Laser ablation of AlN films grown on sapphire substrate

Abstract

Ablation threshold for single-crystal AlN semiconductor films grown epitaxially on sapphire substrate using indigenously built hollow cathode plasma deposition source molecular-beam epitaxy technique is investigated for a number of pulses by varying the fluence value of each pulse. Using a KrF excimer laser (λ=248nm and τ=25ns) as a radiation source, we found that ablation of AlN thin films is a discontinuous process and its onset requires a minimum threshold fluence ≈1.59J∕cm2. The ablation depth is analyzed for different numbers of pulses and for each number as a function of increasing fluence values. The results show that the ablation depth increases linearly with increasing pulse fluence. It is found that the use of a single pulse for ablation at a given value of fluence is more efficient than a large number of pulses at the same value of fluence/pulse. In addition, we investigated the lowest pulse-fluence limit that can sustain ablation on a disordered AlN film surface. We present a theoretical discussion about the laser energy absorption mechanism and also the rate of energy transfer from the conduction-band electrons to lattice phonons which can lead to the ablation of AlN film. It is found that the rate of energy transfer increases linearly with increasing temperature of the electron gas.