Studies of the gap states density in undoped and doped amorphous hydrogenated silicon

The density of the gap states in undoped, and in phosphorusand antimony-doped, n-type a-Si:H has been determined from absorption coefficient measurements. A constant photocurrent method was used to determine the value of optical absorption in a low absorption region. A quantitative model has been suggested to determine the density of states (DOS) within a gap below the Fermi level, from the spectral dependence of the absorption coefficient. The model is based on a Gaussian shaped maximum, connected with the dangling bonds, between the exponential valence and conduction band tails. The doping raises this maximum in the DOS.