Photo-dynamic XPS for investigating photoinduced voltage changes in semiconducting materials
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The main motivation of this Ph.D. study is investigation of the photoinduced voltage changes in semiconductive materials with X-ray Photoelectron Spectroscopy (XPS). For this purpose, we have developed a technique for recording the shifts in the positions of the XPS peaks in response to different waveforms of electrical and/or optical stimuli for tracing dynamics of the developed potentials originating from intrinsic or extrinsic factors of the semiconductive materials such as charging/discharging, photoconductivity, surface photovoltage, band-bending/flattening/inversion, etc. Within this purpose, the surface photovoltage behaviors of n- and p-type doped Si and GaN samples are examined with the photo-dynamic XPS, to follow the behavior of the bandbending under photoillumination in both static and dynamic fashions. The band inversion effects are clearly observed on the n- and p-Si samples in the presence of a dielectric silica overlayer and on the p-GaN sample due to variation of the illuminating laser energies Moreover, the extent of the dopant dependent XPS peak shifts of the n- and p-Si samples are assessed after correction of their surface photovoltage values. A laser patterned silicon wafer with a high-power near infrared fiber laser is also investigated. While the patterned silica domains have identical chemical composition with the non-patterned regions, an investigation with dynamic XPS clearly reveals distinct dielectric characteristics of the patterned domains. Electrical parameters of CdS thin film are extracted by dynamic XPS with and without photoillumination. The photo-dynamic XPS technique has also provided useful information by disentanglement of processes; charging/discharging, photoconductivity, and surface photovoltage. Furthermore, location (space) dependent resistance and chemical profile of a CdS based Light Dependent Resistor (LDR) is also probed during realistic operational conditions, by utilizing spatially resolved XPS analysis (in the area mapping mode). In addition, with the XPS mapping analysis defects and malfunctioning sites/domains have been located under various experimental and preparation conditions.
Light Dependent Photoresistor