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High efficiency white LEDs with single-crystal ZnO current spreading layers deposited by aqueous solution epitaxy

Arthur H. Reading, Jacob J. Richardson, Chih-Chien Pan, Shuji Nakamura, and Steven P. DenBaars

Open Access Open Access

Abstract

Heteroepitaxial ZnO transparent current spreading layers with low sheet resistances were deposited on GaN-based light emitting diodes using aqueous solution phase epitaxy at temperatures below 90°C. The performance of the LEDs was analyzed and compared to identical devices using electron-beam evaporated indium tin oxide transparent current spreading layers. White LEDs with ZnO layers provided high luminous efficacy–157 lm/W at 0.5A/cm2, and 84.8 lm/W at 35A/cm2, 24% and 50% higher, respectively, than devices with ITO layers. The improvement appears to be due to the enhanced current spreading and low optical absorption provided by the ZnO.

©2011 Optical Society of America

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Figures (4)
Fig. 1
Fig. 1 (a) Schematic of a TLM structure used to characterize the ZnO/p-GaN contact resistance. (b) Schematic of the mesa structure used to create the LED devices. (c) Representative I-V behavior of TLM structure in (a) of a ZnO film with a 500°C seed layer anneal.

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Fig. 2
Fig. 2 (a) 2θ-scan showing ZnO and GaN (002) reflections. (b) ω-scan of ZnO (0002) reflection, indicating crystal quality. (c) ϕ -scan around ZnO ( 10 1 ¯ 1 ) peak, and (d) ϕ-scan of GaN ( 10 1 ¯ 1 ) peak confirming heteroepitaxial relationship between GaN and ZnO.

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Fig. 3
Fig. 3 (a) Current-voltage curves of the LED devices show the ZnO and ITO voltages with different contact geometries. Scale schematics show mesa (l. gray), p-contact (d. gray) and n-contacts (black). (b) DC and pulsed (5% duty cycle) EQE performance of the 4 devices, and (c) DC and pulsed (5% duty cycle) WPE performance of the 4 devices.

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Fig. 4
Fig. 4 (a) Illuminated LED strips operating under 100mA current injection. (b) Color contrast intensity images (rotated) showing current spreading in 1mA increments. (c,d) Intensity line scans along center of ZnO and ITO devices. The mesas of the high-aspect ratio LEDs were 150μm × 950μm.

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Tables (3)

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Table 1 Effect of different ZnO seed-layer annealing conditions. All anneals were done using a 20% O2-80% N2 ambient for 15min in a rapid thermal annealing (RTA) system. Testing was performed after second layer ZnO deposition.

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Table 2 Effect of post-annealing on ZnO sheet resistance (Rsh). All anneals were done in a 20% O2/80% N2 ambient for 15 minutes. Measurements were done on the same wafer after consecutively higher-temperature anneals.

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Table 3 Comparison of luminous efficacies for Type 3 white LEDs encapsulated in silicone dome and covered in outer layer of yellow YAG phosphor in silicone. Measurements were made under pulsed current injection with a 10% duty cycle.

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