High mobility Al-doped ZnO grown by reactive pulsed magnetron sputtering

Increasing the power conversion efficiency of modern thin film solar cells based on absorbers like silicon (Eg=1.11 eV), CuIn_xGa_(1-x)Se_2 (Eg=1.0-1.5 eV) and CdTe (Eg=1.44 eV) is one of the major goals of research devoted to photovoltaics. These cells rely on transparent electrodes made of transparent conductive oxides (TCO) whose high transmittance and low resistivity result in a high short circuit current and fill factor enabling high cell efficiency.

In TCOs absorption in the near infrared (NIR) and visible spectral region is caused by the free electrons and is influenced by their density and mobility inside the host lattice. Due to this inter-relation of optical and electrical properties TCOs with highest carrier mobility at moderate electron densities (~5x10²⁰ cm^-3) are required to simultaneously reach low resistivities and high NIR transmittance. Furthermore the deposition process used should be cost effective and scalable to large area substrates.

Therefore a reactive magnetron sputtering method using metallic Zn/Al alloy targets was developed to achieve high carrier mobilities (µ~45 cm²/Vs) in ZnO:Al (AZO) thin films [1], which is comparable to values achieved with other methods like RF magnetron sputtering or pulsed laser deposition.
Mass spectrometry and high accuracy capacitive pressure sensing together with a variation of the magnetron discharge parameters allowed for a fine control of oxygen partial pressure (p_O2). The dependence of the films electrical properties on the substrate temperature (T_S), the Al content in the sputter targets and p_O2 have been investigated systematically by Hall-effect measurements. Spectroscopic ellipsometry was used to determine film thickness and optical properties. The film structure, morphology and elemental composition was analysed by various methods including AFM, XRD, X-TEM, ERDA and RBS. Analysis of the film composition together with Hall-effect data was used to estimate Al donor activation.

Results show that there are optimum values of T_S and p_O2 at which films with resistivities down to ~2.3x10^-4 Ohm*cm and free electron densities of ~6.0x10²⁰ cm^-3 were achieved. A shift of these optimum growth parameters and the resulting film properties with the target Al content has been detected. The observed limit of mobility in polycrystalline AZO is discussed in terms of ionized impurity scattering and clustering as well as grain boundary limited transport.
Recent investigations of Al concentration in the films and local bonding structure, revealed by XANES seem to explain the well known deterioration of resistivity in AZO at elevated substrate temperatures or after annealing.