Electron drift mobilities measurements and conduction band tail states in hydrogenated amorphous silicon-based materials (effects of germanium and carbon alloying)

We have measured the temperature-dependence of the electron drift mobility using the time-of-flight technique for a series of undoped hydrogenated amorphous silicon-germanium alloys with bandgaps spanning the range 1.47-1.72 eV and undoped hydrogenated amorphous silicon-carbon alloys with bandgaps ranging from 1.75-1.90 eV. We also developed new techniques for analyzing dispersion effects in such measurements which permitted us to compare essentially all previous measurements with our own. We draw two main conclusions. First, there is substantial agreement between laboratories for the reductions in the electron drift mobility due to bandgap modification by germanium and carbon alloying. Second, we are able to account for most features of the germanium data using a standard multiple-trapping model invoking only variations of an exponential conduction bandtail width {dollar}Esbsp{lcub}C{rcub}{lcub}0{rcub}{dollar}; we find a fair linear correlation between this width and the optical bandgap {dollar}Esb{lcub}T{rcub}{dollar}. The effects of alloying upon the microscopic mobility and the attempt-to-escape frequency were relatively minor. This simple fitting procedure was less successful for the carbon alloys, which need further study.; We also measured the electroabsorption in amorphous silicon-based solar cells to assess the relationship of electroabsorption and built-in potentials. We constructed an apparatus for these measurements incorporating extensive signal averaging to improve reproducibility. We found a highly precise, linear relationship of electroabsorption {dollar}delta T{dollar} upon external reverse bias V; {dollar}delta T propto{dollar} (V-{dollar}Vsb{lcub}0{rcub}{dollar}), as expected from previous work. The parameter {dollar}Vsb{lcub}0{rcub}{dollar} is usually identified with the built-in potential {dollar}Vsb{lcub}bi{rcub}{dollar}; however, {dollar}Vsb{lcub}0{rcub}{dollar} shows significant dependence on measuring wavelength. We ascribe this effect to the role of the {dollar}psp{lcub}+{rcub}{dollar} layer in p-i-n solar cells, and propose a procedure for obtaining {dollar}Vsb{lcub}bi{rcub}{dollar} from the wavelength-dependent measurements.