Synthesis And Characterization Of ZnO Films By Ultrasonic Spray Pyrolysis

Zinc oxide (ZnO) is one of the most important multifunctional materials with direct band gap of 3.37 eV and exciton binding energy up to 60 meV which can raise efficient excitonic emission above room temperature. Therefore, it is a promising material that can effectively emit light. Besides, due to the high chemical stability, high transmittance, low deposition temperature, non-toxic and high material availability, ZnO is considered as a promising material used in solar cells, flat panel displays, especially in light emitting diodes(LEDs) and laser diodes(LDs).The research on ZnO mainly focus in nanostructure ZnO, transparent conductive ZnO film and p type ZnO film. Among these three research fields, the preparation of p type ZnO film is most important and difficult. Because once p type ZnO was successfully prepared with high quality, ZnO can be used to fabricate LEDs and LDs instead of GaN. Nitrogen (N) is recognized as the best element to substitute oxygen (No) to prepare p type ZnO. However, the amount of No concentration is limited and the hole activation energy is relatively high. Thus, N-Al co-doped method is widely use to lower Madelung energy as well as hole activation energy in ZnO. Therefore, the thesis pays attention to the effects of growth conditions on the preparation of N-Al co-doped ZnO film using Ultrasonic Spray Pyrolysis (USP), in order to better understand the preparation of p type ZnO film.N-Al co-doped ZnO films were deposited on glass substrates at various substrate temperatures including 300℃,350℃,400℃and 450℃using USP method. The X-ray diffraction (XRD) results show that all the films deposited at various temperatures are hexagonal wurtzite ZnO. The full width at half maximum (FWHM) of (002) diffraction peak decreases as the increase of substrate temperature, indicating that the size of ZnO crystalline increases with substrate temperature. The AFM images show that the size of ZnO particles increases gradually as the increase of substrate temperature, which is well consistent with XRD results. Therefore, a conclusion can be made that the size of ZnO increases gradually as the increase of substrate temperature.Fluorescence Spectrometer was used to measure the photoluminescence (PL) properties of ZnO films. According to the PL spectra, it is obvious that PL property of the films is extremely sensitive to the substrate temperature. PL spectra of the films deposited at low temperature shows strong defect-related emission, indicating that the films were deposited with low crystalline quality and high defects concentration. When the film deposited at 450℃only ultraviolet emission is observed in the PL spectra. The disappearance of green emission caused by oxygen vacancy (VO) implies that the concentration of donated defects decreases as the increase of substrate temperature, which will benefit the preparation of high quality p type ZnO. Since ordinary glass substrate will melt if the temperature is further increased,450℃is regarded as the most appropriate temperature.Single crystalline Si (100) wafer was used as substrate to grow N-Al co-doped ZnO films with various Al concentrations. The spectra show that all the samples only exhibit untraviolet emission centered at around 382 nm. As the increase of Al concentration, the intersity of untraviolet emission increases. The reason why it increases with Al concentration is probably that the concentration of Alzn-No increases with Al doping concentration, leading to the increase of electron-hole pair concentration. Therefore, ultraviolet emission increases because of the increase of electron-hole pair radiation. This result is meaningful to improve the emission intensity of ultraviolet emitter.N-Al co-doped ZnO films with various thicknesses including 80 nm,235 nm,545 nm,700 nm and 1440 nm, were deposited on glass substrates. The thickness effect on the evolution of structure and optoelectronic properties of N-Al co-doped ZnO films as well as the influence of N-Al co-doped on the photoluminescence of ZnO films were investigated. The XRD and Field Emission Scanning Electron Microscropy (FESEM) results show that with the increase of film thickness the grain size increases and the grain shape changes from regular hexagonal sheet-like to wedge-shaped, even pyramidal. The changes in the shape of ZnO grains with the thicknesses of films indicate that growth model of ZnO crystalline changes with the film thickness.The PL spectra illustrate that there is an obvious red-shift for the emission center from ultraviolet to blue region, and the intensity of defects emission increases with the increase of thickness. Considering that there are so many kinds of defects acting as emission centers in ZnO film and the PL spectrum is emission band rather than separated emission peaks, we deduce that the emission band is composed of different kinds of emission peaks, including free excitonic emission, bound excitonic emission and defect recombination emission. The energy of defect recombination emission is lower than that of free excitonic emission. Therefore, from the red shift of emission center, a conclusion can be made that the intensity of defect emission increases with the film thickness.Additionally, Al doped ZnO transparent conductive films were also deposited on glass substrates. Before adjusting the precursor solution, the lowest resistivity obtained is 4.1Ω·cm with transparency of 85%-90%. The value of resistivity is a bit high. However, after changing the precursor solution, the resistivity of as-grown and post-grown ZnO film obtains as low as 1.93×10-2Ω·cm and 3×10-3Ω·cm, while the transparency increases up to 93.97%. This result will benefit the application of ZnO as condutive transparent film.