Fabrication And Properties Of ZnO Thin Films

Recently, thin films prepare technique is developed fast because ofdevelopment of materials sciences in micro-electronics field and it is important inadvanced technique and new materials field. Investigated of preparation techniqueof thin film is very urgent because of its variety. One important advantage of ZnOis that it is a II–VI semiconductor of wurtzite structure with a wide direct-band-gapof 3.3 eV at room temperature. The most unique property of ZnO is its largeexciton 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 at room temperature even in bulk crystals. Owing to theseproperties, ZnO is considered as a promising material for light-emitting devicesand semiconductor lasers with low thresholds in the UV region, such aslight-emitting diodes and laser diodes. ZnO films with high intensity emission havebeen deposited by using several growth techniques, such as metal-organic chemicalvapour deposition (MOCVD), molecular beam epitaxy (MBE), sol–gel deposition,rf magnetron sputtering and reactive DC sputtering, oxidation of metal zinc film,pulse laser deposition (PLD). Pulsed laser deposition (PLD) technique is moreuseful in obtaining high quality thin films of metal oxide materials compared withother techniques, because of its advantage of simple hardware, atomic-layer controlobtained by adjusting the laser energy density, the pulse duration and repetition rate.For these practical reasons, PLD technique has been wildly applied for theformation of the high quality thin films.In our article, we adopted first method of ZnO thin films deposited on Si(111)and quartz glass at different substrate temperature by laser (wavelength of 1064 nm)ablation of Zn target in oxygen active atmosphere. The influence of substratetemperature and oxygen pressure on the surface morphology andphotoluminescence property was studied. In PLD, in general the excimer lasershave been widely used to grow ZnO thin films because of its shorter wavelengthand small pulse duration, and high surface quality ZnO films can be obtained withfew and small droplet.In consideration of low-cost preparation of ZnO film, in the present work, weapplied a very common Nd-YAG Laser of 1064nm (which is much cheaper thanthe excimer Lasers) and a Zn target (which is much cheaper than ZnO target) todecrease the price of ZnO thin films. Many reports thought the surface of ZnO thinfilms by Nd-YAG Laser (wavelength of 1064nm) have a lot of big droplets. It ispossible to obtain relatively high quality ZnO thin films through control the processparameters including laser energy density, oxygen pressure, substrate temperatureand substrate species, so that to study the most optimize processing parameter,surface morphology, preferal orientation. We adopted second method of Zn thinfilms deposited on quartz glass by laser (wavelength of 1064 nm) ablation of Zntarget in argon atmosphere, and Zn thin films were oxidized with different oxygenpressure at different annealing temperature. In general, annealing temperature islower than melted temperature of metal Zn, or graded oxygen methods wereadopted. In our article, annealing temperature is higher than melted temperature ofmetal ZnOptical emission mechanism of ZnO thin films were studied until now. In ourarticle, we studied structure, defects, Optical emission mechanism and prepareprocessing. And effect of texture and varities defects on optical properties isstudied to obtained high quality ZnO thin films. We had made several conclusionsthrough theory and experiment investigation:(1) (101) oriented ZnO thin films can be obtained at large laser energy density(for example:43～53J/cm2)with low substrate temperature by PLD, and thin filmssurface have a lot of big droplets. Surface of ZnO thin films deposited at smalllaser energy density have few and small droplets(2) (002) oriented ZnO thin films can be obtained at small laser energydensity(for example:31J/cm2)with high substrate temperature(500oC) and lowoxygen pressure(11Pa) by PLD. The large and uniformity grains were obtained andthin films surface is smooth. ZnO thin films deposited under the conditions havetypical luminescence behavior. Random oriented ZnO thin films can be obtainedwith low substrate temperature(200oC) and high oxygen pressure(30～60Pa) byPLD. (3) The intensity of UV emission from ZnO thin films obtained with laser energydensity of 31J/cm2 and oxygen pressures of 11Pa at substrate temperature of 400oCis the strongest, and the intensity ratio of UV emission to deep level emission is thelargest (41). Theory investigation indicated that intensity of UV emission dependedon crystal structure and deep level emission is related to defects in ZnO thin films.The electron transitions from the bottom of the conduction band to the antisiteoxygen OZn mainly contribute to the deep level emission of PL spectra of ZnOfilms. (4) ZnO thin films obtained on quartz glass with small laser energy density(31J/cm2) and low oxygen pressure (11Pa) at low substrate temperature(100oC～250oC) have obviously (002) orientation, and this is the obvious at 200oC. Thelarge and uniformity grains can be obtained. ZnO thin films deposited under theconditions have typical luminescence behavior. (5) Intensity of UV emission increased with the increasing of substratetemperature as substrate temperature is below 200oC; Intensity of UV emissionincreased as substrate temperature is above 200oC. In case of films deposited underthis conditions, the average optical transmittance values of films were higher than70% in visible wavelength range. (6) The ZnO thin films on quartz glass substrate obtained by the oxidation ofmetal Zn film at 550oC with the oxygen pressure from 50Pa to 23000Pa show astrong single violet emission, and intensity of UV emission increased with theincreasing of oxygen pressure. Theory indicated that the violet emission of PLspectra of the ZnO thin films be attributed to the energy transition of electronicsfrom the upside of the valence band to the interstitial zinc (Zni) level. The fact thatviolet emission peak position shifted to higher energy interval as oxygen pressureincreases can be attributed to the increase of zinc vacancy defects and the decreaseof interstitial zinc defects in the ZnO films. (7) The ZnO thin films on quartz glass substrate obtained by the oxidation ofmetal Zn film with the oxygen pressure of 50Pa at the annealing temperatures of500oC～700oC show a strong single violet emission. Intensity of violet emission...