Anisotropic Drift Mobility in Hydrogenated Amorphous Silicon.

The electrical transport of excess carriers for undoped hydrogenated amorphous silicon (a-Si:H) was measured using the photocurrent response to a 4 nsec impulse of laser illumination. Measurements were made of the dependence of the transport process upon the direction of the applied electric field, temperature, dangling bond defect density in the bulk and interfacial regions, bias illumination, type of substrate and interfaces, type of electrical contacts and their separation. Time-of-flight effects were clearly observed in all cases. The data show that drift mobilities can be determined from the transient photocurrents measured parallel and perpendicular to the substrate of the hydrogenated amorphous silicon films. The major finding of this research was that the electron drift mobility is anisotropic. This observation is unexpected from previous theoretical descriptions of transport in amorphous semiconductors. However, it rationalizes the common observation of a 100 fold difference between the electron mobility-lifetime products obtained using different experimental techniques. Measurements of the effects of optical bias, contacts, substrates and interfaces do not account for this anisotropy. It is speculated that microstructure might be involved. Several new techniques were developed in the course of this work. Bulk absorbed impulse illumination was used as a probe of the bulk in time-of-flight experiments on specimens with both gap (interdigital) and sandwich electrode geometries. Time-of-flight was detected through thin glass ribbons to determine the effects of different types of substrates. Surface region defects were grown by intense surface absorbed illumination for the purpose of reducing the effects of interfacial built-in fields. Surface absorbed bias illumination was used to investigate the surface region and its mobility-lifetime product. Fourier transform photocurrent techniques were used to investigate the influence of contact effects upon the transient photocurrents.