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 amo rphous-to-polycrystalline silicon (polysilicon) phase transition, and the effect on as-crystallized films of a simple post-hydrogenation tre atment. The polysilicon thin films were produced by low-temperature (6 00-degrees-C) furnace crystallization of undoped hydrogenated amorphou s silicon (a-Si:H). Other parameters, such as the onset of crystalliza tion, degree of amorphization and average grain size were determined b y 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 em ployed to examine carrier transport mechanisms. We observe a Meyer-Nel del relationship between the d.c. conductivity pre-factor sigma0 and a ctivation 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 c onsistent with a shift of the Fermi-level position by 0.06 eV towards the conduction band. Additionally, from an analysis of the transient p hotoconductivity, a reduced density of localized states is inferred.