Photocarrier Transport and Localized States in Microcrystalline Silicon

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Photocarrier transport and localized states in undoped hydrogenated microcrystalline Si have been investigated by means of the modulated photocurrent technique. The experiments show that the photocarrier drift mobility varies systematically with the crystalline grain size, the volume fraction, as well as the transport direction. The drift mobility measured in the coplanar electrode configuration increases with the volume fraction while a lower mobility is observed for smaller grains. The measurement in the sandwich arrangement reveals that the drift mobility in the direction perpendicular to the substrate is an order of magnitude higher than parallel to the substrate. The observations indicate that transport takes place via a grain-to-grain carrier delivery, and reflects the connectivity and the geometry of grains included in the material. These effects are discussed with two basic models. The effective-medium model brings into light the percolation process involved in the transport, and satisfactorily accounts for the drift mobility increase with the volume fraction observed in the experiment. The lower drift mobility for smaller grains proves that the grain boundary bottlenecks the percolation transport. The boundary-limited random-walk model reasonably links the grain geometry to the transport anisotropy. Apart from the variation of drift mobility magnitude, the frequency spectra and generation rate dependence are also extensively discussed in comparison with those of hydrogenated amorphous Si. The comparative study suggests that dangling bond states contained in these materials have a very similar character. Based on the numerical simulation as well as on a direct evaluation from the measurements, we interpret the experimental results as indicating that common to both materials, the charged dangling bond state possesses a higher capture efficiency than neutral ones.