Effect Of The Substrate Temperature On The Optical Properties Of ZnO Films At The Low Power

ZnO is a new-type semiconductor with a direct wide-band-gap of 3.37eV at RT. It has a high exciton binding energy of 60meV and hexahedron wurtzite structure at the air condition. The feasibility of using excitionic lasers of ZnO at RT has been demonstrated. It has been investigated extensively because of its interesting electrical, optical and piezoelectric properties making suitable for many applications such as light emitting diodes, photodetectors, electroluminescence, transparent conductive film, surface acoustic waves device and so on.ZnO films have been grown by various deposition methods including magnetron sputtering, pulsed laser deposition, molecular beam epitaxy, metal organic chemical vapor deposition, spray pyrolysis and sol-gel process. Magnetron sputtering method is widely used by the researchers because the equipment is simple, low cost, easy operation, high rate of deposition, the requirement of base temperature is lower and good film adhesion, its ingredients in a certain extent controllable, etc.In this thesis, we further researched the effect of preparation temperature on the structure and optical property of ZnO films deposited by magnetron sputtering with DC at the low power. In this way some experimental data and the theoretical basis are provided for the application of ZnO films. The results are summarized as follows:1. The influence of substrate temperature on the property of ZnO films have been studied on Si substrates. It is found that the FWHM (half high peak width) of ZnO (002) decreased gradually and the grain size grew up, which indicate that the crystallization of the ZnO films have been improved with the increasing of substrate temperatures in the range from RT to 400℃.We also studied the influence of substrate temperature on the photoluminescence (PL) properties of ZnO films by using fluorescence spectrometer. Blue peak at 440 nm and 485 nm, green peak at 527 nm of the films have been observed. With the rising of the growth temperature, the intensity of the blue peak increased sharply. It is concluded that the blue peak located at about 440 nm was mainly attributed to Zni, the blue peak located at about 485 nm may correspond to the electron transition from deep oxygen vacancy level to the valence band, the green emission peak located at about 527 nm corresponds to the transition from the bottom of the conduction band to the local level composed of oxide misplaced defects. The growth temperature is mainly via changing the kind of the defects and concentrations to influence the luminescence properties of the ZnO films.2. The influence of substrate temperature on the property of ZnO films have been studied on glass substrates. It is found that the FWHM of ZnO (002) is smallest and the grain size is biggest, which indicate that the crystallization of the ZnO films is best at 400℃. The suitable substrate temperature is conducive to enhance the quality of the crystallization of ZnO films.By studying the light absorption and transmission of the ZnO films, we found that all the samples have a large light absorption in the UV region. In the visible region, the average transmittance of the samples exceeded 94% when the substrate temperatures were 200℃and 300℃, respectively. This is because the band gap of ZnO is higher than the energy of the visible region. So the light absorption is mainly free carrier absorption in the visible region. However, the ZnO films,mobility rate of free carrier is so high that the absorption coefficient of free carrier is very small. Finally, we also calculated the optical band gap of the ZnO films by using the quantum confinement model and the corresponding theory. The results are in excellent agreement with the experimental values.3. We also studied the influence of preparation temperature on the light absorption and transmission of the ZnO films on glass substrates. It is found that all the samples showed a strong transmission in the visible region. Especially the sample①RT and sample③preparation in RT, annealed in 275℃, the average transmittance exceeded 90%. Simultaneously, we also discovered that the ZnO films possessed a strong transmittance exceeding 100% in the ultraviolet range from 200nm to 300nm. The transmittance exceeded 400%, especially the sample①and③respectively. The phenomenon may due to the low power of the sputtering, defects of the film and the quadratic luminescence under the shortwave excitation. It is only an initial guess and the specific reason need for further research.