Electrical properties of reactively sputtered ZnO thin films with a wide range of Al impurity concentrations

Despite the fact, that thin films of transparent conductive oxides (TCOs) are already in use as transparent electrodes for thin film solar cells, there are still questions whether it is possible to further improve their electrical and optical properties. Moreover the fundamental mechanisms that govern the incorporation and activation of dopants in the host lattice are only partially understood. Therefore this study is focused on Al doped ZnO (AZO) as one of the most used TCOs. By relating parameters of film growth to structure and properties the abovementioned questions were addressed. In particular it is of big interest to increase the Hall mobility while maintaining a high free carrier concentration to obtain highly conductive films which show low absorption in the near IR and visible spectral range.

The AZO films were grown using reactive pulsed magnetron sputtering (RPMS) on fused silica and sapphire substrates. The oxygen partial pressure during deposition was precisely controlled by exploiting the gettering of the reactive gas by the sputtered metal at constant oxygen gas flow. This was combined with different substrate temperatures (T_S=RT-550 °C) and Al concentrations (c_Al=0.5-8.7 at %). The film structure was studied by X-ray diffraction (XRD), cross-sectional TEM and atomic force microscopy, while the elemental composition was determined by elastic recoil detection analysis. Further characterization was done by spectroscopic ellipsometry and Hall-effect measurements in van-der Pauw geometry.

It was shown for every Al concentration there is a maximum Hall mobility which can be reached at a certain optimum substrate temperature and oxygen partial pressure which are interrelated. The recent achievement of a high Hall mobility of 46 cm²/Vs at a free electron density of 6.0x10²⁰ cm^-3 in reactive pulsed magnetron sputtered (RPMS) ZnO:Al demonstrates that the RPMS technique is suitable for the production of high quality TCO films [1].

A comparison of the results with data from other publications reveals an underlying physical limit of mobility in polycrystalline TCO films in general. For different growth conditions and target Al concentrations the relation between the mobility and free electron density can be explained in terms of ionized impurity scattering, impurity clustering and grain boundary limited transport.
Moreover epitaxial undoped and Al doped ZnO films were grown on sapphire substrates and studied by extensive XRD analysis. It was found that these films possess a high degree of in-plane orientation with a rotation of their hexagonal lattice by 30° with respect to the substrate.

[1] S. Cornelius et al, Appl. Phys. Letters 94, 042103 (2009)