STUDY OF SOME OXIDE FILMS FOR SURFACE PASSIVATION OF GALLIUM ARSENIDE

Abstract

Gallium arsenide is an attractive electronic device material because of its high electron mobility, high resistivity, high saturation velocity, low diffusivity and wide and direct energy bandgap. It has wide applications' in low noise optical and microwave devices such as lasers, solar cells, light emitting diodes, gunn oscillators, avalanche diodes, field effect transistors, leading to the feasibility of manufacturing microwave and optical integrated circuits. Inspite of their impressive performance, the. advancement of gallium arsenide devices is delayed due to the nonavailability of an ideal insulating film for surface passivation. Therefore, there is a pressing need for developing the technique of growing an insulating film on gallium arsenide that meets ail the requirements of surface passivation. The present thesis deals with the problems of growth and deposition of insulating film on gallium arsenide and studied their suitability for surface passivation of semiconductor. The objective of the thesis were met by experimental investigations on the behaviour of a number of gallium arsenide MOS structures,the gate insulators of which were the insulating films under consideration. Accurate characteri zation of MOS structures requires a high quality of ohmic contact. A process of growing such ohmic contacts by alloying technique has been developed. The ohmic contacts were formed by vacuum evaporation of 90% of In (99.999% purity) and 10% of gold (99.999% purity). The deposited indium film was 2000 A in thickness, while the gold film was about 300 A in thickness. The metallic films were then alloyed in vacuum for 30 seconds at a temperature of 500 C. The ohmic contacts thus formed were characterized by studying their I-V characteristics and calculating the contact resistances. The results indicate (0

that the contacts made by this technique are of acceptable quality. Gallium arsenide - Native oxide - Aluminium (MOS) structures were fabricated. In this structure a layer of native oxide grown by wet anodization process, formed the gate oxide. The electrolytic bath used for anodization was composed of 3% tarteric acid buffered with ammonium hydroxide and diluted with ethylene glycol. Properties of these MOS structures were found to be repro ducible, but the interface state densities were high. This was confirmed . after the process was repeated several times and anodization carried out at different voltages. The use of alumina layer for surface passivation of semiconductors was found to be more promising, as it has high breakdown voltage, lower dissipation factor, higher radiation resistance and low ion migration effect at elevated temperatures. In view of this, a process of depositing high quality alumina layers by reactive evaporation has been developed. The process was standardized by fabricating several MIM (A1-A1_0,-A1) structures. It has been observed that the quality of the deposited alumina layer depends on deposition para meters such as oxygen pressure, source to substrate distance, deposition rate, annealing time, annealing temperature, substrate temperature etc. All these parameters were optimized one by one, by conducting several experiments. The quality and the reproducibility of the aluminium oxidedielectric layers as grown by reactive evaporation were examined by measuring their electrical parameters. The measured parameters are resistivity, dielectric constant and break-down field. Frequency dependance of the capacitance and conductance of the MIM structure was also studied. All these results are reported and discussed in the thesis. Compositional analysis of the deposited dielectric layer were also carried out. The composition of such a layer was found to be uniform over its surface and in its entire bulk. The technique

of depositing alumina films by reactive evaporation was then extended to the fabrication of GaAs-MOS structures having high quality low loss alumina layer as a gate insulator. By means of series of experimental studies, effect of various process parameters on the properties of the insulator was studied. Based on these studies, important parameters like post deposition annealing temperature and annealing time were optimized. The suitability of the Al-O, layer for surface passivation of gallium arsenide was analyzed by studying C-V, G-V and I-V curves and from the measurement of interface state densities of the GaAs-MOS structures. The experimental observations show that a reactively deposited alumina layer is a superior surface passivation layer than a native oxide layer. When compared with a native oxide, an alumina layer is found to have the advantage of having higher breakdown voltage and lower interface state density. It has been reported that double oxide structures in MIS diodes have shown several improvements over normal native oxide MOS diodes. An alumina layer which is either grown or deposited on top of the native oxide offers higher temperature stability to the latter. This is because the alumina layer acts as an over layer, preventing the loss of As or As203 from the native oxide. Keeping this in view MIS diodes with double layer oxides namely native oxide - A1203, Native oxide - SiO and A1203 - SiO were fabricated and studied. These structures were analyzed for surface passivity of gallium arsenide. The analysis reveal that, improved electrical characteristics espe cially, dielectric - semiconductor interface properties can be achieved by forming double layer oxide. Such double layer oxides have been found to result in surface states of larger time constants. This makes the double layer oxides more suitable for surface passivation of GaAs for high-speed and microwave applications.