A density-driven phase transition between semiconducting and metallic polyamorphs of silicon

Abstract

Amorphous and crystalline forms of silicon are well-known, tetrahedrally coordinated semiconductors. High-pressure studies have revealed extensive polymorphism among various metallic crystal structures containing atoms in six-, eight- and 12-fold coordination. Melting silicon at ambient or high pressure results in a conducting liquid, in which the average coordination is greater than four (ref. 3). This liquid cannot normally be quenched to a glass, because of rapid crystallization to the diamond-structured semiconductor. Solid amorphous silicon is obtained by synthesis routes such as chemical or physical vapour deposition that result in a tetrahedrally bonded semiconducting state. It has long been speculated that the amorphous solid and the liquid could represent two polymorphic forms of the amorphous state that are linked by density- or entropy-driven transformations. Such polyamorphic transitions are recognized to occur among several different types of liquid and glassy systems. Here we present experimental evidence for the occurrence of a density-driven polyamorphic transition between semiconducting and metallic forms of solid amorphous silicon. The experiments are combined with molecular dynamics simulations that map the behaviour of the amorphous solid on to that of the liquid state.

Latent ion tracks in amorphous silicon

“…We suggest the overdense ion track core could conceivably contain frozen-in structural remnants of the molten HDL Si phase, potentially in the form of the pressureinduced high-density amorphous (HDA) Si phase reported by McMillan et al 27 The rapid cooling rate associated with resolidification of an ion track may well be sufficient to quench in metastable HDA Si. Alternatively, SAXS determinations of core-shell structures for ion tracks in a-SiO 2 12 or a-Ge 13 were consistent with an underdense core and overdense shell, which can be attributed to radially outward material flow.…”

High-density amorphous phase of silicon carbide obtained under large plastic shear and high pressure

“…4 Similarly, 3C SiC transforms to the rock salt structure in DAC ∼100 GPa. 3 Pressure-induced amorphization in SiC was not observed experimentally, in contrast to some molecular dynamic (MD) simulations [5][6][7] and to amorphization in materials with a similar structure, like ice, 8 Si, 9 silica, 10 and AlPO 4 11 (see the review in Ref. 12).…”

“…Amorphous SiC obtained in MD simulations from melt also had a lower density than the crystalline phase. 16 While the existence of various amorphous phases of SiC among existing forms cannot be excluded (similar to polyamorphism in ice, 8,19 Si, 9,20 and carbon 21 ), low-density amorphous (lda) SiC has a lower density than crystalline SiC. Then, direct pressure-induced amorphization to lda-SiC is impossible because of its density is lower and its enthalpy is higher 16 than those for crystalline SiC.…”

Microscopic identification of the order parameter governing liquid–liquid transition in a molecular liquid

“…This means that there can be more than two liquid states for a single-component substance. Despite its counterintuitive nature, there have recently been many pieces of experimental and numerical evidence for the existence of LLT, for various liquids such as water (1)(2)(3)(4)(5), aqueous solutions (6)(7)(8), triphenyl phosphite (9-12), l-butanol (13), phosphorus (14), silicon (15,16), germanium (17), and Y 2 O 3 -Al 2 O 3 (18,19). This suggests that the LLT may be rather universally observed for various types of liquids.…”