GaN-based LEDs have generally made use of ITO transparent contacts as current-spreading layers for uniform current injection. However, the high raw material and processing costs of ITO layers have generated interest in potentially cheaper alternatives. In this work, zinc oxide transparent layers were fabricated by a low-cost, low-temperature aqueous epitaxial growth method at 90°C for use as transparent contacts to GaN LEDs on c-plane sapphire, and on semipolar bulk GaN substrates. Low-voltage operation was achieved for c-plane devices, with voltages below 3.8V for 1mm2 broad-area LEDs at a current density of 30A/cm 2. Blue-green LEDs on 202¯1¯-plane GaN also showed low voltage operation below 3.5V at 30A/cm2. Ohmic contact resistivity of 1:8 × 10−2Ωcm2 was measured for films on (202¯1) p-GaN templates. Ga-doped films had electrical conductivities as high as 660S/cm after annealing at 300°C. Optical characterization revealed optical absorption coefficients in the 50–200cm −1 range for visible light, allowing thick films with sheet resistances below 10Ω/□ to be grown while minimizing absorption of the emitted light. Accurate and reproducible etch-free patterning of the ZnO films was achieved using templated growths with SiOx hard masks. A roughening method is described which was found to increase peak LED efficiencies by 13% on c-plane patterned sapphire (PSS) substrates. In addition, ZnO films were successfully employed as laser-cladding layers for blue (202¯1) lasers, with a threshold current density of 8.8kA/cm 2.
|Advisor:||DenBaars, Steven P.|
|Commitee:||Dagli, Nadir, Nakamura, Shuji, Speck, James S.|
|School:||University of California, Santa Barbara|
|School Location:||United States -- California|
|Source:||DAI-B 75/08(E), Dissertation Abstracts International|
|Subjects:||Electrical engineering, Optics, Materials science|
|Keywords:||Gallium nitride (GaN), Lasers, Light-emitting diodes (LEDs), Solid-state lighting, Transparent conductive oxides (TCOs), Zinc oxide (ZnO)|
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