Compositional characterization of microwave plasma a-Si: H films

The concentration of hydrogen in amorphous hydrogenated silicon films prepared by microwave glow discharge decomposition of silane has been measured as a function of several fabrication parameters: substrate temperature, deposition rate, and partial pressures of silane and of argon. Hydrogen concentration profiles have been obtained by two techniques: elastic recoil detection (ERD) with a 30 MeV 35Cl beam, and by the resonant nuclear reaction 15N + 1H → 12C + 4He + γ. Both offer very high depth resolution (in the order of 100 Å) and have revealed important inhomogencities in chemical composition, both at the free surface and the substrate – film interface. Using standard infrared absorption measurements between 1950 and 2150 cm−1, the intensities of the lines normally associated with SiH, SiH2, and SiH3 bonds were measured. As has been previously reported for reactively sputtered films for some fabrication conditions the sum of the three intensities is not consistent with the total hydrogen concentration obtained from the nuclear measurements. The average hydrogen concentration decreases with increasing substrate temperature, changes little with deposition rate, and increases as the silane to argon ratio is increased. These observations are related to the chemical reactions which take place in the plasma, and at the plasma–film interface during film growth.