VUV laser (157 nm) chemical vapor deposition of high quality amorphous hydrogenated silicon: gas phase chemistry and modelling

High quality amorphous hydrogenated silicon (a-Si:H) films were deposited from disilane using a fluorine laser (157 nm) in a parallel configuration. The deposition of a-Si:H films at low source gas partial pressures (3–10 μbar) was possible due to the high photon flux and the relatively high absorption cross section of disilane at this wavelength. Regarding its properties, the material obtained by VUV laser CVD is comparable to high quality PECVD films deposited at similar substrate temperatures. Typical values of a sample grown at a substrate temperature of 270°C are a dark conductivity of 5.0 × 10−11 S/cm, photoconductivity of 6.3 × 10−5 S/cm, Urbach energy of 59 meV, optical band gap energy of 1.62 eV, a hydrogen content of 11 at% and a SiH2/SiH concentration ratio of 0.08. We developed a one-dimensional model, which describes the gas phase processes, to improve our understanding of the deposition process. This model is based on the data obtained by mass spectrometric analysis of the exhaust gas taking into account the diffusion, the gas flux, a broad spectrum of chemical reactions and the adsorption of species on the film surface and the reactor walls as well as their interaction with the laser radiation.