Research On The Preparation And Properties Of P-type ZnO Films And ZnO Based P-n Junctions

Zinc oxide (ZnO) is a wide band gap II-V semiconductor material with direct band gap of 3.37eV and hexagonal wurtzite structure. It can be used in many areas due to its piezoelectric, thermoelectric, gas sensing and optoelectronic multiple properties. Its high exciton binding energy (60meV at RT), which is much higher than RT heat energy (26meV) and also higher other wide band gap semiconductors (25meV for GaN, 22meV for ZnSe), will theoretically favor efficient UV excitonic emission process, therefore, it is potential candidate for application in short-wavelength optoelectronic devices. However, to realize the ZnO-based devices, the first step is to fabricate high quality n and p type ZnO and ZnO based p-n homojunction. The n-type ZnO is easily available by doping III elements(Al, Ga), which can fabricate the film with 10-4?cm, while it has been prove that it is very difficult to obtain p-type ZnO due to the self-compensating effect of native defects and /or H incorporation, which impede the development of ZnO-based devices. In this study, we have tried two new approaches to obtain p-type ZnO films by doping As via Zn3As2 with magnetron sputtering: (i) by sputtering Zn3As2/ZnO target, (ii) co-sputtering pure Zn3As2 and ZnO targets. We characterized the structural, electrical and optical properties of the films in our experiments using XRD, SEM, AFM, Hall, XPS, EDS, PL and UV-Vis methods and discussed the effects of grown and post-annealed conditions on the properties. The p-type ZnO films with a carrier concentration of 1016-1017cm-3 were fabricated. It is difficult for As atoms to submit O sites and form AsO acceptors in ZnO structure since the ionic radius is much larger than that of O. In the paper, we proposed tentatively the interpretation for the electrical behaviors of the As-doped ZnO films according to the AsZn-2VZn acceptor model from thermaldynamics and considered that the conduction type of As-doped ZnO films was dependent on the substrate temperature and the films exhibited p-type conduction with proper substrate temperature. In addition, proper thermal treatment for ZnO:As films may change their electrical behavior from n type to p type. XPS analysis was carried out for p-type ZnO films to study the bonding state of As in the films, indicating that As was in its oxidization state. The result showed that As replaced Zn site instead of O site and formed acceptor defects, which helped to understand the microscopic structure of As in As-doped ZnO and also supported the AsZn-2VZn acceptor model. Based the fabrication of p-type ZnO films, we prepared ZnO based p-n homojunctions and the I-V curves of the junctions showed clear rectifying behavior. Also prepared ZnO based p-n heterojunction on p-type Si substrate that exhibited typical rectifying behavior. The structural, electrical and optical properties of the p-n junctions were studied in the paper. In addition, we fabricated the MgZnO:As films by magnetron sputtering and studied the effects of grown and post-annealed conditions on the structural, electrical and optical properties of the films and considered that the Mg dopant played an important role in determining the conduction type of the films.The ionic radius of Mg(0.57?)is about that of Zn(0.60?)and it is easier to occupy Zn sites for Mg than As in the films, leading to the difficulty to form AsZn-2VZn acceptor, therefore, it is rather difficult to fabricate the p-type MgZnO:As films.