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IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 2, FEBRUARY 2008
161
Dark-Current Suppression in
Metal–Germanium–Metal Photodetectors Through
Dopant-Segregation in NiGe—Schottky Barrier
H. Zang, S. J. Lee, W. Y. Loh, J. Wang, K. T. Chua, M. B. Yu, B. J. Cho, G. Q. Lo, and D.-L. Kwong
Abstract—We demonstrate, for the first time, the application of dopant-segregation (DS) technique in metal–germanium–
metal photodetectors for dark-current suppression and high-speed
performance. Low defect density and surface smooth epi-Ge
(∼300 nm) layer was selectively grown on patterned Si substrate
using two-step epi-growth at 400 ◦ C/600 ◦ C combined with a thin
(∼10 nm) low-temperature Si/Si0 .8 Ge0 .2 buffer layer. NiGe with
DS effectively modulates the Schottky barrier height and suppresses dark current to ∼10−7 A at −1 V bias (width/spacing:
30/2.5 µm). Under normal incidence illumination at 1.55 µm, the
devices show photoresponsivity of 0.12 A/W. The 3 dB bandwidth
under −1 V bias is up to 6 GHz.
Index Terms—Dark current, dopant segregation (DS), germanium, optical communications, photodetectors, selective epitaxial.
I. INTRODUCTION
ILICON-BASED optoelectronic device is a promising candidate to replace the III–V compounds semiconductors devices due to its low cost, ease of process, and potential integration with CMOS compatibility. Ge photodetectors on Si
substrate have attracted tremendous research interest, since
surface-smooth Ge epitaxial layer can be realized on Si wafer using two-step Ge growth method [1], [2]. However, the two-step
epi-grown Ge layer still suffers from high threading-dislocation
density (∼9.5 × 108 cm−2 ), which seriously degrades device
performances. High-quality Ge layer with low threading dislocation density (≤6 × 10−6 cm−2 ) can be achieved by two-step
growth combined with an intermediate ultrathin SiGe buffer
layer [3]. Metal–semiconductor–metal photodetectors (MSMPDs) have been demonstrated with ease of fabrication, low
detector capacitance, and large device bandwidth as its main
advantages [4]. However, MSM-PDs exhibit high dark current,
especially, for metal–germanium–metal photodetectors (MGMPDs), since the Schottky barrier height (SBH) scales with
S
Manuscript received September 14, 2007; revised October 30, 2007. This
work was supported by the Science and Engineering Research Council, Agency
for Science, Technology, and Development (ASTAR), Singapore. The review
of this letter was arranged by Editor P. Yu.
H. Zang and J. Wang are with the Institute of Microelectronics, Singapore
117576, Singapore (e-mail: g0500085@nus.edu.sg; g0600134@nus.edu.sg).
W. Y. Loh, K. T. Chua, M. B.Yu, G. Q. Lo, and D.-L. Kwong are
with the Institute of Microelectronics, Singapore 117685, Singapore (e-mail:
lohwy@ime.a-star.edu.sg; chuakt@ime.a-star.edu.sg; mingbin@ime.a-star.edu.
sg; logq@ime.a-star.edu.sg; kwongdl@ime.a-star.edu.sg).
S. J. Lee (corresponding author) and B. J. Cho are with the SiliconNano-Device-Laboratory, Department of Electrical and Computer Engineering (ECE), National University of Singapore, Singapore 117576 (e-mail:
elelsj@nus.edu.sg; elebjcho@nus.edu.sg).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2007.914095
energy band gap. To suppress MGM-PD dark current, some
solutions were proposed such as amorphous Ge Schottky barrier enhancement layers [5], asymmetric area electrodes [6],
and asymmetric metal electrodes [4]. At the same time, SBH
modulation by dopant segregation (DS) has been proposed for
high-performance Schottky source/drain MOSFETs by Kinoshita
et al. [7] with enhanced drive current and suppressed OFF-state
leakage [8], [9].
In this letter, we demonstrate, for the first time, the application
of DS in MGM-PD for dark-current suppression and high speed.
With simultaneous n- and p- type DS in two NiGe electrodes
using a single-step self-aligned germanidation process, hole and
electron SBHs are modified preferentially, resulting in reduced
dark currents. In addition, low sheet resistance and contact resistance of metal-germanide result in high-speed performance
under low reverse bias.
II. EXPERIMENT
Starting with (1 0 0) p-type Si (∼8–15 Ω cm), SiO2 (150 nm)
was deposited, patterned, and dry/wet etched to open circular
windows (20 µm in diameter) for subsequent Ge selective
deposition. Before epitaxy growth, the wafers were cleaned
in SC1 (NH4 OH:H2 O2 :DI ∼ 1:2:10] and diluted HF (1:200).
Subsequently, ∼10-nm Si and 25-nm thick Si0.8 Ge0.2 buffer
layer was grown at 350 ◦ C–400 ◦ C, and Ge was deposited in an
ultrahigh vacuum epi-chamber. For the two-step Ge process, Ge
seed was first grown at a low temperature of 400 ◦ C followed
by main growth of Ge at a temperature of 550 ◦ C–600 ◦ C
[3]. Atomic force microscope measurements show a root-meansquare surface roughness of 0.425 nm (10 × 10 µm2 ). The Ge
was patterned, and then, implanted with As (1 × 1015 cm−2 /
12 keV) in n-type electrode area and boron (1 × 1015 cm−2 /
12 keV) in p-type electrode region. One part of the wafer is
not implanted for non-DS-MGM PD fabrication. After SiO2
deposition, patterning, and etch to expose the Ge, 20 nm Ni
was deposited by sputtering for DS-NiGe electrodes formation.
Germanidation was carried out by rapid thermal annealing
(RTA) at 350 ◦ C. Unreacted Ni was removed by nitric acid.
Finally, Al deposition, patterning, and etching were performed
for metallization formation.
III. RESULTS AND DISCUSSION
Fig. 1(a) shows high-resolution transmission electron microscopy (HR-TEM) image of the Ni-Germanide layer formed
in epi-Ge. Energy dispersive spectroscopy analysis on the Nigermanide shows formation of mono-nickel germinade, NiGe,
0741-3106/$25.00 © 2008 IEEE
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IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 2, FEBRUARY 2008
Fig. 2. Dark current comparison between MGM-PDs with and without DS
and measured the photocurrent at a wavelength of 1.55 µm for DS-MGM-PD.
Fig. 3. Richardson plots of current of NiGe/Ge (1 0 0) MGM Schottky junction
(width: 188 µm) at V = −0.1 V; the inset is the temperature dependent I–V
curves.
Fig. 1. (a) HR-TEM image of the layers of NiGe/Ge. (b) Schematic diagram
of cross section of vertical incidence. (c) SEM of DS-MGM-PD with 2.5 µm
spacing between two electrodes. The SEM scale is 20 µm × 20 µm.
which has been reported to show low resistivity of 15 µΩcm
[10]. It is observed that the NiGe forms sharp and smooth
interface and exhibits epitaxial growth due to low lattice
mismatch [11]. Fig. 1(b) shows schematic diagram of cross
section of vertical incidence. Fig. 1(c) is the scanning electron
micrograph (SEM) image of MGM-PD with 2.5 µm spacing
between two electrodes. Fig. 2 compares the dark-current performances between MGM-PDs with and without DS in the NiGe
contact electrodes. The MGM-PDs with and without DS were
fabricated on the same wafer with identical process for fair comparison. I–V for MGM-PD without DS shows a back-to-back
diode behavior. Due to Fermi level pinning effect between NiGe
and Ge, hole SBH is 0.07–0.1 eV for NiGe/p-Ge [8] and electron SBH is 0.6 eV for NiGe/n-Ge [8], [9]. The epi-Ge on Si
exhibits p-type characteristics due to the structural defects in
Ge, which leads to acceptor-type states near the valence band
edge [5], [14]. So, the major component of the high dark current
is the hole injection over the Schottky barrier. The MGM-PD
with DS-NiGe shows a dark current of 10−7 A at −1 V, which
is ∼3–4 orders of magnitude lower than that of MGM-PD without DS. The NiGe/Ge with As (1 × 1015 cm−2 ) segregation can
lower the electron barrier height up to 0.1 eV [8]. The reason
why DS can be used in dark-current suppression is that Asincorporated Schottky junction can increase the effective hole
barrier height [7]. For similar reason, NiGe with boron segregation can suppress the electron current, even though electron
contributes only a minor part in dark current in the case of
MGM-PDs. At forward bias, the currents are significantly increased, because the segregated As lowers the electron SBH
and segregated boron lowers the hole SBH, as used similarly
in SB-MOSFET [7]. Fig. 2 also shows DS-PD photocurrent at
a wavelength of 1.55 µm. The laser input power is 3.5 mW at
1.55 µm. The normal incidence responsivity of PD is 0.088 A/W
at 0 V, 0.12 A/W at −1 V, and 0.14 A/W at −2 V. The ability
of the device to operate at photovoltaic status (0 V bias) provides high SNR. The responsivity is limited by the area of the
MGM-PD active region and Ge thickness (300 nm). To accurately extract the hole barrier height of NiGe/Ge contact, the I–V
curves of MGM Schottky junction (As, boron segregated separately) were measured at different temperature ranging from
ZANG et al.: DARK-CURRENT SUPPRESSION IN MGM-PDs THROUGH DS IN NiGe
Fig. 4. The 3 dB bandwidth of DS-MGM-PD as a function of reverse bias.
The full bandwidth is achieved at −1 V. The insert is the temporal response of
the DS-MGM-PD at −1 V bias to a 80 fs laser pulse at wavelength of 1.55 µm.
The FWHM is 56 ps at −1 V bias.
TABLE I
SUMMARY OF METAL-SEMICONDUCTOR-METAL PHOTODETECTORS
PERFORMANCE. THE DS-MGM PD IN THIS LETTER SHOWS LOWEST DARK
CURRENT AND HIGH SPEED AT LOW REVERSE BIAS
163
56 ps at −1 V. The 3 dB bandwidth of the MSM Ge photodetector
is close to 6 GHz at −1 V. Table I compares the performance of
DS-MGM-PD in this letter with the other published MSM-PDs
and lateral p-i-n PD results. DS-MGM-PD shows lowest dark
current and highest speed as compared to previous reported
MGM-PD and comparable dark current to reported lateral Ge
p-i-n PD. In addition, DS-MGM-PD shows high speed at low
reverse bias, which is desirable for low power consumption and
high SNR. Compared to the lateral Ge p-i-n PD with the same
electrodes spacing (2.5 µm) in [16], the DS-MGM-PD shows a
larger bandwidth, and its bandwidth is less bias dependent. This
is due to lower resistance of NiGe [10] as compared to Ge n- and
p-junction. The lateral Ge p-i-n [17] shows a significant higher
speed as compared to the DS-MGM-PD in this letter. This is due
to smaller electrodes spacing of the PD in [17]. Since the bandwidths of lateral PDs are strong electrodes spacing dependent,
the bandwidth of DS-MGM-PD can be improved by scaling
electrodes spacing.
IV. CONCLUSION
We demonstrate MGM-PD on Si substrate with suppressed
dark current using DS method. DS-MGM-PD shows fast photo
response up to ∼56 ps and low dark current 10−7 A at −1 V
bias, indicating that DS is a promising technique for future Ge
photodetector application.
ACKNOWLEDGMENT
The authors would like to thank the staff of SemiconductorProcess-Technology Laboratory, Institute of Microelectronics,
Singapore, for their assistances in sample preparation used in
this letter.
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25 ◦ C to 75 ◦ C (see Fig. 3 inset). The barrier height was extracted using thermionic-emission theory [13]. From the slope
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Fig. 4 shows the 3 dB bandwidth of DS-MGM-PD extracted
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