Origin of Persistent Photoconductivity in Modulation Doped Amorphous Silicon Multilayers
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ORIGIN OF PERSISTENT PHOTOCONDUCTIVITY IN MODULATION DOPED AMORPHOUS SILICON MULTILAYERS S.C. AGARWAL* AND S. GUHA Energy Conversion Devices, Inc., 1675 West Maple Road, Troy, Michigan 48084 *On leave from the Physics Department, Indian Institute of Technology, Kanpur 208 016, India ABSTRACT We show that persistent photoconductivity (PPC) observed in npn ... multilayers of hydrogenated amorphous silicon (a-Si:H) and compensated thin a-Si:H films have a common origin. It is caused by special centers lying in the upper half of the mobility gap. The centers are not related to the presence of argon during deposition. Similarities of PPC between the layered and compensated films are shown. We also find that PPC in compensated samples, is not entirely a surface effect. Experiments are suggested to decide whether these centers are P-B complexes or can arise from the presence of boron alone. INTRODUCTION It has been realized, for some time now, that the persistent photoconductivity (PPC) observed in modulation doped amorphous silicon multilayers after a brief light exposure, can not be explained entirely in terms of an increase in the life time of photogenerated carriers separated by the internal pn fields [1-6]. Alternatively, trapping in special centers is thought to be responsible for this effect. The origin of these centers is, however, far from established. We have found that compensated samples also show PPC effect qualitatively similar to the npnp... multilayers [2,3]. This prompted us to speculate that they originate either from the formation of P-B complexes or are boron related defects [2,3]. Su et al. [5], on the other hand, found that the presence of argon during the preparation of these multilayers was necessary for the observation of PPC. Hundhausen and Ley [4] proposed that these centers are hole traps (AX centers) in the p regions of the samples. Finally, Kakalios and Fritzsche also concluded that these centers are related to the presence of boron [6]. Here, we present additional results which show the similarity and the differences between the PPC observed in compensated and the doping modulated multilayers. We find that argon is not essential for the observation of PPC. We suggest ways to find out whether presence of boron alone can cause PPC or whether doping with phosphorous is essential. EXPERIMENTAL The films are grown by the glow discharge method described before [2]. Our samples are about 4000A thick. The layered samples consist of 6n and 5p type layers, each of thickness =360A, doped with phosphorous and boron respectively. Graphite electrodes, in coplanar configuration, are used. The compensated samples are ohmic. In contrast, the layered samples are non ohmic even for fields as small as lOV/cm. Upon application of the field, the dark current, parallel to the layers, varies with time and is found to settle down to a smaller equilibrium value in a few hours. We believe that the states and heterogeneities at the interfaces are responsible for this effect
[2].
Persistent photoconductivity was studi
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