Study of Nonpolar GaN/ZnO Heterostructures Grown by Molecular Beam Epitaxy Chiao-Yun Chang, † Huei-Min Huang, † Yu-Pin Lan, † Tien-Chang Lu,* ,† Li-Wei Tu, ‡ and Wen-Feng Hsieh † † Department of Photonics, National Chiao Tung University, Hsinchu 30050, Taiwan ‡ Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan ABSTRACT: The growth mechanism and characteristics of the nonpolar GaN/ZnO heterostructure grown on the r-plane sapphire substrate by using molecular beam epitaxy were studied. The crystal interaction between GaN and ZnO epitaxial layers was clarified by using transmission electron microscopy and X-ray diffraction. A new epitaxial relationship of ZnGa 2 O 4 (220)//GaN (101̅3̅) in the normal surface direction was obtained in the GaN/ZnO heterostructure. It was believed that the formation of ZnGa 2 O 4 (220) was due to the recrystalliza- tion of the ZnO layer with Ga atoms, which in turn resulted in the formation of semipolar-oriented GaN. In addition, the main optical transition in the GaN/ZnO heterostructure was attributed to the existence of the interface states and new ZnO:(Ga,N) alloys. ■ INTRODUCTION GaN-based wide-bandgap semiconductors have attracted much attention in the application of optoelectronic devices. 1−3 However, owing to the lattice mismatch between the epitaxial layer and substrate, the growth of GaN-based materials is usually accompanied by a high-defect density to limit the device performance. Therefore, selection of a suitable substrate for GaN-based optoelectronic devices will play a significant role. ZnO was regarded as a candidate for the substrate because of similar physical properties and amenability to conventional chemical wet etching. In particular, the lattice mismatches between the wurtzite GaN and ZnO are only 1.9% along the c- axis direction and 0.4% along the a-axis direction, respectively. It indeed has the potential to achieve the high quality GaN epitaxial layer. The related growth method and characteristics have been widely studied, and the heterostructures consisted of GaN and ZnO were also extensively applied. 4−7 However, the conventional GaN-based devices always suffered from the internal electric field effects along the c-axis direction and caused spatial separation of electron and hole wave functions that in turn gave rise to the restriction of carrier recombination efficiency. Thus, the semi/nonpolar-oriented GaN-based epitaxial layer and heterostructure grown on the r-plane sapphire substrate, 8 γ-LiAlO 2 , 9 and (100) MgAlO 2 10 have been demonstrated to effectively improve the carrier recombi- nation efficiency and to reduce the polarization effects. At present, the growth condition and the related optical and structural properties in the semi/nonpolar GaN/ZnO hetero- structure were not yet clarified. In our previous work, 11 we have demonstrated that the GaN/ZnO heterostructure grown on the a-plane GaN template showed the intermediate phase at the interface, due to volatility issues at high temperatures. The intermediate phase appeared even if the growth temperature was relatively low. Therefore, in this work we made efforts to clarify the interaction between GaN and ZnO layers and its influences and characteristics. ■ EXPERIMENTAL SECTION The GaN/ZnO heterostructure was grown by the molecular beam epitaxy (MBE) system. First, the 2.0 μm thick a-plane GaN template was grown on the r-plane sapphire using the metal organic chemical vapor deposition (MOCVD). This was followed by deposition of an a- plane ZnO film of about 400 nm thickness at 520 °C using pulsed laser deposition. Then, the GaN epitaxial layer of nominal 200 nm thickness was grown by MBE at 620 °C of the substrate temperature. After the growth, the structural variation and surface morphology for the GaN/ ZnO heterostructure were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), as shown in Figure 1. The thickness of the ZnO epitaxial layer was unexpectedly diminished from 400 to 150 nm. Some air−voids between the GaN and ZnO interface appeared, and the surface morphology variation of the GaN thin films was observed to be full of GaN grains. Since we have known that the growth temperature of the GaN epitaxial layer was higher than that of the ZnO epitaxial layer and the single-step growth was used without any buffer layer, the decomposition of the underneath ZnO epitaxial layer could be redecomposed and the Ga and N atoms could interact with the Zn and O atoms. Therefore, we used the high-resolution-X-ray diffraction (HR-XRD) and trans- Received: April 5, 2013 Revised: June 11, 2013 Published: June 13, 2013 Article pubs.acs.org/crystal © 2013 American Chemical Society 3098 dx.doi.org/10.1021/cg400497r | Cryst. Growth Des. 2013, 13, 3098−3102