ELECTRONIC MOBILITY GAP STRUCTURE AND THE NATURE OF DEEP DEFECTS IN AMORPHOUS-SILICON GERMANIUM ALLOYS GROWN BY PHOTO-CVD

Amorphous hydrogenated silicon-germanium alloys grown by photo-CVD hav e been studied using a variety of steady-state and transient junction- capacitance techniques. The dependence of the electronic mobility gap structure, the density of deep defects, and the carrier trapping prope rties on the germanium content has been investigated systematically. T he Urbach tail slope is found to be nearly constant over the whole all oy range. The dominant defect band is found to track the midgap energy position, and the density of deep defects increases exponentially wit h increasing germanium content. These results are fully consistent wit h weak bond breaking theories and suggest that the quality of amorphou s silicon-germanium alloys and a-Ge:H is inherently inferior to pure a -Si:H materials.