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METASTABLE OPTICAL ABSORPTION AND PARAMAGNETISM IN HYDROGENATED AMORPHOUS SILICON J. M. VINER, C. LEE, W.D. OHLSEN AND P. C. TAYLOR Department of Physics, University of Utah, Salt Lake City, UT 84112

ABSTRACT After rapid cooling from 210°C a characteristic electron spin resonance (ESR) and a characteristic below-gap optical absorption have been observed to grow with time at room temperature in hydrogenated amorphous silicon (a-Si:H). Both the increased ESR and the increased optical absorption anneal at temperatures above about 100°C. These unexpected results may have important consequences for our understanding of the electronic properties of a-Si:H.

INTRODUCTION The fact that hydrogen can diffuse in hydrogenated amorphous silicon (a-Si:H) has been known for some time [1], but the possibility that this diffusion could contribute to the metastabilities in this material has only recently been considered in any detail [2,3,4]. In doped films-rapid quenching from elevated temperatures appears to increase the density of deep gap states [2,3], but in undoped films the situation is less clear [2,4]. Our previous measurements on undoped films of a-Si:H have shown that films rapidly quenched from 210'C exhibit a reduced paramagnetism as measured by ESR. In time this paramagnetism increases while the films are stored in the dark at room temperature. These changes are one manifestation of changes in the densities of electronic states, or in the occupancy of localized electronic states, as a function of quenching rate or of time spent below some characteristic temperature T *. Above T thermal equilibrium in the electronic system occurs on a laboratory time scale, while below T* there are departures from equilibrium. As the temperature decreases below T* the approach to thermodynamic equilibrium occurs exponentially more and more slowly. In this paper we describe measurements of the optical absorption below the optical gap in a-Si:H. These measurements, which employ photothermal deflection spectroscopy (PDS), were performed at room temperature after rapid quenching from 210°C.

EXPERIMENTAL PROCEDURE Cooling rates of at least several hundred K/s were obtained by quenching the samples in ice water or alcohol. Experimental details have been described in a previous publication [4]. After rapid quenching the samples were mounted in the PDS apparatus where they were kept in the dark (except during measurements of the optical absorption) for the duration of the experiment. Because there are variations in the absolute values of the optical absorption of 10 to 30% across the surface of a typical film of a-Si:H, the films were never moved during an experiment. Precautions were also taken to insure that the increases in the optical absorption were not due to optically induced changes which could have occurred during the measuring process. The samples used in this study were a subset of those which were employed in our previous ESR measurements [4].

Mat. Res. Soc. Symp. Proc. Vol. 118. '1988 Materials Research Society

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