Quantification of Al and Ga electrical activation in ZnO films grown by reactive magnetron sputtering

In this work Al doped ZnO (AZO) and Ga doped ZnO (GZO) polycrystalline and epitaxial thin films were grown on fused silica and on c-plane sapphire, respectively, by reactive magnetron sputtering using a wide range of target dopant content cT from 0.7 to 8.7 at%. A special method of discharge operation was used in order to establish a precise control of metal to oxygen content in the films [1]. The film thickness and optical properties were determined using a combination of Spectroscopic Ellipsometry and Spectral Photometry. The results of optical modelling using a Parameterized Semiconductor Model (PSEMI) and a Drude term agree well with optical transmission measurements. The electrical resistivity (ρ), Ne and µ were obtained via Hall-effect measurements in van-der-Pauw geometry. The high quality of optimized polycrystalline films is characterized by ρ<4x10-4 Ωcm and µ>40cm²/Vs (epitaxial films µ>55cm²/Vs ) and a visible transmission of ~90%. Microstructural properties such as macrostrain, crystallite size, microstrain, texture and mosaicity were investigated by X-ray diffraction.
This paper is focused on the systematic study of the effects of Al and Ga dopant incorporation in ZnO on the abovementioned properties and the effective dopant activation rate in dependence of the growth parameters. Therefore the film composition has been quantified by a combination of ion beam analysis techniques. In the case of AZO the Al content was obtained by Rutherford backscattering spectrometry (RBS). However, due to the limited mass resolution Ga and Zn are indistinguishable in the RBS spectrum and only the total metal content of the samples is measured. Recently we were able to quantify also the Ga to Zn ratio by using Particle induced X-ray Emission (PIXE). Both Al and Ga show a pronounced systematic variation of their concentration in the films (cF) with substrate temperature and target dopant content. In general cF > cT due to the preferred desorption of surplus Zn during the film growth. These results explain the typically observed Ne(TS) dependence in AZO and GZO. Furthermore it is deduced that the dopant activation rate is constant with an absolute value of ~35% for AZO and ~50% for GZO below a certain critical cF value. At even higher dopant concentrations the film electrical and structural properties deteriorate strongly, which gives rise to the commonly observed optimum substrate temperature behaviour. In extreme cases this may even lead to insulating films, containing a substantial fraction of secondary phases [2]. Finally, implications for the physical limits of electrical transport in ZnO will be discussed.