Growth and Characteristics of a-Plane GaN/ZnO/GaN Heterostructure
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Growth and Characteristics of a-Plane GaN/ZnO/GaN Heterostructure Chiao-Yun Chang1, Huei-Min Huang1, Yu-Pin Lan1, Tien-Chang Lu1᧦, Hao-Chung Kuo1, Shing-Chung Wang1 and Li-Wei Tu2, Wen-Feng Hsieh1
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Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30050, Taiwan 2 Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan Abstract The crystal structure of a-plane GaN/ZnO heterostructures on r-plane sapphire was investigated by using the XRD and TEM measurment. It was found the formation of (220) ZnGa2O4 and crystal orientation of semipolar (10 13) GaN at GaN/ZnO interface. The epitaxial relation of normal surface direction are the sapphire (1 102) // a-GaN (1120) and ZnGa2O4
(220) // semi-polar GaN (10 13) . Beside, the emission peak energy of ZnO appears shift about 60 meV in the GaN/ZnO/GaN heterostructures due to the re-crystallization of ZnO layer with Ga or N atom and the formation of the localized state. Introduction
The ZnO has attracted extended attention because of the superior material characteristics in optoelectronics and piezoelectronics[1]. Furthermore, the physical properties of ZnO greatly resemble GaN, which include the lattice constant and thermal expansion coefficient, crystal structure and energy bandgap. The lattice mismatches between the wurtzite structure of GaN and ZnO are 0.4% for the a-axis direction and 1.9% along the c-axis direction. Based on their similar lattice constant, the related GaN/ZnO -based heterojunctions are suitable to realize high performance optoelectronic devices[2]. Therefore, it has the potential to grow along the non-polar orientation direction such as a-plane (1120) and m-plane (1 100) for the growth of GaN epilayer on ZnO layer. However, the growth mechanism and the optical properties of the GaN/ZnO interface are still not yet clear. This work investigates the growth and characteristics
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of a-plane GaN/ZnO epitaxial structures, which can be applied to various optoelectronic devices.
Experimental procedure
The a-plane GaN/ZnO heterostucture was grown on r-plane sapphire. First of sample, a 1.5-ȝm-thick a-plane GaN layer was grown on r-plane sapphire at the temperature of 1100 °C by using the metal organic chemical vapor deposition (MOCVD). Next, a-plane ZnO epitaxial layer was deposited at the temperature of 550 °C by using the plasma-assisted molecular beam epitaxy (PAMBE) system. Afterwards, the GaN was grown on the ZnO template at the temperature of 620 °C by molecular beam epitaxy (MBE). The optical properties of GaN/ZnO/GaN heterostucture were investigated by the photoluminescence (PL) measurment and time-resolved photoluminescence (TRPL) measurment. The crystal orientation and the interface elements of composition for GaN/ZnO heterostucture were determined using high solution x-ray diffraction (XRD) measurement and TEM measurements. Results and discussion
The PL spectrum of GaN/ZnO heterostucture on r-plane sapphire at room temperature were shown in
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