Optical properties of ultra-thin low pressure chemically vapor deposited silicon films

Abstract
Ultra-thin silicon films, with thicknesses approximately 10 nm, were low pressure chemically vapor deposited (LPCVD) on fused silica substrates at 0.23 Torr and temperatures 550, 610 and 620 °C, by silane decomposition. Transmission and reflection spectra of these films were recorded within the energy range 6.2 to 0.5 eV, throughout of which they were transparent. From these measurements the energy variation of the complex dielectric function, ε(E)=ε₁(E)+iε₂(E) of the films, was extracted. It was found that the overall shapes of ε₁(E) and ε₂(E) were similar with those for thicker LPCVD Si films. More precisely, structures attributed to the E₀, E₁, E₂ and E₁ transitions of crystalline silicon, were shown on the ε₁ and ε₂ spectra of samples deposited at 610 and 620 °C at nearly the same energies as in crystalline Si. The above structures were not shown on the corresponding spectra for the sample at 550 °C. Absorption threshold and gap of these ultra-thin samples, coincided with those for thicker ones deposited at similar conditions. Analysis of the dielectric function of the films with the aid of the effective medium approximation, has shown that samples deposited above 600 °C contained smaller fractions of crystalline material than thicker ones, deposited at such temperatures. It was concluded that quantum effects, related to the confinement of electronic wave functions, do not cause significant changes to the electronic band structure of ultra-thin LPCVD Si films relative to that of thicker films.