Microcrystallisation in Si:H: the effect of gas pressure in Ar-diluted SiH4 plasma

Using noble gas argon as a diluent of SiH4 in RF glow discharge, undoped μc-Si:H thin films have been developed at a low power density of 30 mW/cm2. It has been found that the gas pressure is a critical factor for the growth of μc-Si:H films. Undoped μc-Si:H films having σD∼10−6 S/cm and ΔE<0.57 eV have been obtained at and above a critical pressure of 0.8 Torr. When the RF power density is increased to 90 mW/cm2, a more crystalline as well as highly conducting (σD∼10−4 S/cm) μc-Si:H film has been achieved at a deposition rate of 30 Å/min, which is much higher than that attained from H2-diluted SiH4 plasma, by conventional approach. The crystallinity of the films has been identified by the sharp Raman peak at ∼520 cm−1 and a large number of micrograins in the TEM micrographs. The metastable state of Ar, denoted as Ar*, plays the crucial role in inducing microcrystallisation by transferring its de-excitation energy at the surface of the growing film. A mechanism has been proposed to explain the dependence of the formation of μc-Si:H film on the working gas pressure in the plasma.