ZnO/AlGaN ultraviolet light emitting diodes.
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ZnO/AlGaN ultraviolet light emitting diodes. D.M.Bagnall, Ya.I.Alivov1, E.V.Kalinina2, D.C.Look3, B.M.Ataev4, M.V.Chukichev5, A.E. Cherenkov2, A. K.Omaev4 Electronics and Computer Science, University of Southampton, UK 1 Institute of Microelectronics Technology, RAS, Chernogolovka, Moscow dist., 142432 Russia 2 A.F.Ioffe Physico-Technical Institute, RAS, St. Peterburg, Russia 3 Semiconductor Research Center, Wright State University, Dayton, Ohio 45435, U.S.A. 4 Institute of Physics, Daghestan Scientific Centre of RAS, Makhachkala, 367003 Russia 5 Department of Physics, M.V.Lomonosov Moscow State University, Moscow, Russia
ABSTRACT Ga doped n-type ZnO layers are grown using plasma assisted chemical vapor deposition on Mg doped p-type AlGaN epitaxial layers grown by hydride vapor phase epitaxy to form nZnO/p-AlGaN heterojunction light emitting diodes. I-V characteristics clearly show rectifying behavior with a threshold voltage of ∼3.2 V and intense ultraviolet electroluminescence with peak emission at 390 nm. The dominant emission mechanism is found to result from hole injection from the p-type AlGaN into the n-type ZnO. Significant emission up to 500 K is observed indicating possible applications in harsh environments.
INTRODUCTION Zinc Oxide, with a direct bandgap of ∼3.3 eV is a promising semiconductor for the fabrication of ultraviolet light emitting diodes (LEDs) suitable for operation in harsh environments and at high temperatures [1]. ZnO has some properties that are perhaps more favorable than those other wide band semiconductors such GaN and SiC, these include a large exciton binding energy of ∼60 meV, higher quantum efficiency, greater resistance to high-energy radiation [1,2]. However, despite much progress in ZnO technology in recent years, high quality p-type ZnO and therefore, the manufacture of ZnO homojunctions, continues to be problematic. For now, the unique properties of ZnO might be best exploited by constructing heterojunctions with ZnO active regions. GaN makes an excellent buffer layer for ZnO, because the crystal structures of the two materials are identical, and the lattice constants are quite close [3] GaN can also be doped ptype.GaN and AlN have band-gap energies of 3.4 eV and 6.2 eV, and lattice mismatches with ZnO of 1.8 % and 4.4 %, respectively [4] For AlGaN alloys the lattice parameters are assumed to be linearly dependent on Al content and in our case, Al0.12Ga0.88N, the lattice mismatch is estimated to be 2.2 %.
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EXPERIMENTAL DETAILS Commercial, n-type 6H-SiC wafers were used as substrates. First of all, epitaxial n-type GaN buffer layers of 0.2-µm thickness were grown by hydride vapor phase epitaxy (HVPE). Then, Mg-doped, p-type AlGaN epitaxial layers of 0.8-µm thickness and 12% Al content were grown, also by HVPE. Aluminum compositions in the AlGaN epilayers were estimated using xray diffraction, and the concentration of uncompensated acceptors, determined by Hg-probe measurements and analysis of capacitance-voltage characteristics of Cr Schottky barriers, was fo
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