Evaluation of microstructures and carrier transport behaviors during the transition process from amorphous to nanocrystalline silicon thin films

Abstract

Hydrogenated amorphous Si thin films were prepared by plasma-enhanced chemical vapor deposition technique. As-deposited samples were thermally annealed at various temperatures to obtain nanocrystalline Si. The microstructures and carrier transport behaviors were evaluated during the transition process from amorphous to nanocrystalline structures. Raman scattering spectroscopy and Fourier-transform infrared spectroscopy were used to characterize the changes in microstructures and bonding configurations. It is found that hydrogen is completely effused from the film at the annealing temperature of 600 °C, while crystallization occurs at around 700 °C. The carrier transport characteristics in nanocrystallized films are different from those in the amorphous Si films. The carrier transport in the amorphous silicon films is strongly influenced by the defect states resulting from the effusion of hydrogen. The dual activation energies are found in temperature-dependent conductivity results which can be attributed to the two different conduction paths in the samples. In the case of the nanocrystallized Si films obtained by high temperature annealing, the transport process is accounted for in the framework of a three-phase model comprised of amorphous and nanocrystalline phases and the grain boundary in the films.