ИСТИНА

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 PBE with plane wave basis set. The formation of these (SiH)X groups is the consequence of tetrahedral Si symmetry of MHS and can be the result of a conservation of local ordering in AHS [1]. The presence of similar (SiH)X groups in the voids of amorphous hydrogenated silicon (AHS) explains macro concentration of H…H pairs separated by 1.8 Å [2] according to NMR data (1019 – 1020 cm-3). Such H…H pairs were obtained in di-vacancies of AHS but the authors mentioned that it is not typical for higher size vacancies [3]. At the absence of the evidences of dominant concentration of di-vacancies this result can be considered as additional confirmation of the local ordering in AHS [1] and closer similarity between the vacancies in MHS and AHS. [1] Larin A.V., Milyaeva D.V., Rybakov A.A., Bezrukov D.S., Trubnikov D.N., Mol. Phys., 112, 956 (2014). [2] D.C. Bobela, T. Su, P.C. Taylor, and G. Ganguly, J. Non-Cryst. Solids, 352, 1041 (2006). [3] S. Chakraborty, D.A. Drabold, Physical Review B 79, 115214 (2009)