Synthesis and Atomic and Electronic Structure of New Si−Ge−C Alloys and Compounds

Abstract

The synthesis and characterization of completely novel binary and ternary alloy semiconductors and ordered phases based on C, Si, and Ge are discussed in this review. Metastable compound semiconductors with ordered structures, which include stoichiometric SiGe, Si 4 C, Si 3 GeC 4 (sphalerite), Ge 4 C, (Si 2 Ge)C x , and (Ge 2 Si)C x (x ) 5%), are described. Materials systems include diamond-structured silicon-germanium solid solutions with dissolved carbon (Si 1-x-y Ge x C y ), monocrystalline Ge 1-x C x hybrids of Ge, and C-diamond and related Si-containing random alloy systems. The Si 4 C and Ge 4 C materials incorporate the corresponding tetrahedra that are linked together to form a diamond-cubic structure related to Si. The Si 3 GeC 4 phase is related to sphalerite and (Si 2 Ge)C x has a new P3 hm1 structure formed by Ge-Si-Si ordering along the diamond 〈111〉 direction. These compounds offer the prospect of band gaps wider than that of Si; in some cases, the band gaps are expected to become direct. This report emphasizes an approach that combines novel precursor chemistries and modern deposition techniques (ultrahigh-vacuum chemical-vapor deposition) to develop heteroepitaxial, device-quality inorganic materials. Important highlights of recent research based on conventional deposition methods are also summarized.

Theory of metastable group-IV alloys formed from CVD precursors

“…The Ge-C bond length is 2.00 Å, which lies between those in C(GeBr 3 ) 4 (2.05 Å) and C(GeH 3 ) 4 (1.97 Å). 10 Although no stable GeC phase exists at ambient conditions, as a privilege for theorists, we can still do the calculation for this idealized compound and we find that compared to that of GeC, the Ge-C bond length in Ge 4 C is stretched by 2%, similar to Si-C. The overstretched Si-C and Ge-C bonds as compared to their binary counterparts is expected in these strained systems.…”

“…On one hand, the small radius of carbon might help reduce the lattice mismatch between band-gap-engineered Si/Ge materials and the silicon substrate; on the other hand, the presence of strongly electronegative carbon atoms, along with the strain introduced by them, offers a unique way to tailor the electronic structure with a compelling possibility of obtaining direct-gap group-IV semiconductors whose lattices match silicon. 16 With newly developed ultrahigh-vacuum chemical-vapor deposition ͑UHV-CVD͒, 10,11 group-IV compounds such as Si 4 C and Ge 4 C, which contain 20 at. % of carbon, have been synthesized.…”

“…Although ordered microcrystalline Si 4 C and Ge 4 C have been successfully synthesized, 10,11 their structural properties remain largely unknown. Here we assume a minimal unit cell for structural optimizations.…”

Synthesis of novel compounds with cation ordered fluorite and α-PbO2 related structures by oxidation of Sn2Nb2O7 pyrochlore

“…Amorphous solids from the solution route and gas molecules in CVD process function as precursors to produce metastable phases; for instance, the metastable phases of perovskite-type YMnO 3 [1], tungsten bronzes [2], and Si-Ge-C semiconducting compounds possessing ordered structures [3], respectively have been prepared from the metal citrates, the metal peroxo-polytungstates, and C(SiH 3 ) 4 and C(GeH 3 ) 4 molecules. In the CeO 2 -ZrO 2 system, when cooled from a high temperature, crystalline cubic phases function as a precursor to produce a metastable tetragonal t phase through diffusionless transformation [4].…”

Novel Si–Ge–C superlattices and their applications

“…These values make it impossible to grow thick pseudomorphic layers of Si 4 C, but it is feasible to grow the few atomic planes necessary to construct short period SLs strained to Si. In addition, these ordered alloys have a larger critical thickness, at least 7 to 8 monolayers [13], than what one could expect from the lattice constant derived from Vegard's law [16].…”