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