【作者】 何晶晶；

【导师】 吴雪梅； 诸葛兰剑；

【作者基本信息】 苏州大学 ， 材料物理与化学， 2011， 硕士

【摘要】 ZnO薄膜材料因具有无毒、源材料丰富、相对生长温度低和在强H等离子体环境中性能稳定等特点获得了广泛研究和应用。其中ZnO:Al (AZO)薄膜由于电阻率低,在可见光区域透过率高而备受关注,被广泛地应用于硅基太阳电池前电极。然而,硅基太阳电池除了要求前电极具有优异的光电性能以外,还要求薄膜表面具有凹凸起伏的绒面结构,实现有效的光散射从而提高电池的光电转换效率。本文采用射频磁控溅射法在硅和石英衬底上成功制备了不同溅射功率下的AZO薄膜,研究了溅射功率对AZO薄膜结构及光电性能的影响,后又采用湿法刻蚀方法对AZO薄膜表面进行织构化处理,研究不同的刻蚀时间对薄膜表面形貌的变化及相同的刻蚀时间下不同溅射功率制备的AZO薄膜刻蚀行为的差异,得出的研究结果如下:一、XRD结果显示,所有AZO薄膜均显示了明显的(002)峰C轴择优取向,随着功率的增加,衍射峰的强度和半高宽减小,衍射峰向大角度方向移动,这是由于半径较小的Al3+(0.054nm)部分取代了ZnO晶格中半径较大的Zn2+(0.074nm),因此AZO薄膜的晶面间距d(002)减小,(002)面的衍射角向大角度方向移动。SEM表面形貌图显示,随着溅射功率的增加,薄膜表面由圆点状向瓦片状转变,颗粒逐渐变大,AFM结果显示,薄膜的均方根粗糙度(RMS)在3.33-9.54nm范围内。SEM截面图显示,薄膜垂直于基片生长,随着溅射功率的增加,薄膜由柱状生长方式变为片状生长方式。XPS结果显示,Zn以Zn2+形式出现在AZO基体中,Al2p峰的结合能在74.0eV附近,处于单质金属Al的电子结合能(72.3eV)和Al2O3的电子结合能(74.6eV)之间,说明Al离子进入到ZnO的晶格内形成了掺铝氧化锌化合物。二、采用霍尔系统测试了AZO薄膜的电学性质,发现随着溅射功率的增加,AZO薄膜的载流子浓度和霍尔迁移率均增加了,从而导致薄膜的电阻率降低,最小值为1.37×10-2?cm。采用紫外-可见光分光光度计测试了AZO薄膜在可见光区域的透过率,所有薄膜在可见光区域的透过率均在80%以上,溅射功率为200W的样品的平均透过率最高,达86%。此外,所有的AZO薄膜的透射曲线都有一个明显的特征吸收边,且随着功率的增加,样品的特征吸收边向短波方向移动,即产生了吸收边蓝移的现象。三、初沉积的AZO薄膜的表面比较平滑,首先采用体积分数为0.5%的稀盐酸对具有最优的光电性能的170W和200W的样品进行湿法刻蚀,刻蚀时间为10-30s。SEM图显示刻蚀后样品表面呈现坑状形貌,随着刻蚀时间的增加,坑越深越大,腐蚀时间30s的时候AZO薄膜刻蚀后坑状形貌最为明显。因此,对溅射功率在70-200W之间的所有AZO薄膜进行30s的刻蚀,发现不同溅射功率下的AZO薄膜刻蚀后显示出不同的坑状形貌,表现为坑的大小、深度、分布密度不同,其中溅射功率为170W的AZO薄膜腐蚀后表面分布的坑特征尺寸达500nm,分布均匀,在500nm处的绒度高达40.6%,具有优异的光散射性能。更多还原

【Abstract】 ZnO thin films have attracted much attention because of its excellent characteristics such as non-toxicity, low cost, low growth temperature and high stability in the atmosphere of H plasma. ZnO:Al (AZO) transparent and conducting thin film is widely used as a front contact in solar cells because its low resistivity and high transmittance in the visible region. However, in order to improve the efficiency, the thin film solar cells require the AZO films have not only the excellent optical and electrical properties but also an adequate surface texture to provide an efficient light scattering.In this paper, we investigated the structure, optical and electrical properties of AZO films prepared by RF-magnetron sputtering with various RF power, then applied a post deposition wet etching technique to initially smooth AZO films and observed that the effect of the as-grown structure on etching behavior in diluted HCl. The following is the major results:1. The XRD spectra reveal that all the AZO films deposited at various RF sputtering power clearly exhibited the c-axis orientation with (002) diffraction peaks.With increasing the RF power, the peak intensities and the Full Width at Half Maximum of (002) peak decreased. The SEM images show that as the RF power was increased, the surface morphology gradually transferred from dot-like to tile-like and the grain got larger. From AFM images, the root mean square roughness values (RMS) of AZO films with the variety of RF sputtering power from 70W to 200W was found to be 3.33-9.54nm. In addition, the SEM cross-sectional images show that a transition from columnar to granular growth is observed. The AZO films deposited at 70W-120W exhibit a columnar crystal structure, however, at higher RF powers ranging from 150-200W the AZO films show a granular crystal structure. The XPS analysis shows that the Al 2p spectrum of the AZO thin film was successfully monitored. The binding energy of the AZO films is around 74.0eV, which is lower than the binding energy (74.6 eV) of Al2O3 films. The core line of Zn 2p exhibited high symmetry and the binding energy of Zn 2p3/2 remains at about 1021.9 eV, which confirms that the majority of Zn atoms remain in the same formal valence state of Zn2+ within the ZnO matrix.2. The electrical properties of the AZO films deposited on quartz glass substrates were measured by Hall measurements in the van der Pauw configuration at RT. As the RF power was increased, the carrier concentration increased, finally reached the maximum values of 1.27×1020 cm-3 at the RF of 200W. It can also be seen that as the RF power was increased the Hall mobility increased obviously, and reached the maximum values of 3.6 cm2/Vs at the RF of 200W. Finally, the increases in the carrier concentration and the Hall mobility caused the reducing of resistivity of AZO films. A minimum resistivity of 1.37×10-2?cm was detected for the AZO film deposited at the RF of 200W. The transmission spectra show that all the films exhibit a high transmittance in the visible range and a sharp fundamental absorption edge. The transmittance of AZO films on glass gradually increases with an increasing power and gets the maximum value of 86% at the power of 200W. In addition, the UV absorption edge shifted to a shorter wavelength with an increasing power. The Eg of the AZO films deposited at the RF power from 70W to 200W are varied among 3.34–3.43 eV, which is a little larger than that of pure ZnO due to Burstein-Moss shift.3. The films deposited at 170W and 200W had the optimum opto-electrical property and then were surface textured by a post-deposition chemical etching with 0.5% HCl for 10-30s. The SEM images show that all these films developed a craterlike surface morphology and the crater became larger and deeper as the etching time was increased. The light scattering property of AZO films was researched by calculated spectral haze. The AZO film etched 30s had the optimal light scattering property due to its suitable craterlike surface morphology. Then, all the AZO films deposited at the RF power of 70-200W were etched by 0.5% HCl for 30s. After a post-deposition chemical etching, these films developed different surface textures depending on their structural properties. The AZO film deposited at 170W and etched 30s had the optimal light scattering property due to its suitable craterlike surface morphology with feature size of ~500nm, the haze ratio at 500nm reached as high as 40.6%.更多还原