PHYSICAL-PROPERTIES OF SEMIINSULATING POLYCRYSTALLINE SILICON .2. OPTICAL STUDIES OF THIN-FILMS

The optical properties of semi-insulating polycrystalline silicon (SIP OS), i.e., SiOx with O less-than-or-equal-to x less-than-or-equal-to 2 are studied in the spectral range from the infrared to the UV region. The refractive index n (x,E) and the absorption coefficient alpha(x,E ) are evaluated as a function of the oxygen content x and the photon e nergy E. The actual shape of the n(x) and alpha(x) curves are determin ed by two counteracting effects: (1) the increase of n(x) and alpha(x) caused by the transition from c-Si to a-Si of the Si grains in SIPOS in the range 0.02 less-than-or-equal-to x less-than-or-equal-to 0.2, a nd (2) the decrease of n(x) and alpha(x) due to the increasing insulat ing character of SIPOS with increasing x. The infrared vibrational mod es of bulk SIPOS are described on the basis of an oscillator model wit h effective parameters. Both, the vibrational frequencies nu1(x) of th e asymmetrical stretching motion and the total oscillator strength S(x ) increase with increasing x. While nu1(x) can be qualitatively descri bed on the basis of a simple mixture model, this is neither the case f or the shape of the frequency-dependent conductivity sigma(nu,x) nor f or S(x). Both, sigma(nu,x) and S(x) depend sensitively on the real def ect structure consisting of Si grains embedded in the oxide matrix con sisting Of SiO2, SiO, and other suboxides.