Electrical Characterization of Isotype n-ZnO/n-GaN Heterostructures
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Electrical Characterization of Isotype n-ZnO/n-GaN Heterostructures Yahya Alivov1, Xiao Bo1, Sena Akarca-Biyikli1, Fan Qian1, Daniel Johnstone2, Olena Lopatiuk-Tirpak3, Leonid Chernyak3, Cole Litton4, and Hadis Morkoç1 1 Electrical Engineering, Virginia Commonwealth University, 601 W. Main Street, Richmond, VA, 23284 2 SEMETROL, Chesterfield, VA, 23838 3 Physics Department, University of Central Florida, Orlando, FL, 32816 4 Air Force Research Laboratory-retired (AFRL/MLPS), Dayton, OH, 45433
ABSTRACT Electrical properties of n-ZnO/n-GaN isotype heterostructures obtained by rfsputtering of ZnO films on GaN layers grown by metal-organic vapour phase epitaxy are discussed. Current-voltage (I-V) characteristics of the n-ZnO/n-GaN diodes revealed highly rectifying behavior with forward and reverse current densities ~1.42×102 A/cm2 and ~2.4×10-4 A/cm2, respectively, at ±5 V. From the Arrhenius plot built using temperature dependent current-voltage characteristics (I-V-T) an activation energy 0.125 eV was derived for the reverse bias leakage current path, and 0. 618±0.004 eV for the band offset from forward bias measurements. From electron-beam induced current measurements the minority carrier diffusion length in ZnO was estimated in the range 0.125-0.175 µm, depending on excitation conditions. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073 V.
INTRODUCTION Devices capable of functioning in harsh environments are desirable. From this point of view ZnO, a wide band gap semiconductor (Eg=3.3 eV, T=300 K), is a very promising material due to e.g. large exciton binding energy (∼60 meV), radiation hardness, availability of bulk crystal and ease of ZnO film growth. 1 Although p-ZnO growth technology is still a hotly debated issue, development of ZnO heterojunction based devices is of interest nonetheless because such devices have advantages over the former owing to carrier and optical confinement, lower diffraction losses, and reduction of threshold current in injection devices. A number of reports are available on the anisotype ZnO heterostructures prepared by growing n-type ZnO films on different p-type materials.2 N-ZnO/p-(Al)GaN heterostructures have been of particular interest because ZnO and GaN have relatively close lattice parameters and physical properties.3,4,5,6,7 A study of the properties of isotype n-ZnO/n-(Al)GaN type heterostructures is also important because, in particular, isotype heterostructures (depending on their band alignment) can also have strong diode-like rectifying behavior comparable to that of anisotype p-n heterojunctions. In addition, transparent highly conductive ZnO layers can form the basis for very good ohmic contacts to GaN. An n-ZnO/n-GaN being a part of the p-GaN/n-ZnO/n-GaN double heterostructure may be of interest for high efficiency optoelectronics devices. Therefore, investigation of the current transport mechanisms in
n-ZnO/n-GaN heterostructures is warranted. In this work we report on the electrical properties of n-ZnO/n-GaN h
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