Photoinduced midgap absorption in tetrahedrally bonded amorphous semiconductors

Transient photoinduced optical absorption (PA) was used to study the transport, trapping, and recombination of excess electrons and holes in hydrogenated amorphous semiconductors with tetrahedral bonding. The materials studied were $a$-Si:H, $a$-Ge:H, $a$-GaAs:H, and the binary alloy systems $a-{\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$:H and $a-{\mathrm{Si}}_y{\mathrm{C}}_{1-y}$:H prepared by sputtering or glow discharge. In materials with optical energy gaps $E_g\gtrsim 1.25$ eV the absorption arises from transitions between carriers trapped at deep-lying defects and the band edge. This mechanism leads to a threshold in the induced absorption spectrum below $E_g$. In those compounds with $E_g\lesssim 1.25$ eV the PA spectrum consists of a single symmetric band whose shape can be well explained if absorption is due to photon-assisted hopping of small polarons bound to defects with 0.32—0.40-eV binding energy. The transition between these two spectral shapes as composition is varied appears to be discontinuous. Recombination of these excitations is found to follow bimolecular diffusion-limited kinetics that involves dispersive transport. The time-averaged mobility in $a$-Si:H was on the order of ${10}^{-3\mathrm{}}$ ${\mathrm{cm}}^2$/V s at room temperature and decreased with decreasing temperature as $\exp \left(\frac{T}{T_1}\right)$. A model for this unusual temperature dependence is proposed.