Contributions to Collected Editions
S. Schulz, D. Chaudhuri, M. O'Donovan, S. Patra, T. Streckenbach, P. Farrell, O. Marquardt, Th. Koprucki, Multi-scale modeling of electronic, optical, and transport properties of III-N alloys and heterostructures, in: Proceedings Physics and Simulation of Optoelectronic Devices XXVIII, 11274, San Francisco, California, USA, 2020, pp. 416--426, DOI 10.1117/12.2551055 .
Preprints, Reports, Technical Reports
D. Abdel, P. Vágner, J. Fuhrmann, P. Farrell, Modelling charge transport in perovskite solar cells: Potential-based and limiting ion vacancy depletion, Preprint no. 2780, WIAS, Berlin, 2020.
Abstract, PDF (1067 kByte)
From Maxwell--Stefan diffusion and general electrostatics, we derive a drift-diffusion model for charge transport in perovskite solar cells (PSCs) where any ion in the perovskite layer may flexibly be chosen to be mobile or immobile. Unlike other models in the literature, our model is based on quasi Fermi potentials instead of densities. This allows to easily include nonlinear diffusion (based on for example Fermi--Dirac, Gauss--Fermi or Blakemore statistics) as well as limit the ion vacancy depletion (via the Fermi--Dirac integral of order -1). The latter will be motivated by a grand-canonical formalism of ideal lattice gas. Furthermore, our model allows to use different statistics for different species. We discuss the thermodynamic equilibrium, electroneutrality as well as generation/recombination. Finally, we present numerical finite volume simulations to underline the importance of limiting ion vacancy depletion.
O.C. Ernst, D. Uebel, S. Kayser, F. Lange, Th. Teubner, T. Boeck, Revealing all states of dewetting of a thin gold layer on a silicon surface by nanosecond laser conditioning, Preprint no. 2777, WIAS, Berlin, 2020, DOI 10.20347/WIAS.PREPRINT.2777 .
Abstract, PDF (1817 kByte)
Dewetting is a ubiquitous phenomenon which can be applied to the laser synthesis of nanoparticles. A classical spinodal dewetting process takes place in four successive states, which differ from each other in their morphology. In this study all states are revealed by interaction of pulsed nanosecond UV laser light with thin gold layers with thicknesses between 1 nm and 10 nm on (100) silicon wafers. The specific morphologies of the dewetting states are discussed with particular emphasis on the state boundaries. The main parameter determining which state is formed is not the duration for which the gold remains liquid, but rather the input energy provided by the laser. This shows that each state transition has a separate measurable activation energy. The temperature during the nanosecond pulses and the duration during which the gold remains liquid was determined by simulation using the COMSOL Multiphysics software package. Using these calculations, an accurate local temperature profile and its development over time was simulated. An analytical study of the morphologies and formed structures was performed using Minkowski measures. With aid of this tool, the laser induced structures were compared with thermally annealed samples, with perfectly ordered structures and with perfectly random structures. The results show that both, structures of the laser induced and the annealed samples, strongly resemble the perfectly ordered structures. This reveals a close relationship between these structures and suggests that the phenomenon under investigation is indeed a spinodal dewetting generated by an internal material wave function.
The purposeful generation of these structures and the elucidation of the underlying mechanism of dewetting by ultrashort pulse lasers may assist the realisation of various technical elements such as nanowires in science and industry.
D. Uebel, S. Kayser, T. Markurt, O.C. Ernst, Th. Teubner, T. Boeck, Fast Raman mapping and in situ TEM observation of metal induced crystallization of amorphous silicon, Preprint no. 2772, WIAS, Berlin, 2020, DOI 10.20347/WIAS.PREPRINT.2772 .
Abstract, PDF (2098 kByte)
Crystalline silicon is grown onto an amorphous silicon (a-Si) seed layer from liquid tin solution (steady state liquid phase epitaxy, SSLPE). To investigate the crystallization of embedded a-Si during our process, we adapted Raman measurements for fast mapping, with dwell times of just one second per single measurement. A purposely developed imaging algorithm which performs point-by-point gauss fitting provides adequate visualization of the data. We produced scans of a-Si layers showing crystalline structures formed in the a-Si matrix during processing. Compared to scanning electron microscopy images which reveal merely the topography of the grown layer, new insights are gained into the role of the seed layer by Raman mapping. As part of a series of SSLPE experiments, which were interrupted at various stages of growth, we show that plate-like crystallites grow laterally over the a-Si layer while smaller, randomly orientated crystals arise from the a-Si layer. Results are confirmed by an in situ TEM experiment of the metal-induced crystallization. Contrary to presumptions, initially formed surface crystallites do not originate from the seed layer and are irrelevant to the final growth morphology, since they dissolve within minutes due to Ostwald ripening. The a-Si layer crystallizes within minutes as well, and crystallites of the final morphology originate from seeds of this layer.
D. Abdel, Comparison of Scharfetter--Gummel schemes for (non-)degenerate semiconductor device simulation, 20th International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD) (Online Event), September 14 - 18, 2020, Turin, Italy, September 16, 2020.
P. Farrell, Numerical methods for innovative semiconductor devices (Online Talk), Technische Universität Dresden, Institut für Wissenschaftliches Rechnen, May 27, 2020.
N. Dropka , P. Farrell, S. Kayser, N. Rotundo, Numerics for innovative semiconductor devices -- An outlook, German Conference on Crystal Growth, München, March 11 - 13, 2020.
P. Farrell, D. Peschka, Challenges in drift-diffusion semiconductor simulations, Finite Volumes for Complex Applications IX (Online Event), Belgien, Norway, June 15 - 19, 2020.