Crystallized mixed-phase silicon films for thin film transistors on glass substrates

Abstract

The low temperature crystallization of mixed-phase silicon films deposited by low pressure chemical vapor deposition was investigated. The mixed phase consists of crystalline regions embedded in an amorphous matrix, with the crystalline fraction being a function of the deposition temperature and the deposition rate. In this material, the nucleation rate of new crystallites during annealing is higher than in a completely amorphous film, resulting in a shorter crystallization time and a smaller average grain size. To compensate for the reduction in the grain size, a composite structure was proposed consisting of two successive layers; a mixed-phase layer and an amorphous layer. Crystallized mixed-phase films were used for the fabrication of polysilicon thin film transistors on new Corning (Code 1735) glass substrates at a maximum processing temperature of 560 °C. A field-effect mobility of 20 cm2/V s and a threshold voltage of 5.1 V were obtained even before hydrogenation.