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UBC Theses and Dissertations

Characterization of semi-insulating liquid encapsulated Czochralski gallium arsenide Hui, David C. W.


This thesis consists of a study of several qualification techniques for SI LEC GaAs and the application of these techniques to various ingots. For use on the starting material before any doping procedures, the technique of studying the semi-insulating properties by monitoring the activation energy of dark resistivity with temperature was investigated. Experiments were performed on both ring dot as well as cloverleaf samples. Different activation energies for the dark resistivity were observed for temperatures above and below 290 K. Also, ingots with different background impurity concentrations were tested. Another technique applicable to the undoped starting material is Optical Transient Current Spectroscopy (OTCS). The occurrence of 'negative' peaks was simulated using a depletion layer model. The results showed that under certain conditions a recombination centre can produce a positive peak, a negative peak, or both a positive and a negative peak. Further analysis of the negative peaks led to the formulation of a field enhanced injection model to explain their occurrence. More than one negative peak was observed experimentally. In addition, the effect of different electrode structures on OTCS experiments was investigated. The effect of polarity on negative peaks was studied using ring dot structures and was found to agree with the proposed model. Some peculiar anomalies which were observed in investigating OTCS led to the discovery of a photocurrent memory effect with decay time constants of the order of minutes at a temperature of 266 K. This memory effect was found to be associated with surface modifications. The effects of surface passivation with Na₂S were investigated. The method of normalizing the OTCS peak height with photocurrent was investigated. A microscopic spatial analysis tool, scanning OTCS, with a spot size of about 2 µm was developed in order to probe the spatial variation of deep levels and compare with that of dislocations or other defects. An experiment on an abraded surface was performed using the scanning OTCS and showed that the negative peak does indeed correlate with mechanical damage. Wafer performance during implantation doping is an important qualification test. Comparisons between standard furnace annealing and rapid thermal annealing were performed. A comparison of the estimated percentage activation using C(V) measurements with that from Hall measurements, with and without a correction for the surface depleted region, was performed. The C(V) analysis technique, used in the industry to obtain doping profiles of implanted wafers, was studied. The effect of using serial and parallel measurement modes was investigated. Simulations of C(V) measurements on implanted devices by solving the Poisson-Boltzmann equation for the charge distribution under different biases were performed. The limitation of the C(V) profiling technique in detecting sharp dopant profiles was investigated. A system for quick analysis of the percentage of activation using a mercury probe was designed. The effect of serial and parallel analysis of the impedance measured by the mercury probe on the estimated dopant profile was investigated. The effect of different electrode structures (Schottky to Schottky as compared to Schottky to Ohmic) on estimated doping profiles was studied. The mobility profile as a tool for qualification was investigated. The effect of surface states on mobility was studied. A crucial factor in wafer qualification is the uniformity of transistor characteristics across the wafer. In order to test this on a wafer, thousands of transistors have to be measured. A technique of perforating measurements automatically with consistency is needed. An automatic probing station for measuring large arrays of transistors was engineered. Tests on arrays of transistors were performed to investigate the effect of different fabrication processes, in particular the amount of surface etch, on the uniformity of threshold voltage.

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