Low-voltage blue light emission from n-ZnO/p-GaN heterojunction formed by RF magnetron sputtering method Yanwei Shen a , Xiang Chen a , Xiaoqin Yan a, * , Fang Yi a , Zhiming Bai a , Xin Zheng a , Pei Lin a , Yue Zhang a, b, * a State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China b Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 100083, China article info Article history: Received 19 August 2013 Received in revised form 24 November 2013 Accepted 15 December 2013 Available online 27 December 2013 Keywords: ZnO film ZnO/GaN heterojunction RF magnetron sputtering Electroluminescence abstract High quality n-ZnO/p-GaN heterojunction was fabricated by growing highly crystalline ZnO epitaxial films on commercial p-type GaN substrates via radio frequency (RF) magnetron sputtering. Low-voltage blue light emitting diode with a turn-on voltage of w2.5 V from the n-ZnO/p-GaN heterojunction was demonstrated. The diode gives a bright blue light emission located at w460 nm and a low threshold voltage of 2.7 V for emission. Based on the results of the photoluminescence (PL) and electrolumines- cence (EL) spectra, the origins of the EL emissions were studied in the light of energy band diagrams of ZnOeGaN heterojunction, and may attribute to the radiative recombination of the holes in p-GaN and the electrons injected from n-ZnO, which almost happened on the side of p-GaN layer. These results may have important implications for developing short wavelength optoelectronic devices. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Short-wavelength light emitting diodes (LEDs) have many ap- plications, such as flat-panel display and solid-state lighting [1]. Due to its wide direct-bandgap (3.37 eV) and large exciton binding energy (60 meV, much larger than that of GaN (25 eV)) at room temperature, ZnO has aroused worldwide researchers’ much more interest in developing the next generation of short-wavelength LEDs and laser diodes (LDs) [2]. However, up to now, the ZnO ob- tained by various synthesis methods, suffering from its self- compensating effect and low solubility of the acceptor dopants, exhibits n-type conductivity, and development of ZnO homo- junction LEDs and LDs has been constrained [3,4]. Thus, hybrid pen heterojunctions, which are based on n-ZnO and p-type thin layers such as p-Si [5], p-NiO [6], p-SrCu 2 O 2 [7], p-polymer [8e11], have been widely studied, but high quality heterojunctions based on them are always rather difficult to obtain due to large lattice mismatch. Since GaN has the similar crystallographic and electronic properties with ZnO [12,13], it becomes one of the best p- type candidate material to fabricate ZnO-based heterojunction light-emitting devices. It is found that those aforementioned het- erojunctions usually need a seeding layer, which as an interface layer is detrimental to the light emission and extraction from the device, while ZnO/GaN heterojunction doesn’t need a seeding layer and so can achieve a better junction. To date, for the purpose of producing high-quality ZnO/GaN heterojunction, different material growth methods have been tried, for example, molecular beam epitaxy (MBE) [14], chemical vapor deposition (CVD) [15,16], solu- tion method [17,18]. Probably because of the difference in quality of the ZnO films and corresponding junctions, large differences exis- ted in the devices’ EL emission peak position, threshold emission voltage and emission efficiency [14,16,17,19e21]. As a well- developed film technique, RF magnetron sputtering procedure is rather simple, low cost and controllable. The ZnO/GaN film LEDs are much more facile to be fabricated using RF magnetron sputtering, meanwhile, improved light extraction efficiency and tunable EL spectrum can be realized by incorporating a 2D photonic crystals [22e26], an electron blocking layer like MgO [27], or a green and red light emission quantum dots layer [28]. In our work, high- quality ZnO epitaxial films on commercial p-type GaN substrates via RF magnetron sputtering at room temperature were grown and low-voltage n-ZnO/p-GaN blue LEDs were fabricated. Since a better heterojunction quality has been achieved, a strong blue light * Corresponding authors. State Key Laboratory for Advanced Metals and Mate- rials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail addresses: xqyan@mater.ustb.edu.cn (X. Yan), yuezhang@ustb.edu.cn (Y. Zhang). Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.12.011 Current Applied Physics 14 (2014) 345e348