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

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 degrees 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, epsilon(E)=epsilon(1)(E)+i epsilon(2)(E) of the films, was extracted. It was found that the overall shapes of epsilon(1)(E)and epsilon(2)(E) were similar with those for thicker LPCVD Si films. Mole precisely, structures attributed to the E-0, E-1, E-2 and E-1 transitions of crystalline silicon, were shown on the epsilon(1) and epsilon(2) spectra of samples deposited at 610 and 620 degrees 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 degrees 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 degrees 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.