by means of the Kolmogorov-Smirnov test, while the WD failed in many of the cases studied. In addition, PHD allows to extract information about intermediate probabilistic states that occur in the forming process and the transition probabilities between them; in this manner, we can deepen on the conductive lament formation physics. In particular, the number of intermediate states can be related to the device variability.

(24) Toward Reliable Compact Modeling of Multilevel 1T-1R RRAM Devices for Neuromorphic Systems
E. Pérez-Bosch Quesada, R. Romero-Zaliz, E. Perez, M.K. Mahadevaiah, J. Reuben, M.A. Schubert, F. Jimenez-Molinos, J.B. Roldan, Ch. Wenger
Electronics (MDPI) 10(6), 645 (2021)
(KI-PRO)
In this work, three different RRAM compact models implemented in Verilog-A are analyzed and evaluated in order to reproduce the multilevel approach based on the switching capability of experimental devices. These models are integrated in 1T-1R cells to control their analog behavior by means of the compliance current imposed by the NMOS select transistor. Four different resistance levels are simulated and assessed with experimental verification to account for their multilevel capability. Further, an Artificial Neural Network study is carried out to evaluate in a real scenario the viability of the multilevel approach under study.

(25) Toward Reliable Compact Modeling of Multilevel 1T-1R RRAM Devices for Neuromorphic Systems
E. Pérez-Bosch Quesada, R. Romero-Zaliz, E. Perez, M.K. Mahadevaiah, J. Reuben, M.A. Schubert, F. Jimenez-Molinos, J.B. Roldan, Ch. Wenger
Electronics (MDPI) 10(6), 645 (2021)
(Total Resilience)
In this work, three different RRAM compact models implemented in Verilog-A are analyzed and evaluated in order to reproduce the multilevel approach based on the switching capability of experimental devices. These models are integrated in 1T-1R cells to control their analog behavior by means of the compliance current imposed by the NMOS select transistor. Four different resistance levels are simulated and assessed with experimental verification to account for their multilevel capability. Further, an Artificial Neural Network study is carried out to evaluate in a real scenario the viability of the multilevel approach under study.

(26) Toward Reliable Compact Modeling of Multilevel 1T-1R RRAM Devices for Neuromorphic Systems
E. Pérez-Bosch Quesada, R. Romero-Zaliz, E. Perez, M.K. Mahadevaiah, J. Reuben, M.A. Schubert, F. Jimenez-Molinos, J.B. Roldan, Ch. Wenger
Electronics (MDPI) 10(6), 645 (2021)
(NeuroMem)
In this work, three different RRAM compact models implemented in Verilog-A are analyzed and evaluated in order to reproduce the multilevel approach based on the switching capability of experimental devices. These models are integrated in 1T-1R cells to control their analog behavior by means of the compliance current imposed by the NMOS select transistor. Four different resistance levels are simulated and assessed with experimental verification to account for their multilevel capability. Further, an Artificial Neural Network study is carried out to evaluate in a real scenario the viability of the multilevel approach under study.

(27) Variability and Energy Consumption Tradeoffs in Multilevel Programming of RRAM Arrays
E. Perez, M.K. Mahadevaiah, E. Perez-Bosch Quesada, Ch. Wenger
IEEE Transactions on Electron Devices 68(6), 2693 (2021)
DOI: 10.1109/TED.2021.3072868, (Neutronics)
Achieving a reliable multi-level programming operation in resistive random access memory (RRAM) arrays is still a challenging task. In this work, we assessed the impact of the voltage step value used by the programming algorithm on the device-to-device (DTD) variability of the current distributions of four conductive levels and on the energy consumption featured by programming 4-kbit HfO2-based RRAM arrays. Two different write-verify algorithms were considered and compared, namely, the incremental gate voltage with verify algorithm (IGVVA) and the incremental step pulse with verify algorithm (ISPVA). By using the IGVVA, a main trade-off has to be taken into account since reducing the voltage step leads to a smaller DTD variability at the cost of a strong increase in the energy consumption. Although the ISPVA can not reduce the DTD variability as much as the IGVVA, its voltage step can be decreased in order to reduce the energy consumption with almost no impact on the DTD variability. Therefore, the final decision on which algorithm
to employ should be based on the specific application targeted for the RRAM array.

(28) Variability and Energy Consumption Tradeoffs in Multilevel Programming of RRAM Arrays
E. Perez, M.K. Mahadevaiah, E. Perez-Bosch Quesada, Ch. Wenger
IEEE Transactions on Electron Devices 68(6), 2693 (2021)
DOI: 10.1109/TED.2021.3072868, (Total Resilience)
Achieving a reliable multi-level programming operation in resistive random access memory (RRAM) arrays is still a challenging task. In this work, we assessed the impact of the voltage step value used by the programming algorithm on the device-to-device (DTD) variability of the current distributions of four conductive levels and on the energy consumption featured by programming 4-kbit HfO2-based RRAM arrays. Two different write-verify algorithms were considered and compared, namely, the incremental gate voltage with verify algorithm (IGVVA) and the incremental step pulse with verify algorithm (ISPVA). By using the IGVVA, a main trade-off has to be taken into account since reducing the voltage step leads to a smaller DTD variability at the cost of a strong increase in the energy consumption. Although the ISPVA can not reduce the DTD variability as much as the IGVVA, its voltage step can be decreased in order to reduce the energy consumption with almost no impact on the DTD variability. Therefore, the final decision on which algorithm
to employ should be based on the specific application targeted for the RRAM array.

(29) Study of Quantized Hardware Deep Neural Networks Based on Resistive Switching Devices, Conventional versus Convolutional Approaches
R. Romero-Zaliz, E. Perez, F. Jimenez-Molinos, Ch. Wenger, J.B. Roldan
Electronics (MDPI) 10(3), 346 (2021)
DOI: 10.3390/electronics10030346, (NeuroMem)
A comprehensive analysis of two types of artificial neural networks (ANN) is performed to assess the influence of quantization on the synaptic weights. Conventional multilayer-perceptron (MLP) and convolutional neural networks (CNN) have been considered by changing their features in the training and inference contexts, such as number of levels in the quantization process, the number of hidden layers on the network topology, the number of neurons per hidden layer, the image databases, the number of convolutional layers, etc. A reference technology based on 1T1R structures with bipolar memristors including HfO2 dielectrics was employed, accounting for different multilevel schemes and the corresponding conductance quantization algorithms. The accuracy of the image recognition processes was studied in depth. This type of studies are essential prior to hardware implementation of neural networks. The obtained results support the use of CNNs for image domains. This is linked to the role played by convolutional layers at extracting image features and reducing the data complexity. In this case, the number of synaptic weights can be reduced in comparison to conventional MLPs.

(30) Study of Quantized Hardware Deep Neural Networks Based on Resistive Switching Devices, Conventional versus Convolutional Approaches
R. Romero-Zaliz, E. Perez, F. Jimenez-Molinos, Ch. Wenger, J.B. Roldan
Electronics (MDPI) 10(3), 346 (2021)
DOI: 10.3390/electronics10030346, (Total Resilience)
A comprehensive analysis of two types of artificial neural networks (ANN) is performed to assess the influence of quantization on the synaptic weights. Conventional multilayer-perceptron (MLP) and convolutional neural networks (CNN) have been considered by changing their features in the training and inference contexts, such as number of levels in the quantization process, the number of hidden layers on the network topology, the number of neurons per hidden layer, the image databases, the number of convolutional layers, etc. A reference technology based on 1T1R structures with bipolar memristors including HfO2 dielectrics was employed, accounting for different multilevel schemes and the corresponding conductance quantization algorithms. The accuracy of the image recognition processes was studied in depth. This type of studies are essential prior to hardware implementation of neural networks. The obtained results support the use of CNNs for image domains. This is linked to the role played by convolutional layers at extracting image features and reducing the data complexity. In this case, the number of synaptic weights can be reduced in comparison to conventional MLPs.

(31) Study of Quantized Hardware Deep Neural Networks Based on Resistive Switching Devices, Conventional versus Convolutional Approaches
R. Romero-Zaliz, E. Perez, F. Jimenez-Molinos, Ch. Wenger, J.B. Roldan
Electronics (MDPI) 10(3), 346 (2021)
DOI: 10.3390/electronics10030346, (KI-PRO)
A comprehensive analysis of two types of artificial neural networks (ANN) is performed to assess the influence of quantization on the synaptic weights. Conventional multilayer-perceptron (MLP) and convolutional neural networks (CNN) have been considered by changing their features in the training and inference contexts, such as number of levels in the quantization process, the number of hidden layers on the network topology, the number of neurons per hidden layer, the image databases, the number of convolutional layers, etc. A reference technology based on 1T1R structures with bipolar memristors including HfO2 dielectrics was employed, accounting for different multilevel schemes and the corresponding conductance quantization algorithms. The accuracy of the image recognition processes was studied in depth. This type of studies are essential prior to hardware implementation of neural networks. The obtained results support the use of CNNs for image domains. This is linked to the role played by convolutional layers at extracting image features and reducing the data complexity. In this case, the number of synaptic weights can be reduced in comparison to conventional MLPs.

(32) Influence of Variability on the Performance of HfO2 Memristor-based Convolutional Neural Networks
R. Romero-Zaliz, E. Perez, F. Jimenez-Molinos, Ch. Wenger, J.B. Roldan
Solid State Electronics 185, 108064 (2021)
DOI: 10.1016/j.sse.2021.108064, (Total Resilience)
A study of convolutional neural networks (CNNs) was performed to analyze the influence of quantization and variability in the network synaptic weights. Different CNNs were considered accounting for the number of convolutional layers, size of the filters in the convolutional layer, number of neurons in the final network layers and different sets of quantization levels. The conductance levels of fabricated 1T1R structures based on HfO2 memristors were considered as reference for four or eight level quantization processes at the inference stage of the CNNs, which were previous trained with the MNIST dataset. We also included the variability of the experimental conductance levels that was found to be Gaussian distributed and was correspondingly modeled for the synaptic weight implementation.

(33) Influence of Variability on the Performance of HfO2 Memristor-based Convolutional Neural Networks
R. Romero-Zaliz, E. Perez, F. Jimenez-Molinos, Ch. Wenger, J.B. Roldan
Solid State Electronics 185, 108064 (2021)
DOI: 10.1016/j.sse.2021.108064, (KI-PRO)
A study of convolutional neural networks (CNNs) was performed to analyze the influence of quantization and variability in the network synaptic weights. Different CNNs were considered accounting for the number of convolutional layers, size of the filters in the convolutional layer, number of neurons in the final network layers and different sets of quantization levels. The conductance levels of fabricated 1T1R structures based on HfO2 memristors were considered as reference for four or eight level quantization processes at the inference stage of the CNNs, which were previous trained with the MNIST dataset. We also included the variability of the experimental conductance levels that was found to be Gaussian distributed and was correspondingly modeled for the synaptic weight implementation.

(34) Influence of Variability on the Performance of HfO2 Memristor-based Convolutional Neural Networks
R. Romero-Zaliz, E. Perez, F. Jimenez-Molinos, Ch. Wenger, J.B. Roldan
Solid State Electronics 185, 108064 (2021)
DOI: 10.1016/j.sse.2021.108064, (NeuroMem)
A study of convolutional neural networks (CNNs) was performed to analyze the influence of quantization and variability in the network synaptic weights. Different CNNs were considered accounting for the number of convolutional layers, size of the filters in the convolutional layer, number of neurons in the final network layers and different sets of quantization levels. The conductance levels of fabricated 1T1R structures based on HfO2 memristors were considered as reference for four or eight level quantization processes at the inference stage of the CNNs, which were previous trained with the MNIST dataset. We also included the variability of the experimental conductance levels that was found to be Gaussian distributed and was correspondingly modeled for the synaptic weight implementation.

(35) Raman Shifts in MBE-Grown SixGe1-x-ySny Alloys with Large Si Content
J. Schlipf, H. Tetzner, D. Spirito, C.L. Manganelli, G. Capellini, M.R.S. Huang, C.T. Koch, C.J. Clausen, A. Elsayed, M. Oehme, S. Chiussi, J. Schulze, I.A. Fischer
Journal of Raman Spectroscopy 52(6), 1167 (2021)
(Dfg-QWIP)
We examine the Raman shift in Silicon-Germanium-Tin alloys with high Silicon content grown on a Germanium virtual substrate by molecular beam epitaxy. The Raman shifts of the three most prominent modes, Si-Si, Si-Ge and Ge-Ge, are measured and compared with results in previous literature. We analyze and fit the dependence of the three modes on the composition and strain of the semiconductor alloys. We also demonstrate the calculation of the composition and strain of SixGe1-x-ySny from the Raman shifts alone, based on the fitted relationships. Our analysis extends previous results to samples lattice-matched on Ge and with higher Si content than in prior comprehensive Raman analyses, thus making Raman measurements as a fast and non-destructive characterization technique accessible for a wider compositional range of these ternary alloys.

(36) THz Intersubband Electroluminescence from N-Type Ge/SiGe Quantum Cascade Structures
D. Stark, M. Mirza, L. Persichetti, M. Montanari, S. Markmann, M. Beck, T. Grange, S. Birner, M. Virgilio, C. Ciano, M. Ortolani, C. Corley, G. Capellini, L. Di Gaspare, M. De Seta, D.J. Paul, J. Faist, G. Scalari
Applied Physics Letters 118(10), 101101 (2021)
DOI: 10.1063/5.0041327, (FLASH)
We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of Δf/f≈0.2 . Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on a single quantum well active region employing a vertical optical transition, and the observed spectral features are well described by non-equilibrium Green's function calculations. The presence of two peaks highlights a suboptimal injection in the upper state of the radiative transition. Comparison of the electroluminescence spectra with a similar GaAs/AlGaAs structure yields one order of magnitude lower emission efficiency.

(37) Behavioral Model of Dot-Product Engine Implemented with 1T1R Memristor Crossbar Including Assessment
J. Wen, M. Ulbricht, E. Perez, X. Fan, M. Krstic
Proc. 24th International Symposium on Design and Diagnostics of Electronic Circuits and Systems (DDECS 2021), 29 (2021)
DOI: 10.1109/DDECS52668.2021.9417070, (KI-PRO)

(38) Threading Dislocation Reduction of Ge by Introducing a SiGe/Ge Superlattice
Y. Yamamoto, C. Corley, M.A. Schubert, M.H. Zoellner, B. Tillack
ECS Journal of Solid State Science and Technology 10(3), 034005 (2021)
DOI: 10.1149/2162-8777/abea5e
The influence of introducing a SiGe/Ge superlattice (SL) between Ge layers and Si substrate for the sake of the reduction of the threading dislocation density (TDD) without additional annealing is investigated. By introducing the SiGe/Ge SL and optimizing the layer stack, the TDD of the Ge layer becomes ~1/3. In the case of 2.8 μm thick Ge without introducing the SiGe/Ge SL, the TDD at the surface is 7.6 × 10cm−2. A slight TDD reduction is observed by introducing a Si0.2Ge0.8/Ge SL between the Si substrate and the Ge layer. By inserting 5, 10 and 20 cycles of Si0.2Ge0.8/Ge, the TDD is reduced to 7.1 × 108, 5.9 × 108 and 5.3 × 108 cm−2, respectively. The lateral lattice parameters of these SLs are ~5.656 Å, which is a smaller value compared to that of bulk Ge, indicating plastic relaxation by misfit dislocation formation. Further TDD reduction is realized with increasing Si concentration in the SiGe/Ge SL without changing the cycle of the SL. However, surface roughening due to pit formation occurs if the Si concentration in the SL is higher than 50% because of increased strain at the interfaces between SiGe and Ge. With increasing SiGe and Ge thickness ratio in the SL layer and maintaining periodicity and cycles, the TDD is reduced to 2.8 × 108 cm−2 without degrading the surface roughness. This improvement is related to a relaxation of the SiGe/Ge SL by plastic deformation.

(39) Threading Dislocation Reduction of Ge by Introducing a SiGe / Ge Superlattice
Y. Yamamoto, C. Corley, M.A. Schubert, M.H. Zoellner, B. Tillack
ECS Journal of Solid State Science and Technology 10(3), 034005 (2021)
DOI: 10.1149/2162-8777/abea5e
The influence of introducing a SiGe / Ge superlattice (SL) between Ge layers and Si sbstrate for the sake of the reduction of the threading dislocation density (TDD) without additional anealing is investigated. By introducing the SiGe / Ge SL, the TDD of the Ge surface becomes ~1/3. In the case of 2.8 μm thick Ge without introducing the SiGe / Ge SL, the TDD at the surface is 7.6×108 cm-2. A slight TDD reduction is observed by introducing a Si0.2Ge0.8 / Ge SL between the Si substrate and the Ge layer. By inserting 5, 10 and 20 cycles of Si0.2Ge0.8 / Ge, the TDD is reduced to 7.1×108, 5.9×108 and 5.3×108 cm-2, respectively. The lateral lattice parameters of these SLs are ~5.656Å, which is a smaller value compared to that of bulk Ge, indicating plastic relaxation by misfit dislocation (MD) formation. Further TDD reduction is realized with increasing Si concentration in the SiGe / Ge SL without changing the cycle of the SL. However, surface roughening due to pit formation occurs if the Si concentration in the SL is higher than 50% because of increased strain at the interfaces between SiGe and Ge. With increasing SiGe and Ge thickness ratio in the SL layer and maintaining periodicity and cycles, the TDD is reduced to 2.8×108 cm-2 without degrading the surface roughness. This improvement is related to a relaxation of the SiGe/Ge SL by plastic deformation.

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