The role of the interface structure on the growth of nonpolar (10 10) and semipolar (11 2 2) ZnO on (112) LaAlO 3 substrates Jr-Sheng Tian, Yue-Han Wu, Wei-Lin Wang, Tzu-Chun Yen, Yen-Teng Ho, Li Chang n Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan article info Article history: Received 7 June 2013 Accepted 19 June 2013 Available online 26 July 2013 Keywords: Structural Epitaxial growth Interfaces LaAlO 3 ZnO abstract Growth of nonpolar (10 10) and semipolar (11 2 2) ZnO on (112) LaAlO 3 (LAO) substrates can be obtained by annealing the substrate surface in vacuum and oxygen ambient conditions prior to ZnO deposition, respectively. We investigated the origin of the two different growth relationships by inspecting their interface in atomic scale using high angle annular dark field scanning transmission electron microscopy. (10 10) ZnO was grown on flat (112) LAO surface due to the similar atomic configurations and small lattice mismatch between them at the interface, and (11 2 2) ZnO was grown on a faceted surface with (001) LAO and (110) LAO facets on which accommodation growth of both (11 20) ZnO and (000 1) ZnO are consistent with [1 100] ZnO //[1 10] LAO . & 2013 Elsevier B.V. All rights reserved. 1. Introduction Wurtzite zinc oxide (ZnO), a non-toxic, transparent and wide- direct bandgap semiconductor, is promising for optoelectronic appli- cations due to the large exciton binding energy (60 meV) [1]. Since wurtzite structure has no inversion center, polarization could cause detrimental effect on the luminescent properties [2,3]. To reduce such an effect, wurtzite materials grown along nonpolar and semi- polar orientations have been intensively studied [4,5]. Owing to the lack of large size and cheap ZnO substrates, growth of nonpolar and semipolar ZnO is usually carried out on foreign substrates [6–9]. LaAlO 3 (LAO), a perovskite structure which is R 3c (trigonal lattice) at room temperature and Pm 3m (cubic lattice) at tem- perature above 540 1C, has been used as substrate to grow various oriented ZnO. For simplicity, R 3c LAO can be treated as pseudo- cubic with lattice parameter of 3.791 Å. Recently, we have demon- strated the growth of nonpolar (11 20), (10 10), and (13 40) oriented ZnO on (001), (112), and (114) LAO substrates, respectively [7,10,11]. Also, semipolar (11 2 2) ZnO can be deposited on LAO- buffered (112) (LaAlO 3 ) 0.29 (Sr 2 AlTaO 6 ) 0.35 (LSAT) substrate [12]. These results represent that the structure and chemistry of LAO substrates are favorable for achieving epitaxial ZnO films. How- ever, it is of interest to find out why ZnO growth on (112) LAO substrate exhibits a different orientation from that on (112) LAO/ LSAT. In this report, we first show that (11 2 2) ZnO can also be obtained on (112) LAO substrates by adopting the same condition as for the growth of (11 2 2) ZnO on (112) LAO/LSAT [12]. Next, we investigate the interface structures of nonpolar (10 10) and semipolar (11 2 2) ZnO on LAO substrates with their epitaxial relationships. 2. Experimental ZnO films were grown by pulsed laser deposition using a KrF excimer laser. A laser fluence of ∼3 J/cm 2 on a sintered ZnO target and 99.999% oxygen as ambient gas were adopted for the growth. Prior to ZnO deposition, (112) LAO substrates were annealed at two different ambient conditions, vacuum and oxygen pressure of 100 mTorr, both at 850 1C for 1 h. All the deposited ZnO films were then grown in the same condition at 750 1C and 50 mTorr for 30 min with pulse rate of 5 Hz. Growth orientations of the ZnO films were investigated by using X- ray diffraction (XRD) (Cu K α1 ). The interface structures of ZnO/LAO were studied by using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) to reveal the Z-contrast due to the difference in atomic number. The HAADF images were acquired in a JEOL ARM 200F microscope which provides STEM image resolu- tion of 0.08 nm. The STEM specimens were prepared by a conven- tional method consisting of mechanical grinding and ion milling. 3. Results and discussion XRD patterns in Fig. 1 show that the growth orientation of the ZnO film on (112) LAO substrates with vacuum anealing is in pure nonpolar (10 10) ZnO , whereas it is in semipolar (11 2 2) ZnO on the oxygen-annealed LAO. As the ZnO films were deposited in the same condition, the growth difference may be associated with the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$- see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.07.069 n Corresponding author. Tel.: +886 3 573 1615, fax: +886 3 572 4727. E-mail address: lichang@cc.nctu.edu.tw (L. Chang). Materials Letters 109 (2013) 237–239