Abstract
Polar and nonpolar ZnO thin films were deposited on MgO (001) substrates under different deposition parameters using oxygen plasma-assisted molecular beam epitaxy (MBE). The orientations of ZnO thin films were investigated by in situ reflection high-energy electron diffraction and ex situ X-ray diffraction (XRD). The film roughness measured by atomic force microscopy evolved as a function of substrate temperature and was correlated with the grain sizes determined by XRD. Synchrotron-based X-ray absorption spectroscopy (XAS) was performed to study the conduction band structures of the ZnO films. The fine structures of the XAS spectra, which were consistent with the results of density functional theory calculation, indicated that the polar and nonpolar ZnO films had different electronic structures. Our work suggests that it is possible to vary ZnO film structures from polar to nonpolar using the MBE growth technique and hence tailoring the electronic structures of the ZnO films.PACS: 81; 81.05.Dz; 81.15.Hi.
Highlights
ZnO film has attracted much attention due to its various applications such as short wavelength lasers, vacuum fluorescent or field-emission displays, highpower high-frequency devices, and light-emitting diodes [1,2,3,4]
According to the orientations of the polar (0001) and nonpolar (10-10) planes as illustrated in Figure 1b, these results indicate that the polarization of the ZnO film can be controlled through the substrate temperature
For films grown below 320°C, a strong (0002) peak at 34.5° representing polar c-plane was instead identified; for films grown above 420°C, nonpolar and polar planes coexist
Summary
ZnO film has attracted much attention due to its various applications such as short wavelength lasers, vacuum fluorescent or field-emission displays, highpower high-frequency devices, and light-emitting diodes [1,2,3,4]. High-quality ZnO films are usually grown on expensive and hexagonal substrates such as ZnO, GaN, and sapphire and tend to be polarized, leading to builtin electric field in device structures known as the quantum-confined Stark effect [5]. To overcome this disadvantage, there is an emerging interest of growing nonpolar ZnO thin films. When ZnO thin films are deposited on the cubic substrate of MgO (100), nonpolar m-plane (10-10) and polar c-plane (0001) of ZnO can be grown by molecular beam epitaxy (MBE) and pulsed laser deposition, respectively [6]. We show that both polar and nonpolar ZnO thin films can
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