Structures And Optical Properties Of ZnO Thin Films Fabricated By PLD

ZnO is a II–VI semiconductor of wurtzite structure with a wide direct-band-gap of 3.3 eV at room temperature. The most unique property of ZnO is its large exciton binding energy of 60 meV. Because of this large binding energy, the exciton is stable at room temperature even in bulk crystals. Owing to these properties, ZnO is considered as a promising material for light-emitting devices and semiconductor lasers with low thresholds in the UV region, such as light-emitting diodes and laser diodes. ZnO films with high intensity emission have been deposited by using several growth techniques, such as metal-organic chemical vapour deposition (MOCVD), molecular beam epitaxy (MBE), sol–gel deposition, rf magnetron sputtering and oxidation of metal zinc film, pulse laser deposition (PLD). Pulsed laser deposition (PLD) technique is more useful in obtaining high quality thin films of metal oxide materials compared with other techniques, because of its advantage of simple hardware, atomic-layer control obtained by adjusting the laser energy density, the pulse duration and repetition rate. For these practical reasons, PLD technique has been wildly applied for the formation of the high quality thin films. In our article, we adopted the method of ZnO thin films deposited on Si(111)and quartz glass at different substrate temperatures by laser (wavelength of 1064 nm) ablation of Zn target in oxygen active atmosphere.The excimer laser (pluse duration of 10ns) and high purity ZnO target (99.99% purity) have been widely used to grow ZnO thin films.In consideration of low-cost preparation of ZnO thin films, we applied a very common Nd-YAG Laser of 1064nm (which is much cheaper than the excimer Laser) and a Zn target (which is much cheaper than ZnO target) to decrease the price of ZnO thin films. Many reports thought the surface of ZnO thin films by Nd-YAG Laser have a lot of big droplets. However, it is possible to obtain relatively high quality ZnO films through control the process parameters including laser energy density, oxygen pressure, substrate temperature and substrate species. We adopted XRD,SEM,FESEM,AFM,PL and optical transmittance spectra to show and analyze the structures,surface morphologies and optical properties of ZnO thin films.Optical emission mechanism of ZnO thin films were studied until now. In our article, we studied structures, defects, optical emission mechanism and prepare processing to obtained high quality ZnO thin films. We had made several conclusions through theory and experiment investigation:(1) (101) oriented ZnO thin films can be obtained at large laser energy density (43~53J/cm2) with low substrate temperature by PLD on Si (111), and thin films surfaces have a lot of big droplets. Surfaces of ZnO thin films deposited at small laser energy density (31～38J/cm2) have few and small droplets.(2) ZnO thin films obtained at small laser energy density (31J/cm2) with high substrate temperature (400oC) and different oxygen pressures by PLD on Si (111) substrates have typical luminescence behavior. The film deposited at 11Pa has (002) orientation and small average grains. The FWHM of UV emission from ZnO thin films obtained with oxygen pressures of 11Pa is the smallest, and the intensity ratio of UV emission to deep level emission is the largest (41). The electron transitions from the bottom of the conduction band to the antisite oxygen OZn mainly contributes to the deep level emission of PL spectra of ZnO films.(3) ZnO thin films obtained at 31J/cm2 laser energy density with 11Pa oxygen pressures and different substrate temperatures by PLD on Si (111) substrates have different surface morphologies and typical luminescence behavior. The large and uniformity columnar grains grown vertical to the substrate surface were obtained and the surface of thin films with (002) orientation is smooth at 500 oC substrate temperature. The FWHM of the UV PL spectra is the narrowest (140meV) and the intensity of UV emission is the strongest. Theory investigation indicated that intensity of UV emission depended on crystal structure and deep level emission is related to defects in ZnO thin films.(4) ZnO thin films obtained on quartz glass with small laser energy density (7 J/cm2) and low oxygen pressure (11Pa) at low substrate temperature (100 oC~ 250 oC) have obviously (002) orientation. High-quality ZnO films obtained at different substrate temperatures have nanocrystalline structure with grain size in the range of about 30-45 nm.(5) The ZnO thin films show ultraviolet emission and accompanied deep-level emission in PL spectra. However, the ZnO films deposited at substrate temperature 200 oC exhibits strong UV emission with no deep-level emission due to its good crystalline and low intrinsic defects.(6)The average optical transmittance values of films deposited under the condition were higher than 70% in visible wavelength range. The band gap energies (Eg) of the ZnO films deposited at different substrate temperatures are different. The band gap energies (Eg) obtained from both the optical transmittance spectra and the PL spectra are in coincidence with each other, and both agree well with the band gap energy of ZnO single crystal (3.3eV).