Structure And Properties Of N Doped ZnO Films

ZnO is a semiconductor with direct wide band gap of3.37eV. Due to its large exciton binding energy (60meV), ZnO is considered to be the ideal material to fabricate ultraviolet (UV) light emitting devices in the future. With the efforts about ten years, however, ZnO based light emitting devices are still not achieved a breakthrough. One of reasons is the difficulty to prepare high-quality p-type ZnO thin films as the intrinsic ZnO has a higher background of electron carriers. On the other hand, most of acceptors in ZnO are in the deep level and relatively low solubility, hence lacking of suitable acceptor doping elements. The stable and reproducible p-type ZnO thin films have become the bottleneck of the development of ZnO.Nitrogen is believed to be a candidate of doping elements to prepare p-type ZnO. In this work, N-doped ZnO films are grown with pulsed laser deposition (PLD) assisted by nitrogen ion beam or plasma. Using x-ray diffraction, transmittance spectra, photoluminescence, and Hall measurements, the structure, optical and electrical properties are studied. The following topics are focused in thesis,(1) The influence of sapphire substrate treated with different methods on the growth and properties of ZnO films;(2) N-doped ZnO films grown with PLD assised by nitrogen ion beam;(3) N-doped ZnO films grown with PLD assised by nitrogen plasma. It is found that the treatment of sapphire substrate has an obvious influence ob the growth of ZnO films. When the substrate was annealed at800℃in oxygen ambient and then cleaned by ion beam, the crystallinity of the ZnO films was greatly improved. The films grown the treated substrates have the transmittance up to90%. Compared to the films grown on the substrates without treament, the photoluminescence (PL) intensity is increased by3times while the roughness decreases. For N-doped ZnO films, we demonstrate that PLD assisted by both of nitrogen ion beam and N plasma can produce the ZnO films with p-type conductance. For the PLD assised by nitrogen ion beam, the energy of N ions was found to be the critical factor to control the p-type conductance of ZnO films. At the ion energy of200V, the N-doped ZnO film was p-type conductive. At the ion energy of400V, however, the film will transit to be n-type conductive due to the increase of defects in the near band edge (NBE). The crystallinity of the films can be improved after annealing, but the NBE defects cannot be eliminated obviously. Moreover, the films will be converted to n-type conductive due to desorption of N atoms from the films. For films grown with PLD assisted by N plasma, dissociation ratio or the production of atomic nitrogen is suggested to be responsible for the p-type conductance of ZnO films. In the case of discharge with lower voltage, the as-grown films were n-type conductive. At the discharge voltage of650V, the film was found to have p-type conductance with the carrier concentration of6.532×1017cm-3and hole mobility of44.7cm2/Vs. It is found that N incorporation does not change the crystal orientation of ZnO thin films. However, the PL efficiency was decreased considerabley and transmittance is lowered. For N-doped ZnO films, either doping with ion beam or N plasma, the optical band gap was found to red-shift obviously. At the discharge voltage of650V, the band gap of p-type ZnO films is determined to be3.126eV.