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Abstract
We investigated the influence of free-standing GaN (FS-GaN) substrates on the performance of ultraviolet light-emitting-diodes (UV-LEDs) grown on top by atmospheric pressure metal-organic chemical vapor deposition. High-resolution double-crystal x-ray diffraction (HRDCXD) analysis demonstrated high-order satellite peaks and clear fringes between them for UV-LEDs grown on the FS-GaN substrate, from which the interface roughness was estimated. In addition, the full width at half maximum of the HRDCXD rocking curve in the (0002) and the (101¯2) reflections were reduced to below 90 arc sec. The Raman results indicated that the GaN-based epilayer of strain free was grown. Additionally, the effect of the FS-GaN substrate on the crystal quality of the UV-LEDs was examined in detail by transmission electron microscopy (TEM). The TEM characterizations revealed no defects and V-pits were found in the scanned area. Based on the results mentioned above, the light output power of UV-LEDs on the FS-GaN substrate can be enhanced drastically by 80% and 90% at 20 and 100 mA, respectively. Furthermore, an ultralow efficiency degradation of about 3% can be obtained for the UV-LEDs on the FS-GaN substrate at a high injection current. The use of an FS-GaN substrate is suggested to be effective for improving the emission efficiency and droop of UV-LEDs grown thereon.
Highlights
The 350 to 400 nm InGaN-based ultraviolet light-emitting diodes (UV-LEDs) have received enormous attention for practical applications, such as biosensors, high density storage, and short-distance optical fiber communications.[1,2,3] In addition, the white-light LEDs can be realized by using InGaN-based UV-LEDs chip excited the phosphor of red, green, and blue (RGB).[4,5,6] Compared with the case of white-light emission from blue LEDs chip coated with yellow phosphors, many researches pointed out that has many excellent properties, such as lower wavelength shift as increasing injection current, high luminous efficiency, white point determined by phosphor only, better color rendering and stable light color.[7]
The epitaxial structure of the UV-LEDs investigated is depicted in Fig. 1, comprising a 2.5-μm-thick n-GaN epilayer grown at 1150°C, a ten-period InGaN/ InAlGaN multiple quantum wells (MQWs) active layer grown at 830°C, 15-nm-thick Mg-doped Al0.3Ga0.7N and 10nm-thick Mg-doped Al0.1Ga0.9N electron-blocking layers were grown at 1030°C, a 55-nmthick p-GaN layer was grown at 1030°C, and a 5-nm-thick p-InGaN contact layer was grown at 830°C
This result shows that the defect density of the UV-LEDs can be reduced dramatically by introducing free-standing GaN (FS-GaN) as a substrate for epitaxy of InGaN/InAlGaN MQWs UV-LEDs
Summary
The 350 to 400 nm InGaN-based ultraviolet light-emitting diodes (UV-LEDs) have received enormous attention for practical applications, such as biosensors, high density storage, and short-distance optical fiber communications.[1,2,3] In addition, the white-light LEDs can be realized by using InGaN-based UV-LEDs chip excited the phosphor of red, green, and blue (RGB).[4,5,6] Compared with the case of white-light emission from blue LEDs chip coated with yellow phosphors, many researches pointed out that has many excellent properties, such as lower wavelength shift as increasing injection current, high luminous efficiency, white point determined by phosphor only, better color rendering and stable light color.[7]. In order to overcome the above mentioned problem, our group has replaced the AlGaN barrier with an InAlGaN quaternary material in an InGaN/AlGaN multiple quantum wells (MQWs) system, which demonstrated an improvement of 55% in output power and a droop of 13% in efficiency.[11] it is believed that the optical and electrical properties of InGaN-based UV-LEDs were very sensitive to the defect density in the epilayer, suggesting a high crystalline quality epilayer could dramatically improve the device’s performance.[12,13] The recent availability of free-standing GaN (FS-GaN) substrates with a low defect density (about 106∕cm2) could perhaps facilitate this development, which can enhance the light output power, the IQE, and the EQE of InGaN-based UV-LEDs.[14,15,16]
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