Growth of a-axis ZnO ﬁlms on the defective substrate with different O/Zn ratios: A reactive force ﬁeld based molecular dynamics study Y.L. Liu a , M. Babar Shahzad a,b , Y. Qi a,⇑ a Institute of Materials Physics and Chemistry, School of Sciences, Northeastern University, Shenyang 110819, China b Institute of Metals Research, Chinese Academy of Sciences, Shenyang 110016, China article info Article history: Received 18 August 2014 Received in revised form 4 December 2014 Accepted 5 December 2014 Available online 13 December 2014 Keywords: Zinc oxide Non-polar thin ﬁlms Atomic scale structure Point defects Molecular dynamics simulations abstract The understanding of the growth process and formation mechanism of non-polar ZnO ﬁlms in atomic- scale is crucial in adjusting and controlling the ﬁlm deposition conditions. Using the advanced reactive force ﬁeld based molecular dynamics method, we theoretically studied the effect of O/Zn ratios (8/10– 10/8) on the quality of ZnO ﬁlms. The comprehensive investigation of energy and temperature ﬂuctuation proﬁle, radial distribution function, the sputtering and injecting phenomenon, and layer coverage indi- cated that the ﬁlm grown under stoichiometric conditions possesses the optimized quality. Furthermore, the auto-transformation ability of the substrate from defective to perfect stacking was presented and dis- cussed by comparing to the perfect structure. The instant ﬁlm growth conﬁgurations, atomic layer snap- shots, and the interfacial morphology evolution were provided step-by-step to reveal the defect type and initial ﬁlm nucleation and growth mechanism. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction ZnO has drawn a great deal of interest as a potential material for optoelectronic applications in short-wavelength light-emitting diodes (LEDs), piezoelectric transducers [1], and 3D ﬂexible semi- conductor networks [2] owing to its wide band-gap (3.37 eV), large excitation binding energy (60 meV) [3] and large bendability to high curvature [4,5]. Also, with a large electro-mechanical coupling factor and a high piezoelectric coefﬁcient, ZnO thin ﬁlms are good candidates for high frequency and low loss surface acoustic wave (SAW) devices [6,7]. Generally, most of ZnO-based devices are grown along the polar c-axis direction, perpendicular to the sub- strate, which under normal conditions is the most preferable growth orientation for ZnO ﬁlms. However, the spontaneous piezo- electric polarization along the c-axis can induce a large built-in electrostatic ﬁeld, then separating the overlapping of electron and hole wave functions in the quantum well, leading to a deteri- oration of the internal quantum efﬁciency of light emitting devices, a phenomenon known as the quantum conﬁned stark effect (QCSE) [8,9]. Moreover, the special application of ZnO as the surface wave devices in liquids requires a ﬁlm growing with c-axis tilted away from the normal to the ﬁlm surface [10,11]. Therefore, the growth of a-plane (1 1  20) or m-plane (1 0  1 0) non-polar ZnO ﬁlms has drawn huge attention as an alternative for overcoming the QCSE problem [12–14] and extending the application of SAW devices in liquids. To date, the epitaxial growth of non-polar ZnO thin ﬁlms mostly relies on the ﬁlm growth techniques [15–27], growth conditions [11,28–41] and element doping methods [42–45]. The ﬁlm growth techniques mainly include atomic layer deposition [15], (plasma- assisted) molecular beam epitaxy [16–20], pulsed laser deposition [21,22], oblique-angle sputtering deposition [23], chemical vapor deposition [24,25], the chemical solution growth method [26], and the hydrothermally-grown method [27]. The non-polar m- or a-plane ZnO ﬁlms has been obtained using these growth techniques on different substrates, such as ZnO [17,24], sapphire [15,16,19,21,22,33,38,41,43,45], MgO [18,20], LiGaO2 [25,3 0,31,37], GaN [21,24,27,32], GaAs [36], Al2O3 [32,37], SiTiO3 [44], Si [28,29,42] and Glass [23,26]. Besides the growth methods, growth conditions could also affect the microstructure, defects, interface, and surface morphology, leading to different electrical and optical properties of the ﬁlms. Great efforts have been devoted to study the effect of growth conditions on the thin ﬁlm properties, such as temperature and surface treatment conditions [28–31,36], deposition rate [32,37], incidence angle [11], substrate plane [33], partial pressure [38] and controlling ﬂux ratio of the elemental sources [34,35,39–41]. The group II/VI elemental ratio is one of the most important growth parameters among the above mentioned factors, which can greatly inﬂuence the interface http://dx.doi.org/10.1016/j.jallcom.2014.12.018 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +86 2483678479; fax: +86 2483683674. E-mail address: qiyang@imp.neu.edu.cn (Y. Qi). Journal of Alloys and Compounds 628 (2015) 317–324 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom