Internal (SiH)X groups, X = 1–4, in microcrystalline hydrogenated silicon and their IR spectra on the basis of periodic DFT modelling

Abstract

Vibrational Si–H frequencies were calculated on the basis of density functional theory (DFT) using periodic boundary conditions for N-Si voids, N < 8, in microcrystalline hydrogenated silicon (MHS) and (100), (110), and (111) slabs of 8, 5, and 8 layers, respectively, with the dangling bonds being saturated with hydrogen atoms. The slabs are considered as the models of inter-grain boundaries (IGB) in MHS. The N-Si voids of different shapes have been obtained via random deleting N silicon atoms. It was shown that the high stretching modes (HSM) of Si–H vibrations, which are usually assigned to SiHX, appear also due to (SiH)X groups, X = 2–4, in the N-Si voids. No such (SiH)X groups were formed with X > 1 at the IGB. The low stretching modes (LSM) are thus assigned to Si–H groups presented at both N-Si voids and IGB. Similar relative stability of the voids is obtained with two different DFT approaches, i.e., B3LYP with atomic basis set and Perdew-Burke-Ernzerhof (PBE) with plane wave basis set. This result allows a simple interpretation of usually small IHSM/(ILSM + IHSM) intensity ratio as a consequence of minor concentration of any voids in device quality MHS.