Effect of temperature on the growth of a -axis ZnO films: a reactive force field-based molecular dynamics study
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derstanding film initiation and growth mechanisms at the atomic level is crucial to obtain high-quality nonpolar ZnO films. Using the advanced reactive force field-based molecular dynamics method, we theoretically studied the effect of substrate temperature (350–950 K) on the quality, layer develop mechanism and defect formation of ZnO films. Investigation of the energy, radial distribution function, layer coverage, sputtering and injecting phenomena indicated that the present films grown at 500–600 K possessed the optimal quality. Further investigation of the growth condition, instant film profiles, interfacial microstructure evolutions and layered snapshots revealed that, addition of atoms on newly formed localized films can induce some partially bonded or extruded atoms out of the film plane. Further adherence of depositing atoms to these unstable or extruded atoms induces the initiation and growth of a new layer.
I. INTRODUCTION
The properties and application of c-axis ZnO has been widely researched and developed. However, its spontaneous piezoelectric polarization along the c-axis ZnO film can induce a large built-in electrostatic field and then separate the overlapping of electron and hole wave functions in the quantum well; this phenomenon, known as the quantum confined stark effect (QCSE), can lead to the deterioration of the internal quantum efficiency of light-emitting devices.1,2 Also, the special application of ZnO as a surface acoustic wave (SAW) device in liquids requires a film growing with the c-axis tilted away from the normal to the film surface.3,4 Therefore, the growth of nonpolar ZnO films such as a-(1120) or m-(1010) plane has drawn attention as an alternative way for overcoming the QCSE problem and extending the application of SAW devices in liquids. In general, the (0002) polar surface is the most preferable growth direction because of its low surface energy; the non polar a- or m-plane is the next preferable growth directions. Therefore, the preparation of nonpolar films was much more difficult than that of polar films. The difference between polar and nonpolar ZnO film is whether the c-axis is perpendicular or parallel to the surface of substrate. To realize the stable nonpolar film growth, a transition from polar to nonpolar growth is necessary. Researchers need to explore the conditions which may not only inhibit the polar film growth but Contributing Editor: Susan B. Sinnott a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.491
also promote the non-polar crystal growth. Different film growth conditions5–24 were adopted to grow nonpolar ZnO thin films on different substrates and to explore the related properties and microstructure characteristics. Such as the temperature and surface treatment,5–19 the deposition rate and the incidence angle,4 the partial pressure,20 the substrate plane21 and the elemental source flux ratio.22–24 Temperature is one of the most important growth parameters because of its significant influence on interfacial microstructure, surface mo
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