Prepared Of ZnO Thin Film By Pulsed Laser Deposition And Study On Optoelectronic Properties

Zinc oxide (ZnO) is an interesting wide band gap (3.3 eV at room temperature)Ⅱ-Ⅵsemiconductor material, which has a much larger exciton binding energy (60 meV) than GaN (~25 meV) and ZnSe (22 meV). ZnO has wurtzite structure, and its lattice parameters are a=0.3249 nm and c=0.5206 nm. ZnO possess excellence transparent conductivity, piezoelectricity, photoelectricity, gas sensitivity, voltage sensitivity, and they can be integrated with some semiconductor materials readily. ZnO films have many potential applications such as surface acoustic wave devices , planar optical waveguides, transparent electrodes, ultraviolet photodetectors, piezoelectric devices, varistors, UV-LEDs (light emitting diodes) and–LDs (laserdiodes) and gas sensors etc. ZnO is used to make UV-emitter and UV laser in the range of short wavelength, which play a very important role for improving light-noting density and accumulate-adopt vilocity of light information. The visible emission and ultraviolet lasing emission of ZnO have been the subject of much research up to now. It is receivable generally that the UV emission centre could be an exciton transition. However, the exact origin of the visible emission is not well understood and is still debatable owing to different intrinsic defects, such as oxygen vacancies, Zn vacancies (Vzn), oxygen interstitial (Oi), Zn interstitials, and antisite oxygen (Ozn) et al. Many different techniques such as sputtering, metal organic chemical vapor deposition (MOCVD), pulsed laser deposition, laser molecular beam epitaxy (LMBE), e-beam evaporation, spray pyrolysis, sol-gel, film oxygenation have been employed to fabricate ZnO films. Among numerous deposition techniques, PLD is a newly advanced film growth technique developed in recent years. Its advantages for fabricating ZnO films by PLD method are that the growth is easily controllable by changing the experimental parameters, and high quality films can be produced at lower growth temperature. We fabricate ZnO thin films with preferencel c-axis orientation by PLD method. The gowth procedure, characteristics and influence factors of ZnO thin films are discussed. The thin films with uniformity, compact and perfect c-orientation and good transparency in visible range have been obtained under optimum conditions.The primary research contents in this paper are as follows.1. The ZnO thin films with c-axis orientation were fabricated on Si (111), quartz and sapphire substrates by PLD technique. We found that the sapphire is the best substrate material among all materials.2. The effect of growth temperarue on crystalline quality of ZnO film were analyzed by XRD, SEM, AFM and PL spectrum. We achieved single crystal ZnO thin films with the best crystallization, uniform grains and intense UV emission on various substrate at temperature 400-500℃.3. We fabricated ZnO thin films under various laser energy at substrate temperature of 400℃in oxygen ambient pressure of 0.13 Pa and found that high quality films can be obtained under laser energy of 200 mJ.4. As the ZnO thin films were fabricated in oxygen pressure of 0.1- 60 mTorr, we found that the optimum crystallization and photoluminesence do not presented at same oxygen pressure, and the sensitivity of XRD and PL spectra to crystalline structure are different. The dominant factor affectting PL spectra is point defects of ZnO crystal sach as O vacancies and Zn interstitials. The XRD is more sensitive to crystalline defects such as dislocation, grain boundary and crystalline orientation et al.5. The fabricated thin films were annealed in nitrogen gas, oxygen gas and vaccum. The effects of various annealing ambient on structure and photoluminesence of ZnO films are disccused. The annealing treatment can improve the crystal and UV emission of the film, but various defects can be caused. The thin films annealed in oxygen gas possess the best crystal, the least surface and point defects. 6. We found that the blue emission sited at 460 nm derives from the electronic transition from donor energy level of Zni to acceptor level of Vzn by the analysis of PL spectra of ZnO thin films fabricated at various conditions.7. We fabricated ZnO thin films by using ceramic and powder ZnO target, and found that the best growth temperature of ZnO film prapared by using powder target at same condition is 600℃. This reveals that the growth efficiency of ZnO film by employing ceramic target is better than that of powder target.