B. K. Ghosh

Vibrational excitations in a-Si1−xCx:H alloys

Abstract We report here the results of a first calculation of the vibrational excitations of hydrogenated a-Si 1−x C x :H alloys using a cluster Bethe lattice method (CBLM). The study has been made for two types of distributions of constituent atoms of the alloy: a random sequence and a chemically ordered one. The infrared data of a-Si 1−x C x :H alloys can be well understood if one assumes the occurrence of a random distribution of Si and C atoms.

Vibrational excitations in a-Si3N4:H(D) alloys

The authors report here the first results of a theoretical study of the vibrational excitations in the amorphous Si3N4 and a-Si3N4:H(D) alloys using a cluster Bethe lattice method. The a-Si3N4 network has a nearest-neighbour tetrahedral coordination of N atoms around the Si atom and a threefold planar coordination of Si atoms around the N atom. The computed phonon density of states for a-Si3N4 and a-Si3N4:H alloys is in very good agreement with the available infrared and Raman data. The study shows the occurrence of the mono- and dihydrides both at the Si and N atoms. Detailed infrared and Raman measurements need to be performed on the a-Si3N4:H alloys for the different concentrations of H atoms, especially in the low-frequency region, to elucidate the microscopic structure of the alloys.

Vibrational excitations in pure and fluorinated silane chains

Abstract We have studied the vibrational excitations of the various fluorine hydrogen complexes in amorphous silicon using a cluster Bethe lattice formalism and also the polysilane like configurations. The calculated high frequencies are seen to be in excellent agreement with the infrared and Raman data. Some calculated peaks appearing near 214 cm−1 and 600 cm−1 have not been detected so far.

Phonons in a-SiC:F alloys

Abstract A theoretical study of the vibrational excitations induced by F atoms in amorphous SiC has been made in a cluster Bethe lattice method formalism. The calculated results are in agreement with the available infrared data. However, two predicted peaks near 200 and 1000 cm −1 have not been detected so far.

Phonons in Fluorinated Amorphous Silicon Alloys

Recently1 we have employed a cluster Bethe lattice method to make a comprehensive study of the density of states for the electrons and phonons in the amorphous silicon-fluorine alloys. In all, there have been three CBLM calculations for phonons2 and one OLCAO calculation for electrons3 for a-Si:F alloys. The present CBLM calculation both for the phonons and electrons have the following new and specific features: 1) The two calculated peaks near 214 and 300 cm−1 which are in agreement with the infrared data4 arise from the isolated SiF and SiF2/SiF3 units/ respectively from the same value of the angle bending force constant for the Si-Si-F bond angle for the first time. 2) The present calculation predicts for the first time a peak near 647 cm−1 from the interacting SiF2 units which has been seen in the infrared data of Shimada et al4.None of the earlier calculation3 has shown it. 3) For the first time, all the peaks observed in the photoemission data5 have been explained in terms of the SiFn (n = 1, 3) units. The earlier OLCAO calculation of Ching3 has predicted three peaks against the observed four peaks for SiF3 unit. 4) For the first time, the electronic structure for the two adjacent interacting SiF and SiF2 units have been presented. The calculation is very much needed as some extra peaks in the infrared can only be understood in terms of these interacting units. The calculated structure is compatible with the available photoemission data and thus supports the occurrence of the interacting SiF2 units. 5) In conclusion, on the basis of the available infrared and Raman data the present results for the first time establish conclusively without any ambiguity the occurrence of isolated SiF, SiF2 units and the interacting SiF2 units in a-Si:F alloys.