Photoinduced non-linear optical diagnostic of SiN_xO_y/Si〈111〉 interfaces

Abstract

Anisotropic (elliptically polarized) photoinduced second harmonic generation (PISHG) in $\text{SiN}{}_{\mathrm{x}}\text{O}{}_{\mathrm{y}}\text{/}\text{Si}\text{〈1}\text{1}\text{1〉}$ 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 $\text{354}\text{nm}$ as well as doubled frequency YAG–Nd laser wavelength $\text{(λ=530}\text{nm}\text{)}$. The YAG : Nd pulse laser $\text{(λ=1.06}\text{μm}\text{;}\text{W=30}\text{MW}\text{;}\text{τ=10}$–$\text{50}\text{ps}\text{)}$ 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 SiN_xO_y films were synthesized using a technique of chemical evaporation at low pressures. Films with thickness varying between 10 and $\text{30}\text{nm}$ and with an N/O ratio between 0 and 1 were obtained. Electrostatic potential distribution at the $\text{Si}\text{〈1}\text{1}\text{1〉}$–SiN_xO_y 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 role 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 fine structure spectroscopic and linear optical method for films with the N/O ratio higher than 0.50.

Introduction

Due to their technological importance, silicon oxynitrides have, recently, been the subject of increasing research interest [1]. However, several important aspects remain unclear. The most important problem which is yet to be resolved involves determining the parameters of the trapping electron levels in relation to the nitrogen/oxygen concentration (N/O). The traditional electroconductance methods are restricted in this case because the relatively low mobility of the trapping carriers limits electroconductivity. Neither the extended X-ray absorption fine structure (EXAFS) nor the Auger method, nor other photoemmission methods are sufficiently sensitive to distinguish particular contributions of the trapping levels playing a major role in the optoelectronic properties.

One suitable method here could be a non-linear optical (NLO) technique, specifically second harmonic generation (SHG). An important advantage of this method is that it is very quick and non-destructive. Recently, the SHG methods have been used widely for the investigation of SiO₂–Si interfaces and SiO₂ films [2], [3]. It is known that the SHG signal originates both from the $\text{Si}\text{(1}\text{1}\text{1)/}\text{SiON}$ interface dipole contribution (caused by the loss of inversion symmetry in the near-surface layer), and from the bulk electric quadruple contribution [4], [5]. These contributions are usually comparable, but there is always the problem of separating them. This is particularly important in the case of SiON–$\text{Si}\text{〈1}\text{1}\text{1〉}$ interfaces. In the present work, we have tried to separate the contributions and to check the possibility of applying SHG in order to determine the SiON film content. We eliminated the need to rotate the specimen relative to the pumping light polarization [6] by using an elliptically polarized UV light beam. This technique, as well as the method of specimen control are described in Section 2.

Theoretical simulations of the NLO with respect to different film thickness and N/O ratio were carried out using ab initio molecular dynamic simulations of the near-surface and intra-bulk structural fragments in Section 3. We compare theoretical approaches with experimentally obtained data. Sensitivity of the proposed method, both to the N/O ratio as well to the film thickness, is demonstrated. The particular role played by electron-phonon interactions is demonstrated. Several discrepancies between experimentally obtained and theoretically calculated non-linear susceptibilities and inter-atomic distances are analyzed.