FABRICATION AND EVALUATION OF VARIOUS METAL CONTACTS TO n-GALLIUM ARSENIDE

Abstract

It has been shown that a p--n junction device has a maximum cut-off frequency of about 1.5 GHz. It is, therefore, not suitable for high frequency applications due to the inherent disadvantage of minority carrier storage in the forward direction of conduction. Also, various techniques for making p-n junction devices have been discussed showing the high cost of infrastructure and the resulting devices. An attempt was, therefore, made to study the metal-semiconductor rectifier systems to Si, GaAs and InP since these could work to about 100 GHz, a frequency range not covered by p-n junction devices. Further, the cost of infrastructure and the device works out to be much lower compared to p-n device. The device, thus, has great potential for cheap and better devices which can replace p-n junction devices in similar applications. The metal-semiconductor diodes can be used in a variety of applications both electrical and optical such as FET, solar cells, lasers, photo-diodes, photodetectors, mixers etc. In the present work, results on the fabrication and testing of Al and Au Schottky barrier diodes on Si and GaAs, respectively, have been reported. Ohmic contacts have been made using Indium metal and subsequent alloying. Current voltage and capacitance-voltage characteristics of the diodes and ohmic contacts have been measured. The experimental results have been analysed to obtain the built-in potential, iv carrier concentration and potential barrier for the contacts fabricated. An attempt has been made to test the work function model on GaAs which seems to work well with clean GaAs samples. The results on alloyed and non-alloyed metal-contacts have been explained on the model of electron tunneling through the transparent layer after alloying. It is also inferred that in GaAs, the Au work function is reduced by--' 0.07 eV due to Schottky effect while this lowering could not be observed for MIS type structure in InP. Previously reported results on Au diodes to n-InP have been reanalysed. The surface state density in InP can be considerably reduced by growing a native oxide layer prior to Au contact for the diode. The work function model has been tested on the experimental results and an excellent agreement has been found between the theory and experimental results. Thermionic model does not seem to hold good for InP contacts. The observed change in capacitance of Silicon diodes with the measurement frequency of the bridge has opened the possibility of studying deep energy levels in semiconductors by this method.