Fabrication and Optical Properties of ZnO Thin Films with Various Orientations and their Nitrogen-Doping Behavior

Zinc oxide, a wide band gap (3.37 eV at room temperature) semiconductor, is a suitable material for short wavelength optoelectronic devices such as light-emitting diodes and lasers in the blue and ultraviolet regions. However, p-type doping of ZnO is not as easy. Nitrogen is regarded to be the more soluble group-V impurity also having the shallowest acceptor level relative to P and As. The doping bottleneck is generally due to intrinsic defects, unintentional hydrogen impurity, and low N solubility. ZnO films can be fabricated using thermal evaporation, metal organic chemical vapor deposition, pulse laser deposition, and molecular beam epitaxy. In this work, a dual plasma technique comprising zinc arc plasma and mixed oxygen and nitrogen gas plasma is used to produce undoped and N-doped ZnO thin film. Undoped ZnO films with various high orientations of (1000), (1100). and (0001) were fabricated under different substrate biases. Some interesting properties of these as-deposited and annealed ZnO films were studied including C-exciton absorption at high energy edge, Raman scattering effect in different crystalline orientation, and photoluminescence (PL) behavior with different intrinsic defects measured at room temperature and low temperature (11K). Our results shows that the ionized zinc atoms and oxygen plasma bond easily and the desirable stoichiometry can be achieved. At the same time, nitrogen can be used to dope the ZnO thin film to optimizing the properties. The stability of Ndoping was determined by cathodoluminescence and x-ray photoelectron spectroscopy. Our results reveal unstable Zn-N bonds in the as-deposited Ndoped ZnO film and the results are in good agreement with our theoretical calculation aiding our understanding of the N-doping mechanism in ZnO.