氧分压对磁控溅射制备ZnO薄膜的结构及光学特性的影响

Influence of Oxygen Partial Pressure on the Structural and Optical Properties of ZnO Films Deposited by Radio-frequency Magnetron Sputtering

采用射频反应磁控溅射法在玻璃衬底上成功制备出具有c轴高择优取向的ZnO薄膜,利用X射线衍射及紫外-可见吸收和透射光谱研究了氧分压变化对ZnO薄膜的微观结构及光吸收特性的影响。结果表明,当工作气压恒定时,用射频反应磁控溅射制备的ZnO薄膜的生长行为主要取决于成膜空间中氧的密度,合适的氧分压能够提高ZnO薄膜的结晶质量;薄膜在可见光区的平均透过率达到90%以上,且随着氧分压的增大,薄膜的光学带隙发生了一定程度的变化。采用量子限域模型对薄膜的光学带隙作了相应的理论计算,计算结果与对样品吸收谱所作的拟合结果符合较好,二者的变化趋势完全一致,表明ZnO纳米晶粒较小时,薄膜光学带隙的变化与量子限域效应有很大关系。

ZnO is surely one of the most promising semiconductor materials in the future in the fields of optoelectronics, photochemistry and microelectronics, etc. Many techniques such as radio frequency or direct current sputtering, ion plating and chemical vapor deposition have been employed to prepare ZnO thin films, the sputtering process is the most promising method for depositing ZnO films among these processes. The advantages of sputtering are the simple apparatus, high deposition rate, low substrate temperature, good surface flatness, transparency and dense layer formation. The properties of ZnO films are generally affected by the preparation conditions, such as deposition methods, working pressure, and the types of substrates, etc. In order to do a systematic research about effect of the oxygen/argon ratio on the microstructure and the optical absorption properties, ZnO thin films with c-axis preferred orientation were prepared on glass substrates by r.f. co-reactive magnetron sputtering technique, influence of oxygen optical absorption properties of ZnO thin films were studied by X-ray partial pressure on the microstructure and diffraction （XRD） and UV-Vis absorption spectra. The XRD patterns of the five ZnO samples were measured by XRD equipment. The figure which embo- died the relation of Full Width at Half Maximum （FWHM） of the diffraction peak and grain size of the five samples as a function of oxygen partial pressure was given out. The XRD patterns and the figure showed that relative intensity of （002） diffraction peak of the samples increased when the oxygen/argon ratio was under 14： 10, but it then decreased when the oxygen/argon ratio near 18： 10. The figure showed that the FWHM decreased and the grain size of the samples increased when the oxygen/argon ratio was under 6： 10, but it then represented opposite trend when the oxygen/argon ratio was bigger than 6： 10, and the sample which were prepared at the oxygen to argon ratio of 6：10 had the biggest crystal grain size and the least FWHM. The results showed that when the working pressure was kept in constant, the growth behavior of the ZnO thin film was mainly decided by the density of oxygen in the space where the sample was deposited, and the crystallization of the ZnO thin films was promoted by desirable oxygen partial pressure. In addition, absorption spectra of the five ZnO samples deposited at different oxygen partial pressures have been measured at room temperature, the films possessed a transmittance of about 90% in the visible region. The figure embodied （ahu）^2 versus hu of ZnO films grown at different oxygen partial pressures was given out, too. It was concluded that with the increase of oxygen partial pressure, the band gap exhibited different values. Furthermore, theoretical calcula- tions of the optical band gap were carried out using the quantum confinement model, the theoretical calculated values are in good agreement with the extrapolated results. It indicated that the change of band gap had great relationship with quantum confinement effect when the ZnO nanocrystals was small. The conclusion above is of great importance in preparing high-quality ZnO thin films, improving transmittance, accommodating absorption edge of ZnO films when it is used as transparent conducting layer.