GaN-based light-emitting diodes via the hierarchical structure: SiO
2nanorods/p-GaN microdomes
§ 6-1 Introduction
Nitride-based light emitting diodes (LEDs) have been utilized in various
applications, such as displays, cell phones, and general lighting.[1] The huge
commercial success is the main driving force of the intensive research efforts in this
field. As the internal quantum efficiencies (IQE) of InGaN-based quantum wells have
been well above 60% due to the advance of epitaxial growth technologies, the
external quantum efficiencies (EQE), being mostly below 50%, are still in need of
effective approaches for further improvement. One of the hurdles to be overcome is
the large difference in refractive index (n) between the air (n=1) and GaN (n~2.5) [2],
leading to a very limited critical angle, θc~23o, for light escape from the
air/semiconductor interface. Numerous studies on light extraction enhancement of
GaN LEDs had been reported.[3-5] Surface roughening is among the popular methods.
Roughening GaN surface via wet or dry etching technologies has recently been
reported.[6-8] It is demonstrated that the textured surfaces can result in enhanced light
trapping by the scattering effects on the roughened surface. Furthermore, IQE of
LEDs can also be improved by the texturing of p-GaN, which is due to the relaxation
of in-plane strain and consequent reduction of piezoelectric field.[9,10]
Moreover, a variety of nanostructures have been introduced to avoid the abrupt
change of refractive index at air/device interface to reduce the interface reflection,
leading to improved light extraction/harvesting efficiency. For examples, the
improved light-harvesting of solar cells can be realized using nanostructures as ZnO
nanorods [11-13], Si nanowires [14], SiO2nano-honeycombs [15], etc. On the other
hand, the light emission intensity of GaN LEDs can be enhanced via ZnO nanorods
[16-19], micropillars [20] and pyramid structures [21,22] due to the improved light
extraction. Among the materials for light-harvesting nanostructures, SiO2 is a
promising candidate due to its intermediate refractive index (n~1.55 at 460 nm) [23]
between air and GaN, the low absorption in visible region [24,25], and the wide
bandgap of 8.9 eV [25], which is well above the emission energies of nitride-based
LEDs. As Yik Khoon et al. reported [26], SiO2 nanostructures obtained by the
fabrication with photoresist-free and wafer-scale-uniformity capabilities are
particularly applicable to photon extraction from LEDs. Besides, it is recently found
that SiO2 nanostructures can be produced by a nature lithography technique utilizing
simple annealing process.[27] Therefore, SiO2nanorods arrays (NRAs) produced by a
self-assembled Ag nanoparticles and dry etching method hold a great promise for
solid-state lighting, and yet relevant studies has been scarcely reported. It is worth to
be mentioned that hierarchical structures integrating nano- and micro-scale roughness
usually present superior and unique functionalities, which are expected to bring
additional advantages beyond single structures.[28,29] For example, the hierarchical
ZnO NRA/silica microsphere structure is investigated to have large light scattering
efficiency due to the further light-scattering enhancement by ZnO NRAs.[30] The
reported nano-/micro- structure is a promising candidate for photovoltaic and
optoelectronic device applications.
In this study, we fabricated the hierarchical SiO2 NRA/p-GaN microdome
structure on GaN-based LEDs, in order to improve the light extraction efficiencies.
Bare p-GaN microdomes are regarded as scattering centers, which help enhance the
LED light output by 16.7 % at 20 mA. Upon the microdomes, the added SiO2NRAs
can further reduce the internal total reflections by mitigating the index change of
air/GaN interface. LEDs employing the hierarchical structure exhibit an excellent
improvement of output intensity by up to 36.8 %, which is attributed to the efficient
scattering effect on the surface texture and the improved impedance match between
air and GaN. Finite-difference time-domain (FDTD) simulations solving Maxwell’s
equations are performed to simulate the light propagation across the nanostructure
interfaces. The results show that the hierarchical surface structure leads to additional
photons escaping from the air/device interface, and thus results in the enhanced light
extraction. The proposed concept in this study should benefit the development of
nitride-based LEDs and many other optoelectronic devices.
§ 6-2 Experiment
The MQW blue LEDs were grown by metal–organic chemical vapor deposition
on c-plane sapphire substrates. The layer structures consist of fifteen periods of
intentionally undoped In0.15Ga0.85N (2.4 nm)/GaN (14 nm) MQWs, sandwiched by a
2.5-μm n-type (Si-doped) and a 0.2-μm p-type (Mg-doped) GaN layer. The free
carrier concentrations for n-type and p-type GaN are around 2×1018 cm-3 and 5×1017
cm-3, respectively. Ammonia (NH3), trimethylgallium (TMG, for n-GaN and p-GaN),
triethylgallium (TEG, for MQWs), and trimethylindium (TMI) were used as the
precursors. Surface morphologies of p-GaN were controlled by TMGa flows and
substrate temperatures. For flat p-GaN, TMGa flow rate was 45 μmol/min and
substrate temperature was in the range of 950-1100 °C; for p-GaN microdomes,
TMGa flow rate was increased to more than 55 μmol/min and substrate temperature
was decreased to lower than 920 °C. In device fabrication, indium tin oxide (ITO)
was deposited by electron beam evaporation on p-GaN to form transparent ohmic
contacts. The 1×1 mm2 diode mesas were then defined by chlorine-based plasma
etching. The contacting scheme consists of fingered Ti/Al/Ni/Au metal grids with the
thicknesses of 20/400/20/2000 nm deposited on the ITO and n-GaN. To fabricate a
hierarchical structure, SiO2NRAs were performed on the p-GaN microdome LED via
the same procedures in chapter 5 (see Fig. 5.1).
§ 6-3 Results and Discussion
Fig. 6.1(a) and the inset are respectively the 45-degree-tilted and the
cross-sectional SEM images of p-GaN microdome surface. The geometric dimensions
of these p-GaN microdomes are 530 250 nm in height and 600 370 nm in
diameter. Fig. 6.1(b) and the inset are the 45-degree-tilted and the cross-sectional
images of SiO2 NRA/p-GaN microdome surface, respectively. These vertically
aligned SiO2NRAs are around 400 nm high and the diameters range from 50 to 150
nm.
Figure 6.1 (a) and the inset are 45-degree tilted and cross-sectional surface images of p-GaN microdome LED respectively. (b) and the inset are 45-degree tilted and cross-sectional surface images of SiO2NRA/p-GaN microdome LED respectively.
The electrical characteristics (I－V curves) of the LEDs are presented in the inset
of Fig. 6.2. The devices with p-GaN microdomes and SiO2NRAs/p-GaN microdomes
respectively show the forward voltages of 3.17 V and 3.21 V at the driving current of
20 mA, which are slightly smaller than that (3.23 V) of the flat device. The result is
attributed to the improved ohmic contact resistance due to the increased contact area
of microdomed surfaces. Furthermore, the slightly higher operation voltage of
NRA/microdome LED than bare microdome LED is attributed to the additional
increased series resistances, which are possibly caused by the additional surface
recombination centers and damage occurring during the RIE process of SiO2NRAs.
Fig. 6.2 shows the light intensity as a function of injection current (L－I curve) for Figure 6.1 (a) and the inset are 45-degree tilted and cross-sectional surface images of p-GaN microdome LED respectively. (b) and the inset are 45-degree tilted and cross-sectional surface images of SiO2NRA/p-GaN microdome LED respectively.
The electrical characteristics (I－V curves) of the LEDs are presented in the inset
of Fig. 6.2. The devices with p-GaN microdomes and SiO2NRAs/p-GaN microdomes
respectively show the forward voltages of 3.17 V and 3.21 V at the driving current of
20 mA, which are slightly smaller than that (3.23 V) of the flat device. The result is
attributed to the improved ohmic contact resistance due to the increased contact area
of microdomed surfaces. Furthermore, the slightly higher operation voltage of
NRA/microdome LED than bare microdome LED is attributed to the additional
increased series resistances, which are possibly caused by the additional surface
recombination centers and damage occurring during the RIE process of SiO2NRAs.
Fig. 6.2 shows the light intensity as a function of injection current (L－I curve) for Figure 6.1 (a) and the inset are 45-degree tilted and cross-sectional surface images of p-GaN microdome LED respectively. (b) and the inset are 45-degree tilted and cross-sectional surface images of SiO2NRA/p-GaN microdome LED respectively.
The electrical characteristics (I－V curves) of the LEDs are presented in the inset
of Fig. 6.2. The devices with p-GaN microdomes and SiO2NRAs/p-GaN microdomes
respectively show the forward voltages of 3.17 V and 3.21 V at the driving current of
20 mA, which are slightly smaller than that (3.23 V) of the flat device. The result is
attributed to the improved ohmic contact resistance due to the increased contact area
of microdomed surfaces. Furthermore, the slightly higher operation voltage of
NRA/microdome LED than bare microdome LED is attributed to the additional
increased series resistances, which are possibly caused by the additional surface
recombination centers and damage occurring during the RIE process of SiO2NRAs.
Fig. 6.2 shows the light intensity as a function of injection current (L－I curve) for
the three devices. The LED with SiO2NRA/p-GaN microdome structure exhibits the
highest emission intensities in the entire current range, which is believed to result
from the enhanced scattering effects on the hierarchical structure and the smoothened
transition of refractive index from GaN to air provided by the SiO2NRAs [31-33],
both of which greatly improve light extraction from the device.
Compared with flat LED, the emission intensities of bare microdome LED and
hierarchical LED exhibit the enhancements of 16.7 % and 36.8 % at 20 mA,
respectively. The enhancements of the two textured surfaces are attributed to the
improved light extraction or EQE caused by the roughened morphologies. Surface
roughening with p-GaN microdomes not only increases the total emission area but
also increases the variation of the optical incident angles at the interfaces between
GaN and air, which is believed to reduce the undesired total internal reflections within
GaN. Besides the enhancement of light extraction efficiency, the p-GaN roughening
can further improve the IQE of LEDs which has recently been disscussed.[3,9,34]
Accordingly, it is believed that the microdomed p-GaN can increase the LED light
output due to the improved light extraction and IQE. For the hierarchical structure, the
added SiO2 NRAs can be regarded as an intermediate medium with the gradient
refractive index further mitigating the abrupt index change between air and p-GaN
microdomes. This effective medium effect additionally suppresses the Fresnel
reflections from device to air. Consequently, the hierarchical structure results in the
most light extraction of LEDs.
Figure 6.2 Light-output intensity versus injection currents (L－I curves) of flat, p-GaN microdome, and SiO2NRA/p-GaN microdome LEDs. The inset is the corresponding I－V characteristics.
In order to reveal the light propagation across the roughened surface structures,
the distributions of electromagnetic fields within the device structures were simulated
by FDTD analysis. We modeled three different geometric features of the LEDs with
flat, p-GaN microdome, and SiO2 NRA/p-GaN microdome surfaces. Fig. 6.3(a)－(c)
visualize the time-averaged TE-polarized electric field intensity distributions, |Ey|, for
GaN blue LEDs with different surface conditions at 460 nm wavelength. All of the reflections from device to air. Consequently, the hierarchical structure results in the
most light extraction of LEDs.
Figure 6.2 Light-output intensity versus injection currents (L－I curves) of flat, p-GaN microdome, and SiO2NRA/p-GaN microdome LEDs. The inset is the corresponding I－V characteristics.
In order to reveal the light propagation across the roughened surface structures,
the distributions of electromagnetic fields within the device structures were simulated
by FDTD analysis. We modeled three different geometric features of the LEDs with
flat, p-GaN microdome, and SiO2 NRA/p-GaN microdome surfaces. Fig. 6.3(a)－(c)
visualize the time-averaged TE-polarized electric field intensity distributions, |Ey|, for
GaN blue LEDs with different surface conditions at 460 nm wavelength. All of the reflections from device to air. Consequently, the hierarchical structure results in the
most light extraction of LEDs.
Figure 6.2 Light-output intensity versus injection currents (L－I curves) of flat, p-GaN microdome, and SiO2NRA/p-GaN microdome LEDs. The inset is the corresponding I－V characteristics.
In order to reveal the light propagation across the roughened surface structures,
the distributions of electromagnetic fields within the device structures were simulated
by FDTD analysis. We modeled three different geometric features of the LEDs with
flat, p-GaN microdome, and SiO2 NRA/p-GaN microdome surfaces. Fig. 6.3(a)－(c)
visualize the time-averaged TE-polarized electric field intensity distributions, |Ey|, for
GaN blue LEDs with different surface conditions at 460 nm wavelength. All of the
calculated values are normalized to the ones of the excitation source. It can be seen
that the emitted field intensities (in air) of both p-GaN microdome and SiO2
NRA/p-GaN microdome devices are enhanced. Compared with the flat LED of Fig.
6.3(a), the p-GaN microdome structure, marked in Fig. 6.3(b), results in strong field
intensities on the roughened surface, indicating enhanced light scattering. Moreover,
in Fig. 6.3(c), one can see that the regions of SiO2NRAs/p-GaN microdomes further
strengthen field intensities inside the microdomes and NRAs, and lead to the most
scattering centers on the device surface. The strong scattering and re-bouncing of light
within SiO2NRAs/p-GaN microdomes signal an improved light extraction from the
device internal, which are supported by the observation that the light trapped inside
the device in Fig. 6.3(c) is less than that in Fig. 6.3(b) and this results in less field
intensity leaking out from the backside (through n-GaN and sapphire). In other words,
integrating SiO2 NRAs with p-GaN microdomes not only help light propagate across
the interfaces by avoiding the abrupt index transition from GaN to air but also
reinforce the field distribution within the roughness by increasing the light scattering
through the surfaces. As a result, this hierarchical structure leads to the most light
extraction.
Fig. 6.3(d) shows the normalized optical power, integrated over the marked
regions, as a function of time for the three LEDs. Steady-state power values for the
three devices are 0.647, 0.693 and 0.791, respectively. Compared with the flat LED,
the optical power enhancement of the two textured LEDs are 7.1 % and 22.3 %,
respectively. This indicates that these textured nanostructures can increase the number
of the photons leaving LEDs, and thus enhance light extraction efficiency.
Figure 6.3 Time-averaged and normalized TE electric field distribution, |Ey|, simulated by FDTD analysis with different surface conditions at 460 nm wavelength, (a) Flat, (b) p-GaN microdome, (c) SiO2 NRA/p-GaN microdome, (d) Normalized optical power, integrated over the framed regions, as a function of times for the three LEDs.
three devices are 0.647, 0.693 and 0.791, respectively. Compared with the flat LED,
the optical power enhancement of the two textured LEDs are 7.1 % and 22.3 %,
respectively. This indicates that these textured nanostructures can increase the number
of the photons leaving LEDs, and thus enhance light extraction efficiency.
Figure 6.3 Time-averaged and normalized TE electric field distribution, |Ey|, simulated by FDTD analysis with different surface conditions at 460 nm wavelength, (a) Flat, (b) p-GaN microdome, (c) SiO2 NRA/p-GaN microdome, (d) Normalized optical power, integrated over the framed regions, as a function of times for the three LEDs.
three devices are 0.647, 0.693 and 0.791, respectively. Compared with the flat LED,
the optical power enhancement of the two textured LEDs are 7.1 % and 22.3 %,
respectively. This indicates that these textured nanostructures can increase the number
of the photons leaving LEDs, and thus enhance light extraction efficiency.
Figure 6.3 Time-averaged and normalized TE electric field distribution, |Ey|, simulated by FDTD analysis with different surface conditions at 460 nm wavelength, (a) Flat, (b) p-GaN microdome, (c) SiO2 NRA/p-GaN microdome, (d) Normalized optical power, integrated over the framed regions, as a function of times for the three LEDs.
Fig. 6.4 indicates the normalized radiation profiles of the three LED samples. The
corresponding viewing angles of flat, microdome, and NRA/microdome LEDs are
112°, 114 ° and 115°, respectively. The wider and stronger emission cone of
NRAs/microdomes LEDs is in line with the enhanced scattering effects shown in Fig.
6.3(c), confirming the excellent scattering and light-extraction abilities of the
NRAs/microdomes [16]. This lambertian light emission might be beneficial to the
applications in general lighting.
Figure 6.4 Radiation patterns of the three LEDs under a 20 mA injected current.
Fig. 6.4 indicates the normalized radiation profiles of the three LED samples. The
corresponding viewing angles of flat, microdome, and NRA/microdome LEDs are
112°, 114 ° and 115°, respectively. The wider and stronger emission cone of
NRAs/microdomes LEDs is in line with the enhanced scattering effects shown in Fig.
6.3(c), confirming the excellent scattering and light-extraction abilities of the
NRAs/microdomes [16]. This lambertian light emission might be beneficial to the
applications in general lighting.
Figure 6.4 Radiation patterns of the three LEDs under a 20 mA injected current.
Fig. 6.4 indicates the normalized radiation profiles of the three LED samples. The
corresponding viewing angles of flat, microdome, and NRA/microdome LEDs are
112°, 114 ° and 115°, respectively. The wider and stronger emission cone of
NRAs/microdomes LEDs is in line with the enhanced scattering effects shown in Fig.
6.3(c), confirming the excellent scattering and light-extraction abilities of the
NRAs/microdomes [16]. This lambertian light emission might be beneficial to the
applications in general lighting.
Figure 6.4 Radiation patterns of the three LEDs under a 20 mA injected current.
§ 6-4 Summary
In summary, we fabricated two surface-textured LEDs respectively with p-GaN
microdomes and SiO2 NRAs/p-GaN microdomes. Using self-assembled Ag
nanoparticles as etching masks and subsequent RIE, the fabricated SiO2NRA/p-GaN
microdome LED enhances light output intensity at 20 mA by up to 36.8 %, as
compared with flat LED. The SiO2 NRAs provide an effective refraction index to
reduce the total internal reflection at the air/GaN interface and the microdomes lead to
stronger light scattering effect, increasing the light extraction efficiency of LEDs. The
presented concept and manufacturing technique of surface roughening would be a
viable way to enhance the light extraction efficiency for nitride-based LEDs.
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