Investigation of surface and interface phenomena in silicon and silicon dioxide systems

Sammanfattning: The characterization of a thin silicon dioxide (SiO2) film close to the silicon dioxide -silicon interface is very important in understanding and improving solid state electronic and optoelectronic devices. The refractive index, the dielectric constant, and the density of oxide film, are all related to the chemical composition and porosity (voids) of the films. These properties, especially in the silicon - silicon dioxide transition region, are important for electronic devices. Experimentally they can be easily determined by the ellipsometry technique. A very low refractive index and anomalous depth profile of the refractive index were observed on very thin oxide films in our work. Our efforts to understand the mechanisms behind this abnormal observation are presented in this thesis.Starting from fundamental ellipsometry equations, we find an approach to analytically inverse partial derivatives of extracted film parameters over experimentally measured ellipsometric angles, avoiding the divergence problem in numerical inversion. This method is used to calculate derivatives of any transparent film on a silicon substrate, which gives a general description of how small errors propagate from measurement to the film parameters (linear approximation). Parameters of an ultrathin film are extremely sensitive to measurement errors. The accuracies decrease rapidly with decreasing film thickness; tts relation to the refractive index of the film is more complicated than its relation to the thickness. Some kind of films like silicon nitride (n = 2.0) can be measured down to several angstroms, but the order of several nanometers is the limit for most other materials, including silicon dioxide.Besides the ellipsometric angles, other parameters can also affect the determination of the film thickness and the refractive index. They are discussed by extending the basic analyses. We derived partial derivatives of substrate parameters and incident angle over ellipsometric angles. Again we find high sensitivities to these errors on very thin film. Rather small deviations from the nominal substrate parameter and incident angle can result in large errors in film parameters. The difference is that these errors are systematic errors and can be corrected. We discussed how to eliminate these systematic errors. In all cases we find that ψ is more important than Δ in error propagation.Non linear analyses are necessary in the region very close to the interface where the errors are relatively large. Computer simulations are carried out on an abrupt model, reproducing the scattered profile after correcting the systematic errors. By applying this analyzing method on experimental results, the error enveloping curves can be used to find out random errors in ellipsometric angle measurements, which are 0.02° in ψ and 0.04° in Δ for our instrument.Auger electron spectra (AES) measurement confirmed the abruptness of both thermal and PECVD silicon dioxide - silicon substrate interface, supporting our analyses on ellipsometric measurements. The widths of the transition region are determined to be 1.8 and 2.8 nm for thermal and PECVD oxide, respectively.In the last part of the thesis we discuss another semiconductor surface / interface phenomenon, i.e., the frequency dependent capacitance of semiconductor / electrolyte junctions. By applying fractal geometry to rough surfaces and analyzing the movement of low mobility ions in solution, the often observed power-law frequency dependence of capacitance is ascribed to the contribution of constant phase angle impedance. The power-law exponent can be related to the fractal dimension if the semiconductor surface can be described by fractal geometry. This explanation is in agreement with the experimental facts.