Dependence of the vibrational spectra of amorphous silicon on the defect concentration and ring distribution

Abstract

The Raman spectra of nine 216-atom models of amorphous silicon (a-Si) are calculated using the bond polarizability approximation of Raman scattering. These a-Si models, generated by the activation relaxation technique, have different concentrations of coordination defects, ring statistics and local strain distributions, which cause changes in the vibrational density of states and the Raman scattering. Analysis of the vibrational modes indicates that an increase in the number of coordination defects leads to an increase in the high-frequency localization and to mixing of the TA modes with other high-frequency modes. Calculation of partial Raman spectra indicates that five-coordinated Si atoms enhance the high-frequency part of the LO Raman peak at about 400 cm −1 and lead to characteristic band at about 600 cm −1 on the high-frequency side of the TO Raman peak. For their part, the three-coordinated Si atoms contribute to the low-frequency part of the LO peak. A weak correlation between the number of four-membered rings and the intensity of the LO Raman peak is also established although there is no correlation between the number of three-and four-membered rings and the total strain energy.

Structure and electronic states in a-Si:H thin films

“…It is seen from Table 1 that as T s increases from 100 to 250°C, the ratios of I LA /I TO and I LO /I TO decrease from 0.257 and 0.590 down to 0.230 and 0.574, respectively. It was reported that responsible for the LA mode are primarily the closed rings configuration and the three-coordinated defects of atoms [13,17]. In our present study, the decrease in LA strength can therefore be attributed to the reduction of the silicon coordination defects and the closed rings in the amorphous network due to a higher T s .…”

“…The TA mode intensity at about 150 cm -1 is considered to be proportional to the density of dihedral angle fluctuations, reflecting the medium-range order (MRO) of amorphous network [12]. The existence of coordination defects is necessary for the presence of the LA band at 300 cm -1 and the LO band at 410 cm -1 [13]. The peaks at about x 2LA = 610 cm -1 and x 2TO = 960 cm -1 are overtones of the main a-Si x LA and the main a-Si x TO , respectively.…”

“…The decrease in C TO is closely related to the decreasing bonded hydrogen content in thin films, which is confirmed by the FTIR results in ''Hydrogen bonding in thin films'' section. The LA mode and the LO mode are related to defects in the thin films, and the more the defects the larger the values of I LA /I TO and I LO /I TO ratio [13]. It is seen from Table 1 that as T s increases from 100 to 250°C, the ratios of I LA /I TO and I LO /I TO decrease from 0.257 and 0.590 down to 0.230 and 0.574, respectively.…”

Structure and physical properties of paracrystalline atomistic models of amorphous silicon

“…PC-K1EDIP1, which has no D 1 or D 2 defects, exhibits a much higher p c () at the 90 cm Ϫ1 band edge. The main difference in the participation ratio between the PC models and the WWW CRN model 37 as well as a-Si models generated by the activation-relaxation technique ͑ART͒, 47 investigated previously, 36,48 is that the CRNs do not show low-frequency localization. However, CRN models of a-Si produced with the Biswas, Grest, and Soukoulis method 42 also exhibit low-frequency localization on undercoordinated atoms, which implies that such features are not specific to the PC structure, or the SW potential or EDIP.…”

“…They are located predominantly on D 3 atoms. Such modes are not observed in CRN models generated by the WWW 36 or ART 48 methods. The general character of the total phase quotient q ⌺ (), shown in Fig.…”