Hybridization in Electronic States and Optical Properties of Covalent Amorphous Semiconductors
- PDF / 301,007 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 51 Downloads / 218 Views
ABSTRACT The electronic structure and optical properties of covalent amorphous semiconductors are theoretically studied with special attention to the s-p hybridization in electronic states and the spatial correlation in their mixing. One-dimensional tight binding model is used in which the interatomic transfer energy of an electron between nearest neighbor atoms depends linearly on their interatomic distance. All the electronic states are numerically calculated for a 150-atom system and the ensemble average is taken over 10 samples. Following results have been obtained. As the degree of randomness increases, the degree of hybridization decreases and rearrangements in the covalent bonds take place. The width of the band gap decreases but the gap remains rather long compared to a case where the spatial correlation is neglected. There appears a characteristic peak in the optical absorption spectrum, which reflects central peaks in the partial (s- or p-) density of states in the valence and conduction bands and is related to an -lectron localization caused by the spatial correlation. INTRODUCTION Amorphous semiconductors are attractive materials because of randomness. It has been pointed out that randomness induces several interesting phenomena: electron localization [1], band tailing and the mobility edge [2], Urbach-tail in the optical absorption spectrum [3], and so on. Theoretical studies usually use the tight-binding model and calculate electronic states, by a tractable approximation such as the coherent potential approximation, or by an exact numerical calculation. Most of the previous studies have been devoted to clarify the effects of the sitediagonal randomness and a few studies discussed the off-diagonal randomness, where the transfer energy of an electron is modulated. If we simulate the spatial distribution of atoms in real amorphous materials within the tight-binding model, the correlation in the fluctuation must be taken into account both for the site-diagonal and the site off-diagonal randomness. Let us imagine a case where an atom happens to move right, then the electron-transfer energy will increase to the right-side neighbor atom and decrease to the left-side neighbor atom. If displacements of several atoms result in a particular atom to be isolated in space, the energy gain by the covalent bond between hybridized orbitals will decrease and this may affect the hybridization and then the stability of the covalent bonding. The purpose of the present paper is to clarify the effect of the correlation in the offdiagonal randomness to the hybridization and the stability of the covalent bonding. We will calculate the optical absorption spectrum and discuss how it reflects the spatial correlation of atoms and the hybridization. 309 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
THEORY Let us consider a one-dimensional array of N atoms at each site n=l,..., N, which are randomly located along the x-direction (Fig. 1). The n-th atom has a pair of s-orbital ns and pr-orbital i,,, of an el
Data Loading...
