Homogeneous Transparent Conductive Al-doped ZnO Thin Films Deposited by Reactive Direct Current Magnetron Sputtering

Abstract

Reactive direct current sputtering of Al-doped ZnO films faces a serious drawback of lateral heterogeneity of the electrical properties of the films due to bombardment of the film by negative oxygen ions. By adopting appropriately large target-substrate distance and tuning the oxygen flow rate, Al-doped ZnO (AZO) films were reactively sputtered on a glass substrate from Zn0.98 Al0.02 alloyed target. The properties of the films were investigated as a function of the position of the substrate with respect to the magnetron axis, using X-ray diffractometer, UV-Vis-NIR spectrophotometer and four-point probe. Results indicated that, both resistivity and optical transmittance were homogeneous across the substrate positions, irrespective of the oxygen gas flow rate (OFR); due to reduced bombardment by negative oxygen ions and uniformity in the film composition, respectively. There was a transition in the film properties from absorbent and highly conducting to transparent and more resistive, with the increase of the OFR from 3.5 to 6 sccm, which is related to a transition from sub-to-over stoichiometry. With an optimum OFR of 4 sccm, transparent AZO films with a homogeneous minimum resistivity of the order 10-3 Ω cm were obtained across the substrate positions. The film thickness was quite homogeneous, with values between 182 and 218 nm. The optimized films also crystalized in a homogeneous wurtzite structure and (002) preferred orientation along the c-axis. But crystallinity and optical band gap were heterogeneous. The former improved monotonically with increase in substrate position due to relaxation of stress at higher substrate positions, while the latter increased steadily with the increase in the substrate positions due to Burstein-Moss effect. Therefore, the experimental results showed that by adopting appropriately large target-substrate distance and tuning the oxygen gas flow rate, homogeneous transparent conductive AZO films was deposited by reactive direct current sputtering on a glass substrate from Zn0.98Al0.02 alloyed target.