Improvement in light-output efficiency of Near-Ultraviolet InGaN–GaN LEDs Fabricated on Stripe Patterned Sapphire Substrate 指導教授 : 管鴻 教授 報告學生 : 林耀祥 日 期： 97/10/20

Outline Introduction -The output power of GaN-based near ultraviolet stripe patterned sapphire substrate (PSS) LEDs was higher than that of the conventional LEDs. Expertment Result Conclusion

introduction due to the large differences in lattice constant and thermal expansion coefficient between the GaN epitaxial layers and the underneath sapphire substrate, the threading dislocation density are very high that can degrade the light emitting efficiency. In particular for UV LEDs, the emission efficiency is more sensitive to the dislocation density than the green and blue LEDs. Hence, in order to increase the output power of the UV-LED, the reduction of the dislocation density is desirable. There are many techniques such as epitaxial lateral overgrowth (ELOG) ﹑ air-bridges lateral epitaxial growth (ABLEG)and patterned sapphire substrates (PSS) to reduce the dislocation density. Among there, the PSS technique has attracted many attentions recently for its high production yield due to the single growth process without any interruption.

Experiment Epitaxial layers of near UV-LED device were grown on a c-face (0001) PSS in a low-pressure vertical MOCVD system. the widths of the ridges and grooves and the depth of grooves and the slope of sidewall were 3 um, 3um, 1.5um and 75°, respectively.

For comparison, the same epitaxial structure was also grown on a conventional sapphire substrate at the same time. Epitaxial layer consists of 25-nm-thick low temperature GaN buffer layer,500-nm-thick low temperature GaN defect suppression layer, 4-um-thick n-GaN: Si, n-type AlGaN/GaN: Si superlattice, seven pairs of InGaN well layer and GaN barrier layer forming the multiple quantum well active layer, p-type AlGaN/GaN:Mg superlattice layer and 200-nm-thick p-GaN:Mg contact layer.

Result The cross-sectional SEM micrographs of stripe PSS after ICP dry etching and subsequent epitaxy of LED full structure aligned along the sapphire direction are presented in Fig. As the previous report, the groove aligned along this direction can be buried by the GaN layer completely.

The corresponding electroluminescence (EL) spectra with an about 409 nm emitting wavelength for both LEDs. From this figure, the I-Vcharacteristics are very similar for both structures and with respective 3.57 voltage and 3.58 voltage for stripe PSS and conventional LEDs at a driving current of 20 mA. As for the output power, an about 20% enhancement of stripe PSS LED with 6.5mW compared to that of conventional LED with 5.4mW at a driving current of 20 mA. Curves of power-current- voltage (L-I-V) of PSS and conventional LEDs. The insert is the room temperature EL spectra for both LED structures

Conclusion The additional improvement of light output efficiency could be attributed to the elimination of threading dislocations with better epitaxial quality. plots the reliability test results of stripe PSS and conventional LEDs under the stress condition of 55C° and 50 mA. The same trend of the declination of the output power might present the same order of the stripe PSS and conventional LEDs. In summary, the 20% improvement of output power is due to the better epitaxial quality by the reduction of threading dislocation, and that is a good agreement with the simulation result.

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