ZnO Deposited by Magnetron Sputtering Incorporating Si Nanocrystals

Abstract

Zinc oxide has attracted a great deal of interest in recent years with a view to potential photovoltaic applications, particularly as a material for transparent conducting electrodes. The material's abundance and low cost make it a potentially attractive alternative to the current standard technology. Silicon nanocrystals have recently been identified as exhibiting properties potentially suitable for multiple exciton generation (MEG), which could theoretically improve the efficiency of solar cells above the thermodynamic limit on the current standard solar cell design.

Silicon-doped ZnO thin films were deposited by a radio frequency co-sputtering technique, using ZnO and Si targets, onto glass substrates and Si wafers. By adjusting the target power ratio, films with various micro-structures could be obtained. This work began with optimisation of the sputtering parameters for deposition of ZnO thin films of ~100 nm thickness with a high degree of texturisation and grain sizes of ~20 nm or greater, as determined by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Sputter deposition simultaneously from both targets was then performed and XRD, optical transmittance and Hall measurements were used to determine the variation of the resulting films' structural, optical and electrical characteristics with increasing Si/ZnO target power ratio. A Si-doped ZnO film was fabricated with resistivity ~0.001 Ohms cm, carrier concentration of ~10^{20} cm^{-3} and transmittance ~80% or greater in the wavelength range from 400 to 1400 nm. Finally, TEM imaging techniques were used to confirm the precipitation of Si nanocrystals in a highly doped film.