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Fabrication of Metal-Deposited Indium Tin Oxides: Its Applications to 385 nm Light-Emitting Diodes Min Ju Kim and Tae Geun Kim* The School of Electrical Engineering, Korea University, Seoul 136-701, Korea ABSTRACT: We report performance improvements in nearultraviolet (NUV) light-emitting diodes (LEDs) using various metal-doped indium tin oxide (ITO/metals). Metals with an orbital energy gap greater than that of an In atom (e.g., Ti, Ga, Ge, and Al) are deposited on ITO, and subsequent annealing is performed to improve optical transmittance of ITO due to effective bandgap increase via the linear combination of atomic orbitals, as well as electrical conductivity; thus, current spreading via metal-doping effect at the surface of ITO. As a result, the ITO/metals (annealed at 550 °C, 1 min) exhibit 90.5−94.7% transmittance at 385 nm and a specific contact resistance of 2.1−3.0 × 10−3 Ω cm2, whereas the reference ITOs exhibit 76.5−89.5% and 3.2−4.5 × 10−3 Ω cm2, respectively. Compared to NUV LEDs using conventional ITO (60 nm), the InGaN/AlGaInN NUV LED using ITO (110 nm)/metal (3 nm) on average exhibits a 70% increase in light output power at 100 mA and a 2% decrease in forward voltage at 20 mA, with more uniform and brighter emission images. We also identified the origin for the improvement by analyzing the surface of ITO/metals using X-ray photoelectron spectroscopy and Auger electron spectroscopy. This approach could offer a simple, effective way to enhance the overall efficiency of conventional NUV LEDs using ITO. KEYWORDS: light-emitting diodes, ultraviolet, indium−tin-oxide, metal-deposition, transmittance atoms’ diffusion into the ITO top layer;11 this can improve the optical transmittance of ITO via an effective bandgap increase according to the linear combination of atomic orbitals (LCAO),12 in addition to the improvement in the electrical conductivity and current spreading via doping effect at the surface of ITO. This approach should be distinguished from other previous reports on metal-doped ITO.13−15 In this study, we investigate the optical and electrical properties of ITO/ metal as a TCE for NUV LEDs in comparison with those fabricated using ITO. We also fabricate InGaN/AlGaInN NUV LEDs with ITO/metal and conventional ITOs for comparison on the device level.
1. INTRODUCTION Demand for transparent conductive electrodes (TCEs) has been growing, particularly in the area of optoelectronic devices such as light-emitting devices. To be effective across a wide range of applications, TCE films need to have low contact resistance on the semiconductor and high optical transmittance in both the visible and ultraviolet (UV) spectral regions.1−4 However, the external quantum efficiency (EQE) of near-UV (NUV) LEDs is still low (under 45%).5 High UV light absorption and low current spreading6 in the TCE during LED operation are critical reasons for low EQE. Metal-doped ZnO,7 carbon-based nanomaterials,8 oxide-metal-oxide films,9 and Ag nanowires10 have been proposed for improving the electrical and optical properties in the UV spectral region. However, these materials and structures exhibit some limitations owing to a trade-off between electrical conductivity and optical transmittance. For these reasons, some companies have used thin ITO (
