Extrinsic doping in group IV hexagonal-diamond type crystals

TL;DR

This study uses DFT simulations to compare extrinsic doping capabilities of group IV hexagonal-diamond crystals with diamond-like structures, revealing differences in impurity site symmetry and stability for various dopants.

Contribution

It provides the first detailed comparison of extrinsic doping in hexagonal-diamond versus cubic structures, highlighting how dopant valence affects site symmetry and stability.

Findings

Hexagonal-diamond crystals preserve impurity site symmetry differently than cubic ones.

Group III impurities are more easily incorporated in hexagonal-diamond phase for Si and Ge.

Both n- and p-type dopants are more stable in hexagonal-diamond C, especially n-type.

Abstract

Over the last few years, group IV hexagonal-diamond type crystals have acquired great attention in semiconductor physics thanks to the appearance of novel and very effective growth methods. However, many questions remain unaddressed on their extrinsic doping capability and on how it compares to those of diamond-like structures. This point is here investigated through numerical simulations conducted in the framework of the Density Functional Theory (DFT). The comparative analysis for group III and V dopant atoms shows that: i) in diamond-type crystals the bulk sites symmetry ($T_d$) is preserved by doping while in hexagonal crystals the impurity site moves towards a higher ($T_d$) or lower ($C_{3v}$) symmetry configuration dependently on the valence of the dopant atoms; ii) for Si and Ge, group III impurities can be more easily introduced in the hexagonal-diamond phase, whose local $C_{3v}$ symmetry better accommodates the three-fold coordination of the impurity, while n-type impurities do not reveal any marked phase preference; iii) for C, both n and p dopants are more stable in the hexagonal-diamond structure than in the the cubic one, but this tendency is much more pronounced for n-type impurities.