Abstract
High-pressure and -temperature experiments on Ge and Si mixtures to 17 GPa and 1500 K allow us to obtain extended Ge-Si solid solutions with cubic (Ia3) and tetragonal (P4(3)2(1)2) crystal symmetries at ambient pressure. The cubic modification can be obtained with up to 77 atom % Ge and the tetragonal modification for Ge concentrations above that. Together with Hume-Rothery criteria, melting point convergence is employed here as a favored attribute for solid solution formation. These compositionally tunable alloys are of growing interest for advanced transport and optoelectronic applications. Furthermore, the work illustrates the significance of employing precession electron diffraction for mapping new materials landscapes resulting from tailored high-pressure and -temperature syntheses.
Highlights
While cubic diamond-structured silicon is the single most important material in the semiconductor industry, it has an indirect band gap[1] and a fixed lattice constant, constraining it from efficient light emitting applications including most prominently photovoltaics and laser devices
We present examples documenting the Ia3 assignment first, followed by examples documenting the P43212 assignment and examples of our chemical analysis of crystals from both symmetries
0-21 0-12 b about 23 at.% Si, with the P43212 symmetry occurring for Si compositions from 22 at% Si downwards
Summary
While cubic diamond-structured silicon is the single most important material in the semiconductor industry, it has an indirect band gap[1] and a fixed lattice constant, constraining it from efficient light emitting applications including most prominently photovoltaics and laser devices. This constraint remains present for its associated cubic diamond-structured pure germanium[1] and silicon-germanium alloy[2] counterparts because they retain fundamental indirect band-gaps. Between 12 and 13 GPa all four generally accepted criteria for solid solution formation, namely same crystal structure, atomic radii ratios within 15% of each other, similar valencies and electronegativities are fulfilled.[20]
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