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  • Publications 2020

Publications 2020

since January 2020

(1) Investigation of the Oxidation Behavior of Graphene/Ge(001) Versus Graphene/Ge(110) Systems
F. Akhtar, J. Dabrowski, M. Lisker, Y. Yamamoto, A. Mai, Ch. Wenger, M. Lukosius
ACS Applied Materials & Interfaces 12(2), 3188 (2020)
DOI: 10.1021/acsami.9b18448, (Graphen)
The oxidation behavior of Ge(001) and Ge(110) surfaces underneath the CVD grown graphene films has been investigated experimentally and interpreted on the basis of ab initio calculations. Freshly grown samples were exposed to air for more than seven months and periodically monitored by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Raman spectroscopy. The oxidation of Ge(110) started with incubation time of several days, during which the oxidation rate was supposedly exponential. After an ultrathin oxide grew, the oxidation continued with a slow but constant rate. No incubation was detected for Ge(001). The oxide thickness was initially proportional to the square root of time. After two weeks the rate saturated at a value fourfold higher than that for Ge(110). We argue that after the initial phase, the oxidation is limited by the diffusion of oxidizing species through atomic-size openings at graphene domain boundaries and is influenced by the areal density and by the structural quality of the boundaries, whereby the latter determines the initial behavior. Prolonged exposure affected the surface topography and reduced the compressive strain in graphene, from ~ –0.15% to ~ 0.0% on Ge(001) and from ~ –1% to ~ –0.5% on Ge(110). In the last step, both the air-exposed samples were annealed in vacuum at 850⁰C. After annealing, the oxidation of Ge substrates through graphene was reversed by removing the oxygen atoms and thus restoring the original status of graphene/Ge systems. These findings might constitute an important step towards further optimization of graphene/Ge systems.

(2) Disentangling Elastic and Inelastic Scattering Pathways in the Intersubband Electron Dynamics of N-Type Ge/SiGe Quantum Fountains
L. Bagolini, M. Montanari, L. Persichetti, L. Di Gaspare, G. Capellini, M. Ortolani, M. De Seta, M. Virgilio
Physical Review B 101(24), 245302 (2020)
DOI: 10.1103/PhysRevB.101.245302, (FLASH)
n -type Ge/SiGe quantum wells have been suggested as a promising platform for the realization of a Si-compatible THz laser. Focusing on this material system, we have developed a numerical model to describe the intersubband carrier dynamics which restores the equilibrium after pulsed optical excitation in asymmetric coupled Ge/SiGe quantum wells. We take into account inelastic and elastic scattering processes and investigate different quantum-well geometries, doping densities, and excitation regimes. In this configuration space, we disentangle the effect on the overall dynamics of each scattering channel and provide intersubband relaxation times, finding larger values with respect to III-V based materials, thanks to the weaker electron-phonon coupling with respect to III-V compounds. Finally, the model is used to study and optimize the population inversion between the first- and second-excited subband levels and to assess its dependence on the lattice temperature, providing a sound theoretical framework to guide forthcoming experiments.

(3) Influence of Temperature on Growth of Graphene on Germanium
A.P. Becker, Ch. Wenger, J. Dabrowski
Journal of Applied Physics 128(4), 045310 (2020)
DOI: 10.1063/5.0003234, (Graphen)
Growth of high-quality graphene on germanium is to date only reported at growth temperatures near the substrate melting point. Direct integration of graphene growth into technological processes would, however, require a significantly lower growth temperature. Accordingly, we investigated the influence of growth temperature on the quality of graphene on Ge(001), Ge(110), and Ge(111). We found that increased defect density as indicated by Raman spectroscopy correlates with topographically protruding carbon defect clusters as indicated by scanning tunneling microscopy. The Raman quality of graphene on Ge(001) and Ge(110) grown at 850 °C is clearly limited by defects within the relatively large grains and not by grain boundaries, whereas the quality of graphene on Ge(111) additionally suffers from small grain size. We explain the decreased graphene quality by too weak substrate-mediated etching of defective carbon structures. Finally, we discuss potential ways to increase the rate of carbon etching.

(4) Electron Transport Across Vertical Silicon/MoS2/Graphene Heterostructures: Towards Efficient Emitter Diodes for Graphene Base Hot Electron Transistors
M. Belete, O. Engström, S. Vaziri, G. Lippert, M. Lukosius, S. Kataria, M.C. Lemme
ACS Applied Materials & Interfaces 12(8), 9656 (2020)
DOI: 10.1021/acsami.9b21691, (Graphen)
Heterostructures comprising of silicon, molybdenum disulfide (MoS2) and graphene are investigated with respect to the vertical current conduction mechanism. The measured current-voltage (I-V) characteristics exhibit temperature dependent asymmetric current, indicating thermally activated charge carrier transport. The data is compared and fitted to a current transport model that confirms thermionic emission as the responsible transport mechanism across the devices. Theoretical calculations in combination with the experimental data suggest that the heterojunction barrier from Si to MoS2 is linearly temperature dependent for T = 200 to 300 K with a positive temperature coefficient. The temperature dependence may be attributed to a change in band gap difference between Si and MoS2, strain at the Si/MoS2 interface or different electron effective masses in Si and MoS2, leading to a possible entropy change stemming from variation in density of states as electrons move from Si to MoS2. The low barrier formed between Si and MoS2 and the resultant thermionic emission demonstrated here makes the present devices potential candidates as the emitter diode of graphene-base hot electron transistors for future high-speed electronics.

(5) Terahertz Absorption-Saturation and Emission in Germanium Quantum Wells
C. Ciano, M. Virgilio, L. Bagolini, L. Baldassare, A. Pashkin M. Helm, M. Montanari, L. Persichtetti, L. DI Gaspare, G. Capellini, D.J. Paul, G. Scalari, J. Faist, M. De Seta, M. Ortolani
Optics Express 28(5), 7245 (2020)
DOI: 10.1364/OE.381471, (FLASH)
We study radiative relaxation at terahertz frequencies in n-type Ge/SiGe quantum wells, optically pumped with a terahertz free electron laser. Two wells coupled through a tunneling barrier are designed to operate as a three-level laser system with non-equilibrium population generated by optical pumping around the 1→3 intersubband transition at 10 THz. The non-equilibrium subband population dynamics are studied by absorption-saturation measurements and compared to a numerical model. In the emission spectroscopy experiment, we observed a photoluminescence peak at 4 THz, which can be attributed to the 3→2 intersubband transition with possible contribution from the 2→1 intersubband transition. These results represent a step towards silicon-based integrated terahertz emitters.

(6) Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models
C. Ciano, M. Virgilio, L. Bagolini, L. Baldassarre, A. Rossetti, A. Pashkin, M. Helm, M. Montanari, L. Persichetti, L. Di Gaspare, G. Capellini, D.J. Paul, G. Scalari, J. Faist, M. De Seta, M. Ortolani
Photonics 7(1), 2 (2020)
DOI: 10.3390/photonics7010002, (FLASH)
n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron–phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1→3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3.

(7) Liquid Phase Exfoliated Indium Selenide Based Highly Sensitive Photodetectors
N. Curreli, M. Serri, D. Spirito, E. Lago, E. Petroni, B. Martín-García, A. Politano, B. Gürbulak, S. Duman, R. Krahne, V. Pellegrini, F. Bonaccorso
Advanced Functional Materials 30(13), 1908427 (2020)
DOI: 10.1002/adfm.201908427
Layered semiconductors of the IIIA–VIA group have attracted considerable attention in (opto)electronic applications thanks to their atomically thin structures and their thickness‐dependent optical and electronic properties, which promise ultrafast response and high sensitivity. In particular, 2D indium selenide (InSe) has emerged as a promising candidate for the realization of thin‐film field effect transistors and phototransistors due to its high intrinsic mobility (>102 cm2 V−1 s−1) and the direct optical transitions in an energy range suitable for visible and near‐infrared light detection. A key requirement for the exploitation of large‐scale (opto)electronic applications relies on the development of low‐cost and industrially relevant 2D material production processes, such as liquid phase exfoliation, combined with the availability of high‐throughput device fabrication methods. Here, a β polymorph of indium selenide (β‐InSe) is exfoliated in isopropanol and spray‐coated InSe‐based photodetectors are demonstrated, exhibiting high responsivity to visible light (maximum value of 274 A W−1 under blue excitation 455 nm) and fast response time (15 ms). The devices show a gate‐dependent conduction with an n‐channel transistor behavior. Overall, this study establishes that liquid phase exfoliated β‐InSe is a valid candidate for printed high‐performance photodetectors, which is critical for the development of industrial‐scale 2D material‐based optoelectronic devices.

(8) Temperature-Driven Transformation of CsPbBr3 Nanoplatelets into Mosaic Nanotiles in Solution Through Self-Assembly
Z. Dang, B. Dhanabalan, A. Castelli, R. Dhall, K.C. Bustillo, D. Marchelli, D. Spirito, U. Petralanda, J. Shamsi, L. Manna, R. Krahne, M.P. Arciniegas
Nano Letters 20(3), 1808 (2020)
DOI: 10.1021/acs.nanolett.9b05036
Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light-emitting applications. Recent studies have focused on nanoplatelets that are able to self-assemble and transform on solid substrates. However, the mechanism behind the process and the atomic arrangement of their assemblies remain unclear. Here, we present a detailed analysis of the transformation of self-assembled stacks of CsPbBr3 nanoplatelets in solution over a period of a few months by using ex situ transmission electron microscopy and surface analysis. We demonstrate that the transformation mechanism can be understood as oriented attachment, proceeding through the following steps: (i) desorption of the ligands from the surfaces of the particles, causing the seamless atomic merging of nanoplatelet stacks into nanobelts; (ii) merging of neighboring nanobelts that form more extended nanoplates; and (iii) attachment of nanobelts and nanoplates, forming objects with an atomic structure that resembles a mosaic made of broken nanotiles. We reveal that aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr, with the atomic columns of neighboring perovskite lattices shifted by a half-unit-cell, forming Ruddlesden–Popper planar faults. We also show, via in situ monitoring of the nanocrystal photoluminescence combined with transmission electron microscopy analysis, that the transformation is temperature driven and that it can take place within tens of minutes in solution and in spin-coated films. Understanding this process gives crucial information for the design and fabrication of perovskite materials, where control over the type and density of defects is desired, stimulating the development of perovskite nanocrystal structures with tailored electronic properties.

(9) Composition Analysis and Transition Energies of Ultrathin Sn-Rich GeSn Quantum Wells
I.A. Fischer, C.J. Clausen, D. Schwarz, P. Zaumseil, G. Capellini, M. Virgilio, M.C. da Silva Figueira, St. Birner, S. Koelling, P.M. Koenraad, M.R.S. Huang, C.T. Koch, T. Wendav, K. Busch, J. Schulze
Physical Review Materials 4(2), 024601 (2020)
DOI: 10.1103/PhysRevMaterials.4.024601, (Dfg-QWIP)
While GeSn alloys with high Sn content constitute direct group-IV semiconductors, their growth on Si remains challenging. The deposition of a few monolayers of pure Sn on Ge and their overgrowth with Ge using molecular beam epitaxy can be a means of obtaining Sn-rich quantum wells with very high Sn content while maintaining high crystal quality. Here, we provide structural and compositional information on such structures with very high accuracy. Based on our characterization results we theoretically predict transition energies and compare them with experimental results from photoluminescence measurements. Our results constitute the groundwork for tuning the molecular beam epitaxy based growth of Sn-rich quantum wells and dots for applications in electronic and optoelectronic devices.

(10) Design and Simulation of Losses in Ge/SiGe Terahertz Quantum Cascade Laser Waveguides
K. Gallacher, M. Ortolani, K. Rew, C. Ciano, L. Baldassarre, M. Virgilio, G. Scalari, J. Faist, L. Di Gaspare, M. De Seta, G. Capellini, T. Grange, S. Birner, D.J. Paul
Optics Express 28(4), 4786 (2020)
DOI: 10.1364/OE.384993, (FLASH)
The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1-xGex THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 μm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 μm thick active gain region with silver metal of 21 cm−1 at 300 K reducing to 14.5 cm−1 at 10 K. Losses for silicon foundry compatible metals including Al and Cu are also provided for comparison and to provide a guide for gain requirements to enable lasers to be fabricated in commercial silicon foundries. To allow these losses to be calculated for a range of designs, the complex refractive index of a range of nominally undoped Si1−xGex with x = 0.7, 0.8 and 0.9 and doped Ge heterolayers were extracted from Fourier transform infrared spectroscopy measurements between 0.1 and 10 THz and from 300 K down to 10 K. The results demonstrate losses comparable to similar designs of GaAs/AlGaAs quantum cascade laser plasmon waveguides indicating that a gain threshold of 15.1 cm−1 and 23.8 cm−1 are required to produce a 4.79 THz Ge/SiGe THz laser at 10 K and 300 K, respectively, for 2 mm long double metal waveguide quantum cascade lasers with facet coatings.

(11) Effective Reduction of the Programing Pulse Width in Al:HfO2-based RRAM Arrays
O. Gonzalez Osorio, E. Perez, S. Dueñas, H. Castan, H. Garcia, Ch. Wenger
Proc. Joint International EUROSOI Workshop and International Conference on Ultimate Integration on Silicon (EUROSOI-ULIS 2019), (2020)
DOI: 10.1109/EUROSOI-ULIS45800.2019.9041880, (NeuroMem)

(12) Atomic-Scale Insights into Semiconductor Heterostructures: From Experimental Three-Dimensional Analysis of the Interface to a Generalized Theory of Interfacial Roughness Scattering
T. Grange, S. Mukherjee, G. Capellini, M. Montanari, L. Persichetti, L. Di Gaspare, S. Birner, A. Attiaoui, O. Moutanabbir, M. Virgilio, M. De Seta
Physical Review Applied 13(4), 044062 (2020)
DOI: 10.1103/PhysRevApplied.13.044062, (FLASH)
In this manuscript, we develop a generalized theory for the scattering process produced by interface roughness on charge carriers and which is suitable for any semiconductor heterostructure. By exploiting our experimental insights into the three-dimensional atomic landscape of Ge/GeSi heterointerfaces obtained by atom probe tomography, we have been able to define the full set of interface parameters relevant to the scattering potential, including both the in-plane and axial correlation inside real diffuse interfaces. Our experimental findings indicate a partial coherence of the interface roughness along the growth direction within the interfaces. We show that it is necessary to include this feature, previously neglected by theoretical models, when heterointerfaces characterized by finite interface widths are taken into consideration. To show the relevance of our generalized scattering model in the physics of semiconductor devices, we implemented it in a non-equilibrium Green’s function simulation platform to assess the performance of a Ge/SiGe-based THz quantum cascade laser.

(13) Modeling of Plasmonic Semiconductor THz Antennas in Square and Hexagonal Array Arrangements
S. Gruessing, B. Witzigmann, F. Roemer, G. Capellini, C.A. Chavarin, W. M. Klesse, E. Hardt, J. Piehler, C. You, J. Flesch
Proc. SPIE Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XIII (2020), 11279, 1127926 (2020)
DOI: 10.1117/12.2543553, (DFG Group 4 Plasmonics)

(14) Editors' Choice - Precipitation of Suboxides in Silicon, their Role in Gettering of Copper Impurities and Carrier Recombination
G. Kissinger, D. Kot, A. Huber, R. Kretschmer, T. Müller, A. Sattler
ECS Journal of Solid State Science and Technology 9(6), 064002 (2020)
DOI: 10.1149/2162-8777/aba0ce, (Future Silicon Wafers)
This paper describes a theoretical investigation of the phase composition of oxide precipitates and the corresponding emission of self-interstitials at the minimum of the free energy and their evolution with increasing number of oxygen atoms in the precipitates. The results can explain the compositional evolution of oxide precipitates and the role of self-interstitials therein. The formation of suboxides at the edges of SiO2 precipitates after reaching a critical size can explain several phenomena like gettering of Cu by segregation to the suboxide region and lifetime reduction by recombination of minority carriers in the suboxide. It provides an alternative explanation, based on minimized free energy, to the theory of strained and unstrained plates. A second emphasis was payed to the evolution of the morphology of oxide precipitates. Based on the comparison with results from scanning transmission electron microscopy the sequence of morphology evolution of oxide precipitates was deduced. It turned out that it is opposite to the sequence assumed until now.

(15) AC Electrokinetic Immobilization of Organic Dye Molecules
E.-M. Laux, Ch. Wenger, F.F. Bier, R. Hölzel
Analytical and Bioanalytical Chemistry (2020)
DOI: 10.1007/s00216-020-02480-4, (BioBic)
The application of inhomogeneous AC electric fields for molecular immobilization is a very fast and simplemethod that does not require any adaptions to the molecule’s functional groups or charges.Here, themethod is applied to a completely new category of molecules: small organic fluorescence dyes, whose dimensions amount to only 1 nm or even less. The presented setup and the electric field parameters used allow immobilization of dye molecules on the whole electrode surface as opposed to pure dielectrophoretic applications, where molecules are attracted only to regions of high electric field gradients, i.e., to the electrode tips and edges. In addition to dielectrophoresis and AC electrokinetic flow, molecular scale interactions and electrophoresis at short time scales are discussed as further mechanisms leading to migration and immobilization of the molecules.

(16) Influence of Specific Forming Algorithms on the Device-to-Device Variability of Memristive Al-Doped HfO2 Arrays
M.K. Mahadevaiah, E. Perez, Ch. Wenger
Journal of Vacuum Science and Technology B 38(1), 013201 (2020)
DOI: 10.1116/1.5126936, (NeuroMem)
In this work, the influence of specific switching algorithms on device-to-device (D2D) variability of the forming process, in an integrated Al-doped HfO2 1T-1R 4 kbit RRAM array is investigated. The resistive devices are programmed by using two different algorithms: the incremental step pulse and verify algorithm (ISPVA) at different temperatures and the constant amplitude pulse and verify algorithm (CAPVA) at different voltage amplitudes. The stabilized forming currents of both algorithms are compared in terms of their distributions, yields and dispersions. The D2D distributions of the forming voltages of ISPVA and the forming times of CAPVA are fitted by Weibull distributions. The obtained Weibull parameters provide a link with the statistics governing the process. Finally, we discuss the importance of the ISPVA, CAPVA, temperature and voltage amplitudes to improve the reliability of the forming process.

(17) Temperature Dependence of Strain–Phonon Coefficient in Epitaxial Ge/Si(001): A Comprehensive Analysis
C.L. Manganelli, M. Virgilio, O. Skibitzki, M. Salvalaglio, D. Spirito, P. Zaumseil, Y. Yamamoto, M. Montanari, W.M. Klesse, G. Capellini
Journal of Raman Spectroscopy 51(6), 989 (2020)
DOI: 10.1002/jrs.5860
We investigate the temperature dependence of the Ge Raman mode strain–phonon coefficient in Ge/Si heteroepitaxial layers. By analyzing the temperature‐dependent evolution of both the Raman Ge─Ge line and of the Ge lattice strain, we obtain a linear dependence of the strain–phonon coefficient as a function of temperature. Our findings provide an efficient method for capturing the temperature‐dependent strain relaxation mechanism in heteroepitaxial systems. Furthermore, we show that the rather large variability reported in the literature for the strain–phonon coefficient values might be due to the local heating of the sample due to the excitation laser used in μ‐Raman experiments.

(18) Dielectrophoretic Immobilization of Yeast Cells using CMOS Integrated Microfluidics
H. Matbaechi Ettehad, P.S. Zarrin, R. Hoelzel, Ch. Wenger
Micromachines 11(5), 501 (2020)
DOI: 10.3390/mi11050501, (BioBic)
This paper presents a dielectrophoretic system for the immobilization and separation of live and dead cells. Dielectrophoresis (DEP) is a promising and efficient investigation technique for the development of novel lab-on-a-chip devices, which characterizes cells or particles based on their intrinsic and physical properties. Using this method, specific cells can be isolated from their medium carrier or the mixture of cell suspensions (e.g., separation of viable cells from non-viable cells). Main advantages of this method, which makes it favorable for disease (blood) analysis and diagnosis applications are, the preservation of the cell properties during measurements, label-free cell identification, and low set up cost. In this study, we validated the capability of CMOS integrated microfluidic devices for the manipulation and characterization of live and dead yeast cells using dielectrophoretic forces. This approach successfully trapped live yeast cells and purified them from dead cells. Numerical simulations based on a two-layer model for yeast cells flowing in the channel was used to predict the trajectories of the cells with respect to their dielectric properties, varying excitation voltage, and frequency.

(19) Growth of Ge/SiGe Quantum Cascade Heterostructures
M. Montanari, L. Persichetti, C. Ciano, L. Di Gaspare, M. Virgilio, G. Capellini, M.H. Zoellner, O. Skibitzki, G. Scalari, D.J. Paul, T. Grange, S. Birner, O. Moutanabbir, S. Mukherjee, L. Baldassarre, M. Ortolani, M. De Seta
8th International Symposium on Control of Semiconductor Interfaces (ISCSI 2019), 71 (2020)

(20) Programming Pulse width Assessment for Reliable and Low-Energy Endurance Performance in Al:HfO2-based RRAM Arrays
E. Perez, O.G. Ossorio, S. Dueñas, H. Castan, H. Garcia, Ch. Wenger
Electronics 9(5), 864 (2020)
DOI: 10.3390/electronics9050864, (NeuroMem)
The reduction of the pulse width used during the programming of RRAM devices is crucial in order to accomplish fast low-energy switching operations. In this work, several pulse width values between 10 μs and 50 ns were evaluated by using the incremental step pulse with verify algorithm (ISPVA) on Al-doped HfO2 4 kbit RRAM arrays. 1k endurance cycles were initially performed to assess the switching stability. Both conductive levels and voltages needed for switching showed a remarkable good behavior along the 1k reset/set cycles regardless the pulse width considered. Nevertheless, the distributions of voltages as well as the amount of energy required to perform the switching operations were definitely impacted by the change of the pulse width. In addition, the data retention, after the endurance test, was evaluated at 150 oC for 100 hours. Only an extremely slight increase on the degradation rate of 1 μA after 100 hours was reported between samples programmed by using pulse widths of 10 μs and 50 ns. Finally, an endurance performance of 200k cycles without any degradation was achieved on 128 RRAM devices by using programming pulses of 100 ns width.

(21) Programming Pulse width Assessment for Reliable and Low-Energy Endurance Performance in Al:HfO2-based RRAM Arrays
E. Perez, O.G. Ossorio, S. Dueñas, H. Castan, H. Garcia, Ch. Wenger
Electronics 9(5), 864 (2020)
DOI: 10.3390/electronics9050864, (Total Resilience)
The reduction of the pulse width used during the programming of RRAM devices is crucial in order to accomplish fast low-energy switching operations. In this work, several pulse width values between 10 μs and 50 ns were evaluated by using the incremental step pulse with verify algorithm (ISPVA) on Al-doped HfO2 4 kbit RRAM arrays. 1k endurance cycles were initially performed to assess the switching stability. Both conductive levels and voltages needed for switching showed a remarkable good behavior along the 1k reset/set cycles regardless the pulse width considered. Nevertheless, the distributions of voltages as well as the amount of energy required to perform the switching operations were definitely impacted by the change of the pulse width. In addition, the data retention, after the endurance test, was evaluated at 150 oC for 100 hours. Only an extremely slight increase on the degradation rate of 1 μA after 100 hours was reported between samples programmed by using pulse widths of 10 μs and 50 ns. Finally, an endurance performance of 200k cycles without any degradation was achieved on 128 RRAM devices by using programming pulses of 100 ns width.

(22) Intersubband Transition Engineering in the Conduction Band of Asymmetric Coupled Ge/SiGe Quantum Wells
L. Persichetti, M. Montanari, C. Ciano, L. Di Gaspare, M. Ortolani, L. Baldassarre, M.H. Zoellner, S. Mukherjee, O. Moutanabbir, G. Capellini, M. Virgilio, M. De Seta
Crystals 10(3), 179 (2020)
DOI: 10.3390/cryst10030179, (FLASH)
n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device.

(23) Enhanced Thermal Stability of Yttrium Oxide-Based RRAM Devices with Inhomogeneous Schottky-Barrier
E. Piros, S. Petzold, A. Zintler, N. Kaiser, T. Vogel, R. Eilhardt, Ch. Wenger, L. Molina-Luna, L. Alff
Applied Physics Letters 117(1), 013504 (2020)
DOI: 10.1063/5.0009645, (NeuroMem)
This work addresses the thermal stability of bipolar resistive switching in yttrium oxide-based resistive random access memory revealed through the temperature dependence of the DC switching behavior. The operation voltages, current levels, and charge transport mechanisms are investigated at 25 ° 25 ° C, 85 ° 85 ° C, and 125 ° 125 ° C, and show overall good temperature immunity. The set and reset voltages, as well as the device resistance in both the high and low resistive states, are found to scale inversely with increasing temperatures. The Schottky-barrier height was observed to increase from approximately 1.02 eV at 25 ° 25 ° C to approximately 1.35 eV at 125 ° 125 ° C, an uncommon behavior explained by interface phenomena.

(24) Metastable CdTe@HgTe Core@Shell Nanostructures Obtained by Partial Cation Exchange Evolve into Sintered CdTe Films Upon Annealing
I. Rosina, B. Martín-García, D. Spirito, Z. Dang, G. Gariano, S. Marras, M. Prato, R. Krahne, L. De Trizio, L. Manna
Chemistry of Materials 32(7), 2978 (2020)
DOI: 10.1021/acs.chemmater.9b05281
Partial Hg2+ → Cd2+ cation exchange (CE) reactions were exploited to transform colloidal CdTe nanocrystals (NCs, 4–6 nm in size) into CdTe@HgTe core@shell nanostructures. This was achieved by working under a slow CE rate, which limited the exchange to the surface of the CdTe NCs. In such nanostructures, when annealed at mild temperatures (as low as 200 °C), the HgTe shell sublimated or melted and the NCs sintered together, with the concomitant desorption of their surface ligands. At the end of this process, the annealed samples consisted of ligand-free CdTe sintered films containing an amount of Hg2+ that was much lower than that of the starting CdTe@HgTe NCs. For example, the CdTe@HgTe NCs that initially contained 10% of Hg2+, after being annealed at 200 °C were transformed to CdTe sintered films containing only traces of Hg2+ (less than 1%). This procedure was then used to fabricate a proof-of-concept CdTe-based photodetector exhibiting a photoresponse of up to 0.5 A/W and a detectivity of ca. 9 × 104 Jones under blue light illumination. Our strategy suggests that CE protocols might be exploited to lower the overall costs of production of CdTe thin films employed in photovoltaic technology, which are currently fabricated at high temperatures (above 350 °C), using post-process ligand-stripping steps.

(25) Toward a Reliable Synaptic Simulation Using Al-Doped HfO2 RRAM
S. Roy, G. Niu, Q. Wang, Y. Wang, Y. Zhang, H. Wu, S. Zhai, P. Shi, S. Song, Z. Song, Z.-G. Ye, Ch. Wenger, T. Schroeder, Y.-H. Xie, X. Meng, W. Luo, W. Ren
ACS Applied Materials & Interfaces 12(9), 10648 (2020)
DOI: 10.1021/acsami.9b21530, (NeuroMem)
The potential in a synaptic simulation for neuromorphic computation has revived the research interest of resistive random access memory (RRAM). However, novel applications require reliable multilevel resistive switching (RS), which still represents a challenge. We demonstrate in this work the achievement of reliable HfO2-based RRAM devices for synaptic simulation by performing the Al doping and the postdeposition annealing (PDA). Transmission electron microscopy and operando hard X-ray photoelectron spectroscopy results reveal the positive impact of Al doping on the formation of oxygen vacancies. Detailed I–V characterizations demonstrate that the 16.5% Al doping concentration leads to better RS properties of the device. In comparison with the other reported results based on HfO2 RRAM, our devices with 16.5% Al-doping and PDA at 450 °C show better reliable multilevel RS (∼20 levels) performance and an increased on/off ratio. The 16.5% Al:HfO2 sample with PDA at 450 °C shows good potentiation/depression characteristics with low pulse width (10 μs) along with a good On/Off ratio (>1000), good data retention at room temperature, and high temperature and good program/erase endurance characteristics with a pulse width of 50 ns. The synapse features including potentiation, depression, and spike time-dependent plasticity were successfully achieved using optimized Al-HfO2 RRAM devices. Our results demonstrate the beneficial effects of Al doping and PDA on the enhancement of the performances of RRAM devices for the synaptic simulation in neuromorphic computing applications.

(26) Measuring Oxygen and Bulk Micro Defects in Silicon
H. Savin, G. Kissinger, V.-M. Airaksinen
Handbook of Silicon Based MEMS Materials and Technologies, 3rd Edition, Editors: M. Tilli, M. Paulasto-Krockel, M. Petzold, H. Theuss, T. Motooka, V. Lindroos, Chapter 37. Measuring Oxygen and Bulk Microdefects in Silicon, Elsevier, 775 (2020) 
DOI: 10.1016/B978-0-12-817786-0.00037-2, (Future Silicon Wafers)

(27) Ge(Sn) Nano-Island Photodetectors with Plasmonic Antennas
V. Schlykow, C.L. Manganelli, F. Römer, C. Clausen, L. Augel, J. Schulze, J. Katzer, M.A. Schubert, B. Witzigmann, T. Schroeder, G. Capellini, I.A. Fischer
Nanotechnology 31(34), 345203 (2020)
DOI: 10.1088/1361-6528/ab91ef
We report on photodetection in deep subwavelength Ge(Sn) nano-islands on Si nano-pillar substrates, in which self-aligned nano-antennas in the Al contact metal are used to enhance light absorption by means of local surface plasmon resonances. The impact of parameters such as substrate doping and device geometry on the measured responsivities are investigated and our experimental results are supported by simulations of the three-dimensional distribution of the electromagnetic fields. Comparatively high optical responsivities of about 0.1 A W−1 are observed as a consequence of the excitation of localized surface plasmons, making our nano-island photodetectors interesting for applications in which size reduction is essential.

(28) A Comprehensive Study of Charge Transport in Au-Contacted Graphene on Ge/Si(001)
A. Sinterhauf, S. Bode, M. Auge, M. Lukosius, G. Lippert, H. Hofsäss, M. Wenderoth
Applied Physics Letters 117(2), 023104 (2020)
DOI: 10.1063/5.0013802, (Graphen)
We investigate the electronic transport properties of Au-contacted graphene on Ge/Si(001). Kelvin probe force microscopy at room temperature with additionally applied electric transport field is used to gain a comprehensive understanding of macroscopic transport measurements. In particular, we analyze the contact pads including the transition region, perform local transport measurements in pristine graphene/Germanium, and explore the role of the semiconducting Germanium substrate. We connect the results from these local scale measurements with the macroscopic performance of the device. We find that a graphene sheet on a 2 μm Ge film carries approximately 10% of the current flowing through the device. Moreover, we show that an electronic transition region forms directly adjacent to the contact pads. This transition region is characterized by a width of > 100 μm and a strongly increased sheet resistance acting as the bottleneck for charge transport. Based on Rutherford backscattering of the contact pads, we suggest that the formation of this transition region is caused by diffusion.

(29) Nano- and Microscale Apertures in Metal Films Fabricated by Colloidal Lithography with Perovskite Nanocrystals
D. Spirito, J. Shamsi, M. Imran, Q.A. Akkermann, L. Manna, R. Krahne
Nanotechnology 31(18), 185304 (2020)
DOI: 10.1088/1361-6528/ab70f7
We demonstrate patterning of metal surfaces based on lift-off of perovskite nanocrystals that enables the fabrication of nanometer-size features without the use of resist-based nanolithography. The perovskite nanocrystals act as templates for defining the shape of the apertures in metal layers, and we exploit the variety of sizes and shapes that can be controlled in the colloidal synthesis to demonstrate the fabrication of nanoholes, nanogaps and guides with size smaller than the wavelength of light in the visible spectrum. The process can be readily integrated with standard lithography and etching techniques for the creation of more complex structures.

(30) Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy
T.J.Z. Stock, O. Warschkow, P.C. Constantinou, J. Li, S. Fearn, E. Crane, E.V.S. Hofmann, A. Kölker, D.R. McKenzie, S.R. Schofield, N.J. Curson
ACS Nano 14(3), 3316 (2020)
DOI: 10.1021/acsnano.9b08943
Over the last two decades, prototype devices for future classical and quantum technologies have been fabricated using scanning tunneling microscopy and hydrogen resist lithography to position phosphorus atoms in silicon with atomic-scale precision. Despite these successes, phosphine remains the only dopant precursor molecule to have been demonstrated as compatible with the hydrogen resist lithography technique. The potential benefits of atomic-scale placement of alternative dopant species have, until now, remained unexplored. In this work, we demonstrate the successful fabrication of atomic-scale structures of arsenic-in-silicon. Using a scanning tunneling microscope tip, we pattern a monolayer hydrogen mask to selectively place arsenic atoms in the Si(001) surface using arsine as the precursor molecule. We fully elucidate the surface chemistry and reaction pathways of arsine on Si(001), revealing significant differences to phosphine. We explain how these differences result in enhanced surface immobilization and in-plane confinement of arsenic atoms compared to phosphorus, and a dose-rate independent arsenic saturation density of 0.24±0.04 monolayers. Finally, we demonstrate the successful encapsulation of arsenic delta-layers using silicon molecular beam epitaxy, and find electrical characteristics that are competitive with equivalent structures of phosphorus. Arsenic delta-layers are also found to offer improvement in the out-of-plane confinement compared to similarly prepared phosphorus layers, while still retaining >90% carrier activation and sheet resistances of 1.5 kΩ/square, These excellent characteristics of arsenic represent opportunities to enhance existing capabilities of atomic-scale fabrication of dopant structures in silicon, and are particularly important for three-dimensional devices, where vertical control of the position of device components is critical.

(31) Carbon Related Hillock Formation and its Impact on the Optoelectronic Properties of GaN/AlGaN Heterostructures Grown on Si(111)
H. Tetzner, P. Sana, W.M. Klesse, G. Capellini, M.A. Schubert, S.B. Thapa, P. Storck, T. Schroeder, M.H. Zoellner
Applied Physics Letters 116(25), 252101 (2020)
DOI: 10.1063/5.0005484, (GaN HEMT)
The integration of GaN on Si as large scale substrate still faces many hurdles. Besides the large difference in lattice constant and the high thermal mismatch existing between GaN and Si, spiral hillock growth phenomena are common problems in the development of thick GaN layers. In this work, hexagonal hillocks were observed on GaN/AlGaN high-electron-mobility transistor heterostructures grown on Si(111) by metal-organic chemical vapor deposition. The presence of these morphological and structural defects is attributed to the presence of localized contamination at the AlN/Si interface. These carbon-based defects cause highly defective regions in the AlN seed layer and propagating through all the AlGaN buffer layers, inducing the formation of V-shaped pits at the lower AlGaN interfaces. In hillock regions of the wafers, Raman spectroscopy indicates disturbed two dimensional electron gas characteristics resulting from GaN/AlGaN interface roughness and a decreased amount of free carriers in the potential well. Energy-Dispersive X-ray spectroscopy reveals Ga accumulation inside the V-pits and in nanopipes below, which is responsible for defective areas in GaN and following increased GaN growth rate resulting in hillock formation. Photoluminescence measurements confirm the presence of Ga-rich material reducing the inherent Gallium vacancy concentration. Here, the reduced amount of Ga-vacancies acting as shallow acceptor suppress the ultra-violet luminescence band from donor-acceptor pair transition.

(32) Nanocrystals of Lead Chalcohalides: A Series of Kinetically Trapped Metastable Nanostructures
S. Toso, Q.A. Akkerman, B. Martín-García, M. Prato, J. Zito, I. Infante, Z. Dang, A. Moliterni, C. Giannini, E. Bladt, I. Lobato, J. Ramade, S. Bals, J. Buha, D. Spirito, E. Mugnaioli, M. Gemmi, L. Manna
Journal of the American Chemical Society 142(22), 10198 (2020)
DOI: 10.1021/jacs.0c03577
We report the colloidal synthesis of a series of surfactant-stabilized lead chalcohalide nanocrystals. Our work is mainly focused on Pb4S3Br2, a chalco-halide phase unknown to date that does not belong to the ambient-pressure PbS – PbBr2 phase diagram. The Pb4S3Br2 nanocrystals herein feature a remarkably narrow size distribution (with a size dispersion as low as 5%) a good size tunability (from 7 to ∼30 nm), an indirect bandgap, photoconductivity (responsivity = 4 ± 1 mA/W) and stability for months under air. A crystal structure is proposed for this new material by combining the information from 3D electron diffraction and electron tomography of a single nanocrystal, X-Ray powder diffraction and density functional theory calculations. Such a structure is closely related to that of the recently discovered high-pressure chalcohalide Pb4S3I2 phase, and indeed we were able to extend our synthesis scheme to Pb4S3I2 colloidal nanocrystals, whose structure matches the one that has been published for the bulk. Finally, we could also prepare nanocrystals of Pb3S2Cl2, which proved to be a structural analogue of the recently reported bulk Pb3Se2Br2 phase. It is remarkable that one high-pressure structure (for Pb4S3I2) and two metastable structures that had not yet been reported (for Pb4S3Br2 and Pb3S2Cl2) can be prepared on the nanoscale by wet-chemical approaches. This highlights the important role of colloidal chemistry in the discovery of new materials and motivates further exploration into metal chalcohalides nanocrystals.

(33) Ge/SiGe Multiple Quantum Well Fabrication by Reduced-Pressure Chemical Vapor Deposition
Y. Yamamoto, O. Skibitzki, M.A. Schubert, M. Scuderi, F. Reichmann, M.H. Zoellner, M. De Seta, G. Capellini, B. Tillack
Japanese Journal of Applied Physics Pt. 1 59, SGGK 10 (2020)
DOI: 10.7567/1347-4065/ab65d0, (FLASH)
In this paper we deposit structures comprising a stack of 10 periods made of 15-nm-thick Ge multiple quantum wells (MQWs) enclosed in a 15-nm-thick Si0.2Ge0.8 barrier on SiGe virtual substrates (VSs) featuring different Ge content in the 85%–100% range to investigate the influence of heteroepitaxial strain on Si0.2Ge0.8 and Ge growth. With increasing Ge concentration of the VS, the growth rate of Si0.2Ge0.8 in the MQWs increases. Si incorporation into the Si0.2Ge0.8 layer also becomes slightly higher. However, almost no influence of the growth rate is observed for Ge growth in the MQWs. We argue that increased tensile strain promotes the Si reaction at the surface. In the case of Si0.2Ge0.8 growth on Ge, we observe a smeared interface due to Ge segregation during the growth. Furthermore, we observe that the interface width increases with increasing Ge concentration of the VS. We attribute this observation to the increased segregation of Ge driven by increased strain energy accumulated in the Si0.2Ge0.8 layers. We also observe that the MQW layer "filters out" threading dislocations formed in the VS.

(34) Epileptic Seizure Detection Using a Neuromorphic-Compatible Deep Spiking Neural Network
P.S. Zarrin, R. Zimmer, Ch. Wenger, T. Masquelier
Proc. International Work-Conference on Bioinformatics and Biomedical Engineering (IWBBIO 2020), in: Lecture Notes in Bioinformatics, Springer, LNBI 12108, 389 (2020)
DOI: 10.1007/978-3-030-45385-5_34, (Total Resilience)

The building and the infrastructure of the IHP were funded by the European Regional Development Fund of the European Union, funds of the Federal Government and also funds of the Federal State of Brandenburg.