Blue light emission from the heterostructured ZnO/InGaN/GaN
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University
, Wuhan 430072,
People's Republic of China
ZnO/InGaN/GaN heterostructured light-emitting diodes (LEDs) were fabricated by molecular beam epitaxy and atomic layer deposition. InGaN films consisted of an Mg-doped InGaN layer, an undoped InGaN layer, and a Si-doped InGaN layer. Current-voltage characteristic of the heterojunction indicated a diode-like rectification behavior. The electroluminescence spectra under forward biases presented a blue emission accompanied by a broad peak centered at 600 nm. With appropriate emission intensity ratio, the heterostructured LEDs had potential application in white LEDs. Moreover, a UV emission and an emission peak centered at 560 nm were observed under reverse bias.
Nowadays, white light-emitting diodes (WLEDs) have
attracted significant interest for solid-state illumination
due to their low power consumption, long operating
time, and environmental benefits [1-3]. Hence, WLEDs
are the most promising alternatives to replace
conventional light sources, such as backlighting, interior lamps,
and general lightings . Currently, the prevailing
method is to use a blue LED coated with a
yellowemitting phosphor. However, during a long period of
optical pumping, the degradation of the phosphor would
decline the output efficiency of the WLEDs. Another
way to obtain white light is to mix the emissions from
different light sources . In particular, InGaN with a
continuously variable bandgap from 0.7 to 3.4 eV has
attracted considerable interest, and thus, InGaN/GaN
WLEDs are regarded as the most promising solid-state
lighting device which can work in the whole visible and
part of the near UV spectral regions . Some groups
have fabricated dichromatic InGaN-based WLEDs .
However, compared with WLEDs with a mixture of blue,
green and red emissions, they had lower color rendering
With a direct wide bandgap of 3.37 eV and high exciton
binding energy of 60 meV, ZnO is considered as one of
the best electroluminescent materials. However, herein
lays an obstacle of ZnO homojunction diodes, which is
p-type; it is a problem in obtaining high-quality and
stable p-ZnO films. Although some p-n homo-junction
ZnO LEDs have been reported, their electroluminescence
(EL) intensities were very weak [8-10]. As an alternative
approach, heterostructured LEDs have been fabricated
on top of a variety of p-type substrates, such as SrCu2O2
, Si , and GaN . With the advantages of
InGaN and ZnO, it is significant to fabricate ZnO/
InGaN/GaN heterojunctions with blue, green, and red
emissions to obtain white light.
In this work, we report the fabrication of ZnO/InGaN/
GaN heterostructured LEDs. The EL spectra under
forward biases presented a blue emission accompanied by a
broad peak centered at 600 nm. With appropriate
emission intensity ratio, heterostructured LEDs have potential
application in WLEDs. Moreover, a UV emission and an
emission peak centered at 560 nm were observed under
There were two steps to fabricate the ZnO/InGaN/GaN
LEDs (inset of Figure 1). Firstly, InGaN films were
deposited on commercially available (0001) p-GaN
wafers on sapphire by radiofrequency plasma-assisted
molecular beam epitaxy (SVTA35-V-2, SVT Associates
Inc., Eden Prairie, MN, USA). A 7-N (99.99999%) Ga
and 6-N (99.9999%) In were used as source materials.
Nitrogen (6 N) was further purified through a gas
purifier and then introduced into a plasma generator. The
InGaN film consisted of a 150-nm Mg-doped InGaN
layer, a 200-nm intrinsic InGaN layer, and a 400-nm
Si-doped InGaN layer. Secondly, ZnO films were
deposited on the InGaN films by atomic layer deposition
(TSF-200, Beneq Oy, Vantaa, Finland). The detailed
experimental method can be found in our previous work
. In this work, 4,000 cycles were performed, and the
thickness of ZnO films was about 600 nm. In order to
demonstrate the rectifying behavior that originated from
the heterojunction, Ni/Au and In were fabricated as the
p-type and n-type contact electrodes, respectively.
Results and discussion
The photoluminescence (PL, HORIBA LabRAM HR800,
HORIBA Jobin Yvon S.A.S., Longjumeau, Cedex, France)
measurements were conducted at room temperature in
the wavelength range of 350 to 700 nm to analyze the
optical properties of n-ZnO films, InGaN films, and
p-GaN substrates. In order to assess the performance of
the heterostructured LEDs, current-voltage (I-V) and EL
measurements were carried out at room temperature.
The rectifying behavior with a turn-on voltage of about
2 V is observed in the I-V curve (Figure 1).
The room-temperature PL spectra of the ZnO, InGaN,
and GaN layers are presented in Figure 2. As shown, the
PL spectrum of p-GaN was dominated by a broad peak
Figure 1 I-V curve of ZnO/InGaN/GaN heterostructure. Inset
shows the sketch map of the structure.
centered at about 430 nm, which can be attributable to
the transmission from the conduction band and/or
shallow donors to the Mg acceptor doping level .
Fringes were observed in the spectrum on account
of the interference between GaN/air and sapphire/
GaN interfaces . The spectrum of InGaN:Si was
dominated by a peak centered at about 560 nm. Because
the total thickness of the intrinsic InGaN film and the
Si-doped InGaN film was about 600 nm, the influence of
Mg doping in InGaN cannot be observed from the PL
spectrum. The spectrum of ZnO displayed a dominant
sharp near-band-edge emission at 380 nm, and
deeplevel emission at around 520 nm was not observed.
Deep-level emission has been reported to be caused by
oxygen vacancies. Therefore, it indicated few oxygen
vacancies existing in the ZnO films .
The EL spectra of ZnO/InGaN/GaN heterojunction
LED under various forward biases are shown in Figure 3a.
The EL spectra were collected from the back face of the
structure at room temperature. As shown in Figure 3a,
with a forward bias of 10 V, a blue emission located at
430 nm was observed. Compared with the PL spectra, it
can be easily identified that it originated from a
recombination in the p-GaN layer. With bias increase, the
blue emission peak shifted toward a short wavelength
(blueshift). Note that mobility of electrons is faster than
holes. Therefore, with low bias, electrons were injected
from the n-ZnO side, through the InGaN layer, to the
p-GaN side, and little recombination occurred in the
n-ZnO and InGaN layers. With bias increase, some holes
can inject to the n-ZnO side. Hence, the intensity of
emission from the ZnO increased, and as a result, the
blue emission peak shifted toward a short wavelength.
Additionally, with the bias increase, a peak centered at
600 nm was observed, as shown in Figure 3a. Compared
Figure 3 EL spectra of ZnO/InGaN/GaN heterojunction LED under forward various biases (a) and multi-peak Gaussian fitting (b). The
fitting are from experimental data at the range of 500 to 700 nm.
Figure 4 CIE x and y chromaticity diagram.
with the PL spectra, the peak is not consistent with
pGaN, ZnO, and InGaN:Si. The peak under the bias of 40
V is thus fitted with two peaks by Gaussian fitting
(Figure 3b). The positions of two peaks are 560 and 610
nm, respectively. The emission peak at 560 nm matches
well with the PL spectrum of InGaN:Si. However, the
emission peak at 610 nm cannot be found in the PL
spectra. The PL emission of intrinsic GaN was at 360 nm, and
GaN:Mg changes to 430 nm due to transmission from the
conduction band and/or shallow donors to the Mg acceptor
doping level. Hence, the peak centered at 610 nm might be
from the Mg-doped InGaN layer .
Figure 4 illustrates the possibility of white light from
the ZnO/InGaN/GaN heterostructured LEDs by the
Commission International de l'Eclairage (CIE) x and y
chromaticity diagram. Point D is the equality energy
white point, and its CIE chromaticity coordinate is (0.33,
0.33). Because the points from 380 to 420 nm on CIE
chromaticity diagram are very close, point A is used to
represent the blue emission from p-GaN and ZnO.
Points B and C represent emissions from InGaN:Si and
InGaN:Mg, respectively. As shown in Figure 4, triangle
ABC included the white region defined by application
standards. Therefore, theoretically speaking, the white
light can be generated from the ZnO/InGaN/GaN LED
with the appropriate emission intensity ratio of ZnO,
InGaN:Si, InGaN:Mg, and p-GaN. Therefore, when the
EL intensity ratio (380:560:610 nm) was adjusted to
4:5:1, white light was observed from the LED, and its
CIE chromaticity coordinate was (0.33, 0.33). Calculating
the EL spectrum under the bias of 40 V, the EL intensity
Figure 5 EL spectrum of the ZnO/InGaN/GaN heterojunction
LED under the reverse bias.
ratio (380:560:610 nm) was about 36:1:4, and point
E represented emission of the LED. Hence, in order to
fabricate WLEDs, the EL intensity of InGaN should be
enhanced. In other words, the internal quantum
efficiency of the InGaN layers should be improved.
Improving the crystalline quality and increasing the
carrier concentration of the p-InGaN and n-InGaN
layers are the efficient ways to achieve higher internal
Furthermore, the EL spectrum under a reverse bias of
40 V is presented in Figure 5. It is much different from
that under the forward biases. The EL spectra show a
blue emission accompanied by a broad peak centered at
600 nm under forward biases, whereas two emissions
(380 and 560 nm) appeared under reverse bias.
Obviously, they are attributed to ZnO and InGaN:Si,
respectively. The EL mechanism under reverse bias probably is
the impact excitation .
In conclusion, we have fabricated heterostructured ZnO/
InGaN/GaN LEDs. The EL spectra under forward biases
show a blue emission accompanied by a broad peak
centered at 600 nm. The peak at 600 nm was deemed to
be the combination of the emissions from Si-doped
InGaN at 560 nm and Mg-doped InGaN at 610 nm.
Counted with the CIE chromaticity diagram, white light
can be observed in theory through the adjustment of the
emission intensity ratio. Furthermore, a UV emission
and an emission peak centered at 560 nm were observed
under reverse bias. This work provides a simple
way using the emission from ZnO, Mg-doped InGaN,
Si-doped InGaN, and p-GaN to obtain white light in
theory. With the appropriate emission intensity ratio,
ZnO/InGaN/GaN heterostructured LEDs have potential
application in WLEDs.
TW fabricated the ZnO thin films, performed the measurements of
heterostructures, and wrote the manuscript. HW analyzed the results and
wrote the manuscript. ZW fabricated the InGaN thin films. CC helped to
grow and measure the heterostructures. CL supervised the overall study. All
authors read and approved the final manuscript.
This work is supported by the National Natural Science Foundation of China
(NSFC) under grant numbers 10904116, 11074192, 11175135, and J0830310,
and by the foundation from CETC number 46 Research Institute. The authors
would like to thank HH Huang and BR Li for their technical support.