Fabrication And Properties Of Nano-ZnO Thin Films Prepared By Sol-Gel Process

In recent years, with the development of materials sciences inmicro-electronics field, the film technique is developed fast. As animportant part of material science, it has been widely emphasized and itsapplication field is extended continuously, it plays an important role inadvanced technique and new materials field. Investigated of preparationtechnique of film is very urgent because of its variety.ZnO is a direct wide-band gap semiconductor optical-electronic materialwith many applications, such as ultraviolet laser, low-loss high-frequencysurface acoustic wave (SAW) devices, buffer layers for III-nitride growth,and transparent electrode et al, which make it more attractive to manyresearch groups for a long time. Especially in 1996, the upsurge in ZnOresearch is coming due to the realization of the light pumped excitonemitting and discover of self-formed cavity. Subsequently, some groupsmake more progress in preparation of the p type ZnO and p-n devices, thework in multi-element alloy materials were also expanded.One importantadvantage of ZnO is that it is a II–VI semiconductor of wurtzite structurewith a wide direct-band-gap of 3.3 eV at room temperature. The mostunique property of ZnO is its large exciton binding energy of 60 meV,which is much larger than those of GaN (28meV), ZnSe (20meV) and ZnS(39meV). Because of this large binding energy, the exciton is stable in bulkcrystals even at room temperature. Owing to these properties, ZnO isconsidered as a promising material for light-emitting devices andsemiconductor lasers with low thresholds in the UV region, such aslight-emitting diodes and laser diodes. ZnO films with high intensityemission have been deposited by using several growth techniques, such asmetal-organic chemical vapour deposition (MOCVD), molecular beamepitaxy (MBE), rf magnetron sputtering and reactive DC sputtering,oxidation of metal zinc film, pulse laser deposition (PLD) , sol–geldeposition .Sol-gel deposition (SGD) technique is more useful in obtaininghigh quality thin films of metal oxide materials compared with othertechniques,as thin films derived from sol-gel technology have theadvantage of simpleness, low temperature application, suiting for coatingdifferent substrate materials of versatility and complex geometrieseffectively, more and more scientists play attention to this.In our experiment we prepared the crack-free and uniform nano-ZnOfilms on the surface of quartz glass substrate by sol-gel method. Dippingcoating method was applied. Films were formed through coating, drying,sintering. Scanning Electron Microscopy was used to analyze themorphology of the films. X-ray diffraction was used to investigate themicrostructure of the films. DTA studies were carried out to analyze theenergy chance and phase transition behavior of the films. Based on sol-gelmethod, precursor composition, times of coating, sintering temperature andtime for shelved are fitted together, we changed the parameters of theseprocesses and obtained the most optimal films.Drying is the most important and difficult stage, which affects themorphology, roughness, and configuration property. Of all the factors,temperature plays a most important part, so choosing a good drying methodis a keystone of this paper. To obtain the crack-free and uniform ZnO films,we choose to discharge solvent fast at the initial drying stage at 80℃.Thesystem became dense and viscous at the end. Then shut off thepower ,also dry the Sol till with it down to the room temperature.Post-thermal annealing, as one common means to improve films' quality,at high temperature can make ZnO bonds undergo a relaxation process tocause network reconstruction. So we investigated the influence ofpost-thermal annealing on ZnO films at different temperature. 1. XRD showthat annealing effectively improved the quality of ZnO films. Afterannealing, the intensity of (002) ZnO diffraction was increased by severaltimes. In addition, the FWHM of XRD decreased obviously. 2. PL spectrashow that: As for sample, after annealing, the intensity of ultravioletluminescence increased obviously with the energy position of about3.31eV. However, the intensity of green luminescence in the rang of490-530 (2.53eV～2.34eV) also increased after annealing. The ratio of UVluminescence intensity and green luminescence intensity increased by 26,which also means the increase of ZnO optical quality. 3. FE-SEM ofsamples show that as-grown sample had the smallest roughness(15nm).After annealing, the roughness of sample reach up to 41 nm. Wethink this is mainly attribute to the relaxation process of tensile strain ofZnO bonds at the high temperature and the reconstruction of the network.Therefore the mean grain size and roughness increased.4. All the ZnO thinfilms on quartz glass substrate obtained by sol-gel method at differenttemperature show a strong single violet emission(002),especially at 500℃with a high increased intensity of UV emission. Theory indicated that theviolet emission of PL spectra of the ZnO thin films be attributed to theenergy transition of electronics from the upside of the valence band to theinterstitial zinc (Zni) level. The fact shows that it is attributed to the increaseof zinc vacancy defects and the decrease of interstitial zinc defects in theZnO films.