Low Resistance Optically Transparent Contacts to p-type GaN Using Oxidized Ni/Au and ITO for LED Application
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Low Resistance Optically Transparent Contacts to p–type GaN Using Oxidized Ni/Au and ITO for LED Application C. H. Lin, D. L. Hibbard, A. Au, H. P. Lee, Z. J. Dong1, F. J. Szalkowski1, J. Chen1 and C. Chen1 Department of Electrical & Computer Engineering, University of California, Irvine CA 92697 1 Alpha AXT, Monterey Park CA ABSTRACT We report on a high transparency low resistance contact to p-GaN composed of a thin oxidized Ni/Au bilayer overcoated with indium tin oxide (NiO/Au/ITO). The NiO/Au/ITO layer shows a specific contact resistivity, ρc, of 1.8 x 10-3 Ω-cm2 that is nearly ten times lower than conventional Ni/Au annealed under N2. Measurements on fully processed LEDs with a NiO/Au/ITO current spreading layer (CSL) show an operating voltage of around 4 V at 20 mA that is comparable to LEDs fabricated with a conventional Ni/Au CSL and a dramatic improvement over the previous ITO data. LED top surface light emission through the NiO/Au/ITO CSL is shown to be greater than that from LEDs with a conventional semi-transparent Ni/Au CSL. Taken together, these results demonstrate the feasibility of using NiO/Au/ITO as a CSL for high performance GaN LEDs. INTRODUCTION GaN has demonstrated its great potential for application in a variety of light emitting semiconductor devices operating in the green to ultraviolet wavelength region including light emitting diodes (LEDs) and laser diodes. However, due to its wide band gap and the low doping level of p-GaN, low resistance ohmic contact to p-GaN is difficult to achieve. In addition, due to high resistivity in the p-GaN cap layer itself, carriers flowing from the top contact are not uniformly distributed in the lateral direction. This current crowding under the p-contact yields a device of low efficiency with nonuniform light emission. The introduction of an optically transparent CSL allows more homogeneous current injection to the active layer and, thereby, more efficient generation and emission of photons across the device. Thus, fundamental improvement in the CSL can significantly enhance the performance of GaN-based LEDs. It is important to note that an effective CSL must therefore concurrently possess three basic characteristics: low sheet resistance, low contact resistance to p-GaN, and high optical transparency at the wavelength of application. Conventional GaN-based LEDs currently employ as a CSL a thin, semitransparent Ni/Au bilayer. Around 40 Å of each metal are typically deposited and subsequently annealed under N2. These contacts exhibit ρc values of around 1-2 x 10-2 Ωcm2 [1]. Ho et al measured the level of optical transmission through a Ni/Au (100 Å /50 Å) bilayer on BK-7 glass, annealed at 500ºC under N2, as only around 30-35% between 400-500 nm [2]. While this amount of transparency may be enhanced by decreasing the G4.8.1
metal thicknesses, these layers may be too thin to accommodate the high lateral current densities anticipated for the next generation of larger area LEDs. Two groups have recently published promising results in the effort to develop imp
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