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- ItemTowards the Growth of Hexagonal Boron Nitride on Ge(001)/Si Substrates by Chemical Vapor Deposition(Basel : MDPI, 2022) Franck, Max; Dabrowski, Jaroslaw; Schubert, Markus Andreas; Wenger, Christian; Lukosius, MindaugasThe growth of hexagonal boron nitride (hBN) on epitaxial Ge(001)/Si substrates via high-vacuum chemical vapor deposition from borazine is investigated for the first time in a systematic manner. The influences of the process pressure and growth temperature in the range of 10−7–10−3 mbar and 900–980 °C, respectively, are evaluated with respect to morphology, growth rate, and crystalline quality of the hBN films. At 900 °C, nanocrystalline hBN films with a lateral crystallite size of ~2–3 nm are obtained and confirmed by high-resolution transmission electron microscopy images. X-ray photoelectron spectroscopy confirms an atomic N:B ratio of 1 ± 0.1. A three-dimensional growth mode is observed by atomic force microscopy. Increasing the process pressure in the reactor mainly affects the growth rate, with only slight effects on crystalline quality and none on the principle growth mode. Growth of hBN at 980 °C increases the average crystallite size and leads to the formation of 3–10 well-oriented, vertically stacked layers of hBN on the Ge surface. Exploratory ab initio density functional theory simulations indicate that hBN edges are saturated by hydrogen, and it is proposed that partial de-saturation by H radicals produced on hot parts of the set-up is responsible for the growth.
- ItemHeteroepitaxy of group IV materials for future device application(Bristol : IOP Publ., 2023) Yamamoto, Yuji; Wen, Wei-Chen; Tillack, BerndHeteroepitxy of group IV materials (Si, SiGe, and Ge) has great potential for boosting Si-based novel device performance because of the possibility for strain, band gap/Fermi-level engineering, and applying emerging artificial materials such as a superlattice (SL) and nanodots. In order to control group IV heteroepitaxy processes, strain, interface, and surface energies are very essential parameters. They affect dislocation formation, interface steepness, reflow of deposited layers, and also surface reaction itself during the growth. Therefore, process control and crystallinity management of SiGe heteroepitaxy are difficult especially in the case of high Ge concentrations. In this paper, we review our results of abrupt SiGe/Si interface fabrication by introducing C-delta layers and the influence of strain on the surface reaction of SiGe. Three-dimensional self-ordered SiGe and Ge nanodot fabrication by proactively using strain and surface energies by depositing SiGe/Si and Ge/SiGe SL are also reviewed.
- ItemVertical alignment control of self-ordered multilayered Ge nanodots on SiGe(Bristol : IOP Publ., 2023) Wen, Wei-Chen; Schubert, Markus Andreas; Tillack, Bernd; Yamamoto, YujiSelf-ordered multilayered Ge nanodots with SiGe spacers on a Si0.4Ge0.6 virtual substrate are fabricated using reduced-pressure chemical vapor deposition, and the mechanism of vertical ordering is investigated. The process conditions of Ge and SiGe layer deposition are H2-GeH4 at 550 °C and H2-SiH4-GeH4 at 500 °C-550 °C, respectively. By depositing the SiGe at 550 °C or increasing Ge content, the SiGe surface becomes smooth, resulting in vertically aligned Ge nanodots to reduce strain energy. Ge nanodots prefer to grow on the nanodot where the SiGe is relatively tensile strained due to the buried Ge nanodot underneath. By depositing at 500 °C and lowering Ge content, checkerboard-like surface forms, and the following Ge nanodots grow at staggered positions to reduce surface energy. The Ge nanodots are laterally aligned along the elastically soft 〈100〉 direction without pre-structuring resulting from the strain distribution.