Characterizing Reverse-bias Electroluminescence of InGaN/GaN LEDs
Hsiang Chen1, Chyuan-Haur Kao2, Tien-Chang Lu3
1
Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Puli, Taiwan, R.O.C.
2
Department of Electronic Engineering, Chang Gung University, Kwei-Shan, Taiwan, R.O.C.
3
Department of Photonics Nationonal Chiao Tung Univers-ity, Hsinchu, Taiwan, R.O.C.
Corresponding e-mail:hchen@ncnu.edu.tw Phone: 886-49-2910960 ext 4909
Keywords: InGaN LED, Reverse Bias Luminescence, Electroluminescence, Leakage Current, Hot Carrier Induced Emission
Abstract
The reverse-bias operation of the InGaN LED
device can shed a light on device reliability problems.
Our goal is to use X-ray fluorescent (XRF) element
analysis, 2D electroluminescence (EL) observations, and
electrical measurements to visualize the current leakage
and evaluate the fresh device performance. Hot electroninduced emissions due to a leakage current may be a
mechanism of the reverse-bias emission. The reverse-bias
light emission is relevant to reliability problems because
of its combination of optical characterization and
electrical performance. Besides, a nondestructive
screening method has been built to detect the leakage
current path around the metal contact and
evaluate fresh LED device quality.
combinational study of material analysis and electrical
performance, the reverse-bias electroluminescence behavior
which was caused from the hot electron induced emission
was proven to be relevant to the device performance.
INTRODUCTION
The InGaN/GaN light emitting diode is usually biased
in the forward bias condition functioning as a semiconductor
light source. Recently, several groups studied the reliability
issues from the LED device under reverse bias. Different
from other groups, we observe the reverse-bias
electroluminescence which is close to undetectable from
fresh devices since the behavior of the InGaN LED device in
the reverse-bias condition can also shed a light on device
reliability problems. During the reverse-bias operations,
field dependent tunneling current at low voltage and local
impact ionization in high electric field regions dominate the
leakage current which affects device electrical properties [1]
[2]. Our goal is to use noninvasive optical characterization
techniques including 2D electroluminescence measurements
and 2D XRF (X-ray Fluorescence) element analysis to
explore potential reliability problems on the InGaN LED
device performance. The electroluminescence (EL) light
emission behavior under forward-bias operations and
reverse-bias operations has been examined [3][4]. The
reverse-bias leakage and the forward–bias sub-threshold
current measurements provide additional information to
further explore the potential reliability problem [5]. Under a
Fig. 1 The structure of optical detection systems
INSTRUMENT SETUP
Instead of examining the device with destructive
material analysis, we use noninvasive characterization
techniques including reverse-bias EL measurements and
XRF element analysis. The principles of these three optical
characterizations are similar - collecting light emission from
the device. The differences are the wavelength of the
collected light and the excitation source. Therefore, we use
Fig. 1 to illustrate the functions of the three systems. The
structures of the three optical characterization instruments
are the same as shown in Fig 1. As for the reverse-EL
detection, we bias the device on the stage under reverse bias
and collect the light emission of wavelength between 0.3μm
to 1μm from the device. Furthermore, in a similar optical
system structure, when we incorporate the X-ray excitation
source and collect the stimulated X-ray radiation from the
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un-biased device. 2D element distribution images of the
device can be presented. Through collecting light of
different wavelength, EL images and element distributions
can be juxtaposed to investigate the electrical and material
properties of the device.
DEVICE FABRICATION
The InGaN/GaN MQW LED is grown by metal organic
chemical vapor deposition (MOCVD) on a c-face 2-inch
sapphire (0001) substrate. The device structure consists of a
30 nm GaN nucleation layer, a 2-μm-thick un-doped GaN,
2-μm-thick Si-doped n-type GaN, 100-nm-thick active
layers, 50-nm-thich Mg-doped AlGaN electron blocking
layer, and a 15-nm-thick Mg-doped p-GaN layer. The
InGaN/GaN MQW active region consists of ten pairs of 3nm-thick InGaN well layers and 7-nm-thick GaN barrier
layers. After partially etching the sample down to n+ layer, a
230 nm surface indium-tin oxide (ITO) layer is deposited
onto the sample surface to function as the transparent contact
layer (TCL). Besides, Ti/Al/Ti/Au contact is evaporated onto
the exposed n-type GaN layer to function as the n-type
electrode. The InGaN/GaN MQW structure.
(a)
(b)
(c)
Fig. 2 (a) Forward-bias EL (white bright area) and reversebias EL (in red) in a superposition image (b) Forward-bias
EL (in yellow) and Ga distribution(in purple) areas in a
superposition image (c) Reverse-bias EL (in red) and Au
XRF distribution (in blue) areas in a superposition image.
EXPERIMENTS AND DISCUSSION
Fig. 2(a) shows comparison of forward-bias EL (white
bright area) and reverse-bias EL (in red) in a superposition
image. The reverse-bias emission area is in the border of the
forward-bias emission area close to the non-emission area.
To analyze the emission area, comparison of forward-bias
EL and Ga XRF distribution areas (the chip area) and
comparison of reverse-bias EL and Au XRF distribution area
(the metal contact) in two superposition images are shown
in Fig. 2(b) and Fig. 2(c). The reverse-bias emssion occurs in
the metal contact and the chip overlapped area.
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V (V)
(a)
(b)
Fig. 3 (a) Reverse-bias leakage current and reverse-bias
emission area percentage over the whole chip versus the
reverse-bias voltage (b) The graph of the natural logarithm
of the light intensity from one point and the average light
intensity from the whole chip divided by the intensity versus
the reciprocal of the applied voltage.
To investigate the reverse bias emission mechansim
quatitatively, we explore the relationship between emsssion
area, emission intensity, leakage current distribution, and
electric field. During reverse bias operations of the second
type device, as the negative volatge increases, the reverse
bias emssion area is enlarged Moreover, the increase rate of
the reverse bias emission area is close to the increase rate of
the leakage current as shown in Fig. 3(a), indicating that the
current induced EL ( hot-carrier induced EL) may be the
mechanism of the luminescence because the more the
leakage current is, the larger leakage current induced
emission area will be.
In addition, luminescence intensity, leakage current,
and electric field under reverse bias are measured [6]. The
relationship of the different quantities fits the hot electron
induced luminescence equation because the graph of the
nature logarithm of the light intensity divided by the
intensity versus the reciprocal of the applied voltage as
shown in Fig. 3 (b) is close to a straight line with a negative
slope corresponding to equation 1, confirming the hot
electron-induced emission mechanism of the reverse-bias
emission. The reverse-bias emission may imply the presence
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of a local high electric field. The deformation of the
electrode metal may cause a local high electric field to
generate the emission.
approximately 10 μA. Most of the fresh LEDs are similar to
Device C and have no reverse-bias emission. Device C is a
normal fresh device.
(a)
(a)
(b)
(b)
Fig. 5 I-V curves (a) with reverse bias and (b) forward bias.
(c)
Fig. 4 Matlab images of reverse-bias EL of (a) Device A (b)
Device B (c) Device C.
This study also finds that different reverse-bias
emission characteristics relates to device electrical reliability
and forward-bias emission efficiency. The reverse-bias lightemission behavior differs among several fresh devices. At
the same reverse-bias voltage of -10 V, the leakage current
of the device with the largest reverse-bias emission area as
shown in Fig. 4(a) (Device A) is about 100 μA and the
leakage current of the device with no light emission (Device
C) as shown in Fig. 4(c) has the leakage current of 5 nA. The
leakage current of Device B with a small emission area, as
shown in Fig. 4(b), shows the leakage current of Device B is
To build connections between the device performance
and the reverse-bias emission characteristics, HP 4145
measures the I-V curves of the devices with different lightemission characteristics. Fig. 5(a) show the I-V curves under
reverse bias as shown. Fig. 5(b) shows the enlarged I-V
curves in the subthreshold region under forward bias. The
leakage current induces the reverse-bias emission. The larger
the leakage current is, the stronger the light emission will be.
We operate the LED under forward bias functioning as a
light source, and we zoom into the electrical behavior of the
device in the off state, subthreshold region, and fully turnedon state for the forward-bias condition. When the three
devices are in the off state and the applied voltage is below
0.5 V, the currents of the three devices are almost identical.
As the applied voltage increases to 1.5 V in the subthreshold
region, Device A’s subthreshold current is approximately
500 times that of Device B and 50,000 times the intensity of
Device C, implying that the leakage current of the devices is
still present though not detectable by EL emission. As the
applied voltage further increases to 2.5 V in the fully turnedon region, the currents among three devices become similar
again, since the strong band-to-band recombination current
is so large that it has masked the leakage current. The
leakage current still exists and may deteriorate the device
during long-term operations.
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XRF: X-ray fluorescent
EL: Electroluminescence
MOCVD: Metal Organic Chemical Vapor Deposition
MQW: Multiple Quantum Well
ITO: Indium-tin Oxide
TCL:transparent contact layer
Fig. 6 L-I curves
Furthermore, to investigate the relationship between
device performance and the reverse-bias EL, we evaluate the
light emission efficiency of these three different devices by
measuring L-I curves as shown in Fig. 6. While Device A
with the largest leakage current and the strongest reversebias EL has the lowest light emission efficiency, Device C
with the lowest leakage current and no reverse-bias EL has
the highest light emission efficiency. Device B’s
performance is in-between. The results indicates that
stronger reverse-bias EL due to the leakage current induced
emission implies worse device performance.
CONCLUSIONS
The origin of the reverse-bias luminescence from the
fresh device is attributed to hot electron-induced emission.
The reverse-bias EL test can identify location of noticeable
leakage current from the edge or the border of the metal and
the chip area and assess device quality just like plumbers use
soapy water to locate the gas leak from the pipes. These
techniques promise a screening tool to correlate device
failures with fabrication process.
ACKNOWLEDGEMENTS
This work was supported by the National Science
Council, Taiwan, Republic of China, under Contract No.
NSC-98-2221-E-260-006.
REFERENCES
[1] N.C. Chen, et al, Journal of Crystal Growth 311, 994 (2009).
[2] M. Meneghini, et al, Appl. Phys. Lett. 95, 173507 (2009).
[3] H. Chen, Exploration of the potential defects in GaN HEMTs, Verlag
Dr. Müller, Saarbrücken,Germany, 2009.
[4] J. Kikawa, et al, Solid-state Electron 47, 523 (2003).
[5] N. Kovalev, et al, Semiconductors 32, 54 (1998).
[6] K. Hui, et al, IEEE Electron Dev. Lett. 11, 113 (1990).
ACRONYMS
LED: Light Emitting Diode
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