Abstract
The effects of ruby laser irradiation on the electrical and physical properties of coated (Si[3]N[4]) and uncoated semi-insulating GaAs, implanted with selenium ions have been studied. The thermal stability of laser annealed GaAs implanted with tellurium ions has been studied, in the temperature range 200-800°C. Electrical, Rutherford backscattering, electron and optical microscopy measurements coupled with electron microprobe analysis have been used to study the properties of GaAs samples before and after laser irradiation. It has been shown that Si[3]N[4] encapsulants act as a source of n-type doping of the underlying GaAs for Q-switched laser energy densities greater than 0.3 J/cm[2] . Silicon atoms from Si[3]N[4] were shown to be responsible for the doping effect, possibly due to laser induced indiffusion of silicon atoms. It was shown that selenium ions implanted into GaAs to doses less than or equal to 1x10[14]/cm[2] did not become electrically active after Q-switched laser irradiation, with the substrate held at room temperature, most probably due to laser induced compensating defects. However, selenium implants of doses of at least 5x10[13]/cm[2] became electrically active when the substrate was held at elevated temperatures (≥200°C) during laser irradiation. The measured electrical properties which were found to depend on the ion dose and energy, the laser energy density, the number of pulses and the substrate temperature during laser annealing were rather poor. The electron mobilities in all laser annealed samples were lower than predicted by Sze and Irvin. Q-switched laser irradiation of GaAs implanted with tellurium ions, followed by heat treatment at 200°C and above caused a significant reduction in the electron concentration and the extent of the reduction was found to depend on the annealing temperature. Laser induced surface damage was observed, which could be minimised by improving the spatial non-uniformity of energy across the laser beam using a beam diffuser.