Near-infrared Silicon Schottky Photodiodes based on Non-metallic
Materials
Maurizio Casalino
1
, Mariano Gioffrè
1
, Mario Iodice
1
, Sandro Rao
2
and Giuseppe Coppola
1
1
Institute for Microelectronics and Microsystems (CNR), Via P. Castellino n.111, Naples, Italy
2
Università degli Studi “Mediterranea” di Reggio Calabria, Dipartimento di Ingegneria dell'Informazione, delle
Infrastrutture e dell'Energia Sostenibile (DIIES), Via Graziella Feo di Vito, 89122, Reggio Calabria, Italy
Keywords: Internal Photoemission Effect, Photodetector, Fabry-Perot, Near-infrared, Silicon, Graphene.
Abstract: In this work we have investigated the performance of Schottky photodetectors based on materials non-
conventionally used to detect near-infrared wavelengths. In the proposed devices the absorption mechanism
is based on the internal photoemission effect. Both three-dimensional (sputtered erbium and evaporated
germanium) and two-dimensional materials (graphene) have been considered and their performance
compared. Our insights show that silicon Schottky photodetectors have the potentialities to play a key role
in the telecommunications opening new frontiers in the field of low-cost silicon photonics.
1 INTRODUCTION
In order to develop all-Si photodetectors (PDs) and
to take advantage of low-cost standard Si-CMOS
processing technology without additional material or
process steps, a number of options have been
proposed: two-photons absorption (TPA) (Casalino,
2010), incorporation of optical dopants/defects with
midbandgap energy levels into the Si lattice
(Casalino, 2010) and internal photoemission effect
(IPE) (Casalino, 2010). IPE is the optical excitation
of electrons in the metal to energy above the
Schottky barrier and then transport of these electrons
to the conduction band of the semiconductor. The
standard IPE theory is due to Fowler (Fowler, 1931).
However, the Fowler’s theory was obtained without
taking into account the thickness of the Schottky
metal layer. The enhancement of IPE in thin metal
film was theoretically investigated by Vicker who
introduce a multiplicative factor to the Fowler’s
formula (Vickers, 1971). In addition, a further
enhancement in IPE can be obtained due to the
increase of the reverse voltage that lowers the
Schottky barrier increasing the amount of emitted
carriers (Casalino, 2010). Concerning PDs, due to
the very low signal-to-noise ratio, for a long time
IPE-based Si PDs at infrared (IR) wavelengths were
believed usable only at cryogenic temperature.
However, in order to enhance the photoemitted
current with respect to the noise (dark) current,
many structures have been proposed based on:
surface plasmon polaritons (SPP) (Berini, 2012),
micro- and nano-metric optical waveguide
(Goykhman 2012), optical microcavity (Casalino,
2006). Although optical microcavity (Casalino,
2009) has been adopted with success to increase
device performance (Casalino, 2008), efficiency of
Schottky PDs based on conventional metals or
silicides remains two order of magnitude lower than
commercial PDs based on III-V compounds (i.e.,
InGaAs).
In this work we investigate the performance of
Schottky PDs based on materials not conventionally
used to detect near-infrared wavelengths. Both three-
dimensional (sputtered erbium and evaporated
germanium) and two-dimensional materials
(graphene) have been considered and their
performance compared. Our insights show that
silicon devices based on IPE are already suitable for
power monitoring applications and they could play a
key role in the telecommunications opening new
frontiers in the field of low-cost silicon photonics.
Casalino, M., Gioffrè, M., Iodice, M., Rao, S. and Coppola, G.
Near-infrared Silicon Schottky Photodiodes based on Non-metallic Materials.
DOI: 10.5220/0005740902590263
In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 261-265
ISBN: 978-989-758-174-8
Copyright
c
2016 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
261
2 IPE ENHANCEMENT BY AN
OPTICAL MICROCAVITY
In 2008, we experimentally demonstrated the
influence of the optical microcavity on IPE for a
Schottky Si junction (Casalino, 2008). In this work
we proposed a device realized by a resonant cavity
Fabry-Perot structure formed by a dielectric bottom
mirror, a metallic top mirror and, in the middle, a
silicon cavity (Fig. 1). The dielectric bottom mirror
was a distributed Bragg reflector (DBR) formed by
alternating layers of amorphous hydrogenated
silicon (aSi:H) and silicon nitride (Si
3
N
4
) having
λ/4 thicknesses, while, the top mirror was realized
by a copper (Cu) layer working both as absorbing
material and as optical mirror.
Responsivity measurements were carried out in the
range of 1545–1558 nm (step of 0.05 nm) showing a
maximum value of 4.3 µA/W. In addition, a peak
responsivity enhancement, due to the increased
finesse, of about three times, was demonstrated.
Then, in the 2010 we proved that by reducing the
size of the device a further increase in responsivity
of twice (its value was about 8 μA) could be
obtained (Casalino, 2010). Finally, in 2012 we
proved that a significant increase in responsivity
could be obtained by fulfilling the critical coupling
conditions (Casalino, 2012). Main results of
(Casalino, 2012) are reported in Fig. 2 where
responsivity versus the wavelength is characterized
by four distinct peaks deriving from interference
phenomenon. The measured free spectral range of
3.3 nm is in a perfect agreement with the cavity
thickness (100 µm) while the maximum measured
responsivity is 0.063 mA/W. It is worth noting that
although responsivity has been increased due to the
cavity effect, its value remains quite low due to the
intrinsic poor characteristic of the active material
used. In fact, metals are characterized by a high
reflectivity reducing the amount of absorbed photons
that can be converted into electrons. In addition,
carriers excited in states far below the Fermi energy
gets very low probability to overcome the Schottky
barrier. Although efficiency has been increased in
integrated device (Casalino, 2013) it remains quite
poor in vertically-illuminated devices.
Figure 1: Schematic cross section of the PD reported in
(Casalino, 2008).
Figure 2: Responsivity vs wavelength of the device
proposed in (Casalino, 2012).
In this context, the intrinsic limitations of metals
and silicides could be overcome by investigating IPE
through new not conventional metallic materials.
Indeed, in this work, both three-dimensional
(sputtered erbium and evaporated germanium) and
two-dimensional materials (graphene), have been
considered. The advantage in using non-metallic
materials as absorbing active layer is linked to the
fact that metals behaves like mirrors reflecting back
the most part of the incoming radiation. In other
words, only a little amount of the infrared radiation
can be absorbed hindering device efficiency. For
instance our ellipsometric characterizations show
that at 1550 nm reflectivity at Si/Cu and Si/Er
interface are 0.97 and 0.2, respectively, as reported
in Fig. 3. This means that Cu and Er will be able to
absorb 0.03 and 0.8, respectively. Er absorbs much
more than Cu and we will expect that efficiency of
PDs based on Er/Si Schottky junction will be at least
PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology
262
one order of magnitude higher with respect to PDs
based on Cu/Si.
Figure 3: Reflectivity at Er/Si and Cu/Si interfaces.
3 DEVICE FABRICATION AND
PRELIMINAR RESULTS
Devices reported in Fig. 1 have been fabricated
starting from a bi-polished 200-μm-thick very lightly
doped P silicon. Substrate was chosen slightly doped
in order to avoid free carrier absorption. After a
RCA Si cleaning, substrate has been thermally
oxdidized in order to obtain a 100-nm-thick silicon
dioxide (SiO
2
).
The collecting Ohmic contact and the Schottky
contact were both realized on the top of the sample.
The collecting contact was made by a ring of 200-
nm-thick aluminium film, thermally evaporated at
310
-6
mbar and 150 °C, patterned by a SiO
2
wet
etching and lift-off process of a Shipley S1813
photoresist which, deposited by a spin-coater at
4000 rpm, had a thickness of 1.4 µm. Then, an
annealing at 475 °C in nitrogen for 30 min, in order
to get a not-rectifying behaviour, was carried out
(Card, 1976).
On the wafer back side, a multilayer Bragg
mirror was fabricated by Plasma Enhanced
Chemical Vapor Deposition technique (PECVD).
The mirror was composed by a quarter-wave stack
of aSi:H and Si
3
N
4
layers, having nominal
refractive index, at 1550 nm, of 3.52 and 1.82,
respectively. The reflector was realized with five
periods of aSi:H/Si
3
N
4
pairs, whose nominal
thicknesses are 110 nm and 213 nm, respectively.
Silicon nitride was deposited at pressure of 1.2
mbar, temperature of 250 °C, at 30 W of RF power.
In the deposition chamber 10 sccm of NH
3
, 88 sccm
of SiH
4
(5% in He) and 632 sccm of N
2
are flowed.
The deposition rate was about 23 nm/min and the
suited Si
3
N
4
thickness was obtained with a process
time of about 9 min. Amorphous hydrogenated
silicon, instead, was deposited at pressure of 0.8
mbar, temperature of 250°C, power of 2 W and a
SiH
4
(5% in He) flow of 600 sccm. The a-Si:H
deposition rate was about 3 nm/min and the desired
thickness was obtained with a process time of about
35 min. A second photolithographic process has
been carried out in order to realize two large PADs
useful to connect the device to the macroscopic
world. To this aim 5-nm-thick chromium (Cr) and
100-nm-thick- Gold (Au) have been thermally
evaporated at 310
-6
mbar and 150 °C and patterned
by a lift-off process.
Finally, the Schottky contact was fabricated. The
top of the wafer was covered by Shipley S1813
photoresist, exposed and developed in order to
obtain a disk surrounded by the Al ring Ohmic
contact, as shown in Fig. 3.
Figure 4: Top view of the proposed device before
depositing the active material.
Then the active materials was deposited and
eventually patterned. On the device shown in Fig. 4
we have deposited three materials: erbium,
germanium and graphene.
A Radio-Frequency (RF) Sputtering technique
was used for depositing the Erbium (Er) thin film
directly on the device, from a 99,9% pure metal Er
target. The device was placed on the substrate
holder and the deposition chamber was pumped
down to a base pressure of 3·10
–6
mbar before
introducing the process gas (Ar). Er film was then
deposited at r.t., with 30 W RF power, at 2.5·10
–2
mbar pressure, with a constant 40 scan Ar flux and
11 min deposition time. To overcome the target
surface oxidation a 30 min presputtering process at
150 W RF power was necessary before the
Near-infrared Silicon Schottky Photodiodes based on Non-metallic Materials
263
deposition process.
Germanium (Ge) was thermally evaporated at a
pressure of 3
×
10
-6
mbar and at a temperature of 150
°C. Substrate was put in rotation in order to increase
the uniformity of the deposited layer resulting 100-
nm-thick film.
Graphene has been grown on a copper foil at
1000°C in a chemical vapour deposition (CVD)
chamber under the flow of CH
4
and H
2
. After
growth, PMMA has been deposited on top of
graphene followed by etching of the copper foil.
Then, graphene has been transferred onto the silicon
substrate and the PMMA layer has been removed. A
characterization of the Raman spectrum of graphene
transferred onto substrate has been performed in
order to confirm the monolayer nature of the CVD-
grown graphene.
Figure 5: IV characteristic of the Er/pSi PD.
Figure 6: IV characteristic of the Ge/pSi PD.
We have characterized the I-V behaviour of the
three junctions in order to extract the Schottky
barrier height Φ
B
(Aubry, 1994). In all three cases,
the rectifying I-V characteristic confirms the
Schottky nature of the junction. The IV
characteristics of the Er/p-Si and Ge/p-Si Schottky
junction have been reported in Fig. 5 and 6,
respectively.
The potential barrier of Er/p-Si and Ge/p-Si are
0.49 eV, 0.50 eV, respectively. Moreover the
graphene/p-Si Schottky barrier has been measured as
0.55 eV. It can be derived that the cut-off
wavelengths for the aforementioned junction are
2.53 μm, 2.48 μm and 2.25 μm, respectively, and
thus they are able to detect near-infrared
wavelength.
In addition, preliminary responsivity
measurements have been carried out with the
experimental set-up shown in Fig. 7. The IR light
beam emitted by a wavelength tunable laser, has
been collimated, chopped and focused onto the
device by a long working distance 50X microscope
objective providing a beam diameter of about 5 µm.
On the other hand, a 20X collecting objective
microscope placed at the back of the device has been
used to addresses light on an InGaAs Near-Ifrared
CCD showing when the light impinges on the active
material disk and thus simplifying the alignment
procedure. A lock-in amplifier measures the
photocurrent produced by our device. A trans-
impedance amplifier is employed to provide a
reverse bias voltage to the photodetector, and at the
same time for reducing the dark current. The dark
current cancellation circuit realized by using a trans-
impedance amplifier has a limited bandwidth,
however it is adequate for our scope, that is DC or
quasi-static measurements. Incident optical power
was measured with the same experimental set-up,
replacing our PD with a calibrate commercial
InGaAs PD whose responsivity is very close to the
unity.
Figure 7: Experimental set-up for external responsivity
measurements.
PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology
264
Some preliminary measurements show that the
devices based on Ge are characterized by a very low
responsivity of 0.02 mA/W. On the other hand, Er
and graphene are characterized by a responsivity of
about 0.2 mA/W and 0.08 mA/W, respectively.
These values are two and one order of magnitude
higher than the same devices realized with metals,
respectively (Casalino, 2008). Proposed devices
show the potentialities to play a key role in the field
of silicon photonics.
4 CONCLUSIONS
In this paper we have investigated the responsivity
of Schottky photodetectors based on materials non-
conventionally used to detect near-infrared
wavelengths. Both three-dimensional (sputtered
erbium and evaporated germanium) and two-
dimensional materials (graphene) have been
considered and their performance compared. We
have characterized the I-V behaviour of the three
junctions in order to extract the Schottky barrier
height Φ
B
. The potential barrier of Er/p-Si and Ge/p-
Si are 0.49 eV, 0.50 eV, respectively. Moreover the
graphene/p-Si Schottky barrier has been measured as
0.55 eV. It can be derived that the cut-off
wavelengths for the aforementioned junction are
2.53 μm, 2.48 μm and 2.25 μm, respectively, and
thus they are able to detect near-infrared
wavelength. Some preliminary measurements show
that the devices based on Er and graphene are
characterized by a responsivity of about 0.2 mA/W
and 0.08 mA/W, respectively. These values are two
and one order of magnitude higher than the same
devices realized with metals, respectively. Our
insights show that silicon Schottky photodetectors
have the potentialities to play a key role in the
telecommunications opening new frontiers in the
field of low-cost silicon photonics.
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