Photoinduced non-linear optical diagnostic of SiNxOy/Si⟨111⟩ interfaces

Anisotropic (elliptically polarized) photoinduced second harmonic generation (PISHG) in SiNxOy/Si < 111 > films was proposed for contact-less monitoring of specimens with different nitrogen to oxygen (N/O) ratios. As a source for the photoinducing light, we used a nitrogen Q-switched pulse laser at wavelengths of 315, 337 and 354 nm as well as doubled frequency YAG-Nd laser wavelength (lambda = 530 nm). The YAG:Nd pulse laser (lambda = 1.06 mum; W = 30 MW: tau = 10-50 ps) was used to measure the PISHG. All measurements were done in a reflected light regime. We found that the output PISHG signal was sensitive to the N/O ratio and the film thickness. Measurements of the PISHG versus pumping wavelengths, powers, incident angles as well as independent measurements of the DC-electric field induced second harmonic generation indicate the major role played in this process by axially symmetric photoexcited electron-phonon states. The SiNxOy films were synthesized using a technique of chemical evaporation at low pressures. Films with thickness varying between 10 and 30 nm and with an N/O ratio between 0 and 1 were obtained. Electrostatic potential distribution at the Si < 111 > -SiNxOy interfaces was calculated. Comparison of the experimentally obtained and quantum chemically calculated PISHG data are presented. High sensitivity of anisotropic PISHG to the N/O ratio and film thickness is revealed. The rule of the electron-phonon interactions in the dependencies observed is discussed. We have shown that the PISHG method has higher sensitivity than the traditional extended X-ray absorption line structure spectroscopic and linear optical method for films with the N/O ratio higher than 0.50. (C) 2001 Elsevier Science Ltd. All rights reserved.