The Preparation and Characterization of Surface Textured ZnO: Al Thin Films
|Course||Materials Physics and Chemistry|
|Keywords||Textured AZO thin films Transparent and conducting thin film RF sputtering Wet etching Solar cell|
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%.