Tailoring of polar and nonpolar ZnO planes on MgO (001) substrates through molecular beam epitaxy
© Zhou et al; licensee Springer. 2012
Received: 8 September 2011
Accepted: 9 March 2012
Published: 9 March 2012
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.
KeywordsZnO MgO polar nonpolar RHEED XRD XAS
ZnO film has attracted much attention due to its various applications such as short wavelength lasers, vacuum fluorescent or field-emission displays, high-power high-frequency devices, and light-emitting diodes [1–4]. 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 built-in electric field in device structures known as the quantum-confined Stark effect . 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 . In this work, we show that both polar and nonpolar ZnO thin films can be grown on MgO (001) substrates using oxygen plasma-assisted MBE. It is found that the electronic properties of ZnO films are different between polar and nonpolar structures.
The MgO substrates were first degreased by ultrasonic bath in acetone, followed by ethanol. After being introduced into the MBE growth chamber (ultra-high vacuum environment, with a base pressure of 10-9 mbar), the substrates were annealed at 420°C for 60 min while the power of the radiofrequency plasma source was set to 250 W and the oxygen partial pressure maintained at 5 × 10-5 mbar. The detailed experimental process of plasma-assisted MBE can be found elsewhere [6, 7]. During growth of ZnO films, the temperature of elemental zinc source (with a purity of 99.99999%) was maintained at 330°C, and the oxygen partial pressure was kept at 1 × 10-5 mbar with the power of plasma source at 180 W. All the films reported in this work were grown at these conditions for 60 min. The only growth parameter that changed for each film is the substrate temperature, which ranges from 100°C to 480°C. In situ reflection high-energy electron diffraction (RHEED) was used to examine the surface structure of the MgO substrate (before depositing ZnO) and the ZnO films (after the deposition). The film roughness was characterized by atomic force microscopy (AFM). The polar and nonpolar structures of the ZnO films were determined by X-ray diffraction (XRD) using a Cu anode (λKα1 = 1.54056 Å). The electronic structures of the thin films were probed by synchrotron-based X-ray absorption spectra (XAS). The component analysis of XAS was done through first principles' all-electron calculations based on density functional theory (DFT)  using generalized gradient approximations , as implemented in the Wien2k package (Vienna, Austria) . A dense k-point mesh of 22 × 22 × 12 was used to obtain a well-converged charge density, and the projected density of states (PDOS) of ZnO was calculated and compared with the experimental XAS spectra.
Results and discussion
In summary, ZnO thin films were grown at varying substrate temperatures while keeping other growth parameters unchanged. Structural characterization by XRD revealed a sharp (10-10) peak at 31.7° in the 2θ scanning for films grown between 320°C and 420°C, indicating the formation of nonpolar m-plane. 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. In situ RHEED measurements also confirmed that the grown ZnO thin films were mostly of the wurtzite phase. In addition, the polar ZnO film structure was composed of two domains with a rotation angle of 30°. AFM images showed that the roughness of the thin film surface changed with the substrate temperature and was coupled with the evolution of the FWHM values from XRD. The fine structures of XAS spectra indicated that the polar and nonpolar ZnO films had different electronic structures, which was consistent with the microstructure observations and DFT calculations. Our work suggests the possibility of motoring polar and nonpolar ZnO films using the MBE growth technique and hence tailoring the electronic structure of the ZnO films.
atomic force microscopy
density functional theory
full width at half maximum
molecular beam epitaxy
projected density of state
reflection high energy electron diffraction
X-ray absorption spectroscopy
This work is supported by the Specialized Research Fund for the Doctoral Program of Higher Education (grant numbers 20090121120028 and 20100121120026), Natural Science Foundation of Fujian Province, China (grant number 2010J05138), and Program for New Century Excellent Talents in University (NCET) (grant number NCET-09-0680).
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