First-principles insight in structure-property relationships of hexagonal Si and Ge polytypes

Hexagonal SiGe is a promising material for combining electronic and photonic technologies. In this paper, the energetic, structural, elastic, properties of hexagonal polytypes ($2H, 4H$, $6H$) silicon germanium are thoroughly analyzed under equilibrium conditions. For purpose, we apply state-of-the-art density functional theory. The phase diagram, obtained in framework generalized Ising model, shows that diamond structure most stable ambient conditions, but modifications close to boundary, especially Si. Our band calculations using modified-Becke-Johnson--local-density-approximation (MBJLDA) Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functionals predict significant changes states with hexagonality. While Si crystals always semiconductors indirect gaps, Ge have direct gaps. branch-point energies appear fundamental while they below valence maxima. Band alignment based on energy leads type-I heterocrystalline interfaces between polytypes, where electrons holes can be trapped layer higher