CHIN. PHYS. LETT. Vol. 30, No. 3 (2013) 036101
High Response in aTellurium-Supersaturated Silicon Photodiode
*
WANG Xi-Yuan(王熙元)1 , HUANG Yong-Guang(黄永光)1** , LIU De-Wei(刘德伟)1,2 ,
ZHU Xiao-Ning(朱小宁)1 , ZHU Hong-Liang(朱洪亮)1
1
Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences,
P. O. Box 912, Beijing 100083
2
Department of Technology and Physics, Zhengzhou University of Light Industry, Zhengzhou 450002
(Received 28 November 2012)
Single crystalline silicon supersaturated with tellurium are formed by ion implantation followed by excimer
nanosecond pulsed laser melting (PLM). The lattice damaged by ion implantation is restored during the PLM
process, and dopants are effectively activated. The hyperdoped layer exhibits high and broad optical absorption
from 400 to 2500 nm. The n+ p photodiodes fabricated from these materials show high response (6.9 A/W at
1000 nm) with reverse bias 12 V at room temperature. The corresponding cut-off wavelength is 1258 nm. The
amount of gain and extended cut-off wavelength both increase with increasing reverse bias voltage; above 100%
external quantum efficiency is observed even at a reverse bias of 1 V. The cut-off wavelength with 0 V bias is
shorter than the commercial silicon detector. This implies that the Burstein-Moss shift is due to hyperdoping.
The amount of the extended cut-off wavelength increases with increasing reverse bias voltage, suggesting existence
of the Franz–Keldysh effect.
PACS: 61.72.uf, 61.72.U, 85.60.Gz
DOI: 10.1088/0256-307X/30/3/036101
Because of silicon’s indirect band gap of 1.12 eV,
the absorption and photoelectric response reduce
sharply for wavelengths longer than 1100 nm.[1] The
absorption coefficient of crystalline silicon is less than
103 cm−1 for wavelengths longer than 850 nm, therefore commercial silicon photodetectors only have high
responsivity at wavelengths shorter than 850 nm.[2]
Many efforts have been focused on extending the operating wavelength to improve responsivity. In recent years, silicon highly supersaturated with chalcogens beyond solid solubility limits has revealed
strong and broad optical absorption from 400 nm to
2500 nm. These materials have been prepared by
optical hyperdoping,[3−6] laser incorporation,[7] and
ion implantation followed by pulsed laser melting
(PLM).[8−10] However, experimental results[11,12] and
first-principles calculations[13] show that the optical
absorption above 1100 nm for silicon hyperdoped with
sulfur, selenium, and tellurium all drops markedly after thermal annealing, and the decrease amplitude
for tellurium is the lowest. The initial sub-bandgap
optical absorption for silicon hyperdoped with tellurium is larger than sulfur and selenium by theoretical calculation,[13,14] too. Silicon photodiodes
supersaturated with sulfur prepared by femtosecond
laser irradiation in an SF6 atmosphere show high
and broad response,[15] the reason for the high response is considered as a generation-recombination
gain mechanism.[16] Said et al.[17] reported that n+ p
photodiodes fabricated by sulfur or selenium ion implantation followed by pulsed laser melting exhibit
high gain and a broad response wavelength range.
Moreover, insulator-to-metal transition has been ob-
served separately in silicon supersaturated with sulfur
and selenium prepared by ion implantation followed
by PLM.[18,19]
In this Letter, we report an enhancement of both
the wavelength range and photoelectric response exhibited by n+ p photodiodes fabricated by tellurium
ion implantation followed by excimer nanosecond
pulsed laser melting. Pan et al.[10] and Bob et al.[9]
reported silicon supersaturated with tellurium. The
silicon wafers were implanted with high dose tellurium
ions (130 Te+ of 1 × 1016 cm−2 ) and followed by XeCl
308 nm excimer nanosecond pulsed laser melting (50 ns
duration, laser fluence of 0.6–0.7 J/cm2 and 2.3 J/cm2 ,
respectively), the material shows strong sub-bandgap
optical absorption and high free carrier density, but no
high response photodetector has been reported. Our
experimental parameters are different: tellurium ion
implantation dose of 126 Te+ 2 × 1015 cm−2 and KrF
248 nm excimer nanosecond pulsed laser (30 ns duration, laser fluence of 0.3 J/cm2 ). In addition, we use
double side passivation with a back surface point contact structure to reduce the surface recombination.
Our photodiode results and those of Said et al. demonstrate that silicon photodiodes supersaturated with
sulfur, selenium or tellurium can exhibit a high and
broad photoelectric response. This class of materials
has potential applications for photodetectors.
In our experiment, 370-µm-thick polished
Czochralski p-type monocrystalline Si (100) wafers
with resistivity of 30–50 Ω·cm were ion implanted at
room temperature with 245 keV 126 Te+ to a dose of
2 × 1015 ions/cm2 . All wafers were tilted by ∼7∘ off
the incident beam axis to minimize the channelling
* Supported by the Beijing Natural Science Foundation under Grant No 4122080, the National Basic Research Program of China
under Grant No 2012CB934202, and the Chinese Academy of Sciences (No Y072051002).
** Corresponding author. Email: yghuang@red.semi.ac.cn
© 2013 Chinese Physical Society and IOP Publishing Ltd
036101-1
CHIN. PHYS. LETT. Vol. 30, No. 3 (2013) 036101
)
-1
n+ layer
p-Si
SiO2
)
)
-3/2
SK
3/2
100
-3
Metal
ln(
S
W cm
103
102
101
100
10-1
10-2
10-3
(cm
SiO2
sition. The inset is an Arrhenius plot of carrier concentration versus temperature. The linear fit slope
demonstrates a deep level trap state of 0.183 eV (very
close to reported value[23] ). The measured sheet carrier concentration is 1.436 × 1014 cm−2 at 300 K, the
corresponding carrier-to-donor ratio (the ratio of free
carrier density to total Te atom concentration) is
7.18%. The average carrier bulk concentration is approximately 4.1 × 1018 /cm3 , at least 2 orders of magnitude larger than Te solubility in c-Si.[24]
Conductivity (
effect. The maximum implantation depth is approximately 350 nm according to SRIM-2011 software.[20]
For effectively activating dopants, all ion-implanted
wafers were irradiated with a 1 pulse spatially homogenized, KrF pulsed excimer laser (248 nm, 30 ns
pulse duration). The wafers were translated by a
horizontal displacement set to make sure that the
laser spot could irradiate the whole area. The single pulse laser fluence was 0.3 J/cm2 . Laser melting
and the subsequent rapid solidification form a single
crystal region nearly free of defects, which retains
most of the implanted dopants.[21,22] Raman spectral,
optical property, electrical property, and Rutherford
backscattering spectroscopy (RBS) were performed in
sequence. Before each experiment and measurement,
wafers were cleaned ultrasonically.
200
35.0
34.8
34.6
34.4
34.2
34.06
8
1000/
300
400
10
(1/K)
12
500
(K)
Metal
(arb. units)
Figure 1 shows the cross-sectional view of the photodetector. We use these materials to fabricate photodiodes by using a standard complementary metal
oxide semiconductor (COMS) process. The 200 nm
SiO2 films were deposited on both sides by plasma
enhanced chemical vapour deposition (PECVD) for
passivation. The electrode windows were prepared by
lithography and HF solution etching. The 150 nm top
electrode was deposited by thermal evaporation (background pressure 1 × 10−4 Pa), and the 500 nm bottom
electrode was deposited by electron beam evaporation
(background pressure 1 × 10−6 Pa). The electrodes of
two sides were rapid thermal annealed with pure N2
ambient protection to form the ohm contact. Lastly,
the wafers were diced to small units. We measured the
𝐼–𝑉 characteristics and photoresponse at room temperature.
We measured the Hall effect of the Te-doped silicon
wafer annealed by PLM over a temperature range of
80–500 K using van der Pauw technique. The sign of
the Hall coefficient is of n-type, implying the formation
of a p–n junction. Figure 2 shows the temperaturedependent conductivity of Te-supersaturated silicon.
From 500 K to 80 K, the conductivity monotonically
reduces to 1/10 implying semiconductor characteristics rather than metal characteristics. It demonstrates
that the Te-supersaturated silicon in this doping concentration does not establish insulator-to-metal tran-
Fig. 2. (Color online) Temperature-dependent conductivity of Te-supersaturated silicon. The conductivity reduces
monotonically with decreasing temperature, it suggests
semiconductor characteristics rather than metal characteristics. Inset: an Arrhenius plot of carrier concentration
versus temperature, revealing a deep level trap state with
the activation energy of 0.183 eV.
Raman spectral intensity
Fig. 1. (Color online) Cross-sectional view of the Tesupersaturated silicon photodiode. The n+ layer is Tedoped silicon, and 200-nm-thick SiO2 films were deposited
by PECVD on both sides for passivation. The 150 nm
metal film was deposited by thermal evaporation as the
top electrode, another 500-nm-thick metal film was deposited by electron beam evaporation as the bottom electrode.
2000
1500
1000
500
0
1500
1000
500
Silicon wafer
Silicon wafer implanted with Te
0
2000
Te-supersaturated silicon after PLM
1500
1000
500
0
450 460 470 480 490 500 510 520 530 540 550
Wavenumber(cm
-1
)
Fig. 3.
(Color online) Raman spectra of the silicon
wafer, the silicon wafer implanted with Te, and the Tesupersaturated silicon after PLM.
Figure 3 shows the Raman spectra of the silicon
wafer, the silicon wafer implanted with Te, and the
Te supersaturated silicon after PLM (silicon wafer and
Te-supersaturated silicon after PLM were excited by
the same intensity laser). The Raman peak of the silicon wafer is 520.894 cm−1 (FWHM of 1.638 cm−1 ),
the Raman hump from 450 cm−1 to 530 cm−1 of the
silicon wafer implanted with Te is derived from the
amorphous silicon phase[25] and defects induced by
ion implantation, the Raman peak of the Te supersaturated silicon after PLM is 521.003 cm−1 (FWHM
of 1.420 cm−1 ). After PLM, the hump disappears and
036101-2
CHIN. PHYS. LETT. Vol. 30, No. 3 (2013) 036101
the Raman signal peak is almost completely from the
crystalline silicon phase, and the FWHM is even narrower than that of silicon wafer without any processing. It is suggested that PLM is efficient in restoring the crystalline lattice. Moreover, the Raman peak
shifts to 521.003 cm−1 after the PLM process. This
suggests existence of compressive stress in the lattice, which may be due to lattice distortion induced
by plenty of large-radius Te atom incorporation and
not completely recovering the volume reduction (the
volume of liquid silicon is smaller than that of solid
silicon) after the rapid melting-solidification process
during PLM.
Random
Total Te content
100
Substitutional
Te
10
1
500
750
1000
1250
1500
Current (mA)
RBS yield (Counts)
Channelling
1000
ter PLM ranges from 400 nm to 2500 nm. The total
absorptance A was determined from the directly measured reflectance 𝑅 and transmittance 𝑇 , according
to 𝐴 = 1 − 𝑅 − 𝑇 . The absorptance of silicon wafer
without any processing is less than 0.03 from 1200 nm
to 2500 nm. After ion implantation, the absorptance
changes slightly from 400 nm to 1000 nm, the gradient
absorptance line from 1000 nm to 2500 nm implies the
existence of amorphous silicon, which is demonstrated
by Raman spectral measurement above. After PLM,
the absorptance from 400 nm to 1000 nm restores like
silicon wafer without any processing, and the absorptance from 1150 nm to 2500 nm (corresponding to subbandgap of silicon) is approximately 0.25–0.30, which
may arise from direct defect-to-conduction-band transitions induced by Te ion implantation followed by
PLM.[19]
1750
Scattered energy (keV)
Fig. 4. (Color online) Rutherford backscattering spectrometry of Te-supersaturated silicon after PLM.
Voltage (V)
0.8
Fig. 6. (Color online) The current-voltage characteristics
of the Te-supersaturated silicon photodiode.
0.6
Absorptance
50
40
30
20
10
0
-10-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Silicon wafer
implanted with Te
0.4
Te-supersaturated
silicon after PLM
0.2
Silicon wafer
0.0
500
1000
1500
2000
2500
Wavelength(nm)
Fig. 5. (Color online) The absorptance of silicon wafer,
silicon wafer implanted with Te, and Te supersaturated
silicon after PLM ranges from 400 nm to 2500 nm. The
reflectance 𝑅 and the transmittance 𝑇 were measured
directly, the absorptance was calculated by the formula
𝐴 = 1 − 𝑅 − 𝑇.
Figure 4 shows the Rutherford backscattering
spectroscopy (RBS) of Te supersaturated silicon after
PLM. The peak of Te in the C-RBS spectrum originates from substitutional tellurium atoms in the crystalline silicon lattice, the peak of Te in the R-RBS
spectrum originates from the total tellurium atoms
(substitutional and interstitial). The Te atom substitutional rate is approximately 37.9%. Figure 5 shows
the optical absorptance of silicon wafer, silicon wafer
implanted with Te, and Te supersaturated silicon af-
Figure 6 shows the current-voltage characteristics
of the Te-supersaturated silicon photodiode. The forward threshold voltage is 0.55 V. We measured the
responsivity of these photodiodes at a series of reverse
bias voltages, using light from a 300 W halogen tungsten lamp which was filtered through a monochromator with a spectral resolution of 0.1 nm. We scanned
the monochromator in steps of 20 nm from 400 to
1600 nm, keeping the optical power to be on the order of 1 µW at each wavelength. The light was focused, and the illuminated area was much smaller
than the photodiode top active region. The light
was positioned to give the maximum photoresponse
for each photodiode. The monochromatic light was
chopped so that the photodiode response could be
measured using a lock-in amplifier. A commercial
Si photodiode and a commercial InGaAs photodiode
were used to set the output power at each wavelength.
The commercial Si photodiode was used for measurement from 400 to 1000 nm, and the commercial InGaAs photodiode from 1000 to 1600 nm. Contributions from second-order wavelengths for measurement
above 800 nm were eliminated by passing the light
through a high pass filter. In all measurements, light
was incident on the top surface of photodiode at room
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CHIN. PHYS. LETT. Vol. 30, No. 3 (2013) 036101
temperature.[26] Figure 7 depicts the result of measurement. For wavelengths above 1270 nm, the responsivity is almost undetectable due to the background noise signal in the measurement system. At
0 V, the responsivity is low, and the cut-off wavelength
(0.03 A/W) is 1136 nm, shorter than that of a commercial silicon photodiode (1160 nm). The blue shift suggests that the Burstein–Moss effect[27] may be induced
by hyperdoping. The responsivity increases with increasing reverse bias voltage, EQE (external quantum
efficiency) beyond 100% is observed even at −1 V. A
remarkable gain (6.9 A/W at 1000 nm) at −12 V is
exhibited, the corresponding EQE is more than 850%
due to EQE=[1.24 × 𝑅(𝐴/𝑊 )]/𝜆 (µm). In addition,
by increasing reverse bias voltage, the optoelectronic
response of the Te-supersaturated silicon photodiode
extends into the infrared, the amount of the red shift
also increases with the increasing reverse bias voltage,
the cut-off wavelength is 1256 nm at −12 V. The red
shift may originate from the Franz–Keldysh effect[28]
and further study is in progress.
10
-12 V
Responsivity (A/W)
-4 V
-2 V
Q E = 10
0%
and so does the amount of extended cut-off wavelength
into infrared. The responsivity is 6.9 A/W at −12 V
at 1000 nm corresponding to EQE 855%. According
to our high response, Te-supersaturated silicon photodiodes and the result of Said et al., current chalcogen
elements (sulfur, selenium, and tellurium) supersaturated silicon photodiodes all exhibit a high and broad
response. It is illustrated that this class of materials
indeed has novel optoelectronic properties. The strong
gain mechanism may be the generation-recombination
gain mechanism resulted from the high trap density like sulfur supersaturated with silicon,[16] and
there may be no direct relationship between insulatorto-metal transition and high response (EQE>100%).
The present result has potential applications in faint
light signal detection.
The authors would like to acknowledge Jianming
Li and Jiadong Xu of the Semiconductor Integration
Technology and Engineering Center, Institute of Semiconductors, Chinese Academy of Sciences, for ion implantation, Yong Zeng, Yan Zhao and Professor Tao
Chen of the Institute of Laser Engineering, Beijing
University of Technology for PLM, Shaoxu Hu of the
State Key Lab on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences,
for the optoelectronic response testing.
1
-
1
V
Commercial Si
0V
References
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0.1
400
600
800
1000
1200
1400
1600
Wavelength (nm)
Fig. 7. (Color online) Responsivity of Te-supersaturated
silicon photodiode at different reverse bias voltages. The
responsivity of a commercial silicon photodetector at 0 V
is shown for reference, and its responsivity does not vary
with reverse bias voltages.
The EQE>100% is exhibited even at −1 V, and
Raman spectra of Te-supersaturated silicon after PLM
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In summary, we have fabricated n+ p silicon photodiodes with Te ion implantation and followed by
nanosecond excimer pulsed laser melting. The responsivity increases with increasing reverse bias voltage,
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036101-4