3. Implantation Studies

Ion implantation is a key tool that has developed in recent years for the fabrication of devices. It has the added advantage of providing lateral selectivity, appropriate doping area and fluence control along with localised doping. Ion implantation studies have been performed with the goal of doping ZnO with group I and group V elements to make p-type films. The Low-Energy Accelerator Facility at BARC, Mumbai, was used to implant lithium and phosphorus ions. Lithium ions were found to occupy deep acceptor states in ZnO and thus acting as donors in the films. Hence, no acceptor peaks were observed in the films implanted with lithium. On the other hand, when the films were implanted with phosphorus ions, an increase in the acceptor peaks was observed for the implanted and annealed films. However, the films were still n-type in nature. Plasma immersion ion implantation technique was used to implant the sputter-deposited ZnO thin films with phosphorus and nitrogen ions followed by subsequent annealing to remove any implantation-related defects. In both cases, a strong acceptor-bound-exciton (A°X) peak was observed around 3.35 eV, from PL measurements, in the thin films annealed at temperatures of 900–1000 °C confirming that the films have converted into p-type. The acceptor activation energy was calculated to be 125 and 118 eV for phosphorus- and nitrogen-implanted samples, respectively. This proves that phosphorus ion and nitrogen ion act as shallow acceptor levels in ZnO giving rise to p-type behaviour. PL measurements for the p-type samples were performed over a period of time to check the reliability of the p-type films. For the phosphorus-implanted samples, the p-type was found to exist up to a period of 6 months, although the PL intensity was found to reduce drastically. For the nitrogen-implanted samples, the p-type was seen even after a period of 9 months and even the PL peak intensity was similar to the freshly made sample.