Structural and thermodynamic stability of hexagonal-diamond $\text{Si}_{1 - x - y}\,\text{Ge}_{x}\,\text{B}_{y}$ alloys

TL;DR

This study uses density functional theory to analyze the structural and thermodynamic stability of hyperdoped hexagonal-diamond SiGe alloys, revealing their potential for superconductivity and superior hyperdoping stability compared to cubic phases.

Contribution

It provides the first systematic comparison of B doping effects in hexagonal versus cubic SiGe alloys, highlighting their enhanced stability and hyperdoping potential.

Findings

B is more thermodynamically stable in hexagonal phase.

Hexagonal SiGe alloys follow Vegard's law for lattice parameters.

Hyperdoped hexagonal SiGe alloys are thermodynamically stable across all Ge concentrations.

Abstract

Pushing dopant concentrations beyond the solubility limit in semiconductors -- a process known as hyperdoping -- has been demonstrated as an effective strategy for inducing superconductivity in cubic-diamond Si and SiGe materials. Additionally, previous studies have reported that several polytypes of Si may exhibit a type-I superconducting state under high pressure. In this work, we employ ground-state Density Functional Theory simulations to investigate the effects of both low and high B doping concentrations on the structural and thermodynamic properties of hexagonal-diamond SiGe alloys, with a systematic comparison to their cubic-diamond counterparts. Our results highlight three key findings: (i) structural analysis confirms that the lattice parameters of SiGeB alloys adhere to a ternary Vegard's law, consistent with observations in cubic-diamond SiGe alloys. However, at high doping concentrations, B incorporation can locally disrupt the hexagonal symmetry, particularly in the presence of B clustering; (ii) dopant formation energy calculations reveal that B is thermodynamically more stable in the hexagonal phase than in the cubic phase across all Ge concentrations, regardless of the doping level; (iii) mixing enthalpy calculations demonstrate that hyperdoped hexagonal-diamond SiGe alloys are thermodynamically stable across the full range of Ge compositions and that their tendency for hyperdoping is more favorable than that of cubic-diamond SiGe alloys. Taken together, these findings indicate that hyperdoping is experimentally viable in hexagonal-diamond SiGe alloys and, in light of previous evidence, position these materials as a promising platform for the exploration of superconductivity in group IV semiconductors.