Abstract
We studied the thickness dependence of the crystallographic and optical properties of ZnO thin films grown on c-plane sapphire substrate using atomic layer deposition. High-resolution X-ray diffraction (HR-XRD) revealed two peaks at 34.5° and 36.2° in the initial growth stage of ZnO on the sapphire substrate, corresponding to the (002) and (101) ZnO planes, respectively. However, as the thickness of the ZnO film increased, the XRD intensity of the (002) ZnO peak increased drastically, compared with that of the (101) ZnO peak. This indicated that (002) and (101) ZnO were simultaneously grown on the c-plane sapphire substrate in the initial growth stage, and that (002) ZnO was predominantly grown with the increase in the thickness of ZnO film. The ZnO thin film presented an anisotropic surface structure at the initial stage, whereas the isotropic surface morphology was developed with an increase in the film thickness of ZnO. These observations were consistent with the HR-XRD results.
Highlights
Zinc oxide (ZnO) is a promising material owing to its high chemical stability, good electrical property, wide band gap of 3.37 eV, and large exciton binding energy of 60 meV at room temperature.[1]
When the ZnO film thickness increased to more than 8.0 nm, the surface morphologies of the ZnO films with the (002) ZnO plane changed from non-oriented to isotropic surface structure
As the ZnO film thickness increased, the optical bandgap energy of the ZnO films increased from 3.167 eV to 3.236 eV as shown in the inset of Fig. 4
Summary
Zinc oxide (ZnO) is a promising material owing to its high chemical stability, good electrical property, wide band gap of 3.37 eV, and large exciton binding energy of 60 meV at room temperature.[1] These superior properties of ZnO make it suitable for specific optoelectronic applications, such as solar cells, transparent conducting films, and light-emitting diodes.[2,3] There are many reports on the growth of ZnO films using metal-organic chemical vapor deposition, sol-gel method, molecular beam epitaxy, pulsed laser deposition, atomic layer deposition (ALD), sputtering, etc.[4,5,6,7,8,9,10] Among these growth techniques, ALD has been proposed as a promising method to grow high-quality ZnO films This technique is a modified chemical vapor deposition growth technique in which a binary reaction is split into two self-limited surface reactions by separate sequential exposures of the substrate to the chemical precursors.[11]. Many research groups have studied the effect of thickness-dependent optical and structural properties of a ZnO film grown on various substrates such as silicon, sapphire, polymer, and metal substrates with different crystallographic planes.[17,18,19] In this study, we focused on the effect of thickness-dependent crystallographic properties within a very narrow thickness range of ZnO films grown on c-plane sapphire substrates
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