Novel thin-GaN LED structure adopted micro abraded surface to compare with conventional vertical LEDs in ultraviolet light
Chiang et al. Nanoscale Research Letters
Novel thin-GaN LED structure adopted micro abraded surface to compare with conventional vertical LEDs in ultraviolet light
Yen Chih Chiang 2
Chien Chung Lin 1
Hao Chung Kuo 0
0 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University , 1001 University Road, Hsinchu 300 , Taiwan
1 Institute of Photonic System, National Chiao Tung University , No.301, Gaofa 3rd Rd., Guiren Dist., Tainan City 71150 , Taiwan
2 Institute of Lighting and Energy Photonics, National Chiao Tung University , No.301, Gaofa 3rd Rd., Guiren Dist., Tainan City 71150 , Taiwan
In this study, novel thin-GaN-based ultraviolet light-emitting diodes (NTG-LEDs) were fabricated using wafer bonding, laser lift-off, dry etching, textured surface, and interconnection techniques. Placing PN electrodes on the same side minimized the absorption caused by electrodes in conventional vertical injection light-emitting diodes (V-LEDs) and the current spreading was improved. The light output power (700 mA) of the NTG-LEDs was enhanced by 18.3% compared with that of the V-LEDs, and the external quantum efficiency (EQE) of the NTG-LEDs was also relatively enhanced by 20.0% compared with that of a reference device. When the current operations were 1,500 mA, the enhancements of the light output power and EQE were 27.4% and 27.2%, respectively. Additionally, the efficiency droop was improved by more than 15% at the same current level.
Gallium nitride; Light-emitting diode; Vertical injection; Ultraviolet; Textured surface
A wide range of applications use ultraviolet (UV) lamps
as a light source. These applications, such as chemical
ink curing, disease or virus inspection, and air/water
purification, traditionally adapt mercury-based lamps
that are not environmentally friendly. To replace these
mercury-based units, nitride-based UV light-emitting
diodes (UV-LEDs) have recently received considerable
attention because of their light-weight, high-efficiency,
and eco-friendly features [1-4]. However, currently, the
traditional nitride-based UV-LEDs cannot attain extremely
high efficiency. Several improvements, such as AlInGaN
barriers, high-temperature grown AlN buffers, pattern
sapphire substrates, and current blocking layers,
involving wafer epitaxy and layer designs have been
proposed [5-10]. Apart from wafer design and quality
problems, the sapphire-based UV-LEDs are influenced
by the poor thermal dissipation of substrates and low
light extraction efficiency [11-13]. Vertical injection
LEDs (V-LEDs) have been recently demonstrated as
one of the most promising technologies for achieving
superior brightness operation because of their excellent
thermal dissipation [14-20]. In addition to thermal
problems, to strengthen the light extraction efficiency,
the surfaces of the V-LEDs are abraded to enable extra
scattering capability [21-26]. Furthermore, the current
V-LED metal-contact design affords extra absorption
because of layouts and materials. In this study, a novel
layout design combined with innovative fabrication
processes facilitated placing p and n contact metals on the
same side of the main LED lighting area; this effectively
eliminated the aforementioned electrode absorption loss
and maintained the advantages of vertical bonding
architecture. In the following sections, novel thin-GaN-based
ultraviolet light-emitting diodes (NTG-LEDs) that include
such designs and were created through advanced processing
techniques (such as wafer bonding, laser lift-off (LLO), dry
etching, textured surface, and interconnection processes)
In this study, LED wafers were produced by depositing
low-pressure metal-organic chemical vapor (LP-MOCVD)
onto c-face (0001) 2-in.-diameter sapphire substrates. The
Figure 1 Schematic diagrams and scanning electron microscope images of devices. (a, b) Both configurations and (c, d) SEM observations
include appearances and micro abraded surface morphologies (insets).
Figure 2 Process flows of NTG-LEDs. (a) Defined the n-contact area, (b) n-area exposed from insulating layer (c) transferred the structure to
silicon (d) abraded n-surface and deposited the p-electrode.
LED structure comprised a 20-nm-thick GaN nucleation
layer, 0.5-m-thick undoped GaN layer, 2.0-m-thick
Si-doped n-type AlGaN cladding layer, unintentionally
doped active region of 365-nm emitting wavelengths with
six periods of InGaN-AlGaN multiple quantum wells
(MQWs), 0.2-m-thick Mg-doped p-type GaN cladding
layer, and Si-doped n-InGaN-GaN short period
Figure 1 shows a schematic diagram of conventional
V-LEDs and NTG-LEDs. The fabrication processes of the
NTG-LEDs were began to define the several n-contact
vias by using an inductively couple plasma (ICP) to etch
through the MQW to the n-GaN by an ICP etcher. After
removing the photo-resist, a highly reflective ohmic
contact layer of Ni (3 )/Ag (2,000 )/Ti (300 )/Pt (800 )
was deposited on the blank wafer with n-contact vias and
treated by 30-min thermal annealing at 430C. In our
design, the transparent conductive layer (TCL) which usually
consists of indium tin oxide (ITO) is eliminated due to the
light extraction concerns and flip-chip package layout.
The TCL is favorable in conventional lateral blue LED
fabrication, and it provides a highly transparent and
conductive window for the p-side contact. However, in
our devices, the less conductivity of the p-side contact was
flip-chip bonded on a metal layer; thus, the current
spreading was not an issue for the p-side conductivity.
Furthermore, the emission wavelength was demonstrated
in the UV region, where the TCL can absorb as high as
40% of the incident photons. Based on these two
assessments, the TCL configuration was not included in our
design. Subsequently, a metallic barrier layer of TiW
(1,200 )/Pt (500 ) was deposited and covered on the
previous metal layer to prevent the subsequent silver from
migrating to the PN junction, leading to the short
circuiting of the device. An insulating layer of Si3N4 (5,000 )
was deposited on the entire surface through lithography
and buffer oxide etchant (BOE) wet etching processes to
expose the n-contact layer. Next, a Cr (300 )/Pt (500 )/
Au (15,000 ) bonding layer was evaporated to cover the
gap and the entire area. The sample was then bonded onto
a Cr (300 )/Pt (500 )/Au (15,000 )-coated n-type
conducting Si substrate at 350C for 1 h to form the flip-chip
configuration illustrated in Figure 1b. The bonded samples
were subsequently subjected to the LLO process to
remove the sapphire substrate. A KrF excimer laser at a
wavelength of 248 nm and with a pulse width of 25 ns was
used to withdraw the sapphire substrate. The laser with
a beam size of 0.3 mm 0.3 mm was incident from the
polished backside of the sapphire substrate onto the
sapphire-GaN interface to decompose GaN into Ga and
N. After removing the sapphire substrate, the samples
were dipped into a HCl solution to eliminate the residual
Ga from the u-GaN. Details of the LLO process are
described in .
The u-GaN was then etched away to reveal the
n-AlGaN layer by using an ICP dry etcher; a square
mesa (1,150 m 1,150 m) was created using ICP for
current isolation purposes. To improve light output
power and further enhance the light extraction efficiency
of the NTG-LEDs, a micrometer-range random abraded
surface was formed on the n-AlGaN by using a KOH
solution at 80C for 120 s. Finally, the p-electrode contact
area was created through ICP etching, and the process
was completed by depositing a Ti (300 )/Al (1,500 )/
Ni (1,000 )/Au (10,000 ) metal electrode. Figure 1c,d
Figure 3 The 3-D schematic diagrams and cross-sectional SEM image
of detailed n-type via contact hole.
Figure 4 The detailed specification of the metal pattern in both samples.
illustrates the scanning electron microscope images of
the V-LEDs and NTG-LEDs. The insets of Figure 1c,d
illustrate that a micrometer abraded surface is created
on both samples emission area. Figure 2 shows a brief
summary of the process flow. The three-dimensional
schematic structure of NTG-LED with both views (top
and bottom) and cross-sectional SEM image of detailed
n-type via contact hole are illustrated in Figure 3.
Because of the considerable changes in the contact
layout design, the light-emitting areas of the V-LED and
NTG-LED must be compared. The metal grid width for
n-electrode of the V-LED is 7 m, and the diameter of
via for n-type electrode of the NTG-LED is 55 m.
The detailed specification of the metal pattern is shown in
Figure 4. According to these specifications, the percentage
of shadowing effect (Rshadow) is defined, and it can be
expressed as follows:
where Rshadow is the ratio of the shadowing effect,
Anon-emission is the dark area due to the contact metal,
and Aemission is the area of illumination.
The Rshadow of both devices were 3.3% (V-LEDs) and
6.3% (NTG-LEDs). The conventional V-LEDs exhibited
substantial n-type metal on the top surface, whereas the
n-contact layer of the NTG-LEDs was buried beneath
the active region. Therefore, no obvious electrode was
observed in the NTG-LEDs. As compared to V-LEDs,
Figure 5 Forward voltage and light-output power as a function of
current for both LEDs under pulse current injection. The inset shows
the EL spectrum of the two devices.
Improve ~20.0 % @700 mA
Improve ~27.2 % @1.5 A
NTG-LEDs only sacrificed a small corner of emission
area to create the p-type contact area. This feature can
reduced the packaging cost from requiring two gold
wires to one gold wire and lower the absorption from
fewer bonding pads and gold wires. In addition, the test
results indicated that the output power of the NTG
samples substantially improved. Although the NTG
samples had a higher shadowing factor than did the
traditional V-LED, the distributed n-contact considerably
facilitated the injection uniformity; thus, at the same
pumping current, the overall output power of the NTG
device was substantially stronger than that of the
Figure 7 Distributed light pattern demonstrated at different current injections and relation between position x and power density. (a) V-LEDs
and (b) NTG-LEDs.
Figure 8 The illustration of current spreading paths in different devices: (a) V-LED and (b) NTG-LED.
traditional V-LED. The optical and electrical
characteristics were measured at room temperature by using a
manual probing system featuring an integrating sphere
detector and Keithley 2600 (Keithley Instruments Inc.,
Cleveland, OH, USA). To prevent the thermal effect
during continuous DC current from causing a decrease
in light-output power, L-I-V characteristics were
measured under the pulse mode with a 2.5% duty cycle. The
light intensity distribution and cross-sectional intensity
profile were measured by Unices LED beam profile detective
system (Unice E-O Services Inc., Chungli, Taoyuan, Taiwan).
Firstly, both samples were operated at 1,500 mA to fix the
dark level/gain/threshold settings and then gradually reduce
the current to measure the field distribution at the lower
level of injection.
Results and discussion
Figure 5 (inset) illustrates the electroluminescence (EL)
spectra of both devices, indicating that the peak
wavelength of both devices was at 365 nm. The EL intensity
of the NTG-LEDs was higher than that of the reference
device. According to the L-I-V characteristics shown in
Figure 5, the I-V curves for both devices were almost
identical (approximately 3.25 V at 350 mA); this
similarity indicates that the fabrication processes did not
degrade the electrical properties of the devices. In addition,
the L-I characteristics indicate that the NTG-LEDs
demonstrated more favorable linear characteristics during
a high current injection than did the V-LEDs. When the
current was 700 mA, the L-I-V characteristics (Figure 5)
indicated that the measured intensity of the NTG-LEDs
and reference device were 298 and 252 mW,
respectively. When the current was continually increased to
1,500 mA, the power difference increased to 118 mW
(548 mW for NTG-LEDs and 430 mW for V-LEDs).
Although the shadowing area of the NTG-LEDs was
larger than that of the reference (6.3%:3.3%), the PN
electrode layout induced a more favorable current spreading
and led to an efficient output power. This condition
enabled the NTG-LEDs to be enhanced substantially
compared with the conventional V-LEDs [27-29].
As shown in Figure 6, the external quantum efficiency
(EQE) was calculated according to the L-I-V
characteristics. The EQE of the NTG-LEDs and reference device
at the same 700- and 1,500-mA current injections were
12.6% and 10.5%, and 10.3% and 8.1%, respectively. Direct
comparison was performed at the same injection levels,
and the results indicated that the EQEs improved by
20.0% and 27.2% at 700 mA and 1,500 mA, respectively.
One of the major features of the proposed design is the
improved current injection scheme. An efficient current
spreading can lead to uniform light output and prevent
the nonlinear crowding effect and eventual gain reduction.
Figure 7 shows the light intensity distribution of the
devices at various current levels. According to both
the color-contoured planar map (insets) and the
crosssectional intensity profile, the NTG-LEDs exhibited a
more favorable uniformity than did the V-LEDs. The basic
current paths are different between the two cases, as can
be seen in Figure 8. The case (a) (conventional V-LED) is
more like the classical top contact structure which follows
the current spreading formula :
where Ls is the characteristic spreading length, t is the
thickness of the n-layer in our case, nideal is the idealty
factor of the diode, and J0 is the current density at the
edge of the metal contact.
Table 1 Data of max. power, min. power, and uniformity in Figure 7
Max. power (W/cm2)
Min. power (W/cm2)
Max. power (W/cm2)
Min. power (W/cm2)
On the other hand, the arrangement of electrodes
in the case (b) (the NTG-LED) is very similar to the way
we model the current crowding, and the corresponding
spreading length (Ls) is:
Ls utu c ptp tn
where c, p, tp, n, and tn are the p-type specific contact
resistance, the resistivity and thickness of the p-type
layer, and the resistivity and thickness of the n-type layer,
Comparing Equations 2 and 3, one can immediately
observe the injection current dependence on Equation 2
brought by the term J0. As the injection current increases,
the Ls decreases. The reduction of Ls means the uneven
distribution of the current, and thus, the intensity of light
is getting worse. This phenomenon can be seen in the
aforementioned beam profiles. Meanwhile, in Equation 3,
the Ls is quite constant against the injection current, which
can also be observed. Numerical analysis of maximal and
minimal values across 0.7 mm of the center of the devices
indicated the fair comparison of intensity uniformity. As
shown in Table 1, the uniformity was calculated using the
equation (max min)/(max + min). The NTG-LEDs
exhibited a more favorable intensity uniformity than did the
V-LEDs, particularly in high current ranges. The arrayed
n-contact pattern improved the uniformity of the current
injection, thus improving the resulting output light.
In addition to light output and current spreading,
the efficiency droop of the devices was evaluated. The
results indicated that the peak efficiency of both devices
(V-LEDs and NTG-LEDs) were 13.0% and 15.0%,
respectively. The efficiency droop () can be expressed as:
where is the ratio of efficiency droop, peak is the maximum
efficiency, and exp. is the efficiency in different experimental
The calculated droop percentages for both devices
(NTG-LEDs and V-LEDs) at different current levels
(at 700 and 1,500 mA) were 16.2% and 18.8%, and 31.7%
and 37.7%, respectively. Therefore, the efficiency droop
of the NTG-LEDs was enhanced by 16.0% and 18.9%,
respectively, compared with that of the reference device.
A uniform current injection can be attributed to this
In summary, this paper presents a novel flip-chip
architecture for UV-LEDs, called NTG-LEDs. Compared with
the conventional flip-chip structure, the configuration of
the NTG-LEDs eliminated the electrode effect, increased
the light extraction, and improved current spreading.
According to the proposed device design, the
lightoutput power of NTG-LEDs was further enhanced by
18.3% and 27.4% at 700- and 1,500-mA current levels,
respectively. Furthermore, the EQE of the NTG-LEDs
was effectively improved by 20.0% and 27.2% when 700
and 1,500 mA of current were supplied, respectively.
Finally, the efficiency droop was improved by 16.0% and
18.9% at the same current level, respectively. Therefore,
the proposed NTG-LED is promising for use in the next
generation of UV-LEDs.
EL: electroluminescence; EQE: external quantum efficiency; ICP: inductively
couple plasma; LLO: laser lift-off; LP-MOCVD: low-pressure metal-organic
chemical vapor deposition; MQW: multiple quantum wells; NTG-LEDS: novel
thin-GaN-based ultraviolet light-emitting diodes; UV: ultraviolet; UV-LEDs: UV
light-emitting diodes; V-LED: vertical injection light-emitting diodes.
The authors declare that they have no competing interests.
YCC participated in the design of the study and fabricated and measured all
the samples, explained all the measured data, and contributed in the writing
of the manuscript. CCL participated in the revision of the manuscript and
discussion of the results. HCK participated in the discussion of all the results.
All authors read and approved the final manuscript.
This work was supported in part by the National Science Council of Taiwan under
grant numbers NSC101-2221-E-009-046-MY3, MOST103-2622-E-009-008-CC3,
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