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. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Work is reported on three topics relating to problems which hold back the development of GaAs integrated circuits. These topics are deep trapping levels in the starting semi-insulating GaAs, the effect of dislocations on device characteristics, and backgating. (The latter is the influence of voltages on nearby contacts on device performance). Deep trapping levels in undoped semi-insulating liquid-encapsulated— Czochralskl GaAs grown in the <100> direction were characterized using photocurrent deep level transient spectroscopy (photocurrent-DLTS). Three electron levels were found using photocurrent-DLTS in the temperature range 200K to 400K. By using Cr electrodes, instead of Au-Ge electrodes, it became possible to extend the experiment to a higher temperature range than previously used in this laboratory, and hence to observe the trap known as EL2. This trap has not previously been observed in undoped liquid-encapsulated-Czochralski GaAs by using photocurrent-DLTS. The possibility of an effect of dislocations on device characteristics was investigated using a dislocation etch procedure and measurements on an array of MESFET. Due to problems in controlling the fabrication processes, the scatter was such that no correlation between device characteristics and the distance to nearest dislocation would be established. However, scatter of threshold voltage was larger for devices fabricated on areas of honeycomb-like dislocations network as opposed to areas with unconnected wavy lines of dislocation. Backgating (which causes unwanted communication between devices) was investigated in conjunction with substrate conduction measurement. A model was proposed for the effect as present in the devices used in this experiment. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

No description available.

She Tsz Chung William. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1994. / Includes bibliographical references (leaves 127-133). / Chapter 1. --- Introduction / Chapter 2. --- Review of Optical Logic --- p.11-28 / Chapter 2.1 --- All-Optical Approach / Chapter 2.2 --- Optoelectronic Approach / Chapter 2.3 --- Comparison of the Two Approaches / Chapter 3. --- High Speed Photodetectors applied in Optoelectronic Logic Design --- p.29-40 / Chapter 3.1 --- Photoconductive Switch / Chapter 3.2 --- Metal-Semiconductor-Metal Photodetector / Chapter 3.3 --- Design of Simple Logic Gates / Chapter 4. --- Device Fabrication and Characterization --- p.41-59 / Chapter 4.1 --- Design of Basic Structure / Chapter 4.2 --- Fabrication / Chapter 4.3 --- Mounting of Device / Chapter 4.4 --- Characterization / Chapter 5. --- Experimental Technique --- p.60-74 / Chapter 5.1 --- Measurement Procedure / Chapter 5.2 --- Optical Sources / Chapter 5.3 --- Optical Alignment / Chapter 5.4 --- Control of Optical Path Delay / Chapter 5.5 --- Measurement Automation / Chapter 6. --- Demonstration of Optoelectronic Logic Devices --- p.75-110 / Chapter 6.1 --- OR Gate / Chapter 6.2 --- Exclusive-OR Gate / Chapter 6.3 --- Exclusive-NOR Gate / Chapter 6.4 2 --- to 4 Decoder / Chapter 7. --- Discussion --- p.111-124 / Chapter 7.1 --- Improvements / Chapter 7.2 --- Extensions of this Project / Chapter 7.3 --- Prospects and Limitations of this Approach / Chapter 8. --- Conclusion --- p.125-126 / References --- p.127-133 / Appendix / Chapter I. --- List of Instruments --- p.134-136 / Chapter II. --- Properties of GaAs --- p.137 / Chapter III. --- List of Accepted and Submitted Publications during the Period of Study --- p.138

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

No description available.

Thesis (M. Phil.)--University of Hong Kong, 1994. / Includes bibliographical references (leaves 95-99).

Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references.

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.