OPTICAL 2ND-HARMONIC GENERATION - A PROBE OF ATOMIC-STRUCTURE AND BONDING AT SI-SIO2 INTERFACES, AND OTHER CHEMICALLY-MODIFIED SI SURFACES

Optical second harmonic generation (SHG) is a highly surface-sensitive probe for studying crystalline Si surfaces because the inversion symm etry is broken and electric dipole optical SHG processes forbidden in the bulk are allowed. The polarized optical SHG from a perfectly orien ted Si surface is inherently anisotropic, varying periodically as the in-surface projection of the polarization vector of the incident laser is rotated about a normal to the surface. The harmonic contributions to the angular anisotropy from the surface are characteristic of the s urface bonding, and are modified by misorientation, chemical terminati on, as well as thermal treatments. This paper reviews the results of o ur previously reported optical SHG studies on Si(111) wafers with miso rientations of 0-degrees-5-degrees +/- 0.5-degrees in the [112BAR] dir ection for Si-H or Si-O terminated surfaces. Azimuthal anisotropy data are compared with an empirical model for the SHG intensity that is ba sed on (i) the nonlinear response of anharmonic oscillators, and (ii) a phenomenological theory of azimuthal anisotropies expected for diffe rent surface orientations. This model is used as a framework for estim ating ''effective'' resonance energies from single wavelength experime nts, and in particular, for providing insights into the microscopic me chanisms that can contribute to the changes in these resonance energie s with respect to different processing conditions. For example, import ant differences between thermally grown and plasma-oxidized interfaces are identified, and cor-relations between SHG and electrical performa nce of the Si-SiO2 interfaces are discussed.