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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-currentvoltage (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.
Reference
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• 2) Tsunemasa Taguchi, Yamaguchi Univeristy ,Spie’s oemagazine 13
(2003).
• 3) T. Mukai, T. Yamada and S. Nakamura, Jpn. J.Appl. Phys. 37, L1358
(1998).
• 4) I. Kidoguchi, A. Lshibashi, G. Sugahara, Y.Ban, Appl. Phys. Lett.
76(2000) 3768
• 5) A.M. Rokowaki, P.Q. Miraglia, E.A. Prele, S.Eindeldt, R.F. Davis, J.
Cryst. Growth. 241