厚度对未掺杂ZnO薄膜光谱性能的影响

X射线衍射光谱、拉曼光谱和紫外可见透射光谱技术是薄膜材料检测的重要技术手段。通过对薄膜材料光谱性能的分析，可以获得薄膜材料的物相、晶体结构和透光性能等信息。为了解厚度对未掺杂ZnO薄膜的X射线衍射光谱、拉曼光谱和紫外可见透射光谱性能的影响，利用溶胶-凝胶法在石英衬底上旋涂制备了不同厚度的未掺杂ZnO薄膜样品，并对薄膜样品进行了X射线衍射光谱、拉曼光谱和紫外可见透射光谱的检测。首先，通过X射线衍射光谱检测发现，薄膜样品呈现出（002）晶面的衍射峰，ZnO薄膜为六角纤锌矿结构，均沿着C轴择优取向生长，且随着薄膜厚度的增加，衍射峰明显增强，ZnO薄膜的晶粒尺寸随着膜厚的增加而长大。利用扫描电子显微镜对薄膜样品的表面形貌分析显示，薄膜表面致密均匀，具有纳米晶体的结构，其晶粒具有明显的六角形状。通过拉曼光谱检测发现，薄膜样品均出现了437 cm-1的拉曼峰，这是ZnO纤锌矿结构的特征峰，且随着薄膜厚度的增加，其特征拉曼峰强度也增加，进一步说明了随着ZnO薄膜厚度的增加，ZnO薄膜晶化得到了加强。最后，通过紫外可见透射光谱测试发现，随着膜厚的增加，薄膜的吸收边发生一定红移，薄膜样品在可见光区域内的透过率随着膜厚度增加而略有降低，但平均透过率都超过90%。通过对薄膜样品的紫外-可见透射光谱进一步分析，估算了薄膜样品的折射率，定量计算了薄膜样品的光学禁带宽度，计算结果表明：厚度的改变对薄膜样品的折射率影响不大，但其禁带宽度随着薄膜厚度的增加而变窄，且均大于未掺杂ZnO禁带宽度的理论值3.37 eV。进一步分析表明，ZnO薄膜厚度的变化与ZnO晶粒尺寸的变化呈正相关，本质上，吸收边或光学禁带宽度的变化是由于ZnO晶粒尺寸变化引起的。

Effects of Thickness on Spectral Properties of Undoped ZnO Thin Films

X-ray diffraction spectrum, Raman spectroscopy and UV-Vis transmission spectroscopy are important techniques for detecting thin films. By analysing these spectral properties, we can obtain the information of the phase, crystal structure and light transmission properties of the films. The films with different thicknesses were prepared on a quartz glass substrate by sol-gel and spin coating method. The X-ray diffraction, Raman spectroscopy and UV-Vis transmission spectrum detection of the samples were carried out. Firstly, the X-ray diffraction shows that the films exhibit peak (002) crystal plane. The ZnO thin films are hexagonal wurtzite structures and grow along the c-axis preferred orientation. With the increase of the film thickness, the (002) peak is enhanced, and the grain size of the ZnO grows with the increase of the film thickness. Scanning electron microscopy analysis of the film’s surface morphology also shows that the film surface is compact and uniform, with nanocrystalline structure and obvious hexagonal shape. Secondly, the Raman spectrum detection shows that 437 cm-1 Raman peak appears in all samples, which is the characteristic Raman peak of the ZnO wurtzite structure. With the increase of film thickness, the intensity of 437 cm-1 peak also increases, which further indicates that the crystallization of film is strengthened with the increase of film thickness. Finally, It is found that the absorption edge of the film has a certain redshift with the increase of the film thickness based on that of the UV-Vis transmission spectrum. The transmittance of the film in the visible region decreases slightly with the increase of the film thickness, but the average transmittance is above 90%. On this basis, the further theoretical calculation of the experimental results shows that the change of thickness has little effect on the refractive index of the film samples, but the bandgap width becomes narrow with the increase of the thickness of the film. However, the bandgap width of undoped ZnO films is larger than the theoretical value of 3.37 eV. Further analysis shows that the change of ZnO film thickness is positively related to the change of ZnO grain size. Essentially, the change of absorption edge or optical band gap is caused by the change of ZnO grain size.