The role of hydrogen dilution and radio frequency power in the formation of microcrystallinity of n-type Si:H thin film

Abstract

Phosphorus doped n-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared at low power densities suitable for application in solar cells by the usual radio-frequency plasma enhanced chemical vapor deposition method (rf-PECVD, 13.56 MHz). For this purpose hydrogen (H₂) dilution in the silane (SiH₄) and phosphine (PH₃) gas mixture and rf power densities have been varied carefully to produce a plasma condition conducive to the growth of microcrystallinity. The structural properties of the films have been studied by Raman spectroscopy, x-ray diffractometry, transmission electron microscopy, and infrared vibrational spectroscopy. The electrical and optical characterizations have been done by dark conductivity, dark conductivity activation energy, and optical absorption measurements, respectively. Effects of variations of hydrogen dilution and rf power density on the electrical and structural properties of the films have been investigated thoroughly. Film with highest conductivity (32.6 S cm^-1) has been obtained with a H₂/SiH₄ ratio of 90 and rf power density 30 mW/cm². At lower hydrogen dilution (H₂/SiH₄=20), higher power density (180 mW/cm²) is required to produce the desired microcrystallinity. Attempt has been made to explain the results in terms of growth kinetics.