High performance germanium-on-silicon detectors for optical communications

Article abstract of DOI:10.1063/1.1496492

We demonstrate fast and efficient germanium-on-silicon p-i-n photodetectors for optical communications, with responsivities as high as 0.89 and 0.75 A/W at 1.3 and 1.55 μm, respectively, time response <200 ps and dark currents as low as 1.2 μA. Ge was epitaxially grown on Si by chemical vapor deposition, employing a low temperature buffer and cyclic thermal annealing to reduce the dislocation density. The overall performance is well suited for >2.5 Gb/s integrated receivers for the second and third fiber spectral windows.

Full text of DOI:10.1063/1.1496492

High performance germanium-on-silicon detectors for optical communications
Silvia Famà, Lorenzo Colace, Gianlorenzo Masini, Gaetano Assanto, and Hsin-Chiao Luan
Citation: Applied Physics Letters 81, 586 (2002); doi: 10.1063/1.1496492
View online: http://dx.doi.org/10.1063/1.1496492
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 4
22 JULY 2002
High performance germanium-on-silicon detectors for
optical communications
Silvia Fam a` , Lorenzo Colace,a) Gianlorenzo Masini, and Gaetano Assanto
National Institute for the Physics of Matter and Department of Electronic Engineering, University ‘‘Roma
Tre,’’ Via della Vasca Navale 84, 00146 Roma, Italy
Hsin-Chiao Luanb)
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,
Massachusetts 0213
͑
Received 15 April 2002; accepted for publication 30 May 2002͒
We demonstrate fast and efficient germanium-on-silicon p-i-n photodetectors for optical
communications, with responsivities as high as 0.89 and 0.75 A/W at 1.3 and 1.55 ␮m, respectively,
time response Ͻ200 ps and dark currents as low as 1.2 ␮A. Ge was epitaxially grown on Si by
chemical vapor deposition, employing a low temperature buffer and cyclic thermal annealing to
reduce the dislocation density. The overall performance is well suited for Ͼ2.5 Gb/s integrated
receivers for the second and third fiber spectral windows. © 2002 American Institute of Physics.
͓
DOI: 10.1063/1.1496492͔
Thin film germanium photodetectors fabricated on sili-
con substrates have recently gained widespread interest due
to their potential use in low-cost monolithic transceivers for
typically ranged from 30 to 60 nm. Then the reactor tempera-
ture was incresead up to 600 °C, and about 4 ␮m of Ge were
deposited on Si. In order to further reduce the residual dis-
location density, a cyclic thermal annealing ͑ten cycles be-
tween 900 and 780 °C͒ was performed. Details on the mate-
rial growth and its characterization can be found in Ref. 7.
The fabricated devices are p-i-n photodetectors, with
the top n-type contact and the intrinsic layer made of epitax-
ial Ge, while the bottom p-type contact is the highly doped
silicon substrate ͑resistivity 0.008 ⍀ cm͒. For the realization
of the top n contact, phosphorous was implanted at 30 keV
with a dose of 4ϫ1015 cm . The n contact was about 200
nm thick, as evaluated by secondary ion mass spectroscopy.
After ion implantation, the samples were thermally activated
through a 5 min anneal at 600 °C. The p-type substrate was
chosen in order to get the most favorable band alignment for
1
optical communications. The high absorption coefficient at
the wavelengths of interest ͑1.3–1.55 ␮m͒, the good trans-
port properties, and the compatibility with silicon technology
qualify germanium as the ideal candidate for silicon-
compatible near-infrared light detection. To this extent, Ge
films have been epitaxially grown with different technolo-
gies, aiming at reducing the defects caused by the lattice
2
mismatch with silicon. To date, the best detectors have been
Ϫ2
obtained employing a low temperature buffer layer associ-
3
ated with cyclic thermal annealing. More recently, a differ-
ent approach has been demonstrated, based on the growth of
Ge islands embedded in a Si p–n junction. The latter tech-
nology, however, while suggesting the possibility of dark
current reduction, at the moment yields detectors with a lim-
ited conversion efficiency.4
3
the collection of the photogenerated carriers. The incorpo-
,5
ration of one of the doped layers in the substrate eliminates
the need for doping gases during Ge growth, while simplify-
ing the electrical connections to the diode. Round mesas of
Hereby we report on a fast and efficient Ge p-i-n detec-
tor, epitaxially grown on a silicon substrate and capable of
operation at the 2.5 Gb/s ‘‘OC-48’’ standard of optical com-
munications. In order to effectively exploit the compatibility
with silicon technology, the goal of the present work was the
demonstration of photodiodes exhibiting optimal perfor-
mances in terms of both responsivity and speed at low volt-
age biases.
Ϫ4
Ϫ3
2
areas ranging from 1.4ϫ10 to 2.7ϫ10 cm ͑diameters
from 135 to 585 ␮m, respectively͒ were fabricated by stan-
dard photolithography and wet chemical etching, and subse-
quently passivated by spin-on glass. Top and bottom contacts
were photolithographically defined on a sputter-deposited
aluminum layer. Finally, the samples were annealed to obtain
ohmic contacts.
Germanium was epitaxially grown on silicon by ultra-
high vacuum chemical vapor deposition with a base pressure
The photodiodes were characterized in terms of dark
current, speed of photoresponse and responsivity at both 1.3
and 1.55 ␮m. Figure 1 shows the typical dark current density
as a function of voltage bias at room temperature. From the
Ϫ9
of 3ϫ10 Torr. In order to minimize the dislocations asso-
ciated with the large lattice mismatch, a thin relaxed low-
temperature Ge buffer was deposited on Si at 350 °C with 10
2
sccm of GeH ͑15% in Ar͒. The buffer layer was meant to
graph, current densities of 11 and 15 mA/cm at 0.5 and 1 V
4
promote the insertion of dislocations as a mechanism for
strain relaxation ͑rather than island growth͒. Its thickness
are apparent, respectively. In particular, the smallest device
͑135 ␮m in diameter͒ exhibited a dark current of 1.2 ␮A,
which compares well with commercial bulk-Ge photodiodes
of equal area and dark currents in the range 0.1–1 ␮A. The
inset in Fig. 1 displays the dark current versus device area:
the linear scaling indicates a bulk effect at the origin of the
6
2
a͒
Electronic mail: colace@uniroma3.it
Present address: Agilent Technologies, 350 West Trimble Rd. MS90UB
b͒
San Jose, CA, 95131-1008.
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003-6951/2002/81(4)/586/3/$19.00 586 © 2002 American Institute of Physics
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0
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Appl. Phys. Lett., Vol. 81, No. 4, 22 July 2002
Fam a` et al.
587
FIG. 3. FWHM of the photoresponse of the 135 ␮m diameter device vs
reverse bias.
FIG. 1. Dark current density as a function of the applied reverse voltage at
room temperature. Dark current vs device area is shown in the inset.
ferences due to band gap and transport characteristics of the
employed semiconductor.
The devices’ speed of response was investigated by re-
cording the photocurrent generated by a train of 120 ps
pulses at 1.32 ␮m, as produced by a cw actively mode-
locked Nd: yttrium–aluminum–garnet ͑YAG͒ laser at 100
MHz. The diodes were biased through a 50 ⍀ resistor, and
the signal was acquired by a dc-decoupled ͑via a 100 nF
capacitor͒ sampling oscilloscope with a 12 GHz analog
bandwidth. A 180 ps ͓full width at half maximum ͑FWHM͔͒
pulsed photoresponse was measured at a reverse bias of 1 V
on the 135 ␮m device, as visible in Fig. 2. Note that the
residual oscillations are imputable to a nonperfect impedance
matching with the probe. This result is consistent with a RC
limited response, being the measured junction capacitance
and the series resistance 0.7 pF and 250 ⍀, respectively. The
response FWHM is plotted in Fig. 3 versus reverse bias.
Devices with larger areas exhibited proportionally slower re-
sponses; we can therefore infer that the response time is still
dark current ͑probably carrier generation in the intrinsic-Ge
layer͒, while underlines the negligible contribution of surface
leakage, even in small devices.
The measured dark current is larger than that reported in
Ref. 5 for a lower responsivity Ge-island based photodiode,
probably due to a larger dislocation count. To date, the larger
dark current— and the associated shot noise— of Ge-based
versus InGaAs detectors have penalized the former in tele-
communication applications where high sensitivity is re-
quired. However, as the detector diameter is reduced in order
to achieve high speed operation, the contribution of the pho-
todiode on the total receiver noise becomes unimportant as
compared to the transimpedance preamplifier. Moreover, for
very small area devices ͑diameter Ͻ100 ␮m͒, the increasing
perimeter to area ratio makes leakage dominate over
diffusion/generation currents in the mesas, bleaching the dif-
FIG. 2. Time response of the 135 ␮m diameter device to the 120 ps pulse
FIG. 4. Measured responsivity vs reverse bias at 1.32 ␮m ͑continuous line͒
and 1.55 ␮m ͑dashed line͒.
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train from a Nd:YAG laser emitting at 1.32 ␮m.
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588
Appl. Phys. Lett., Vol. 81, No. 4, 22 July 2002
Fam a` et al.
dominated by the junction capacitance, leaving space for fur-
ther improvements by mesa-diameter reductions. The latter
statement is also supported by an estimate of transit time
from mobility, which yields 50 ps at a reverse bias of 1 V.
The measured speed of photoresponse corresponds to a fre-
quency cutoff higher than 2.5 GHz, and therefore enables our
photodetectors to operate according to OC-48 at bit rates
above 2.5 Gb/s.
but, in addition, offers the advantage of complete integrabil-
ity and voltage compatibility with Si-based high-speed elec-
tronics. We believe that these photodetectors will readily find
applications in optical communication systems operating at
and above 2.5 Gb/s.
The authors are grateful to Professor L. C. Kimerling
and his group ͑M.I.T., Boston, MA͒ for providing the semi-
conductor samples. This work was funded in part by the
Italian Space Agency.
Responsivities as high as 0.89 and 0.75 A/W at 1.3 and
1.55 ␮m, respectively, were evaluated using laser diode
sources whose output power was measured by a calibrated
power meter. Responsivity data versus reverse voltage are
shown in Fig. 4. Notice that complete photocarrier collection
1
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͑
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about 0.3 V. An optimal performance in terms of responsivity
and speed, even when operated at low voltages ͑Ͻ1 V͒,
makes these photodiodes perfectly compatible with standard
Si electronics and quite appealing towards their monolithic
integration in modern, low voltage, electronic transceivers.
In conclusion, we demonstrated fast and efficient Ge
p-i-n photodetectors integrated on silicon. Their overall per-
formance compares well with that of commercial devices
4
5
͑
2002͒.
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