  Recent advances in the characterization of hexagonal-diamond silicon (2H-Si)
have shown that this material possesses remarkably different structural,
electronic, and optical properties as compared to the common cubic-diamond (3C)
polytype. Interestingly, despite the wide range of physical properties
analyzed, to date no study has investigated impurity energy levels in 2H-Si.
Here, we present results of ab initio DFT simulations to describe the effect of
p- and n-type substitutional doping on the structural and electronic properties
of hexagonal-diamond Si (2H-Si). We first provide a detailed analysis of how a
given impurity can assume a different local symmetry depending on the host
crystal phase. Then, by studying neutral and charged dopants, we carefully
estimate donors and acceptors transition energy levels in 2H-Si and compare
them with the cubic-diamond (3C) case. In the case of acceptors, the formation
energy is always lower in 2H-Si and is associated with a shallower charge
transition level with respect to 3C-Si. On the other hand, donors prefer the
cubic phase and have transition energies smaller with respect to 2H-Si.
Finally, by employing a simple model based on the 2H/3C band offset diagram, we
prove the physical validity of our findings and we show how holes can be used
to stabilize the 2H-Si phase.
