GaN-based light emitting diode with graphene/indium antimony oxide composite transparent electrode
Authors: Guo Wei-Ling , Deng Jie , Wang Jia-Lu , Wang Le , Tai Jian-Peng ,   Optoelectronics Technology Lab. Ministry of Education, Beijing University of Technology, Beijing 100124, China
Published: December 2020

In recent years, graphene has received wide attention due to its excellent optoelectronic properties and has been applied to transparent electrodes of light-emitting diodes to replace the scarce and expensive indium antimony oxide (ITO), which is a typical current spreading layer in lateral GaN LED. However, there are some problems in graphene transparent electrode, such as the mismatch between graphene work function and p-GaN work function, and difficult-to-form good Ohmic contact with p-GaN, resulting in poor current expansion and high voltage of devices. In this paper, a thin ITO layer is used as an insertion layer between a three-layer graphene transparent electrode and and p-GaN, thereby improving the Ohmic contact between them. And a three-layer graphene/ITO composite transparent electrode LED is prepared and also compared with the pristine three-layer graphene LED. The thickness of ITO is only 50 nm, which is much thinner than the thickness of ITO in conventional LED. The sheet resistance of the prepared three-layer pristine graphene transparent electrode is 252.6 <inline-formula><tex-math id="Z-20191129084901-1">\begin{document}$ \Omega/\Box $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20190983_Z-20191129084901-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20190983_Z-20191129084901-1.png"/></alternatives></inline-formula>, and the sheet resistance of the three-layer graphene/ITO composite transparent electrode is reduced to 70.1 <inline-formula><tex-math id="Z-20191129084814-2">\begin{document}$ \Omega/\Box $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20190983_Z-20191129084814-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20190983_Z-20191129084814-2.png"/></alternatives></inline-formula>. The specific contact resistance between the three-layer pristine graphene transparent electrode and the p-GaN layer is 1.92 × 10<sup>–2</sup> Ω·cm<sup>2</sup>, after the ITO being inserted, the specific resistance is reduced to 1.01 × 10<sup>–4</sup> Ω·cm<sup>2</sup>. Based on the three-layer graphene transparent electrode LED, the forward voltage is 4.84 V at an injection current of 20 mA, while the forward voltage of the three-layer graphene/ITO composite transparent electrode LED is reduced to 2.80 V; under small currents, the ideal factor of the three-layer graphene/ITO composite transparent electrode LED is less than that of the three-layer graphene transparent electrode LED. In addition, with the current increasing, the luminous intensity of the three-layer graphene/ITO composite transparent electrode LED increases, so does the radiant flux, which is because the addition of the ITO thin layer reduces the barrier height at the interface between the three layers of graphene and p-GaN, and the sheet resistance of the composite transparent electrode is also reduced, thereby improving the Ohmic contact between graphene and p-GaN. At the same time, the current spread is more uniform. The composite transparent electrode uses the much less ITO and obtains better optoelectronic performance, and thus providing a feasible solution for the LED transparent electrode.