We have demonstrated the enhancement of the output power of ultraviolet GaN-based light-emitting diodes ͑LEDs͒ by using one-dimensionally nanopatterned Cu-doped indium oxide͑CIO͒/indium tin oxide ͑ITO͒ p-type contact layers. The one-dimensional ͑1D͒ nanopatterns ͑250 nm in width and 100 nm in depth͒ are defined using a TiO 2 1D nanomask fabricated by means of a surface relief grating technique. When fabricated with the nanopatterned p-contact layers, the output power of LEDs is improved by 40 and 63% at 20 mA as compared to those fabricated with the unpatterned CIO/ITO and conventional Ni/ Au contacts, respectively.

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We have investigated high-quality Sb-doped SnO 2 /Ag ohmic contacts to p-GaN for use in flip-chip light emitting diodes ͑LEDs͒. The Sb-doped SnO 2 /Ag contacts produce specific contact resistances of ϳ10 Ϫ4 ⍀ cm 2 upon annealing at 430 and 530°C for 1 min in air. It is shown that InGaN blue LEDs fabricated with the Sb-doped SnO 2 /Ag contacts give a forward-bias voltage of 3.18 V at 20 mA, while LEDs with Ag contacts show 3.36 V. It is further shown that the LEDs made with the Sb-doped SnO 2 /Ag contact layers show higher light output power compared with the LEDs with the Ag contacts.Group III-nitride semiconductors have been extensively investigated because of their technological importance for the fabrication of light emitting diodes ͑LEDs͒, laser diodes, and photo-detectors. [1][2][3] In particular, GaN-based LEDs are important for practical applications such as solid-state lighting and full-color displays. 4,5 Commercially available GaN-based LEDs, however, have some limits to the application for solid-state lighting. For commercial top-emitting GaN-based LEDs grown on sapphire substrates, light is extracted through a semi-transparent current spreading layer on the top of p-GaN, which limits the light extraction efficiency. 4 To overcome this problem, flip-chip LEDs have recently been introduced. 6,7 In this configuration, LEDs are fabricated with highly reflective metals and light is extracted through the transparent sapphire substrate rather than a partially absorbing current spreading layer. Ag and Al are the most commonly used reflectors for flip-chip LEDs, although Ag was shown to be better than Al due to its higher reflectivity and better ohmic behavior. 4,8 However, Ag suffers from poor adhesion to p-GaN and agglomeration upon annealing, which degrade its electrical and thermal behaviors. 4,8,9 Thus, recently, multilayer contacts consisting of a thin oxidized Ni/Au bilayer overcoated with a thick Al or Ag layer were investigated to minimize the drawbacks of single Ag contacts. 8 However, because Au itself absorbs and scatters the extracted light, and contributes to the lowering of light output, Au-free contact layers have been investigated. For example, Gessmann et al., 10 investigating an ITO/Ag contact to p-GaN, showed that LEDs with the new scheme produces significantly improved output power compared to that with the Ni/Au contact. Therefore, in this work, we have also investigated a transparent conducting oxide ͑Sb-doped SnO 2 )-based contact to p-GaN. The Sb-doped SnO 2 layer was employed as a first layer because of its high transmittance of about 95% at 460 nm. The Sb-doped SnO 2 /Ag contacts produce a good ohmic contact with specific contact resistances of ϳ10 Ϫ4 ⍀ cm 2 when annealed at 430 and 530°C in air ambient. The electrical and optical performances of LEDs fabricated with the annealed Sbdoped SnO 2 /Ag contacts are characterized and compared to those of LEDs with the annealed Ag contact.Metallorganic chemical vapor deposition grown 1.0 m thick p-GaN layers (5 ϫ 10 17 cm Ϫ3 ͒ were ultrason...

A Cu-doped In 2 O 3 ͑CIO͒ interlayer was introduced to enhance the electrical and optical properties of Sb-doped SnO 2 ͑ATO͒ p-type electrodes grown by pulsed laser deposition. CIO ͑2.5 nm͒/ATO ͑250 nm͒ contacts become ohmic with specific contact resistance of 2.1 ϫ 10 −3 ⍀ cm 2 and give transmittance of ϳ81% at 400 nm, when annealed at 630°C for 1 min in air. Near-UV ͑400 nm͒ GaN-based light-emitting diodes ͑LEDs͒ fabricated with the CIO/ATO p-electrodes give forward-bias voltage of 3.91 V at injection current of 20 mA and show much higher output power compared to LEDs with conventional Ni/Au p-electrodes.High-brightness GaN-based light-emitting diodes ͑LEDs͒ are of potential importance for solid-state lighting application. For this purpose, the improvement of the luminous efficiency of LEDs is a key factor to be achieved. 1 To increase the luminous efficiency, lowresistance and highly transparent ohmic contacts to p-type GaN should be developed. 1-3 Transparent conducting oxides ͑TCOs͒ have been regarded as potential p-type electrodes, because of their good transparency and moderate resistance. 4,5 Among various TCOs, indium tin oxide ͑ITO͒ is known to be a good p-contact candidate ͑or a p-type current spreading layer͒ for GaN-based LEDs because it has good conductivity and high-transmittance. Indeed, it was shown that ITO-based contacts could serve as promising p-type electrodes for high-performance GaN-based LEDs. 2,6,7 However, ITO has some drawbacks, such as thermal instability and the exhaustion of indium. 4,[8][9][10] Impurity ͑Sb or F͒-doped tin oxide ͑SnO 2 ͒ is an n-type semiconductor having a wide bandgap of 3.6 eV and it has been considered as promising TCOs, 11 which may replace ITO, because of its high thermal and chemical stability. Thus, Sb-or F-doped SnO 2 was used in photovoltaic cells and transparent electrodes. 12 Our group 4 also investigated the electrical and optical properties of pulsed-laserdeposited ͑PLD͒ Sb-doped SnO 2 films ͑ATO͒ and showed that their electrical and optical properties sensitively depend on the oxygen partial pressure. LEDs fabricated with the ATO contacts produced reasonably good performance. In this work, we have attempted to further improve the electrical and optical properties of ATO-based p-type contacts by introducing Cu-doped indium oxide ͑CIO͒ thin layers. It was shown that the CIO/ATO p-contacts give good electrical behaviors when annealed at 630°C in air. Near-UV ͑400 nm͒ LEDs with the CIO/ATO p-contacts produce much better output power compared to those with conventional Ni/Au contacts.1.0 m thick Mg-doped GaN layers ͑with a carrier concentration of 4 ϫ 10 17 cm −3 ͒ were grown using a metallorganic chemical vapor deposition system. The samples were then ultrasonically degreased using trichloroethylene, acetone, methanol, and deionized ͑DI͒ water for 5 min in each step, followed by N 2 blowing. Prior to photolithography, the samples were treated with a buffered oxide etch ͑BOE͒ solution for 20 min and rinsed in DI water. 13 Circular transfer length method ͑C...