| Zinc oxide (ZnO) is a II-VI semiconductor with a wide band-gap of 3.37 eV and a hexagonal wurtzite structure. Due to its large exciton binding energy,60 meV (much larger than the room temperature thermal energy,25meV), it can be considered as a prime candidate for ultraviolet (UV) light emitting diodes (LED) in both room and high temperature. Recently many groups tried to grow p-type ZnO. Some groups have reported successfully fabricating p-type ZnO:N, which is reasonable because nitrogen has a similar ionic radius as oxygen and is easily substitutable. Unfortunately, reliably obtaining p-type ZnO is still problematic. Moreover, the low solubility and the deep acceptor levels of the dopants may yield high resistivity, low carrier concentrations and unstable, making p-ZnO even harder to fabricate. There is a still long way to go to make p-type ZnO practicality. The key to obtain the high quality device is producible stable low resistivity ZnO.The study of ZnO is a hot and hard problem. To explore it, the details of the series studies have been exhibited successively. We focus on the fabrication, structures, and optoelectronic properties of p-type phosphorus-doped ZnO after annealing process. The films were prepared on quartz substrates by r.f. magnetron sputtering method using a 2wt%P2O5(99.99%) mixed ZnO(99.99%) target at 500℃in Ar/O2 for P-doped.The P-doped ZnO films were polycrystalline in nature with preferred (002) orientation. The as-grown samples is high resistivity, After grown, a series of annealing process has been employed. The greatly improved crystallization property of the ZnO:P films has been found during the annealing due to release of the residual stress. The electrical and optical properties were also improved and the p-type was realized after vacuum RTA at 800℃. The investigation of PL and XRD on n-and p-type samples reveal that the phosphorus has been doped into the ZnO film and the acceptor Pzn-2Vzn has been formed.Great effect of the annealing temperature on the film quality and electrical properties have also been predicted. The appropriate annealing temperature exists in which vacuum RTA can induce the conversion of ZnO:P characteristics.Those annealed temperature is low were dominated by zinc interstitials (Zn;) and oxygen vacancies (Vo) donor defects, the P acceptor was not activated which result in n-tpye ZnO. With the higher annealing temperature, zinc interstitials (Znj) and oxygen vacancies (Vo) donor defects decrease, and the increased P accepter defects will fight over the donor defects Zn; and Vo, lead n-p mixed to p-type. When the temperature keeps going up, the concentrations of Vo will increase dramatically. With increasing annealing temperature, the lattice constant c changed small and band gap became wide. The violet peak intensity in PL spectrum is increasing and FWHM became narrow, the crystal quality is improved.Substrate play an important part on obtainning the high quality of the optoelectronic devices.The crystal quality of film grown on sapphire is advanced, but the conductive type shows n-type. The XRD shows that as the annealing temperature growing, diffraction angle of (002) peak increases, implying that P impurities are activated and produced Po and Pzn.From Raman spectrum we noticed the vibrating peaks related with Po which were compensated by a large number of donor defects (such as Pzn). In PL spectrum we find that there is no Vzn related photoluminescence peak. It indicates that it is hard to produce Pzn-2Vzn complex which is recognized as the main reason of p-type.
|
PDF Full Text Request