3 - The electronic density of states

Summary

The description of the electronic properties of a-Si: H starts with the energy distribution of electronic states. Depending on their energy and character, the different states determine the electrical transport, recombination and doping etc. Some effects of disorder in a-Si: H on the electronic states are the broadening of the density of states distribution compared to the crystal to form the band tails; the localization of the band tail states; the reduction of the scattering length to atomic distances; and the loss of momentum conservation in the electronic transitions. The last of these necessitates the replacement of the energy-momentum band structure of a crystalline semiconductor by an energy-dependent density of states distribution,N(E). It is convenient to divide N(E) into three different energy ranges; the main conduction and valence bands, the band tail region close to the band edge, and the defect states in the forbidden gap (see Fig. 1.6). This chapter describes the first two types of state and defects are dealt with in Chapter 4. The distribution of states, N(E), is derived in this chapter and is used throughout the remainder of the book in the analysis of experimental results.

One conclusion from the structure studies in Chapter 2 is that the bonding disorder of a-Si:H is relatively small. The silicon atoms have the same tetrahedral local order as crystalline silicon, with a bond angle variation of about 10% and a much smaller bond length disorder. Fig. 3.1 shows the calculated dependence of the bond energy on bond length and bond angle (Biswas and Hamann 1987).