Effect of nitridation time on structural, optical and electrical properties of InN films grown on c-sapphire substrates by PAMBE

Abstract

We have investigated the effect of nitridation time of the sapphire substrates on the structural, morphological, optical, and electrical characteristics of InN thin films grown by plasma assisted molecular beam epitaxy technique. Three sets of the films were prepared with nitridation times of 1, 2.5 and 3 h, respectively. Full width at half maximum (FWHM) of ω-scans around X-ray diffraction peaks of different symmetric and asymmetric planes were drastically reduced with increasing nitridation time, hence the threading dislocation densities were also reduced by order of magnitude (2.4 × 1011 cm−2 to 6.7 × 1010 cm−2). The atomic force microscopy also showed improvement in surface roughness. But interestingly, on increasing the nitridation time, a prominent blue shift in the photoluminescence (PL) spectra (~ 60 meV) was observed which has also been confirmed by infrared absorption spectroscopy (blue shift of ~ 120 meV). This phenomenon of blue shift of PL peak was attributed to increase in electron concentration (n) in InN layer and was further confirmed by the Hall measurement. Carrier concentration (n) is seen to increase from 6.8 × 1018 cm−3 to 1.8 × 1019 cm−3 while the highest mobility was estimated to be 164 cm2/V-s which is observed to decrease considerably upon longer nitridation time. Thus there is a trade-off between structural and electrical qualities of InN film upon prolonged nitridation.