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  • Publikationen 2020

Publikationen 2020

seit Januar 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) 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.

(3) 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)

(4) 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)

(5) 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.

(6) 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.

(7) 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, 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.

(8) 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)

(9) 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
not known
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)

(10) AC Electrokinetic Immobilization of Organic Dye Molecules
E.-M. Laux, Ch. Wenger, F.F. Bier, R. Hölzel
Analytical and Bioanalytical Chemistry 1 (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.

(11) 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.

(12) 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)

(13) 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.

(14) Towards 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)

(15) 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.

(16) 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)
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/□, 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.

(17) 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)

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