Polysilicon thin Films and Devices Produced by Low-Temperature (600°C) Furnace Crystallisation of Hydrogenated Amorphous Silicon (a-Si:H)

Abstract

ABSTRACTIn this work, we report on the electronic properties of polysilicon thin films and devices realised via furnace crystallisation of undoped a-Si: H. The onset of crystallisation, degree of amorphisation and average grain size are determined by UV reflectivity and electron microscopy. Grain size is found to increase with decreasing a-Si:H substrate temperature, and a maximum areal grain size of 0.4μm2 is obtained. Optical absorption, DC conductivity and transient photoconductivity measurements are employed to examine carrier transport mechanisms. We observe a Meyer-Neldel relationship between the DC conductivity prefactor σ0 and activation energy Eσ. A plasma hydrogenation treatment of the as-crystallised films results in an order of magnitude increase in the DC conductivity and a similar increase in photoconductivity. This is consistent with a shift of the Fermi level position 0.06 eV towards the conduction band. Additionally, analysis of the transient photoconductivity infers a reduced density of states. We discuss the implications of our results for polysilicon TFT optimisation.