We report the InGaN/GaN multiple quantum well vertical light-emitting diodes (VLEDs) operating at λ ∼ 450 nm by the use of laser lift-off and copper electroplating processes. The thermal characteristics of fabricated VLEDs are measured and analyzed in terms of the junction temperature (T j ) using the forward voltage method, which allows us to estimate the thermal resistance (R th ). Between 298 and 378 K, the characteristic temperature is measured to be about 903 K at 350 mA. The far-field patterns of the VLED have a uniform and good near-Lambertian emission. The T j and R th values are also confirmed by the emission peak wavelength shift method. The use of electroplated copper with a high thermal conductivity instead of a sapphire substrate provides much better heat dissipation capability. For a 1 × 1 mm 2 VLED, the low T j value of 305.8 K is obtained with an output power of 191 mW at an injection current of 350 mA at 298 K, exhibiting R th = 7.98 K W −1 .
In this study, we have fabricated the high-power GaNbased vertical light-emitting diodes (VLEDs) by exploiting a thermally stable nano-scaled Ni-Ag-Ni-Au mirror. After being treated at 600 C for 1 min in air ambient, the nano-scaled Ni-Ag-Ni-Au (5/2000/1000/2000 Å) mirror shows the specific contact resistance of cm and its reflectivity has increased from 89.5% to 93.0%. The increment in the reflectivity is due to the diffusion of Ni contact layer into Ag layer. Moreover, the reflectivity of the mirror has hardly deteriorated even after the wafer was thermally bonded with graphite substrate at 320 C for 5 min, meaning that the proposed mirror is thermally reliable. The light output power of the nano-scaled VLED (Chip size: 1 1 mm ) with thermal treatment at 600 C for 1 min is enhanced by 8% as compared to one without thermal treatment at an injection current of 350 mA. To our knowledge, our nano-scaled mirror has the highest reflectivity among other Ag-based multilayered reflectors, and thus it is a promising candidate for high-performance VLED.
Index Terms-GaN, light-emitting diodes (LEDs), light output power, thermally stable mirror, wafer bonding.
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