XE-Series High Temperature Quartz Tank
Sapphire Etching & Beyond
The Accubath Xe was designed with Sapphire Etching in mind but we know there are other processes that will benefit
from the increased chemical reactivity that higher temperatures provide.
Processes that were previously thought to be too slow due to
temperature limitations may now be practical.
Designed to Take the Heat
The liquid in the Accubath Xe tank comes in
contact with nothing but high purity quartz. No Teflon connections,
sensors, or any other parts are used in the process area. Even the built
in condensing coils and automated lid are quartz. READ MORE
Fabrication of High-Power InGaN-Based Light-Emitting Diode Chips on Pyramidally Patterned Sapphire Substrate
Yi-Ju Chen, Cheng-Huang Kuo 1 , Chun-Ju Tun 1 , Shih-Chieh Hsu 2 , Yuh-Jen Cheng 2 , and Cheng-Yi Liu*
Department of Chemical and Materials Engineering, National Central
University, No. 300, Jhongda Rd., Jhongli, Taoyuan 32001, Taiwan,
R.O.C. 1 Department of Optics and Photonics, National Central
University, No. 300, Jhongda Rd., Jhongli, Taoyuan 32001, Taiwan,
R.O.C. 2 Research Center for Applied Sciences, Academia Sinica, 128
Academia Road, Section 2, Nankang, Taipei 115, Taiwan, R.O.C.
In recent years, GaN-based light-emitting diode (LED) have been widely recognized to be the most promising alternative light source for general lighting.1,2) Using
the break through patterned sapphire substrate technique,
Nichia Corporation has achieved high-brightness GaN-based
LEDs with a record-high efficiency of 150 lm/W.3,4) The
enhancement in efficiency of GaN-based LEDs on patterned
sapphire substrates is generally attributed to the improvement in both light extraction efficiency and internal quantum
efficiency.5–9) The improvement in internal quantum eﬃciency is due to the reduction in threading dislocation
density by the achievement of lateral growth of a GaN
epilayer on the patterned sapphire substrate.10–13) The light
extraction efficiency is enhanced by the regular pattern
created on the sapphire substrate, which counteracts the
eﬀect of total internal reﬂection (TIR) at the GaN/sapphire
Numerous patterning features produced on patterned
sapphire substrates by either dry etching or wet etching sapphire, which include circular cavities, square cavities,
hemispherical bumps, and trenched stripes, have been
studied.5,14–17) However, no matter what etching process is
used to create the patterns, a hard-mask (SiO2 in most cases)
lithographic process is required on the ﬂat c-plane sapphire
wafer. In this study, we utilize a mask-free wet-etching
process to produce a so-called nature-patterned sapphire
substrate (n-pss), with a unique pyramidal pattern on the c-
plane sapphire surface. In addition, metal organic chemical
vapor deposition (MOCVD) is used to grow a GaN epitaxial
layer with an LED structure on the n-pss wafer. The optical
and electrical properties of horizontal LED chips fabricated
on the n-pss wafer are characterized in detail.
The mask-free wet-etching process used to produce the
pyramidal n-pss wafers is described in detail in the
following. Before the wet etching process, a SiO2 layer
was deposited by plasma-enhanced chemical vapor deposition (PECVD) on the back side of the c-plane sapphire wafer
to prevent the back side from being etched. The ﬂat back
side of the patterned sapphire wafer also enables good
contact with the bottom of the growth pot in the MOCVD
chamber, which ensures the high quality of the MOCVD
epitaxial process. After deposition of the SiO2 back side layer, the sapphire wafer was immersed in pure H2SO4 . The etching
temperature was controlled at a constant temperature of 320 'C for
periods of 15, 30, and 60 min. After etching, dilute HF solution was
used to remove the SiO2 back-side layer. Finally, the etched sapphire
wafer was successively cleaned by acetone, isopropyl alcohol (IPA), and
distilled water (DI water).
We found that H2SO4 did not
signiﬁcantly etch the sapphire wafer in the thickness direction.
Instead, it was found that a massive cubic etching product phase
completely covered the surface of the sapphire wafer, as shown in Fig.
1(a). X-ray diffraction (XRD) was used to identify the exact compound
phase of the etching-product. As shown in Fig. 1(b), only one strong
peak appears in the XRD diffraction pattern, which matches the standard
XRD diffraction pattern of the Al2(SO4)3-17H2O phase. Dwikusuma et
al. also reported similar ﬁndings, i.e., the formation of the Al2(SO4)3-17H2O phase on sapphire etched by H2SO4. 18) However, Dwikusuma et
al.’s XRD result showed all the diffraction peaks of the Al2(SO4)3-17H2O phase but in the present study, the Al2(SO4)3-17H2O etching
product exhibits an orientation-preferred XRD diffraction pattern.
Faceted pyramids were observed on the n-pss wafers after removal of the
etching product in dilute HCl solution. Figure 1(c) shows an enlarged
scanning electron microscope (SEM) image of the facet pyramids on the
n-pss wafer surface. We found the size of the pyramids on the n-pss
wafer to be reasonably uniform, the height and width of the pyramids
being about 0.2 and 1.5 mm, respectively. In addition, a ﬂat c-plane
sapphire surface appears between the pyramids. This ﬂat c-plane
sapphire surface is of importance since it can provide suitable
nucleation sites for the initial growth of the buffer GaN epilayer on
the patterned sapphire substrate. The average coverage percentage of
the pyramids on the etched sapphire wafer is about 44%. Note that
patterns typically cover about 50 to 70% of patterned sapphire
substrates.5,15,17) In other words, the percentage coverage of the
currently studied pyramidal pattern on the etched sapphire wafer is
less than the typical coverage.
dihedral angle at the bottom of the pyramid was estimated from the FIB
cross-sectional image to be about 39'. The dihedral angle of the side
planes with respect to the c-planes at the top of the pyramids was as
low as 10'. Thus, we believe that the side planes of the pyramids were
not common sapphire planes, such as r, m, or a planes, but were
composed of many high-index planes.
Fig. 2. (a) Tilted SEM view of the pyramids; (b) FIB cross-sectional image of a pyramid.
Fig. 1. (a) Etching-product layer on the etched sapphire surface; (b) diffraction pattern of the Al2(SO4)3-17H2O phase; (c) enlarged SEM image of the facet pyramids on the n-pss wafer surface.
Energy-dispersive spectrometry (EDS)
analysis of the pyramids produce no signal indicative of S atoms; Al
and O are the only two elements that can be detected (the atomic ratio
of Al to O is about 2:3). Therefore, we believe that the
etching-product phase covering the sapphire surface was completely removed by the HCl solution. Glancing XRD was also performed
on the pyramids on the n-pss surface. The glancing XRD diffraction
pattern further conﬁrms that the phase of the pyramids on the sapphire
wafer was the pure sapphire phase (not the etching-product).
2(a) shows a tilted SEM view of the pyramids on the n-pss wafer. We can
clearly see that the four side planes of the pyramid are not ﬂat. The
curve of the side planes of the pyramids, reminiscent of an upper
convex meniscus, was produced in response to the ﬂat c-plane surface.
Figure 2(b) shows a focused ion beam (FIB) cross-sectional image of a
single pyramid on the n-pss wafer surface. The dihedral angle of the
side planes of the pyramid (relative to the c- plane sapphire)
decreases with the height of the pyramid.
This novel pyramidal pattern on
the n-pss wafer was considered to have similar functions to other
patterns reported to be formed on sapphires, which allow a marked
improvement in external quantum efficiency.5–9) This is why it is of
interest to grow epitaxial GaN/InGaN LED structures on pyramidally
patterned n-pss wafers by MOCVD. The LED epitaxial structure includes a
1.8-mm-thick undoped GaN layer and a 2.5-mm-thick Si-doped n-type GaN
cladding layer, an active region emitting light with a 450 nm
wavelength with six periods of InGaN/GaN multiple quantum wells (MQWs),
and a 0.3-mm-thick Mg-doped p- type GaN cladding layer. Figure 3 shows
the results of PL measurement of the GaN epilayer grown on the n-pss
wafer and the standard c-plane sapphire wafer. PL measurement of the
GaN epilayer on the n-pss wafer indicates a higher intensity and
narrower full width at half maximum (FWHM) than those of the GaN
epilayer grown on the standard sapphire wafer. This is indicative of
the higher axial quality of the GaN epilayer grown on the n-pss wafer
(i.e., lower defect level) than that of the GaN epilayer grown on the
standard c-plane sapphire wafer.
GaN LED epilayers grown on the
n-pss wafer and the standard sapphire wafer were fabricated into
horizontal LED chips with dimensions of 1 x 1 mm2 . The luminous–input
current–voltage (L–I–V) curves of LED chips on the n-pss
wafer and the standard sapphire wafer are plotted in Fig. 4. When the
input current is 350 mA, the turn-on voltages of both LED chips are
similar (about 3.6 V). The leakage currents for the LED chips on the
standard sapphire wafer and n-pss sapphire wafer are -1:7 x 10-7 and -9:5 x 10-7 at -5 V, respectively. The above results indicate that the
electrical performance of LED chips fabricated on the n-pss and
standard sapphire wafers is very similar. From the L–I curve (measured
by an integral sphere apparatus), it was revealed that the LED chips on
the n-pss wafer have a higher light output power than those on the
standard sapphire wafer. At an input current of 350 mA, the average
light output power of the LED chips on the n-pss wafer is 37% larger than that of
the LED chips on the standard sapphire wafer.
Fig. 3. (Color online) PL measurements of the GaN epilayer grown on the n-pss wafer and the standard c-plane sapphire wafer.
Fig. 4. (Color online) L–I–V curves of LED chips on the n-pss wafer and the standard sapphire wafer.
In conclusion, a pyramidal pattern was created on a c- plane sapphire substrate by a
mask-free etching process. The PL results show that a GaN LED epilayer
can be success- fully grown on the pyramidal sapphire surface, which
has higher crystalline quality than a GaN epilayer grown on a standard
ﬂat sapphire. For an input current of 350 mA, the average light output
power of LED chips on the pyramidally patterned sapphire substrate is
37% larger than that of LED chips on a standard c-plane sapphire
substrate. We believe the pyramidal structure on the sapphire to be the
key to greatly enhancing the light extraction efficiency and external
quantum efficiency of LED chips.
Acknowledgements The authors would like to thank all the members of the NCU LED research group and the NCU OEM laboratory. We would also like to thank Nai-Wei Hsu for the PL measurements. We also gratefully acknowledge the ﬁnancial support received from the NSC (Taiwan).
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Permission to reproduce the above publication, on the Imtec site has been granted by: Professor Cheng-Yi , Department of Chemical and Materials Engineering