Properties of Post-Hydrogenated Amorphous and Microcrystalline Germanium Films
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PROPLRIIES OF POST-HYDROGENATED AMORPHOUS AND MICROCRYSTALLINE GERMANIUM FILMS JAMES R. WOODYARD+, J. GONZALEZ-HERNANDEZ*, R.T. YOUNG* and J. PIONTKOWSKI* +Department of Electrical and Computer Engineering, Wayne State University, Detroit, Michigan 48202 *Energy Conversion Devices,
Inc., 1675 West Maple Road,
Troy, Michigan 48084
ABSTRACT Amorphous and microcrystalline germanium films prepared by glowdischarge and molecular beam deposition were hydrogenated after deposition, using a 3cm Kaufman ion source. The hydrogen profiles were determined using the N 5p nuclear resonance reaction. We found that the surface region hydrogen concentration depended on ion beam modification of the material, and the bulk concentration was determined by the hydrogenation conditions and deposition conditions. The light and dark conductivities wee measured before and after hydrogenation. Several orders of magnitude change in the ratios of the conductivities were measured under optimum hydrogenation conditions. The aclivation energy for electrical conductivity was measured and found to be dependent on film structure thickness and hydrogenation conditions. INTRODUCTION It is well known that the electronic properties of pure amorphous and polycrystalline Si and Ge materials can be improved by post-hydrogenation [1]. The general view is that post-hydrogenation results in the passivation of defects in the materials. Dangling Si bonds are believed to be the important defects in amorphous films [2] with grain boundary and dislocation defects dominating in polycrystalline materials [3,4]. Experiments have shown that the properties of polycrystalline solar cells are substantially improved with hydrogen passivation using a Kaufman ion source [5,6]. Electrical [7] and chemical [8] measurements have been carried out in an atLempt to determine the hydrogen transport mechanism in polycrystalline malerials; these investigations show that the grain boundary hydrogen transport is more important than intra-granular transport and that transport along dislocation arrays may play a role. While post-hydrogenation of Si has received a great deal of attention, only Pearton et al. have reported studies of the post-hydrogenation of Ge. They showed that 180 C anneals in the presence of atomic hydrogen were effective in passivating sputter-etched induced defects [9]. Their studies on crystalline Ge diodes showed that post-hydrogenation also passivated defects which they associated with dislocations [10]. The work which they carried out in polycrystalline Ge diodes suggested that hydrogen passivated the grain boundaries and had an enhanced diffusivity along them [11]. We have studied the effect of post-hydrogenation on the electrical properties of amorphous and microcrystalline Ge thin films. Measurements of the hydrogen concentrations show that the film structure is important in hydrogenation. Studies of the thickness dependence of the electrical properties lead us to conclude that ion beam materials modification is an effective defect passivation method. EXPE
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