Ab initio Study of the Hydrogen Molecule on ZnO Surfaces
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Ab initio Study of the Hydrogen Molecule on ZnO Surfaces
Po-Liang Liu, Yen-Ting Wu, and Yu-Jin Siao
Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung, Taiwan 402, Republic of China
ABSTRACT We conduct first-principles total-energy density functional calculations to study the interaction of H2 on ZnO surfaces. Four surface models of Zn-terminated (0001)-, O-terminated (0001)-, (10 1 0)-, and (2 11 0)-oriented ZnO planes in the presence of H2 are evaluated. The relative stability of four different surface models is examined as a function of the chemical potentials of oxygen and hydrogen. We find that only surfaces of O-terminated (0001)-oriented ZnO models exhibit active sites for the dissociation of H2, which in turn enables the formation of water from dissociative chemisorption of 2H on the O-terminated ZnO(0001) surface. The surface energy of O-terminated ZnO(0001) surface in the presence of water was found to be negative under the O-rich and H-rich condition. The findings agree with the experimental observations that ZnO epitaxial layers are easily etched by hydrogen at typical growth temperatures.
INTRODUCTION The wurtizite ZnO, a II-VI compound semiconductor, is a wide band gap of 3.37 eV compared to 3.39 eV for wurtizite GaN and large exciton binding energy (60 meV) for leading to the lasing action even upon room temperature [1,2]. Various epitaxy technologies have been employed to grow high quality ZnO thin layer materials on GaN templates, including the metal-organic chemical vapor deposition (MOCVD) [3,4], metal-organic vapor phase epitaxy
(MOVPE) [5], molecular beam epitaxy (MBE) [6-8], atomic layer deposition (ALD) [9], and so on. Hydrogen molecules (H2) is often employed as a carrier gas in MOCVD, MOVPE, and ALD or Hydrogen is present in the Zn precursor. Unfortunately, it has been found that ZnO films tend to disintegrate in H2 ambient, which is commonly ubiquitous during epilayer growth processes [10,11]. Therefore, it is of significant interest to carry out a detailed study of H2 on ZnO surfaces. The dissociation of H2 and the adsorption of H on ZnO surfaces must be taken into account to estimate the interaction of H2 on ZnO surfaces.
COMPUTATIONAL METHODOLOGY AND BACKGROUND In order to provide further insight into the reaction of a H2 gas on ZnO surfaces, the (0001)-, (10 1 0)-, and (2 11 0)-oriented ZnO planes in the presence of H2 will be evaluated by using thermodynamic functions of the chemical potentials μ of the constituents in Models, i.e. μ Zn , μ O , and μ H . The calculation of chemical potentials μ of the constituents used in the present work follows closely our approach described in Refs. 12 and 13. The surface formation energy of ZnO models depends on the chemical potentials μ Zn and μ O and this dependence can be further simplified by eliminating μ Zn using the bulk thermodynamic equilibrium conditions as bulk follows: μ Zn + μ O = μ ZnO , leading to a dependence on μ O only. At a surface, the chemical potentials of O is restricted within the t
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