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 SiH_X, 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 I_HSM/(I_LSM + I_HSM) intensity ratio as a consequence of minor concentration of any voids in device quality MHS.

Keywords:

• microcrystalline hydrogenated silicon
• DFT
• void stability
• IR spectra
• (SiH)_X groups

Acknowledgements

The authors deeply appreciate the help of the referee to reformulate and to improve the initial version of the manuscript. A.V. Larin and A.A. Rybakov thank the financial support of Ministry of Education and Science of Russian Federation (Minobrnauka, GK No. 07.514.11.4150). The authors thank the FUNDP, F.R.S.-FRFC (convention 2.4.617.07.F), for the use of the Namur Interuniversity Scientific Computing Facility Centre (Belgium). The authors are extremely grateful to Computer Complexes ‘Lomonosov’ and ‘Chebyshev’ of Lomonosov Moscow State University for computational time.

Notes

The isolated or connected character of the voids was determined visually controlling large SCs with MOLDRAW code.

Frequencies should be scaled to be compared to the experimental ones that can be also obtained with different values as the HSM and LSM bands.