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UBC Theses and Dissertations

Characterization of semi-insulating liquid encapsulated Czochralski gallium arsenide Katō, Hiroshi


Deep levels in semi-insulating gallium arsenide (SI GaAs) have been associated with effects such as threshold voltage variations, sidegating and low frequency oscillations in transistors fabricated using this material. The distribution of deep levels is not uniform, which is a key concern to IC manufacturers. Techniques such as altering the stoichiometry of the melt, characterization of crucibles and encapsulants used in crystal growth and boule annealing have been used to improve the uniformity of wafers by GaAs suppliers. The work to be described was part of a project in which optical transient current spectroscopy (OTCS) was used to investigate deep levels in GaAs wafers manufactured by Johnson Matthey in Trail, B.C. using a high pressure liquid encapsulated Czochralski (LEC) method. Part of the present work involved development of a scanning OTCS system to map variations across wafers. The inhomogeneities in LEC material display both microscopic and macroscopic features. The dimensions of dislocation networks are in the range of several tens of microns. On a macroscopic scale, the density of dislocations displays a radial dependence with concentrations being higher near the centre and outer edges of a wafer. Dislocations have been suspected to getter impurities. Previously published scanning OTCS experiments had examined macroscopic variations. The goal of the work was to map variations in the magnitude of OTCS signals with lateral resolution comparable to that of dislocation networks. The system consisted of a pulsed laser which was focussed onto the surface of GaAs wafers. “Sandwich” type electrical contacts with one electrode semi-transparent were made to the specimen to monitor the photo-generated current in the sample. The stage used to support the sample was temperature controlled and could be stepped laterally with respect to the light spot in 0.1 micron steps. The magnitude of the exponentially decaying components of the photo-current pulses were examined using a double gated technique. Lateral variations in the OTCS signal similar in scale to dislocation networks were observed. In practice, it was difficult to correlate OTCS signals to an energy level corresponding to deep levels. Moreover the transient signals which were observed using the scanning OTCS apparatus were different from those typically encountered in non-scanning experiments conducted in this laboratory and reported in literature. To examine some of these differences, copper, which has been reported to create several deep levels in GaAs, was intentionally introduced into specimens of SI GaAs and measured using OTCS methods. The remainder of the thesis deals with studies of copper as a contaminant of GaAs wafers. Copper was chosen because it is believed to be present in significant quantities in materials used in device fabrication and has been associated with problems in device performance[Hiramoto, 1988]. The effects of copper in GaAs have been studied by a number of groups[Tin, 1987][Venter, 1992][Moore,1992]. Various means were investigated to introduce minute quantities of copper sufficient to measurably alter electrical characteristics but not change gross features such as the high resistivity or physical appearance of the material. Results from OTCS measurements using a variety of experimental conditions were compared for both copper-treated and “as-received” GaAs substrates. The OTCS signatures found in this work were comparable to those reported in the literature, but aunique signal due to the presence of copper was not determined. Comparisons of copper contaminated and untreated material were also made using cathodoluminescence and by measuring the currentvoltage characteristics of ohmic and rectifying contacts made to the samples. In addition, the variation in current under illumination and in the dark were examined as a function of sample temperature.

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