Effects of microstructure on transport properties of undoped hydrogenated amorphous silicon films

Abstract

Electronic transport properties have been investigated in undoped hydrogenated amorphous silicon (a-Si:H) materials whose microstructure and void fraction are changed by deposition temperature (Ts). The hydrogen content in these materials decreases from 15 to 5 at. % and the void fraction by 14% as Ts is raised from 200 to 350°C. The photo and dark conductivities are measured from 40 to 190°C and extended state electron mobilities are derived from a self-consistent analysis. The room temperature mobilities are found to increase from 0.8 to 30 cm2/V s and become less temperature dependent as Ts increases. These temperature activated mobilities explain the Meyer-Neldel rule [Z. Tech. Phys. 18, 588 (1937)] in a-Si:H materials whose dark conductivity activation energies are greater than 0.4 eV where it cannot be explained by the statistical shift of the Fermi level.