Local Electronic Structure and UV Electroluminescence of n-ZnO:N/p-GaN Heterojunction LEDs Grown by Remote Plasma Atomic
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Local Electronic Structure and UV Electroluminescence of n-ZnO:N/p-GaN Heterojunction LEDs Grown by Remote Plasma Atomic Layer Deposition Jui F. Chien1, Ching H. Chen2, Jing J. Shyue1,3, and Miin J. Chen1,4,* 1
Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, Republic of China. 2 Protrustech Corporation Limited, Tainan, Taiwan, Republic of China. 3 Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, Republic of China. 4 Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei, Taiwan, Republic of China. ABSTRACT Nitrogen-doped ZnO (ZnO:N) films have been prepared by remote plasma atomic layer deposition (RP-ALD) and treated by rapid thermal annealing (RTA) in oxygen atmosphere. The local electronic structures of the (ZnO:N) films were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) at the O K-edge. The XPS reveals the presence of the Zn-N bond in the ZnO:N films, indicating that partial amounts of oxygen sites are occupied by nitrogen species. This is correspondent with the decrease of electron concentration in ZnO:N films with the nitrogen doping concentration, as indicated by the Hall effect measurement. The RP-ALD technique was applied to fabricate the ntype ZnO:N/p-type GaN heterojunction LEDs. Dominant ultraviolet electroluminescence at 371 nm from the ZnO:N layer was observed at room temperature. INTRODUCTION Zinc oxide (ZnO) has attracted great attention as an ultraviolet (UV) material because of its large exciton binding energy (60 meV) and wide direct bandgap (3.37 eV) [1-5]. ZnO has been developed for many applications, such as UV light-emitting diodes (LEDs) [2,3] and transparent thin film transistors (TFTs) [4]. However, undoped ZnO possesses intrinsic n-type conductivity due to various non-stoichiometric defects, for example, Zn interstitials and O vacancies [6]. It has been reported that p-type conductivity can be obtained by doping group-V elements (N, P, and As), as substituting for O sites in ZnO [7-9]. Many techniques have been used to prepare ZnO thin films doped with group-V elements, including metal-organic chemical vapor deposition [10], molecular beam epitaxy [11], pulse laser deposition [12], sputter [13], and atomic layer deposition [14]. Nitrogen is considered as the promising p-type dopant since the ionic radius of nitrogen is close to oxygen, which leads to facile substitution. Thus nitrogen-doped ZnO (ZnO:N) thin films have been applied in ZnO homojunction LEDs [15,16] and active channel layers in TFTs [17]. Therefore, the details of the local electronic structures of the ZnO thin films would be of great interest to understand their electrical properties. Recently, the p-type GaN was considered as a favorable material for the formation of n-ZnO/p-GaN heterojunction devices duo to a smaller lattice mismatch of 1.8%, similar wide bandgap, and identical crystal (würtzite) structure between ZnO and GaN. In this study, remote plasma atomic lay
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