OPTOELECTRONIC PROPERTIES OF POLYCRYSTALLINE SILICON PRODUCED BY LOW-TEMPERATURE (600-DEGREES-C) SOLID-PHASE CRYSTALLIZATION OF HYDROGENATED AMORPHOUS-SILICON

We present novel data on optoelectronic properties observed in the amorphous-to-polycrystalline silicon (polysilicon) phase transition, and the effect on as-crystallized films of a simple post-hydrogenation treatment. The polysilicon thin films were produced by low-temperature (600-degrees-C) furnace crystallization of undoped hydrogenated amorphous silicon (a-Si:H). Other parameters, such as the onset of crystallization, degree of amorphization and average grain size were determined by ultraviolet reflectivity and electron microscopy. The grain size is found to increase with decreasing a-Si: H substrate temperature, and a maximum areal grain size of 0.4 mum2 is obtained. Optical absorption, d.c. conductivity and transient photoconductivity measurements are employed to examine carrier transport mechanisms. We observe a Meyer-Neldel relationship between the d.c. conductivity pre-factor sigma0 and activation energy E(sigma). A plasma hydrogenation treatment of the as-crystallized films results in an order-of-magnitude increase in the d.c. conductivity and a similar increase in photoconductivity. This is consistent with a shift of the Fermi-level position by 0.06 eV towards the conduction band. Additionally, from an analysis of the transient photoconductivity, a reduced density of localized states is inferred.