ZnO/SiC heterojunctions show great potential for various optoelectronic applications (e.g., ultraviolet light emitting diodes, photodetectors, and solar cells). However, the lack of a detailed understanding of the ZnO/SiC interface prevents an efficient and rapid optimization of these devices. Here, intrinsic (but inherently n-type) ZnO were deposited via molecular beam epitaxy on n–type 6H-SiC single crystalline substrates. The chemical and electronic structure of the ZnO/SiC interfaces were characterized by ultraviolet/x-ray photoelectron spectroscopy and x-ray excited Auger electron spectroscopy. In contrast to the ZnO/SiC interface prepared by radio frequency magnetron sputtering, no willemite-like zinc silicate interface species is present at the MBE-ZnO/SiC interface. Furthermore, the valence band offset at the abrupt ZnO/SiC interface is experimentally determined to be (1.2 ± 0.3) eV, suggesting a conduction band offset of approximately 0.8 eV, thus explaining the reported excellent rectifying characteristics of isotype ZnO/SiC heterojunctions. These insights lead to a better comprehension of the ZnO/SiC interface and show that the choice of deposition route might offer a powerful means to tailor the chemical and electronic structures of the ZnO/SiC interface, which can eventually be utilized to optimize related devices.

ZnO layers were deposited on 6H-SiC single crystalline wafers by radio frequency magnetron sputtering. The chemical structure of the ZnO/SiC interface was studied by x-ray photoelectron and x-ray excited Auger electron spectroscopy. A complex chemical structure, involving not only silicon–carbon and zinc–oxygen bonds but also silicon–oxygen and zinc–silicon–oxygen bonds was revealed to form at the ZnO/SiC interface. Based on the comparison with the presumably inert (i.e. chemically abrupt) ZnO/Mo interface, it was concluded that a willemite-like zinc silicate (i.e. Zn2SiO4) interface species develops between ZnO and SiC. The presence of this species at the ZnO/SiC interface will affect the electronic structure of the heterojunction and thus needs to be considered for device optimization.

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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.