POLARIZED ELECTROABSORPTION EFFECT IN HYDROGENATED AMORPHOUS-SILICON ALLOYS AND ITS IMPLICATION FOR BAND-EDGE MOBILITY

A theory is presented to describe the absorption spectrum of amorphous semiconductors in the presence of an external electric field. Besides the isotropic third-derivative electroabsorption (EA) signal, the theory reveals the appearance of a polarization-dependant anisotropic EA signal characteristic of disordered media. Comparing the theoretical and experimental results on the EA effects, as well as the average dipole-matrix. elements, enables evaluations of the mean free path and the mobility. The result shows that the electron mobility reaches about 13 cm(2)/Vs for device-quality undoped a-Si:H, while the hole mobility is about 15% of the electron mobility. Alloying with carbon leads to a continuous reduction of mobility, with the largest drop (10%) for a carbon concentration of 8 at.%, this being in sharp contrast to a less-pronounced effect by germanium alloying.