High mobility transparent conductive Al-doped ZnO thin films by atomic layer deposition

The effects of growth temperature on the microstructure, transport and optoelectronic properties of a series of Al-doped ZnO (AZO) films with thickness of ∼30 nm deposited on polished silicon-(100) and glass substrates by the atomic layer deposition (ALD) were investigated. By adopting an in-situ doping growth scheme the critical length effect associated with adjacent Al2O3 layers commonly encountered in previous ALD growth schemes was avoided and effective Al-doping was achieved with the growth temperature ranging from 100 °C to 300 °C. Experimental results showed that, in general, increasing the growth temperature would result in much improved film crystallinity and carrier mobility, with the average transmittance in the visible wavelength range being exceeding 95% in all cases. In particular, for AZO films grown at 300 °C, an unprecedented mobility of 136 cm2V−1s−1 was obtained, comparing to the typical values of 50-60 cm2V−1s−1 reported previously. The resistivity of these 300 °C films (ρ ≈ 6 × 10−4 Ω-cm), nevertheless, is slightly higher than that of some highly-doped ZnO (ρ ≈ 2-4 × 10−4 Ω-cm) prepared by sputtering methods. The secondary ion mass spectroscopy (SIMS) analyses revealed that hydrogen incorporation is the key in reducing the charge trap density and, hence, resulting in much enhanced carrier mobility. The present results promise a keen competitiveness of AZO with the indium tin oxide (ITO) film for thin-film-transistor (TFT) as well as in photovoltaic device applications.