Interfacial structure of a-plane ZnO grown on r-plane sapphire by pulsed laser deposition Chun-Yen Peng n , Wei-Lin Wang, Yen-Teng Ho, Jr-Sheng Tian, Li Chang n Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan article info Article history: Received 17 October 2012 Accepted 1 December 2012 Available online 8 December 2012 Keywords: Nonpolar ZnO Epitaxial growth Interfaces HRTEM RSM abstract In this study, interfacial structure of a-ZnO with misfit accommodation on r-sapphire at low and high growth temperatures (LT and HT) by pulsed laser deposition is presented. Along ½1 100 ZnO of large lattice mismatch of ZnO with sapphire, TEM examinations show that a-type misfit dislocations are spaced 1.3–2.2 nm on HT-ZnO/sapphire interface, whereas dislocation pairs in spacing of 2.8–3.5 nm are observed for LT-ZnO/sapphire. For smaller lattice mismatch along the ZnO c-axis direction, reciprocal space maps of ð11 2 2Þ ZnO and ð30 30Þ sapphire reflections show that HT-ZnO is nearly fully strained without much relaxation and has a highly coherent interface with sapphire, in contrast with partial relaxation in LT-ZnO. & 2012 Elsevier B.V. All rights reserved. 1. Introduction As a wide direct bandgap wurtzite semiconductor, zinc oxide (ZnO) is an attractive material for potential applications in optoelectronic devices [1–4]. Recently, growth of nonpolar epi- taxial films has attracted a lot of interest due to the lack of the red-shift caused by the quantum-confined Stark effect [5,6]. Because the spontaneous polarization of ZnO is nearly two times of GaN (  0.057 to 0.05 C/m 2 versus  0.029 to  0.022 C/m 2 ) [7,8], ZnO films without polarity along the growth direction are important for light emitting applications. Sapphire is a commonly used substrate for growth of epitaxial thin films. Nonpolar ð11 20Þ a-ZnO epitaxial films grown on ð10 1 2Þ r-sapphire substrates have been achieved by domain matching epitaxy methods [9] using pulsed laser deposition (PLD) [10], sputtering [11], plasma-assisted molecular beam epitaxy [12–13], and metal–organic chemical vapor deposi- tion [14]. From domain matching epitaxial relationships of ½000 1 ZnO ==½10 11 sapphire and ½1 100 ZnO ==½ 12 10 sapphire , anisotropic stresses resulted from anisotropic lattice and thermal mismatches may have significant effects on its crystallinity [15–17]. It has been shown that the misfit is relaxed by regularly spaced misfit dislocations along the large lattice mismatch direction of ½1 100 ZnO , whereas misfit dislocations are hardly seen along the ZnO c-direction due to its small lattice misfit [12,17–19]. In addition, the densities of threading dislocations (TDs) and stacking faults are often observed in 10 8  10 10 cm 2 and 10 4 –10 5 cm 1 , respectively [18–20]. It is known that growth temperature plays an important role on crystallinities and optical properties [11,21–22]. However, the variation of strains and misfit accommodations with different growth temperatures has been rarely studied. In our previous study, we have shown that the transition of the film growth mode may occur at 600 1C for a-ZnO grown on r-sapphire [22]. In this paper, we show that misfit accommodations along in-plane directions are indeed varied with growth temperature from the evidence of transmission electron microscopy (TEM) and x-ray reciprocal space mapping (RSM). 2. Experiments ZnO films were grown on 8  8 mm 2 r-sapphire wafers at 10 mTorr oxygen partial pressure in a Pascal laser-MBE system at 450 1C (LT-ZnO) and 750 1C (HT-ZnO). The KrF excimer laser irradiation with the energy density of about 1–3 J/cm 2 and the repetition rate of 2 Hz was used to ablate a sintered ZnO target. Sapphire substrates were ultrasonically cleaned in acetone before loading into the vacuum chamber. Before PLD growth, 850 1C thermal cleaning of the substrate in vacuum at about 10 6 Torr was done for 30 min. A Philips Tecnai 20 TEM operated at 200 kV was employed to investigate microstructures of ZnO on sapphire. The preparation of cross-sectional TEM (XTEM) specimens was carried out using mechanical grinding process and Ar þ ion milling at 3.5–4.0 kV. The RSM experiments were performed in a PANalytical X’Pert Pro Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.003 n Corresponding authors. Tel.: þ886 3 5731615; fax: þ886 3 5724727. E-mail addresses: cypeng.mse94g@nctu.edu.tw (C.-Y. Peng), lichang@cc.nctu.edu.tw (L. Chang). Materials Letters 94 (2013) 165–168