We should be able to significantly suppress the dark current and thus achieve a much higher photo current to dark current contrast ratio by using such MIS photodetectors.8,9 Previously,
Trang 1InGaN/GaN Multi-Quantum Well Metal-Insulator Semiconductor Photodetectors
with Photo-CVD SiO2 Layers
Ping-Chuan CHANG, Chin-Hsiang CHEN;1
, Shoou-Jinn CHANG, Yan-Kuin SU, Po-Chang CHEN2, Yi-De JHOU, Chun-Hsing LIU2, Hung HUNG and Shih-Ming WANG3
Institute of Microelectronics & Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, R.O.C.
1
Department of Electronic Engineering, Cheng Shiu University, Kaohsiung 830, Taiwan, R.O.C.
2
Department of Electronic and Computering Engineering, Nan Jeon Institute of Technology, Yan-Hsui, Taiwan 737, R.O.C.
3
Department of Electronic Engineering, National Yunlin University of Science and Technology, R.O.C.
(Received September 18, 2003; accepted November 17, 2003; published April 27, 2004)
InGaN/GaN multiple-quantum well (MQW) structure was epitaxial growth by metal-organic chemical vapor deposition
The normal undoped-GaN metal-semiconductor-metal (MSM) potodiodes were also prepared to compare with them It was
under 5 V reverse bias voltage Furthermore, it was found that we can significantly reduce the dark current of this photodiodes
photo current to dark current contrast
[DOI: 10.1143/JJAP.43.2008]
KEYWORDS: InGaN, MQW, MIS photodiode, GaN, MOCVD
1 Introduction
III-V nitride semiconductors are attractive materials that
can be applied in various optoelectronic devices such as
photodiodes, light emitter diodes (LEDs) and laser diodes
(LDs) In fact, nitride-based blue and green LEDs are
already commercially available and have been extensively
used in traffic light source and full color display However,
relatively few reports regarding nitride-based
blue/ultra-violet (UV) photodiodes could be found in the literature, as
compared to nitride-based LEDs.1) Nitride-based
photodi-odes are important devices that can be used in various
commercial and military applications For example, these
devices can be applied in space, medical and environmental
fields Depending on device structure, nitride-based p-n
junction diodes,2) p-i-n diodes,3) p--n diodes,4) Schottky
barrier detectors,5)metal-insulator-semiconductor (MIS) and
metal-semiconductor-metal (MSM) photodetectors6,7) could
all be used to detect blue/UV signal Among these devices,
MSM and MIS photodetectors have an ultra-low intrinsic
capacitance and their fabrication process is also compatible
with field-effect-transistor (FET) based electronics Thus,
one can easily integrate GaN MSM and/or MIS
photo-detectors with GaN FET-based electronics to realize a
GaN-based optoelectronic integrated circuit (OEIC) These
advantages all make nitride-based MSM and MIS
photo-detectors attractive for practical applications Although it is
easier to fabricate MSM photodetectors, the leakage current
of such MSM photodetectors will become very large if the
Schottky barrier height at the metal/semiconductor interface
is small Such a problem could be overcome by inserting a
thin insulating layer in between metal and the underneath
semiconductor We should be able to significantly suppress
the dark current and thus achieve a much higher photo
current to dark current contrast ratio by using such MIS
photodetectors.8,9)
Previously, it has been shown that photo-enhanced
chemical vapor deposition (photo-CVD) can be used to grow high quality SiO2 layers GaN.10,11) In using photo-CVD to grow thin films, selecting proper light source with a radiation spectrum matching the absorption spectra of the reactance gases is very important In this study, we used a deuterium (D2) lamp as the excitation source It is known that D2lamp emits strong ultra violet (UV) and vacuum ultra violet (VUV), which can effectively decompose SiH4 and
O2;.12,13)In addition, such a photo-CVD system offers better control in the oxide region and selective growth is possible GaN MIS photodetectors with such photo-CVD SiO2 layers have also been demonstrated On the other hand, the multi-quantum well (MQW) structure is frequently used as light emitter in LED and LD optoelectronic devices MQW based
on the quantum-confined Stark effect (QCSE) are very promising as the modulators in the self-electro-optic effect device (SEED) technology14)because of their fast switching speed, relatively high photo current, better responsivity characteristic, high extinction ratio, and variable detected wavelength range Therefore, we can use InGaN/GaN (MQW) structure to shift cut-off wavelength and to enhance the responsivity of photodetectors In this paper, we report the fabrication and characterization of InGaN/GaN MQW MIS photodetectors using photo-CVD SiO2as the insulating materials The electro-optical properties of the fabricated photodetectors will also be discussed
2 Experiment Prior to the deposition of SiO2 layers, an InGaN/GaN MQW structure was grown on (0001) sapphire substrates by metal-organic chemical vapor deposition (MOCVD).15–21)
The InGaN/GaN MQW structure consists a 25 nm-thick low temperature GaN nucleation layer, a 1 mm-thick undoped GaN, a 2 mm-thick Si-doped GaN, a 5-period Si-doped InGaN/GaN MQW and a 30 nm-thick GaN cap layer Each InGaN/GaN pair consists a 2.5 nm-thick In0:23Ga0:77N well layer and a 12 nm-thick GaN barrier layer The SiO2 films with various thicknesses were subsequently deposited onto the 30 nm-thick GaN cap layer by photo-CVD with a 150
E-mail address: chchen@csu.edu.tw
Japanese Journal of Applied Physics
Vol 43, No 4B, 2004, pp 2008–2010
#2004 The Japan Society of Applied Physics
2008
Trang 2Watt D2 lamp During SiO2 deposition, the gas ratio was
fixed at SiH4/O2=0.055 under various growth temperatures
Ni/Au metal contacts were then evaporated on top of the
SiO2to serve as the electrodes Figure 1 shows the schematic
diagram of the fabricated InGaN/GaN MQW MIS
photo-detectors The device consists of two inter-digitated contact
electrodes.6) The capacitance-voltage (C-V) characteristics
of the fabricated MIS capacitors were then measured by an
HP 4284B LCR meter An HP-4156 semiconductor
param-eter analyzer was used to measure the current-voltage (I-V)
characteristics of the InGaN/GaN MQW photodiodes in
dark and under illumination For photo current
measure-ments, a He-Cd laser illuminating from the front side of
InGaN/GaN MQW photodiodes was used as the light
source
3 Results and Discussion
Figure 2 shows C-V characteristics (1 MHz) of the
photo-CVD SiO2 grown at various temperatures It was found that
no significant hysteretic was observed as the gate voltage
varied at 0.1 V/s from þ5 V to 20 V and then backs to
þ5V The lack of hysteresis in these C-V curves indicated
that the number of mobile ions in SiO2 layers is negligibly
small Using the standard high frequency capacitance
method,22) we found that the interface state density, Dit,
equals 1:2 1012
cm2
eV1
and 8:4 1011
cm2
eV1
for the photo-CVD SiO2 layers deposited at 150
C and 300
C, respectively The smaller Dit for the photo-CVD SiO2 layer deposited at 300
C could be attributed to the fact that a higher substrate temperature can significantly improve the SiO2/GaN interfacial properties probably through supplying thermal energy to the Si and O atoms However, Dit was increased to 6:4 1012
cm2
eV1
when the substrate tem-perature was increased to 500
C The exact reason for the increase in Dit is not clear yet Possible reasons for such degradation in oxide quality include too fast an oxide growth rate and/or some interface reactions, which occur at high temperatures
Figure 3 shows dark I-V characteristics of the InGaN/ GaN MQW MIS photodetectors with various SiO2 layer thicknesses It was found that the dark current of the InGaN/ GaN MQW photodetectors without this SiO2layer (i.e SiO2
thickness = 0 nm) was higher than that with SiO2 layers This is mainly caused by sequential resonant electron tunneling through barriers of InGaN/GaN MQW structures Such a high dark current may increase detector noise, reduce detectivity, and consume power It is also indicated that the Schottky barrier height of this MQW photodetector was small In order to reduce the dark current, the SiO2layer was inserted as the insulating materials to fabricate InGaN/GaN MQW MIS photodetectors by using photo-CVD system It was also found that dark current decreases with the increase
of SiO2 layer thickness With a 5 V applied bias, we could reduce the dark current from 8:6 103
A to 4:9 1010
A with the insertion of a 53 nm-thick photo-CVD SiO2 layer
In other words, we could reduce the dark current by more than 7 orders of magnitudes Figure 4 shows measured photo current of the InGaN/GaN MQW MIS photodetectors with various SiO2 layer thicknesses It was found that photo current also decreases as the SiO2 layer thickness increases However, the decrease of photo current was much less significant compared to the decrease in dark current With a
5 V applied bias, we could only reduce the photo current by
a factor of about 4:5 103
Such a result suggests that we could significantly reduce the dark current, while still maintain a reasonably large photo current by introducing the photo-CVD SiO2layer into the photodetectors Figure 5 shows photo current to dark current contrast ratio as a function of the SiO2 layer thickness under various applied biases It can be seen clearly that we could always achieve
Ni/Au
SiO2
InGaN/GaN MQW n-GaN u-GaN nucleation Layer Sapphire
Ni/Au
Fig 1 Schematic diagram of the fabricated InGaN/GaN MQW MIS
photodetectors.
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
C HF
Gate Voltage (V)
Tsub=500°C Tsub=150°C Tsub=300°C Ideal C-V curve
Tox=50 nm Frequency=1 MHz Area=1.26X10-3 cm2
Fig 2 C -V characteristics (1 MHz) of the photo-CVD SiO 2 grown at
various temperatures.
10 -11
10 -8
10 -5
10 -2
SiO
2 thickness
Reverse voltage (V)
0nm 7nm 53nm 88nm
Fig 3 Dark I-V characteristics of the InGaN/GaN MQW MIS photo-detectors with various SiO 2 layer thicknesses.
Trang 3the highest photo current to dark current contrast from the
sample with a 53 nm-thick photo-CVD SiO2 layer,
regard-less of the applied bias With a 5 V applied bias, it was found
that we could achieve a high 1:53 103 photo current to
dark current contrast ratio from the photodetector with a
53 nm-thick photo-CVD SiO2 layer Such a value suggests
that photo-CVD SiO2 layer can indeed be used to improve
the performance of the InGaN/GaN MQW MIS
photo-detectors
4 Summary
In summary, InGaN/GaN MQW MIS photodiodes with
photo-CVD SiO2layers were fabricated successfully It was
found that we could significantly reduce the dark current,
while still maintain a reasonably large photo current by
inserting a photo-CVD SiO2 layer in between metal
electrodes and the underneath InGaN/GaN MQW structure
With a 53 nm-thick SiO2 layer, it was also found that we could achieve a high 1:53 103
photo current to dark current contrast ratio
Acknowledgements The authors would like to acknowledge the financial support from the National Science Council for their research grant of NSC 92-2215-E-230-002 and Ministry of Education for the program for promoting university academic excel-lence A-91-E-FA08-1-4
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10 -13
10 -10
10 -7
10 -4
10 -1
0nm 7nm 53nm 88nm SiO 2 thickness
Reverse voltage (V)
Fig 4 Measured photo current of the InGaN/GaN MQW MIS
photo-detectors with various SiO 2 layer thicknesses.
10 0
10 1
10 2
1V 2V 3V 4V 5V
SiO 2 thickness (nm)
Fig 5 Photo current to dark current contrast ratio as a function of the
SiO 2 layer thickness under various applied biases.