ZnO Nanowire/p-GaN Heterojunction LEDs
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1018-EE12-08
ZnO Nanowire/p-GaN Heterojunction LEDs Xinyu Wang1, Jesse Cole1, Amir M. Dabiran2, and Heiko O. Jacobs1 1 Department of Electrical and Computer Engineering, University of Minnesota, 200 Union Street SE, Minneapolis, MN, 55455 2 SVT Associates, Inc., 7620 Executive Drive, Eden Prairie, MN, 55344 ABSTRACT This article reports forward and reverse biased emission in vertical ZnO nanowire/p-GaN heterojunction light emitting diodes (LEDs) grown out of solution on Mg-doped p-GaN films. The electroluminescence spectra under forward and reverse bias are distinctly different. Forward bias showed two peaks centered around 390 nm and 585 nm, while reverse bias showed a single peak at 510 nm. Analysis of the current-voltage characteristics and electroluminescence spectra is presented to determine the transport mechanism and location of electron hole recombination. Reverse bias transport and luminescence are attributed to hot-hole injection from the ZnO nanowires into the GaN film through tunneling breakdown. Forward bias transport and luminescence are attributed to hole injection from the GaN into the ZnO and recombination at defect states inside the ZnO yielding distinct color variations between individual wires. Major resistive losses occurred in the GaN lateral thin film connecting to the vertical ZnO nanowires. INTRODUCTION ZnO is a promising material for exciton-based opto-electronic devices including light emitting (LED) and laser diodes due to its direct band gap of 3.37 eV at room temperature and a large exciton binding energy of 60 meV. Fabrication of LEDs based on ZnO homojunctions, however, has failed due to the lag in production of high quality p-doped ZnO. As a result most work has focused on heterojunction LEDs in particular, polycrystalline n-ZnO thin films grown on single crystal p-GaN substrates [1-4] because of the similar bandgap, small lattice constant mismatch of 1.9%, and identical Wurtzite crystal structure between both materials. While similar, there remains a band offset greater than 1.0 eV which limits injection efficiency at the heterojunction interface[5] to be lower than homojunction devices. More recently the ZnO polycrystalline thin films have been replaced by ZnO nanowires [6, 7] to take advantage of nanowire properties: high crystallinity and fewer grain boundaries when compared with polycrystalline thin films. The results are interesting: One report showed electroluminescence (EL) [6] under reverse bias while the other reported forward bias emission [7]. In this letter, we report on a ZnO nanowire/p-GaN heterojunction LEDs that contrary to previously reported results [6, 7] show EL emissions under both forward and reverse bias. Analysis of the photoluminescence and electroluminescence spectra together with the I-V characteristics suggests different injection mechanisms and locations of electron-hole recombination. Reverse bias results point towards tunneling breakdown where hot carrier injection and recombination predominately occurred in the GaN film. Forward bias emission is attr
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