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Herein, ZnO cap layers are prepared by chemical vapor deposition on Al‐doped ZnO (AZO) films and demonstrate a reduction in the electrical resistivity of the films. When prepared at 600 °C, a continuous ZnO cap layer is formed and leads to an increase in a Hall mobility from 22 to 37 cm2 V−1s−1, resulting in a resistivity of 5.1 × 10−4 Ω cm, which is superior to those of nanoparticle and nanorod morphologies formed at lower and higher substrate temperatures, respectively. Furthermore, the continuous ZnO cap layers successfully prevent decreases in the carrier concentration and Hall mobility during annealing at the temperatures of up to 600 °C in air, resulting in a figure of merit (FOM) of 1.6 × 10−2 Ω−1, which is approximately one order of magnitude better than those of uncapped films annealed in Ar. The improvement is due to the cap layer having proper morphology to provide sufficient protection for restructuring of the AZO grain boundaries, thereby reducing the defect density and sacrificing its structural order to suppress Zn desorption in AZO and environmental oxygen migration into AZO during annealing. Using ZnO as a cap layer also reduces the possibility of introducing unexpected band offset at the interface due to extrinsic elements.

Al-doped ZnO (AZO) is a promising candidate as a transparent conducting electrode. However, the electrical properties of AZO deteriorate greatly after exposing it to excessive heat. This limits the applications of AZO in devices that experience a demanding operation environment. It has been shown that a ZnO cap layer with proper morphology is capable to dramatically improve the thermal stability of AZO. However, the detailed mechanism is not yet clear. A comparison study of the electrical properties of AZO with a ZnO cap layer prepared by magnetron sputtering (MS) at low substrate temperature (70 °C) and chemical vapor deposition (CVD) at high substrate temperature (600 °C) indicates that MS-prepared ZnO is much less effective in protecting AZO from an oxidizing environment under elevated temperature than the CVD-prepared ZnO. The morphology and crystal structures of two types of ZnO/AZO, investigated by a scanning electron microscope and x-ray diffraction, are relatively similar, whereas the atomic structures (e.g., defects) revealed by Raman spectroscopy are rather different. The results suggest that it is difficult to improve the thermal stability of electrical properties of AZO without a proper restructuring process and a ZnO cap layer that could sacrifice its own structural order. The discoveries offer a novel approach to improve the performance of other transparent conducting oxides.