Photoinduced Absorption in Amorphous Tetrahedrally - Semiconductors.

A photoinduced transient absorption (PA) was observed and studied in amorphous semiconductors and alloys prepared by methods which incorporate hydrogen. In a-Si:H the induced adsorption coefficient has the form (DELTA)(alpha)(H/2PI)(omega) = c((H/2PI)(omega) - E(,t))(' 1/2) in the spectral range 0.25 - 1.5 eV. The threshold energy E(,t) is (0.45 (+OR -) 0.09) eV and c (TURNEQ) 21 eV(' 1/2) cm('-1) at 80 K for excitation by (TURN) 1 W/cm('2) of argon laser light. The absorption arises from transitions between holes trapped at deep lying defects and the valence band. In a-Ge:H the PA spectrum consists of an isolated symmetric band and we interpret the absorption mechanism as photon-assisted hopping of small polarons bound to defects with a binding energy of 0.35 - 0.40 eV. Recombination of these excitations is found to follow bimolecular diffusion limited kinetics. The electron drift mobility can be determined from the recombination rate and is a decreasing function of time following pulsed excitation ((mu) (TURN) t('-.3)). The time averaged mobility in undoped a-Si:H is (2.4 (+OR-) 1.7) x 10('-3) cm('2)/V sec at room temperature and decreases with decreasing temperature as exp (T/T(,1)) with T(,1) = 31 K, 67 K, and 106 K in undoped, B and P doped a-Si:H respectively. The unusual time- and temperature-dependences of (mu) are evidence of the important influence of localized states on the transport properties of amorphous semiconductors.