# High-Speed Near Infrared Optical Receivers Based on Ge Waveguide Photodetectors Integrated in a CMOS Process

Authors: Gianlorenzo Masini; Subal Sahni; Giovanni Capellini; Jeremy Witzens; Cary Gunn
Journal: Advances in Optical Technologies
 (2008)

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## Abstract

We discuss our approach to monolithic intergration of Ge photodectors with CMOS electronics for high-speed optical transceivers. Receivers based on Ge waveguide photodetectors achieve a sensitivity of−14.2 dBm (10−12 bit error rate (BER)) at 10 Gbps and 1550 nm.


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## Body

## 1. Introduction

Monolithic
integration of optical components on an Si platform has been pursued for a long time for its
potential advantages over hybrid approaches: lower assembly cost, compactness,
channel count, scalability to high-volume production. Integration in a CMOS
platform, in particular, is very attractive because of the very low
cost-per-unit attainable with this technology.Several optical
building blocks have been demonstrated already on Si: waveguides [1],
high-speed modulators [2],
photodetectors [3],
and complete transceiver systems have been demonstrated as well by Luxtera with
its recently introduced 40 Gbps (4 channel, 10 Gbps each) monolithic optical
transceiver built on an Si CMOS platform [4].While
waveguiding and modulation can be done in Si and do not require the integration
of new materials in the CMOS process, photodetection at long wavelengths (1.3μm and 1.55 μm) requires either Germanium
or a III–V compound such as InGaAs. The former is preferred over the latter due
to its better compatibility with the Si technology in terms of lattice
structure and parameter and contamination concerns. Germanium on Si
photodetectors has been demonstrated by several research groups with high
responsivity and speed and relatively low dark current [5–8],
however, none of the published devices is integrated in a CMOS process along
with standard Si transistors.In this paper,
we first describe our approach to integration of a Ge module for photodetectors
in LuxG, Luxtera's optical-enabled CMOS process based on Freescale's hip7_soi,
then we introduce the world's first monolithic high-speed optical receiver
based on Ge waveguide photodetectors.


## 2. Technology

SiGe alloys are
commonly found in today's Si technologies both bipolar, as the base layer in
heterojunction bipolar transistors (HBTs) [9],
and CMOS, as channel “stressors” for p-channel field effect
transistors [10].
Unfortunately, the absorption coefficient of the low-Ge-content alloys used in
these applications is too weak to be useful for efficient photodetection at 1.5 μm. On the contrary, pure Ge shows very high absorption coefficient up to
1.55 μm, thanks to the direct transitions occurring at the gamma point of its
band structure at energies above 0.8 eV. While the use of 100% Ge fet is
envisioned as a solution to address the mobility degradation induced by the
extreme gate length scaling dictated by the ITRS roadmap [11],
several issues remain to be solved before the first product based on this
technology faces high volume production, with
gate oxide formation and leakage reduction being, probably, the most formidable
[12].
Luckily enough, the use of 100% Ge for photodetectors does not imply the
formation of a high quality, chemically compatible, high-K, ultrathin
dielectric, as it is the case for fets,
and the leakage attainable by Ge on Si diodes is high but still
compatible with high-speed receivers operation [13].Ge can be
epitaxially grown on Si, despite the large difference in lattice parameter
(4%), using buffering techniques [14].
The buffer is used to plastically relax the strain induced by the lattice
mismatch, usually through controlled creation of dislocations, therefore
allowing a layer-by-layer, flat, film growth. Among the different approaches
found in literature, the low-temperature buffering technique introduced in [15]
is very attractive for integration in a CMOS process since it does not require
thick (and long) growths thus simplifying the insertion of the Ge film in the
limited headroom allowed to a CMOS fet. Ge films grown using this technique
have been shown to have less than 107 cm2 dislocation
density, improving consistently after thermal cycling [16],
though the high temperature required by the latter limits its use in a CMOS
flow. Ge epitaxy in a CVD environment is, usually, naturally selective to oxide
thus allowing for selective deposition in the areas where the detector has to
be fabricated using a hard mask. Selectivity can be improved by adding HCl to
the gas flow. When it comes to choose the insertion point of the Ge epitaxy
step within a CMOS process, several factors must be considered: the temperature
profile of the process, the possibility to contact the Ge device using the
standard Si contact module, the salicide sensitivity to thermal treatments, the
availability of a clean Si surface, the presence of dielectric films, and their
interaction on the selective growth of Ge. All these requirements may differ
among different technology nodes and need to be carefully evaluated to ensure a
seamless integration. LuxG, the Ge-enabled Luxtera process, integrates the Ge
epitaxy step at the end of the front-end processing and before the contact
module (see Figure 1). Epitaxy is performed in an RPCVD reactor for
8′′ wafers using GeH4 and hydrogen as the carrier gas.Figure 1
LuxG process flow.While forcing
the detector to share the same contact module of a transistor greatly
simplifies its integration, it also constraints the maximum thickness that can
be used for the Ge epitaxy to a fraction of a micron. This fact rules out the
possibility to integrate surface illuminated detectors which, given the
penetration depth of 1.5μm light in Ge, require an absorption length (i.e.,
thickness) larger than 2 micron. Waveguide photodetectors can be used, instead,
since absorption in these devices occurs along propagation of the optical beam
parallel to the growth surface (see Figure 2), thus transforming the thickness
constraint in a waveguide length requirement. Waveguide photodetectors have
other advantages over surface illuminated ones, the most relevant being the
decoupling of light absorption and photocarrier drift directions which allows
for independent optimization of absorption on one side and collection
efficiency and speed on the other.Figure 2
Absorption and drift directions are decoupled in a
waveguide photodetector allowing to independently optimize efficiency and
speed.


## 3. LuxG Waveguide Photodetectors

LuxG waveguide
photodetectors have a p-i-n structure in which both anode and cathode are
formed on Ge by ion implantation. While other contact configurations, such as
the heterojunction one with one electrode on Ge and the other on Si, have been
explored, the homojunction one proved best in terms of responsivity and speed,
at the cost of a higher leakage. A pictorial view of an LuxG waveguide
photodetector is shown in Figure3. Light coupling from the Si waveguide is
facilitated by the higher refractive index of Ge and total absorption at 1550 nm
occurs in 28 μm which is the detector length. A typical current/voltage
characteristic in the dark and under illumination is
shown in Figure 4. At a
typical operating voltage of 1 V reverse bias, the dark current is 3 μA and the
responsivity is 0.85 A/W. Note that the responsivity at short circuit is 99% of
that achieved in reverse bias thus allowing efficient operation at 0 V, when
very high-speed operation is not required as an example for power monitor
applications. The relatively high dark current of the Ge detector is due to the
presence of defects in the film and at the Si/Ge interface originated by the
lattice mismatch. Frequency response at 1V reverse bias is reported in Figure 5
showing an optical bandwidth in excess of 20 GHz with a well-behaved, single-pole, roll-off.Figure 3
3D view of an LuxG PIN waveguide photodetector.Figure 4
Current-voltage characteristics of an LuxG PIN waveguide
photodetector in the dark (red) and under illumination at a wavelength of 1550 nm
(blue).Figure 5
Frequency response of an LuxG PIN waveguide photodetector
at a wavelength of 1550 nm and 1V reverse bias. An optical bandwidth of 26 GHz
is extrapolated from the single-pole fit.


## 4. LuxG High-Speed Optical Receiver

LuxG waveguide
photodetectors have been monolithically integrated with a full TIA-LA
(transimpedance amplifier-limiting amplifier) CMOS receiver. The TIA is
implemented using a three stages classic feedback architecture with peaking
inductors to enhance the high-frequency response, and the LA is a 5-stage
differential amplifier also using hybrid resistive-inductive loading. More
details on the TIA-LA design can be found in [17].
The TIA was designed for use with a standard stand-alone, surface-illuminated
detector which has a much higher capacitance than a waveguide integrated one.
The potential sensitivity enhancement achievable by increasing the
transimpedance gain of the TIA and enabled by the smaller photodetector
capacitance (estimated to be between 10 and 15 fF) was, therefore, not
exploited in this experiment. A picture of the full receiver during waferscale
test is shown in Figure 6: the RF probes used to pick up the high speed signal
are visible in the top part of the picture while the electrodes used to bias
the TIA-LA and the detector are on the left. The light was coupled by means of
a grating coupler from a fiber array, partially visible in the bottom part of
the picture, to an Si waveguide feeding the photodetector. A typical eye
diagram at 10 Gbps is shown in  Figure 7. The TIA-LA power supply voltage was 1.4 V while the
detector was biased at 0.3 V. Power level at the detector was −10.5 dBm. The
measured bit error rate as a function of the optical power at the detector is
shown in Figure 8 indicating a sensitivity of −14.2 dBm. The input
signal for this experiment was a 10 Gbps, 27 PRBS (pseudo-random
binary sequence).Figure 6
Optical microscope picture of a TIA-LA with LuxG
waveguide photodetectors during waferscale test.Figure 7
Typical eye diagram at 10 Gbps.Figure 8
Bit error rate as a function of optical power for the 10 Gbps
monolithic receiver as measured during waferscale test. Light wavelength is
1550 nm.


## 5. Conclusions

In this paper, we discuss our approach to monolithic
integration of near infrared Ge waveguide photodetectors in a CMOS process.
Waveguide detectors with 0.85 A/W responsivity at 1550 nm and speed in excess
of 20 GHz are shown. A monolithic CMOS receiver using Ge waveguide
photodetectors and operating at 10 Gbps with a sensitivity better than −14 dBm
is also demonstrated.



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*Source: 196572-2008-06-16.xml*