Surface textured ZnO:Al films by rf magnetron sputtering deposition for thin film solar cells

Abstract

Summary form only given. Transparent conductive oxide (TCO) is indispensable as the front electrode for most thin film solar cells. Aluminum-doped zinc oxide films (ZnO:Al) were prepared by RF magnetron sputtering on glass (Corning 1737) substrates as a function of the deposition condition. This system has three targets to enable simultaneous sputtering. One of them was a 4 inch disk type ZnO (99.9% purity) target doped with Al/sub 2/O/sub 3/ (2.5 wt%). The distance between substrate and target was kept at 5 cm. The substrates were first loaded in the load-lock chamber and then carried forward into the main chamber. The substrates were heated in the temperature range of 300-673 K using a halogen lamp and also were rotated using a motor in order to get uniform ZnO:Al films. During the film growth the argon gas pressure was maintained in the range of 0.3-10 mTorr. It was observed that the deposition rate and the grain size were decreased and the surface roughness and the etch rate were increased with the increase of argon pressure. The deposited films were found to be smooth with excellent electrical properties (/spl rho/=1.9/spl times/10/sup -4/ /spl Omega/cm) and high transmittance (>80%) in the wavelength of 400-800 nm. Also, spectral haze (T/sub diffuse//T/sub total/), an indicator of the scattering properties of the etched ZnO:Al films, of above 70% could be achieved at wavelengths of 400-800 nm. The surface morphology and thereby the light scattering properties of the sputtered films were found to vary over a wide range as a function of substrate temperature and gas pressure. At low pressures (/spl les/2 mTorr) and high substrate temperatures (>573 K), the surface morphology of the films exhibited a more dense and compact structure with effective light-trapping, suitable for the fabrication of thin film silicon solar cells.