Orientation-controlled large-grain (≥10 µm) crystal, i.e., quasi-single crystal, Ge-rich (≥50%) SiGe on insulator grown at low temperatures (≤300 °C) are desired for realization of high-performance flexible electronics. To achieve this, the Au-induced crystallization technique using a-SiGe/Au stacked structures has been developed. This enables formation of (111)-oriented large-grain (≥10 µm) Si1−xGex (x ≥ 0.5) crystals on insulating substrates at low temperatures (300 °C). The surface layers of the grown SiGe crystals have uniform lateral composition profiles. By using this technique, formation of quasi-single crystal Ge on flexible plastic sheets is demonstrated. This technique will be useful to realize high-performance flexible electronics.
Compressively strained germanium-on-insulator (c-GeOI) substrates with a definitely reduced defect density are expected to yield superior hole mobilities together with low off-state currents in p-type metal oxide semiconductor field effect transistors (MOSFETs). In order to fabricate c-GeOI wafers, we started with double-side polished Si(0 0 1) substrates and grew, by reduced pressure-chemical vapour deposition, Si0.15Ge0.85 virtual substrates (VS) on the front side. The wafer curvature was compensated thanks to the deposition of a thick Ge layer on the backside. We then grew, after a chemical mechanical polishing, various thickness (37–148 nm) Ge layers. They stayed smooth (root-mean-square roughness <5 Å) and pseudomorphically strained on the VS underneath (perpendicular lattice parameter: 5.681 Å Leftrightarrow bulk Ge lattice parameter: 5.658 Å) up to a 74 nm thickness. These c-Ge layers were then bonded on oxidized Si substrates using the SmartCut™ process. The resulting c-GeOI substrates were of high crystalline quality, compressively strained and smooth. The threading dislocations density (8 × 105 cm−2 Leftrightarrow 1.7 × 105 cm−2 for the SiGe VS template used to strain the c-Ge layers) was indeed 20 times less than the one associated with conventional GeOI substrates obtained from thick Ge epilayers. Pseudo-MOSFET measurements were performed to quantify the hole mobility gain in those c-GeOI substrates. A +150% enhancement compared to conventional silicon-on-insulator substrates was evidenced, validating the interest of these stacks.
Si-based germanium is considered to be a promising platform for the integration of electronic and photonic devices due to its high carrier mobility, good optical properties, and compatibility with Si CMOS technology. However, some great challenges have to be confronted, such as: (1) the nature of indirect band gap of Ge; (2) the epitaxy of dislocation-free Ge layers on Si substrate; and (3) the immature technology for Ge devices. The aim of this paper is to give a review of the recent progress made in the field of epitaxy and optical properties of Ge heterostructures on Si substrate, as well as some key technologies on Ge devices. High crystal quality Ge epilayers, as well as Ge/SiGe multiple quantum wells with high Ge content, were successfully grown on Si substrate with a low-temperature Ge buffer layer. A local Ge condensation technique was proposed to prepare germanium-on-insulator (GOI) materials with high tensile strain for enhanced Ge direct band photoluminescence. The advances in formation of Ge n+p shallow junctions and the modulation of Schottky barrier height of metal/Ge contacts were a significant progress in Ge technology. Finally, the progress of Si-based Ge light emitters, photodetectors, and MOSFETs was briefly introduced. These results show that Si-based Ge heterostructure materials are promising for use in the next-generation of integrated circuits and optoelectronic circuits.
Propanethiol solution (0.1 M) in 2-propanol, octanethiol solution (0.1 M) in 2-propanol and 20% (NH4)2S solution in water were used to passivate the germanium substrates. HfO2 thin films of 150 ALD cycles were then deposited on the passivated germanium substrates. The morphology of the thin films was investigated by X-ray diffraction and it was found that the morphology of the thin films was not affected by the chemical treatments. A lower leakage current density was observed in the passivated samples compared with the witness one. In addition, the interface quality and long-time stress reliability of the passivated samples were improved when the samples were annealed in forming gas ambient.