ULTRAFAST CHARGE RECOMBINATION IN UNDOPED AMORPHOUS HYDROGENATED SILICON

Nonradiative bimolecular recombination of photocarriers in room-temper ature a-Si:H films was studied as a function of the photocarrier densi ty and excitation energy, using ultrafast laser spectroscopy over the range of 150 fs to 4 ns. It is demonstrated that for excitation with p hotons whose energy exceeds the optical gap of a-Si:H, 1.7 eV, the tim e-evolution of transient absorbance (detected at 1.5-1.6 eV) depends o nly on the initial density N-ex of the photocarriers (>10(18) cm(-3)). For N(ex)similar to 5 x 10(18) to 2 X 10(20) cm(-3), the bimolecular decay of the photocarriers is bimodal: In the first few picoseconds th e decay kinetics are controlled by recombination and trapping of free carriers, whereas at later delay times the kinetics are controlled by intraband migration of the trapped charges and their recombination wit h thermally emitted free carriers. An analysis of the decay kinetics i n the framework of the multiple-trapping model (which included mesosco pic nonuniformity of the photocarrier generation) gave the rate consta nts of 2.3x10(-8) and 6x10(-9) cm(-3)/s for the fast and slow componen ts of the recombination kinetics, respectively. This bimodality accoun ts fora considerable spread of bimolecular rate constants found in the literature. Our results seem to be incompatible with the previously s uggested models of Auger process in undoped a-Si:H. This work emphasiz es the need for further theoretical understanding of the recombination mechanisms involved. We argue that in the low-density regime, the dir ect bimolecular recombination of the excess charges can be a ''rare ev ent'' that causes the Staebler-Wronsky effect. [S0163-1829(98)06219-5] .