We have determined that the temperature for desorption of gallium oxide from GaAs increases linearly with oxide thickness, for oxide layers between about 6Å and 26Å thick. The temperature for the oxide desorption ranged from 580°C to 630°C. The wafer temperature was determined from the optical band-gap measured from the diffuse reflectivity of the sample, which was polished on the front surface and textured on the back surface. Different thicknesses of oxide layers were created by varying the exposure time of the GaAs wafers to a low pressure oxygen plasma. The oxide thicknesses were determined by XPS analysis. Desorption experiments were carried out in a VG V80H MBE system under a As4 beam equivalent pressure of 1.5 x 10⁻⁵ ton. Measurement of diffuse light scattering using a HeNe laser shows an abrupt and non-reversible increase in the scattered light intensity during the oxide desorption. This suggests the surface is macroscopically roughened due to inhomogeneous desorption of the oxide. The oxide desorption was also studied by monitoring the secondary electrons produced by the high energy electrons from the RHEED gun. After the gallium oxide desorption there is a reversible, order of magnitude, increase in the number of scattered electrons produced by the incoming primary beam. We interpret this result as evidence for some form of microscopic roughening. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Test pieces of GaAs were cut from Czochralski grown <100> wafers. Prior to deformation the dislocation configuration was established by cathodoluminescence (CL). Etch pits produced by molten KOH on examined crystal surfaces coincided with the CL images. The test pieces were capped with Si₃N₄, heated to between 950 and 1050°C, and plastically deformed by bending. The dislocation configuration after bending was then compared to that of the undeformed crystal. It was observed that heating to 1050°C did not significantly change the as grown cellular dislocation arrays in the crystal. With strain the dislocation configuration changed appreciably. New bands of dislocations were formed, parallel to the bend axis with dislocation free regions between them. Increasing the strain increased the number of bands. Observations were made on undoped crystals with high and low dislocation densities, and Si doped crystals. The luminescent properties of the dislocations were observed to change with heating and strain. As grown, a dislocation imaged as a dark spot surrounded by a bright halo, giving bright dislocation networks. After heating to 950°C samples showed only the dark spots without halos. After deformation, all the new dislocations appeared as dark spots or lines without halos. At very low strains, the original dislocations were still evident but were distinct from the new arrays. In an attempt to correlate the dislocation images with impurity segregation some observations of the samples were made using secondary ion mass spectroscopy (SIMS). The results suggest the possibility of the dark areas in the CL images being associated with the presence of carbon. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The temperature fields and resulting stress fields have been calculated for a growing GaAs crystal produced by the LEC process. The calculations are based in a finite element numerical thermoelastic stress analysis. The calculated temperature fields have been compared to reported experimental measurements with good agreement. The stress fields have been used to calculate the resolved shear stresses, in the growing crystal, from which the dislocation density and distribution were determined. Using the model the effects of a range of growth and environmental parameters on the dislocation density and distribution were determined. Theses parameters include crystal length, crystal diameter, cone taper, boron oxide thickness, gas pressure, solid/liquid interface shape, vertical temperature gradients and others. The results show that the temperature distribution in the gas surrounding the crystal, and the boron oxide thickness, were critical factors in determining the dislocation density and distribution in the crystal. The crystal radius, crystal length and interface curvature also strongly influenced the dislocation configuration. After crystal growth, the dislocation density at the end of the crystal was strongly influenced by the cooling procedure adopted. The dislocation distribution on cross-sections of the crystal exhibited two-fold, four-fold and eight-fold symmetry depending on growth and cooling conditions and position in the crystal. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

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Inhalation of gallium arsenide (GaAs) particulates represent a potential health hazard in the semiconductor industry. Our results showed that GaAs was soluble under a variety of in vitro conditions. Arsenic levels in phosphate buffer filtrates indicated 78% dissolution by 36 hours. The in vivo dissolution of GaAs was dependent upon particle size, time, and route of administration. Intratracheal (i.t.) instillation of GaAs particulates (10-100 mg/kg) to rats resulted in blood arsenic levels of 5-187 ppm at 14-28 days, depending upon particle size. Dissolution doubled as the mean volume particle diameter was halved. Oral administration of GaAs particulates (10-1000 mg/kg) resulted in blood arsenic levels of 3-18 ppm at 14 days. Gallium was not detected in blood at any dose level by any route of exposure. Indices of toxicity that correlated to GaAs exposure were decreased weight gain and porphyria. These effects were maximal at 100 mg/kg GaAs i.t. Uroporphyrin replaced coproporphyrin as the major urinary metabolite. GaAs (10-100 mg/kg i.t.) resulted in an increase in the lung:body weight ratio (136-228%) at 14-28 days, depending upon particle size. Lungs retained 14-42% of the dose as gallium or arsenic. The increase in lung wet weight was not primarily due to edema although pulmonary edema increased in magnitude as particle size decreased. Lung dry weight, DNA, protein, and lipid content were also elevated 14 days after 100 mg/kg GaAs i.t. (large fraction). At this time and dose, major pathological lesions were a thickening in the alveolar wall, pneumonocyte hyperplasia, and interstitial pneumonia. Gallium, as Ga₂O₃ (65 mg/kg), accounted for the increase in lung lipids. Arsenic, as As₂O₃ (17 mg/kg), was responsible for the remaining changes in lung composition observed with GaAs administration. As₂O₃, but not GaAs, resulted in acute fibrosis at 14 days. With 100 mg/kg GaAs i.t. (smaller fraction), proteinosis, edema, mild fibrosis, and increased reticulin formation were observed over 1-28 days in addition to lesions previously described for the larger fraction. These results showed that oral and i.t. GaAs resulted in systematic arsenic intoxication. Intoxication was proportional to in vivo dissolution which was dependent upon particle size. GaAs i.t. was relatively more toxic to rats than an equivalent oral dose. The finding that urinary uroporphyrin levels were greater than coproporphyrin levels may serve as a sensitive, pretoxic indicator of GaAs exposure.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy