Correlation between plasma chemistry, microstructure and electronic properties of Si:H thin films prepared with hydrogen dilution

Structural and transport properties near the amorphous to microcrystalline transition region of Si:H samples deposited from a silane–hydrogen mixture have been studied. The gas pressure during the plasma enhanced chemical vapor deposition process has been varied from 1.0 to 0.1 Torr. The defect density in the subband gap region measured by modulated photocurrent and constant photocurrent methods vary asymmetrically above and below midgap with changes in pressure. The transport properties of the electrons and holes studied by measurement of mobility lifetime product (μτ) and diffusion length (LD), respectively, change in the opposite direction with pressure. The sample deposited at 0.2 Torr exhibits high LD but low μτ product. Moreover, an increase of the band gap was observed with decreasing pressure. These unusual behaviors have been explained on the basis of quantum confinement effect. The changes in plasma chemistry observed by optical emission spectroscopy present an interesting perspective in understanding the evolution of the structural and electronic properties with the changes in pressure.