Vol. 37, No. 10
Journal of Semiconductors
October 2016
Increased effective reflection and transmission at the GaN-sapphire interface of
LEDs grown on patterned sapphire substrates
Wu Dongxue(吴冬雪)1; 2; 3 , Ma Ping(马平)1; 2; 3; Ž , Liu Boting(刘波亭)1; 2; 3 ,
Zhang Shuo(张烁)1; 2; 3 , Wang Junxi(王军喜)1; 2; 3 , and Li Jinmin(李晋闽)1; 2; 3
1 Research and Development Center for Solid State Lighting, Institute of Semiconductors, Chinese Academy of Sciences, Beijing
100083, China
2 State Key Laboratory of Solid State Lighting, Beijing 100083, China
3 Beijing Engineering Research Center for the 3rd Generation Semiconductor Materials and Application, Beijing 100083, China
Abstract: The effect of patterned sapphire substrate (PSS) on the top-surface (P-GaN-surface) and the bottomsurface (sapphire-surface) of the light output power (LOP) of GaN-based LEDs was investigated, in order to study
the changes in reflection and transmission of the GaN-sapphire interface. Experimental research and computer simulations were combined to reveal a great enhancement in LOP from either the top or bottom surface of GaN-based
LEDs, which are prepared on patterned sapphire substrates (PSS-LEDs). Furthermore, the results were compared
to those of the conventional LEDs prepared on the planar sapphire substrates (CSS-LEDs). A detailed theoretical analysis was also presented to further support the explanation for the increase in both the effective reflection
and transmission of PSS-GaN interface layers and to explain the causes of increased LOP values. Moreover, the
bottom-surface of the PSS-LED chip shows slightly increased light output performance when compared to that of
the top-surface. Therefore, the light extraction efficiency (LEE) can be further enhanced by integrating the method
of PSS and flip-chip structure design.
Key words: light output power; transmission; effective reflection; patterned sapphire substrate; light-emitting
diodes
DOI: 10.1088/1674-4926/37/10/104003
PACS: 74.25.Gz; 85.60.Jb
EEACC: 2520D; 2550
1. Introduction
Nowadays, in order to obtain high brightness in GaNbased LEDs, many methods have been widely adopted, including photonic crystal, surface texturing, PSS, flip-chip and
vertical structureŒ1 . The PSS method has been attracting considerable attention due to its high luminous efficiencyŒ2 . This
is because it is an effective and popular method to enhance
both internal quantum efficiency (IQE), by reducing threading dislocations (TDs) density and improving the crystalline
qualityŒ3 5 , and light extraction efficiency (LEE), simultaneouslyŒ6 . Due to the substantial difference in refractive index
between the epitaxial GaN film and air, the relatively small
total internal reflection angle leads to poor LEEŒ7 . Hence,
the patterns can serve as scattering centers for redirecting the
guided light into a randomized distribution of angles that will
enable multiple entries of photons into the escape coneŒ8; 9 .
This means that, in face-up LED structures, the light scattering effect from the patterns explains very well the enhanced
top-surface light output power (LOP) and LEEŒ10; 11 . Yet, limited research studies have been focused on the investigation of
the change in optical parameters, such as reflection coefficient
and transmission coefficient. The transmission coefficient is of
great importance as it is the key feature to decide whether it is
feasible to utilize the PSS technique in flip-chip structures, in
order to obtain higher luminous efficiency.
In this study, the results of both experimental research
and computer simulations reveal a great enhancement in LOP
from either the top or bottom surface of PSS-LEDs, compared
to CSS-LEDs. Following that, a theoretical analysis, which is
based on two fundamental theories, i.e. the Fresnel Equations
and geometrical optics, was performed in order to demonstrate
the increase in effective reflection and transmission at the GaNsapphire interface, which will explain the enhancement in LOP
from both the top and bottom surface of PSS-LEDs. Here, it is
necessary to clarify that the new term “effective reflection” is
different from the “total internal reflection”. The effectively
reflected light refers to the amount of light that is reflected
and then successfully escapes from the top-surface of the LED.
This excludes the total internal reflection light, which cannot
radiate out of the LED devices and is eventually absorbed by
the LEDŒ12 . As an overall comment, due to the existence of
PSS the total internal reflection light that is ultimately absorbed
decreases, however, the effective reflection light and the transmission light increase. The increased effective reflection light
and the transmission light that can radiate out from top-surface
and bottom-surface, respectively, contribute to the enhanced
top and bottom surface LOP. Furthermore, a mass of PSSLED chips is used for separately measuring the top-surface and
bottom-surface LOP. These experimental results testify that the
latter has slightly superior light luminous performance to the
* Project supported by the National High Technology Program of China (No. Y48A040000) and the National High Technology Program of
China (No. Y48A040000).
† Corresponding author. Email: [email protected]
Received 28 January 2016, revised manuscript received 15 April 2016
© 2016 Chinese Institute of Electronics
104003-1
J. Semicond. 2016, 37(10)
Wu Dongxue et al.
Figure 1. (Color online) Schematic illustration of Trace-Pro simulation models: (a) PSS-LED, (b) CSS-LED. 151 91 mm2 (96 96
DPI2 ).
Figure 2. Cross-sectional SEM images of patterns. 127 95 mm (256
256 DPI2 ).
Table 1. The absorbed power by various monitors.
Sample
Monitor
Power (W)
CSS
Top
0.16326
Bottom 1
0.18434
Bottom 2
0.27452
PSS
Top
0.4231
Bottom 1
0.4991
Bottom 2
0.7929
former. It is mainly because the critical angle of GaN surface
is smaller than the sapphire surfaceŒ13 . Therefore, this study
not only demonstrates how the PSS technology can improve
the LOP of flip-chip and face-up structure LEDs, compared to
CSS, but also that the LOP of flip-chip structure PSS-LEDs is
superior to the face-up structure.
2. Simulation and results
To better demonstrate the improvement of PSS on the top
and bottom LOP, the simulation based on Trace-Pro software is
performed. The LED optical simulation models are presented
in Figure 1. The models consist of a p-doped GaN layer with a
thickness of 0.2 m, an n-doped GaN layer with a thickness of
4 m, and the sapphire substrate with a thickness of 150 m.
The interface is a well-ordered array of circular cones with a
diameter of 2.58 m, a height of 1.53 m and a periodicity of
3 m. The patterns shape and parameters were set according to
the actual chips used in the aftermentioned experiment. Additionally, Figure 1(b) shows the CSS-LED which will serve as
a reference in order to reveal the effect of PSS on LOP or LEE.
The total flux power of MQWs was set at 2 W and the
absorption coefficient of the active layer was considered to be
1000 cm 1Œ14 . In Figure 1, the top monitor receives the optical
waves that can be extracted from the top-surface and the bottom
monitor 1 receives the optical waves that can be extracted from
the bottom-surface of the LED. Therefore, the beam flux power
absorbed by the top and bottom monitor 1 represent the LOP
from the top-surface and bottom-surface of LED, respectively.
As clearly shown in Table 1, when considering either the
top-surface or bottom-surface, the LOP of PSS-LED is higher
than that of the CSS-LED. Thus, the simulation results indicate
that both top and bottom LOP are increased due to the presence of PSS. Also, regardless of the use of either PSS or CSS
model, the bottom LOP is always slightly higher than the top
LOP. This also illustrates that the light extraction performance
of the bottom-surface is slightly superior to the top-surface and
that the flip-chip LED has many advantages over the face-up
LED devices for obtaining superior luminous efficiency. Also,
the bottom monitor 2 is embedded in the sapphire layer. It can
receive directly the optical waves that can be transmitted from
the GaN-sapphire interface. As can be seen in Table 1, the value
of power recorded by monitor 1 is smaller than monitor 2. This
is because the transmission light from the GaN-sapphire interface cannot be completely extracted from the sapphire surface
because of total internal reflection at the surface. However, the
value of power recorded by monitor 2 in the PSS model is larger
than in the CSS model. Thus, the result also demonstrates the
increased transmission light at PSS interface.
3. Experimental results and discussion
Regarding the experimental procedure, a mass of PSSLED and CSS-LED chips measured the LOP of the top-surface
and bottom-surface, by implementing the two different packaging methods. As presented in Figure 2, the PSS patterns are
circular cone shaped, with 2.58 m in diameter, 1.53 m in
height and 3 m in period, and featuring a hexagonal array.
Additionally, according to the cross-sectional SEM images, the
inclination angle of patterns was calculated:
1:53
0:7688; ˛ 50ı :
1:99
Regarding micro-PSS, this kind of pattern with the above feature has been widely used for configuration. Many studies
demonstrate that cone-shaped PSS show superior output performance over any other shape and that the geometric parameters used are more effective for LEE in terms of the geometry opticsŒ15 .
Figure 3 shows the top and bottom LOPs as a function of
the injection current, for the LEDs grown on PSS and CSS.
Under an injection current value of 20 mA, the bottom and top
LOP of the PSS-LEDs and CSS-LEDs were 11.28, 9.11, 6.22,
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sin ˛ D
J. Semicond. 2016, 37(10)
Wu Dongxue et al.
Figure 3. (Color online) The top and bottom light output power as a
function of the injection current for LEDs grown on PSS and CSS.
Figure 4. A schematic of the possible interactions of a beam of light
with a surface with different refractive indexes. 143 112 mm2 (96
96 DPI2 ).
and 5.14 mW, respectively. Compared to CSS, the bottom and
top LOP improvement of PSS-LEDs was 81.4% and 77.2%, respectively. In addition, compared to the top surface of the PSSLED and CSS-LED, the LOP improvement from the bottom
surface was 23.8% and 23.5%, respectively. The experimental results demonstrate the LOP values of PSS-LED compared
to CSS-LED and the superior light output performance of the
bottom-surface compared to the top-surface.
4. Discussion
When a beam of light penetrates an interface, the power is
divided into the one that causes reflection and transmission of
the light, as illustrated in Figure 4.
The amount of the two competing power values is decided
by the Fresnel EquationsŒ16 . The value of the reflection coefficient is a function of the refractive index, n, and the angle of
incidence, i , based on the Fresnel formula:
82
r
ˆ
ˆ
ˆ
n
cos
n
1
i
2 1
1 <6
6
RD
r
4
2ˆ
ˆ
:̂ n1 cos i C n2 1
2
r
6 n1 1
C6
4 r
n1 1
n1
n2
n1
n2
2 32
sin i 7
7
2 5
n1
sin i
n2
n1
n2
32 9
>
>
sin i
n2 cos i 7 >
=
7 :
5 >
2
>
;
sin i C n2 cos i >
2
(1)
With the transmission coefficient it is:
T D1
R;
(2)
where n1 is the index of refraction of substrate, and n2 is the
refractive index of the incident region. When the light beam
arrives at the interface of GaN (n1 D 2:4) and sapphire (n2 D
1:7), the R–i curve that is developed is presented in the Figure 5.
The R–i curve shows that, in the range of 0ı –30ı , the
reflection coefficient increases gradually with the increase of
the incident angle. Remarkably, the reflection coefficient curve
hikes from 0.02 to nearly 1, when the incident angle is about
Figure 5. Reflection coefficient as a function of the incident angle of
i : R–i curve. 252 163 mm2 (96 96 DPI2 ).
to reach the critical angle value of the total internal reflection,
about 45ı . Therefore, the larger the incident angle i , the larger
the reflection coefficient and the smaller the transmission coefficient.
According to Snell’s law, the critical angle of total internal reflection is approximately 25ı and 45ı for the GaN/air
and GaN/sapphire interface, respectivelyŒ17 . Hence, the lightemitting ray a1 that propagates upwards from the MQWs, with
a deflection angle over 25ı , is totally reflected by the topsurface. In the case of CSS-LEDs, these photons cannot still
radiate outside the top-surface since they are completely reflected by the planar sapphire substrates and are eventually
absorbed by the MQWs. However, due to the presence of the
cone-shaped patterns layer, part of the reflected upwards photons could be scattered and redirected into a randomized distribution of angles, which enables multiple entries of photons into
the top escape coneŒ18; 19 . Therefore, the analysis described
above explains the increase of the effective reflection at the
expense of the total internal reflection.
Also, when the ray propagates downwards, with the deflection angle over 25ı , it will reach the GaN/PSS interface
layer and the incidence angle will change from the original to 50ı –, as ray b1 shown in Figure 6. Because the angle is
over 25ı , the angle 50ı – is smaller than the angle , namely,
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J. Semicond. 2016, 37(10)
Wu Dongxue et al.
Figure 6. A schematic ray-tracing of light. 254 190 mm (96 96 DPI).
the incident angle decreases. According to the R i curve, the
transmission coefficient increases as incident angle decreases.
Thus, the realization of the increased transmission photons also
comes at the expense of absorption photons that were originally
completely reflected by the CSS-LEDs chip. Especially for the
light beams with a deflection angle over 45ı , the total internal reflection will occur not only on the GaN/air interface but
also on the GaN/CSS interfaceŒ20 without the use of patterning. Therefore, these photons are less likely to radiate outside
the surface and will be eventually absorbed. However, when
this part of the ray is incident on the inclined planes of the patterns, the incident angle is significantly reduced. Hence, the
part of the total internal reflection ray that was originally absorbed is transformed into large amounts of transmitted light.
[2]
[3]
[4]
[5]
[6]
5. Conclusions
The ray that arrives at the GaN/sapphire interface layer is
divided into three categories, i.e. the effective reflection light
that can radiate outside the top-surface, the total internal reflection light that will mainly be absorbed, and the transmission
light that can radiate outside the bottom-surface. Due to the existence of patterned sapphire substrates, the absorbed light decreases and is mainly transformed into transmission light and
light with more effective reflection. Thus, the enhancement of
top and bottom LOP is a fact, due to the increase in effective
reflection and transmission of GaN/sapphire interface. In addition, with either the PSS-LEDs or CSS-LEDs, the bottomsurface shows superior luminous performance, when compared
to the top-surface. Therefore, the LEE can be further enhanced
by integrating the method of patterned sapphire substrates and
flip-chip structure design.
[7]
[8]
[9]
[10]
[11]
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