Publications by Roksolana Kozak
The early growth stage of GaAs by metal organic vapor phase epitaxy on a novel kind of Si substra... more The early growth stage of GaAs by metal organic vapor phase epitaxy on a novel kind of Si substrate is investigated. The substrate consists of nanotips (NTs) fabricated on a Si(001) wafer by means of lithography and reactive ion etching. 3D GaAs nanocrystals are found to nucleate with a probability of 90% on the (n0m), (–n0m), (0nm), and (0–nm) facets (n, m integers) of these NTs. Additionally, in terms of nucleation yield, an average of 2 GaAs nanocrystals in each of those facets is observed. By contrast, facets of type {±nnm} remain virtually free of any 3D nuclei. A simple model based on the kinetics of the growth is used to explain the facet selective 3D nucleation. The model is consistent with a similar selectivity observed on micrometer-sized substrate features.
Nano-heteroepitaxial growth of GaAs on Si(001) by metal organic vapor phase epitaxy was investiga... more Nano-heteroepitaxial growth of GaAs on Si(001) by metal organic vapor phase epitaxy was investigated to study emerging materials phenomena on the nano-scale of III–V/Si interaction. Arrays of Si nano-tips (NTs) embedded in a SiO 2 matrix were used as substrates. The NTs had top Si openings of 50–90 nm serving as seeds for the selective growth of GaAs nano-crystals (NCs). The structural and morphological properties were investigated by high resolution scanning electron microscopy, atomic force microscopy, electron backscatter diffraction, x-ray diffraction, and high resolution scanning transmission electron microscopy. The GaAs growth led to epitaxial NCs featuring a bi-modal distribution of size and morphology. NCs of small size exhibited high structural quality and well-defined {111}–{100} faceting. Larger clusters had less regular shapes and contained twins. The present work shows that the growth of high quality GaAs NCs on Si NTs is feasible and can provide an alternate way to the integration of compound semiconductors with Si micro-and opto-electronics technology.
We present the nanoheteroepitaxial growth of gallium arsenide (GaAs) on nano-patterned silicon (S... more We present the nanoheteroepitaxial growth of gallium arsenide (GaAs) on nano-patterned silicon (Si) (001) substrates fabricated using a CMOS technology compatible process. The selective growth of GaAs nano-crystals (NCs) was achieved at 570 °C by MOVPE. A detailed structure and defect characterization study of the grown nano-heterostructures was performed using scanning transmission electron microscopy, x-ray diffraction, micro-Raman, and micro-photoluminescence (μ-PL) spectroscopy. The results show single-crystalline, nearly relaxed GaAs NCs on top of slightly, by the SiO 2-mask compressively strained Si nano-tips (NTs). Given the limited contact area, GaAs/Si nanostructures benefit from limited intermixing in contrast to planar GaAs films on Si. Even though a few growth defects (e.g. stacking faults, micro/nano-twins, etc) especially located at the GaAs/Si interface region were detected, the nanoheterostructures show intensive light emission, as investigated by μ-PL spectroscopy. Achieving well-ordered high quality GaAs NCs on Si NTs may provide opportunities for superior electronic, photonic, or photovoltaic device performances integrated on the silicon technology platform.
Papers by Roksolana Kozak
Acta Crystallographica Section A Foundations and Advances, 2014
Acta Crystallographica Section A Foundations of Crystallography, 2013
Acta Crystallographica Section E Structure Reports Online, 2014
A single crystal of the title compound, the ternary silicide digadolinium trirhenium penta-silici... more A single crystal of the title compound, the ternary silicide digadolinium trirhenium penta-silicide, Gd2Re3Si5, was isolated from an alloy of nominal composition Gd20Re30Si50 synthesized by arc melting and investigated by X-ray single-crystal diffraction. Its crystal structure belongs to the U2Mn3Si5 structure type. All atoms in the asymmetric lie on special positions. The Gd site has site symmetry m..; the two Mn atoms have site symmetries m.. and 2.22; the three Si atoms have site symmetries m.., ..2 and 4.. . The coordination polyhedra of the Gd atoms have 21 vertices, while those of the Re atoms are cubo-octa-hedra and 13-vertex polyhedra. The Si atoms are arranged as tricapped trigonal prisms, bicapped square anti-prisms, or 11-vertex polyhedra. The crystal structure of the title compound is also related to the structure types CaBe2Ge2 and W5Si3. It can be represented as a stacking of Gd-centred polyhedra of composition [GdSi9]. The Re atoms form infinite chains with an Re-Re distance of 2.78163 (5) Å and isolated squares with an Re-Re distance of 2.9683 (6) Å.
Zeitschrift für Kristallographie - Crystalline Materials, 2015
The term " high-entropy alloys (HEAs) " first appeared about 10 years ago defining alloys compose... more The term " high-entropy alloys (HEAs) " first appeared about 10 years ago defining alloys composed of n = 5 -13 principal elements with concentrations of approximately 100/ n at.% each. Since then many equiatomic (or near equiatomic) single-and multi-phase multicomponent alloys were developed, which are reported for a combination of tunable properties: high hardness, strength and ductility, oxidation and wear resistance, magnetism, etc. In our paper, we focus on probably single-phase HEAs (solid solutions) out of all HEAs studied so far, discuss ways of their prediction, mechanical properties. In contrast to classical multielement/multiphase alloys, only single-phase multielement alloys (solid solutions) represent the basic concept underlying HEAs as mixing-entropy stabilized homogenous materials. The literature overview is complemented by own studies demonstrating that the alloys CrFeCoNi, CrFeCoNiAl 0.3 and PdFeCoNi homogenized at 1300 and 1100 ° C, respectively, for 1 week are not singlephase HEAs, but a coherent mixture of two solid solutions.
European Microscopy Congress 2016: Proceedings, 2016
Ultramicroscopy, 2016
The development of new electro-optical devices and the realization of novel types of transistors ... more The development of new electro-optical devices and the realization of novel types of transistors require a profound understanding of the structural characteristics of new semiconductor heterostructures. This article provides a concise review about structural defects which occur in semiconductor heterostructures on the basis of micro-patterned Si substrates. In particular, one- and two-dimensional crystal defects are being discussed which are due to the plastic relaxation of epitaxial strain caused by the misfit of crystal lattices. Besides a few selected examples from literature, we treat in particular crystal defects occurring in GaAs/Si, Ge/Si and β-SiC/Si structures which are studied by high-resolution annular dark-field scanning transmission electron microscopy. The relevance of this article is twofold; firstly, it should provide a collection of data which are of help for the identification and characterization of defects in cubic semiconductors by means of atomic-resolution imaging, and secondly, the experimental data shall provide a basis for advancing the understanding of device characteristics with the aid of theoretical modelling by considering the defective nature of strained semiconductor heterostructures.
Applied Surface Science, 2006
... Abstract. We have estimated the threading dislocation density and type via X-ray diffraction ... more ... Abstract. We have estimated the threading dislocation density and type via X-ray diffraction andWilliamsonHall analysis to elicit qualitative information directly related to the electrical and optical quality of GaN epitaxial layers grown by PAMBE on 4H-and 6H-SiC substrates. ...
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Publications by Roksolana Kozak
Papers by Roksolana Kozak