Amorphous and nanocrystalline silicon made by varying deposition pressure in PECVD process

Silicon thin films are deposited using plasma enhanced chemical vapor deposition (PECVD) of silane, argon, hydrogen mixture at various pressures in the range of 2-8 Torr. Raman scattering shows these to be amorphous in the pressure range 6-8 Torr, and nanocrystalline in the range 2-4 Torr. The volume fraction of nanocrystals is estimated by fitting the Raman data to three peaks and is found to be similar to 75% for the films deposited at low pressure, density of states of these films was measured. It is observed that the electrical conduction in these films depends on the crystalline volume fraction (rho), estimated from the laser Raman Spectroscopy. Temperature dependence electrical conductivity shows that at lower temperatures thermionic emission dominates for the films with lower rho, whereas, hopping is the main conduction mechanism for the films having high rho. The density of states is estimated from the space charge limited currents (SCLC) observed at high fields. Photoconductivity at room temperature is also measured. The amorphous films are found to be more photosensitive than the nanocrystalline one. In the context of these findings, changes in the properties of silicon from amorphous to nanocrystalline are described. (C) 2009 Elsevier B.V. All rights reserved.