Dense Si_xGe_1–x (0 < x < 1) Materials Landscape Using Extreme Conditions and Precession Electron Diffraction

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.

An Unexpected Cubic Symmetry in Group IV Alloys Prepared Using Pressure and Temperature

“…This dramatic lowering of Im3 m stabilization pressure cannot be attributed to chemical pressure alone because the radius of Ge in the cubic structure with Im3 m symmetry is very similar to that of Sn. Ge promoting Im3 m formation is moreover,counter-intuitive because Ge,for very specific reasons presented below,d oes not form the Im3 m phase at any pressure.T herefore,G es hould hinder,n ot enhance formation of the Im3 m phase.I nG en amely,t he sa nd ps tates are sufficiently close in energy that sp 3 hybridization and the associated tetrahedrally coordinated diamond structure,are favoured. [25] With increasing pressure, sp 3 hybridization only gradually becomes less favourable in Ge with respect to promotion of electrons from the s-state to the p-state.The ensuing higher pressure intermediate bonded and distant from cubic Ge structures with Im3 m symmetry are dictated by this s-p favourability interplay and the diamond structure.…”

“…[1] While solid solution with isostructural Ge facilitates tunability of both lattice constant and band-gap,the SiGe band-gaps remain indirect. [2,3] Conversely, alloying Ge with Sn can provide tunability and direct bandgap formation making this one of the most actively investigated systems for optoelectronic applications. [4][5][6][7] These investigations are mostly confined to thin films because Ge and Sn are immiscible in the bulk at ambient pressure.…”

“…Despite the overwhelming importance of cubic diamond Si in electronic applications, its indirect band‐gap and fixed lattice constant make it considerably less effective for optoelectronic applications [1] . While solid solution with isostructural Ge facilitates tunability of both lattice constant and band‐gap, the SiGe band‐gaps remain indirect [2, 3] . Conversely, alloying Ge with Sn can provide tunability and direct band‐gap formation making this one of the most actively investigated systems for optoelectronic applications [4–7] .…”

Ab initio study of band gap properties in metastable BC8/ST12 SixGe1−x alloys

“…Thus, Si x Ge 1Àx alloys in both the BC8 and ST12 modifications have been synthesized within the compositional range of 0 x 1. 12 It was found that upon pressure release, the ST12 structure is retained at least up to x % 0:25, whereas for x > 0.25, BC8 is formed. Other than this experimental study, we did not encounter other reports on the atomic structure or the electronic properties of these alloys as a function of x.…”

“…Second, we generated randomly distributed supercells for all intermediate compositions, as the experimentally synthesized alloys are expected to be fully disordered. 12 We achieved this by generating special quasi-random structures (SQSs) 16 of 2 Â 2 Â 2 supercells using the SQS algorithm implemented in the USPEX package. 17,18 For all structural relaxations, we performed ab initio static calculations at zero pressure using the projector augmented wave (PAW) approach.…”