Dielectric properties of very thin low pressure chemically vapor deposited silicon films

Abstract

Very 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 of 550, 610, and 620 °C, from silane decomposition. Transmission and reflection spectra of these films were recorded within the energy range 6.2–0.5 eV, throughout 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 to 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 the very thin samples coincided with those for thicker ones deposited under 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 very thin LPCVD Si films relative to that of thicker films.