S Alexandrova

SHG and AFM study of PECVD a-Si :H films

Abstract In the present paper, results are presented on the dependence of the second harmonic generation (SHG) in a-Si:H on the substrate material. The investigated structures were a-Si:H films deposited onto three different substrates, namely, fused silica plates, Corning glass 7059 and soda-lime glass. It was established that the SHG depended substantially on the substrate material. The effect of the intrinsic stress due to the film–substrate mismatch strain has been considered. Investigation of the roughness of the substrate surface as estimated from atomic force microscopy measurements has shown no significant differences. Formation of microvoids in the a-Si:H films is observed with a possible implication as an SHG source.

Stress in silicon dioxide films

Abstract The stress in dry thermal SiO2 has been studied as a function of growth temperature by measuring the curvature of thin oxidized silicon wafers at room temperature. The results show that the curvature of wafers with thin oxide layers is determined by stress in the back-side SiO2. An in situ anneal at the growth temperature leads to relaxation of the initial compressive stress. The stress in the oxide was found to be very sensitive to water content.

Second harmonic generation in α-Si:H thin films: Thickness dependence

The second harmonic generation (SHG) technique has been applied to investigate the nonlinear optical properties of thin films of hydrogenated amorphous silicon on a glass substrate. SHG measurements have been performed in two transmission experimental schemes. Modeling of a film as a three-layer structure and considering the interference and absorption at the SH wavelength have been carried out. The dependence of the nonlinear susceptibility on the film thickness was used to separate the bulk electric dipole contribution.

LPCVD-silicon oxynitride films: interface properties

The interface properties of silicon oxynitride films prepared by low-pressure chemical vapor deposition at a temperature of 860 ° C have been investigated analyzing the capacitance-voltage and ac conductance-voltage characteristics of the metal-SiO x N y -silicon capacitors. Consistent results for the interface trap density have been obtained from single frequency ac conductance technique, approximation C-V method and from the interface density spectrum. The post-deposition annealing results in an improvement of the interface charge properties. The contribution of the interface traps to the estimation of the fixed oxide charge has been discussed which is important for the threshold voltage control in MOS devices.

Second harmonic generation in thin Ge35Sb5S60 films

Generation of the second optical harmonic is investigated in 250 to 1500-nm thick films of Ge35Sb5S60 chalcogenide glass. It is shown that a decrease in quadratic optical susceptibility in thicker films can result from a decrease in the bulk contribution from the electric dipole to the susceptibility and can be used in nonlinear optical diagnostics of the films.

Mechanical stress in plasma-exposed Si–SiO2 structures

Abstract The mechanical stress in thermal SiO2–Si structures exposed to a radiofrequency hydrogen plasma has been investigated. It was found that the compressive oxide stress is released and a small tensile stress appears, which is consistent with a previously observed annealing of a built-in oxide charge. This result applies for oxides with a thickness of about 40nm and above and an oxidation temperature in the range of 850 to 1050°C

XPS study of nanoscale SiOxNy layers synthezised by plasma immersion implantation of nitrogen

Plasma immersion implantation (PII) of nitrogen into Si was used to prepare nanoscale SiOxNy layers. The SiOxNy layers were synthesized in a cycle of PII followed by a high-temperature treatment in O2 ambient. Nitrogen was implanted into the Si substrate with energy of 2 keV and a fluence in the range of 1016 – 1018 cm−2. The implanted structures were subjected to a high-temperature annealing at 1050 °C for 10 or 20 minutes in dry oxygen at atmospheric pressure. X-ray photoelectron spectroscopy (XPS) revealed the chemical bonds and surface layer composition. The atomic concentrations of the SiOxNy constituents were obtained. By a combination with a sputter experiment the N concentration was found to be about 2,6 at. %. The low N content is due to the nitrogen escaping from Si during the oxidizing annealling. The thickness of the SiOxNy layer for the highest implantation fluence did not exceed 10 nm as revealed from ellipsometric and XPS data analyses.

Electrical parameters of thin nanoscale SiOx layers grown on plasma hydrogenated silicon

In the present paper results are presented on electrical characterization of the interface Si/SiOx, formed by oxidation on Si wafers, previously exposed to rf hydrogen plasma. As a tool of investigations multiple frequency C-V and G-V measurements are applied. The data analysis was performed using two-frequency method to extract generalized frequency independent C-V characteristic. Interface trap densities were evaluated from the generalized C-V data by comparison with theoretical data for an ideal interface. A set of localized states, acting as interface traps, was found that characterize the interface of Si to substoichiometric SiOx, layer with x < 2. The dielectric constant of the oxides was calculated from the capacitance in accumulation of the generalized C-V curves. The thickness and the refractive index of the oxide layers were obtained from ellipsometric data analysis assuming the oxide-Si substrate as single layer system. From the data for the dielectric constant and refractive index suggestion is made that the grown oxides on hydrogenated Si contain voids thus reducing the dielectric constant. Correlation with oxide mechanical stress is found.

Annealing of Si surface region modified by plasma immersion implantation of nitrogen

In the present work, the formation of a nano-scale Si surface layer is studied after high-temperature annealing of Si modified by shallow plasma immersion implantation of nitrogen with fluences of 1016 - 1018 cm−2. The implanted profiles of the atomic (N+) and molecular nitrogen (N2+) are modeled by SRIM for different annealing durations taking into account the diffusion process. The presence of Si-O and Si-N bonds is established by Fourier (FTIR) spectral analysis and spectroscopic ellipsometry (SE). The refractive index value measured at 632.8 nm varies between 1.46 and 1.59, corresponding to a low y/x ratio. The models using VIS and IR ellipsometric data reveal formation of nanostructured SiOxNy layer with Si inclusions.

Structure-related strain and stress in thin hydrogenated microcrystalline silicon films

The stress/strain relation for hydrogenated microcrystalline silicon (μc-Si:H) films in the thickness range 10 to 200 nm was studied. It was found from wafer curvature measurements that all deposited films exhibit compressive intrinsic stress, which decreases with film thickness. This finding is in agreement with the stress level seen from the shifts in Raman spectra. The strain was approached through the Raman Mechanical Coefficient for the thin films and was estimated to be −1.4×10−5 (cm−1/MPa). The Raman spectra indicated highly crystalline films. The deconvolution of the spectra into Lorentzian components revealed inclusion of defective nanocrystallites and amorphous phase as well. The fractions of the different constituents were estimated. The contribution of the defective nanocrystallites to the overall stress in the films has been interpreted.