Enhanced optical properties with the assimilation of boron and phosphorus dopant in co-doped ZnO thin film

Inherent properties of wide bandgap (3.37 eV) and high exciton binding energy (60 meV) have helped zinc oxide (ZnO) to claim its potential in the area short-wavelength optoelectronic devices. Furthermore, it exhibits n-type conductivity due to presence of native defects which has restricted its effective utilization in junction devices. Doping ZnO to achieve p-type conductivity has been an area of interest over the last couple of decades. Taking into consideration the limitations imposed by mono-dopant on the p-type behaviour achieved, co-doping has emerged out to be promising technique with the advantage of increasing dopant solubility and reducing ionization energy. In this report we have studied the enhancement in properties of phosphorus doped ZnO thin film with boron as a co-dopant. Doping was done using plasma immersion ion implantation (PIII) technique where phosphorus was implanted for 70 s and subsequently boron for 10s followed by annealing at 800oC for 10 s in oxygen ambient. Low temperature photoluminescence (PL) spectra showed improvement in the acceptor behaviour with donor-acceptor pair (DAP) and free acceptor (FA) peaks observed at around 3.24 and 3.31 eV, respectively for co-doped sample as compared to phosphorus doped sample which did not show these peaks. High resolution x-ray diffraction (HRXRD) showed c-axis (<002<) orientation of the film with increase in peak intensity and angle for the co-doped sample. For co-doped sample, a blue-shift was observed for E2H peaks in Raman spectra with increase in peak intensities suggesting an improvement in the film crystallinity.