Sample dependence of the structural, vibrational, and electronic properties of a-Si:H: A density-functional-based tight-binding study

Abstract

The sample dependence of various properties of hydrogenated amorphous silicon ($a$-Si:H) have been studied with 216 silicon atoms and 24 hydrogen atoms using the density functional based tight binding molecular dynamics simulations. The Si-Si and Si-H pair correlation functions are independent of preparation procedure as well as initial conditions, the H-H pair correlation functions are sample dependent. The distribution of hydrogen atoms in all the samples is nonuniform and depends upon the preparation procedure as well as the initial structure from which the hydrogenated amorphous silicon sample is generated. The Si-Si bond length and Si-Si-Si bond angle distributions are nearly independent of sample preparation procedure, but Si-H bond length distributions are sample dependent. The peaks in the vibrational density of states (VDOS) at high frequencies are in reasonable accord with the experimental results. The positions of the high frequency peaks are found to be dependent on the local environment. While the high frequency vibrational modes related to Si-Si bond vibrations are moderately localized, the vibrational modes related to Si-H bond vibrations in $a$-Si:H samples are highly localized. In samples generated from the liquid quench the free energy is smaller, whereas the entropy and specific heat are larger as compared to that of samples generated by hydrogenation of pure amorphous silicon samples. The total electronic density of states and local density of electronic states (LDOS) at Si atom sites are nearly sample independent, while the LDOS at the H atom sites are dependent on the sample preparation procedure.