Optimisation of ZnO Thin Films

Extensive research is currently carried out on ZnO as a potential material for the fabrication of optoelectronic devices such as photodiodes, laser diodes (LDs) and light-emitting diodes (LEDs) in the ultraviolet region because of its wide bandgap (3.437 eV at 2 K) and a large excitonic binding energy of 60 meV at room temperature. However, because of intrinsic defects such as oxygen vacancies and zinc interstitials, ZnO is intrinsically deposited as an n-type material. Being a II–VI semiconductor compound, group I and group V elements are considered suitable p-type dopants for ZnO. In this monograph, authors concentrate mostly on the different methods undertaken to achieve strong, reliable and stable p-type ZnO films. Ion implantation, being a suitable technique to achieve selective and localised doping, was used to dope ZnO with different group I and group V elements. Lithium and phosphorus ions were implanted using the conventional ion implantation technique, while plasma immersion ion implantation was used to implant phosphorus and nitrogen ions in the ZnO thin films. Such studies would bring interesting revelations in the optoelectronics field for the ultraviolet range and help to produce devices for the commercial market at a much cheaper cost.

Progress in the fabrication of ZnO-based optoelectronic devices lies in producing reproducible, reliable and stable p-type ZnO films, because of the intrinsic n-type nature of the deposited ZnO films. Hence, for successful conversion of its carriers from n-type to p-type, it is desirable that the deposited ZnO film has as low an electron concentration as possible. Moreover, for the fabrication of optoelectronic devices, the films should have very high optical quality. The growth of highly oriented films with the least strain will offer an added advantage in fabricating these devices. Hence, the deposition parameters of temperature and pressure using pulsed laser deposition (PLD) were optimised keeping these things in mind. A substrate temperature of 650 °C and oxygen pressure of 40 mTorr were found to be optimised growth parameters as it had the lowest carrier concentration of 1.01 × 10¹⁷ cm⁻³ a reasonably high Hall mobility of 16.1 cm² V⁻¹ s⁻¹ and also had the highest optical quality. Once the PLD parameters were optimised, hydrogen implantation was carried out to see whether it further enhanced the electrical and optical properties of the thin film. While the Van der Pauw Hall measurements did not reveal any significant changes in the electrical characteristics of the thin films, the optical quality of the implanted films was found to increase by two orders of magnitude when compared to the as-deposited sample. Such an enhancement in the optical luminescence of the ZnO thin films may be helpful in fabricating highly efficient ZnO-based devices.

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.

Once reliable and reproducible p-type ZnO was achieved, different heterojunction and homojunction devices were fabricated. The heterojunction p–n junction diode was fabricated by depositing p-type ZnO films over low resistivity n-Si substrates. The p-type ZnO was achieved by phosphorus as well as nitrogen implantation. The current–voltage characteristics of the fabricated p–n heterojunction diode showed a clear rectifying behaviour with a threshold voltage of 3.3 V was observed for the phosphorus-implanted device. For the nitrogen-implanted device, a threshold voltage of 1.7 V was attained. This low threshold voltage may help to increase the lifetime of the devices. Homojunction LED was fabricated over high resistivity n-Si substrates, such that the effect of substrate conduction is nullified. The p-type ZnO was deposited by the phosphorus and nitrogen implantation as described above. The n-type ZnO was deposited over the p-ZnO by PLD technique using the optimised temperature and oxygen parameters of 650 °C and 40 mTorr, respectively. Room temperature electroluminescence (RTEL) of the fabricated device (phosphorus implantation) clearly depicted the emission of UV light around 3.18 eV. However, EL emission is dominated by a broad emission around 1.8 eV. This observation is based on the presence of deep-level defects in ZnO films. RTEL for the device fabricated by nitrogen-implanted p-ZnO showed a broad emission around 1.8 eV, corresponding to the red emission, is observed from the device. However, no UV emission was observed from the fabricated device.

The summary of the work and future work has been discussed in this chapter.