Enhanced a.c. losses have been observed in thin films of a-Ge and a-Si exposed to low intensity light, of wavelength 633nm, derived from a He-Ne laser. The samples were prepared in sandwich configuration by R. F. sputtering in argon or argon-hydrogen atmospheres. Illumination intensities of 1uWcm-z or less were applied through a semi-transparent gold top electrode. Changes in a.c. conductivity and capacitance of up to 10% were measured at helium temperatures. The optical response at helium temperatures is non-linear. At high intensities, the permittivity increases as I1/4, but at low intensities the response is closer to The temperature dependence of the response is small up to 20K. The recovery to the dark state is non-exponential and usually many hours elapse before no further change can be detected. When 500nm and 800nm light is used, no difference in response can be seen after the different absorption factors of the semiconductor film at these wavelengths have been accounted for. The loss changes induced in the I 1/4 region are similar in pure a-Si and pure a-Ge films but decrease as the hydrogen content of films increases. At low intensities heavily hydrogenated material shows a greater response than pure material. The following model is used to explain the data. Incident photons generate free carriers which are rapidly trapped by deep, clustered defects. The trapped electrons (or holes) are able to respond to the applied a.c. field and contribute an additional loss. The only escape for the trapped electrons at low temperatures is by tunnelling to a neighbouring excess hole. An simple analysis of the appropriate rate equations leads to carrier densities which compare well to the e. s. r. signal in sputtered material. The above model predicts the I 1/2 behaviour at low intensities. During the decay to dark equilibrium, the induced loss is proportional to -log(time). To account for the form of the decay the model is modified to include the fact that, in the dark, the average excited pair separation will increase with time. By postulating a minimum pair separation it is possible to explain the inconsistency of the slow decays to equilibrium and the recombination times estimated from the high temperature loss. However this model fails in that the predicted maximum pair separation is less than the minimum pair separation. This can only be explained by strong carrier self-trapping at the defect site. The reduced response at higher intensities is ascribed to reduced self-trapping for states excited far from the fermi level. The increased self-trapping in hydrogenated material is also reflected in a slower free decay to equilibrium. The active defect is estimated to be approximately 10A in extent which is consistent with the results of the low temperature a.c. field effect. It is suggested that the optically induced loss is derived from a population of correlated pairs of dangling bond states which exist on the internal surfaces of voids
