Electroluminescence of p-GaN/MgO/n-ZnO Heterojunction Light-emitting Diodes
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Electroluminescence of p-GaN/MgO/n-ZnO Heterojunction Light-emitting Diodes XinYi Chen,1 Alan M. C. Ng,1 Aleksandra B. Djurišiü,1 Chi Chung Ling,1 Wai-Kin Chan,2 Wai Keung Fong,3 Hsian Fei Lui3 and Charles Surya3 1 Department of Physics, The University of Hong Kong, Hong Kong, P. R. China 2 Department of Chemistry, The University of Hong Kong, Hong Kong, P. R. China 3 Department of Electronic and Information Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China ABSTRACT Light-emitting diodes (LEDs) based on p-GaN/ZnO heterojunction were fabricated. GaN was deposited on sapphire using metal-organic chemical vapor deposition (MOCVD), and two kinds of ZnO i.e. ZnO thin film deposited by sputtering and ZnO nanorods (NRs) grown by hydrothermal method were used as n-type layer respectively. MgO film with the thickness around 10 nm was deposited by electron-beam deposition to act as an interlayer between GaN and ZnO. Photoluminescence, electroluminescence and I-V curves were measured to compare the properties of GaN based heterojunction LEDs with different architectures. The existence of MgO interlayer as well as the morphology of ZnO obviously influenced the electrical and optical properties of GaN based LEDs. The effect of MgO interlayer on ZnO growth, properties and I-V curves and emission spectra of LEDs is discussed in detail. INTRODUCTION Zinc oxide as a wide band gap (3.37 eV) semiconductor with high exciton binding energy (60 meV) has attracted much research interest in the study of growth mechanism, morphologies, electrical and optical properties [1-4]. It is also promising in a variety of device application such as light emitting diodes (LEDs) [5-21]. Since the stability and reproducibility of p-type ZnO has not been solved yet, GaN based ZnO heterojunctions are very promising candidates for ZnO semiconducting devices because of the relatively low lattice mismatch between GaN and ZnO (~1.8%). Consequently, LEDs based on p-GaN/n-ZnO heterojunction with various device structures fabricated by different approaches have been reported [5-21]. Various emission colors such as UV, violet, blue, yellow and white had been achieved. Furthermore, both light emission under forward [8-21] and reverse [5-7,10,20] bias were also reported with a wide range of turnon voltages. However, among those LEDs, the electroluminescence (EL) of p-GaN/n-ZnO heterojunctions usually exhibit stronger emission from GaN layer than from ZnO layer i.e. the recombination of electron-hole couples usually occurred in GaN layer and ZnO layer just acted as the provider of electrons for GaN layer [16]. To exploit the emission from ZnO layer, a good method is using a dielectric layer between GaN and ZnO the block the electrons in the ZnO layer. Different interlayers such as MgO, polymer and doped highly-resistive ZnO [16-19] have been introduced to modify the EL emission of GaN/ZnO LEDs and several EL emissions or even lasing that might be identified to the contribution of ZnO layer have been reported [16]. In this paper
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