Influence of ZnO Cap Layer Morphology on the Electrical Properties and Thermal Stability of Al-Doped ZnO Films

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 degrees C, a continuous ZnO cap layer is formed and leads to an increase in a Hall mobility from 22 to 37 cm(2) V(-1)s(-1), resulting in a resistivity of 5.1 x 10(-4) omega 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 degrees C in air, resulting in a figure of merit (FOM) of 1.6 x 10(-2) omega(-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.