Transparent Conductive Electrodes For Gan-Based Light Emitting Device
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Date
2016-02
Authors
Ali, Ahmad Hadi
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
Abstract
Electrical contacts which possess high electrical current conductivity and good optical transparency in visible wavelength are very important for the efficiency of optoelectronic devices. This study focuses on transparent conductive electrode (TCE) deposited on GaN-based templates for the application on InGaN light emitting diode (LED) structure. P-type and n-type GaN templates were used as depositing templates for optimization purposes of the TCE. Indium tin oxide (ITO) is used as a top contact layer since ITO offers low electrical current resistivity (~10-4 – 10-3 -cm), high optical transparency in visible wavelength (> 80 %), has high carrier concentration (~1021 cm-3) and good carrier mobility. In order to improve the electrical current resistivity of the ITO layer, thin metal under-layer was deposited between the top ITO layer and the GaN templates. For the p-GaN templates, Ni and Ag thin metal under-layer were deposited under the ITO top layer, whereas for the n-GaN templates, Ti and Al thin metal under-layer were used. The TCE samples were subjected to post-annealing process in order to improve the structural characteristics of the TCE which consequently will improve the electrical and optical characteristics of the TCE. From the optimization process, the best post-annealing condition for the TCE is at temperature of 600C under N2 gas flow of 2 L/min with annealing period of 15 min. The electrical resistivity and optical transmittance at 470 nm of the Ni/Ag/ITO (5 nm / 5 nm / 80 nm) TCE layer on p-GaN after post-deposition annealing were measured as 3.65 10-5 -cm and 97 %, respectively. For the Ti/Al/ITO (5 nm / 5 nm / 80 nm) TCE layer deposited on n-GaN, the electrical resistivity and optical transmittance at 470 nm were measured as 8.61 10-5 -cm and 95 %, respectively. From calculation, the figure of merit (FOM) of the post-annealed Ni/Ag/ITO and Ti/Al/ITO TCE is 9.51 10-2 -1 and 5.91 10-2 -1, respectively, which is better than the as-deposited TCE samples. Besides the TCE optimization, multi quantum-well (MQW) InGaN LED grown by metal organic chemical vapor deposition (MOCVD) on Si (111) substrate was characterized based on its structural and optical properties. It is found that the threading dislocation densities (TDD) is reduced with the insertion of n-Al0.06Ga0.94N/n-GaN strain-layer superlattices (SLS) on stack of n-AlN/n-GaN intermediate layer and producing crack-free GaN epitaxial layers on Si. From photoluminescence results, the emitted photon energy from the LED with SLS under-layer is 2.97 eV corresponding to wavelength of 417 nm. At 20 mA current, the turn-on voltage of the InGaN LED with Ni/Ag/ITO TCE on top of the p-GaN layer and Al contact layer at the bottom of the n-Si substrate is 7.4 V. The high forward voltage is mainly due to the resistance from the Si substrate and AlN-based buffer layer under the active region.
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Keywords
Transparent conductive electrodes