Dielectric properties of very thin low pressure chemically vapor depositedsilicon films

Very thin silicon films, with thicknesses approximately 10 nm, were low pre ssure chemically vapor deposited (LPCVD) on fused silica substrates at 0.23 Torr and temperatures of 550, 610, and 620 degrees C, from silane decompos ition. Transmission and reflection spectra of these films were recorded wit hin the energy range 6.2-0.5 eV, throughout which they were transparent. Fr om these measurements the energy variation of the complex dielectric functi on epsilon( E) = epsilon(1) (E) + 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 to those for thicker LPCVD Si films. More precisely, structur es 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 dep osited at 610 and 620 degrees C at nearly the same energies as in crystalli ne Si. The above structures were not shown on the corresponding spectra for the sample at 550 degrees C. Absorption threshold and gap of the very thin samples coincided with those for thicker ones deposited under similar cond itions. Analysis of the dielectric function of the films, with the aid of t he effective medium approximation, has shown that samples deposited above 6 00 degrees C contained smaller fractions of crystalline material than thick er ones deposited at such temperatures. It was concluded that quantum effec ts, related to the confinement of electronic wave functions, do not cause s ignificant changes to the electronic band structure of very thin LPCVD Si f ilms relative to that of thicker films. (C) 1999 American Institute of Phys ics. [S0021-8979(99)03307-1].