Download Graphene light modulators working at near

Vol. 25, No. 9 | 1 May 2017 | OPTICS EXPRESS 10255
Graphene light modulators working at nearinfrared wavelengths
D. E. AZNAKAYEVA,* F. J. RODRIGUEZ, O. P. MARSHALL, AND A. N.
GRIGORENKO
School of Physics and Astronomy, Manchester University, Manchester, M13 9PL, UK
*
[email protected]
Abstract: We demonstrate a graphene-based electro-absorption modulator with extremely
small modulation volume that can be controlled by low gating voltages 1-3 V and shows light
modulation at wavelengths as short as 900 nm. Our choice of hafnium oxide dielectric
separator gives the possibility to obtain significant electro-optical effect in a simple optical
heterostructure. Having low power consumption, our devices could find a wide range of
applications in telecom industry.
© 2017 Optical Society of America
OCIS codes: (220.0220) Optical design and fabrication; (250.0250) Optoelectronics; (230.4110) Modulators.
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Journal © 2017
https://doi.org/10.1364/OE.25.010255
Received 27 Feb 2017; accepted 21 Mar 2017; published 25 Apr 2017
Vol. 25, No. 9 | 1 May 2017 | OPTICS EXPRESS 10256
1. Graphene based low-voltage modulators
Fast electro-optical modulators with small area footprints, low operating voltages, and low
power consumption are in great demand by the optoelectronic industry. Such modulators can
be created using the unique optoelectronic properties of graphene combined with high-k
dielectrics [1]. High-k metal oxides are promising materials for dielectric spacer in graphenebased optical modulators due to their electrical and optical properties, as well as their
compatibility with the fabrication of CMOS devices. HfO2 is the most promising dielectric to
replace SiO2 due to its high k value, large band gap energy (5.68 eV) [2], thermal stability and
ease of fabrication and integration. Furthermore, HfO2 is an ionic crystal which has low
potential of ionization [3]. Through strong ionic bonding and high Madelung energy, pure
stoichiometric HfO2 is stable and unlikely to engage in chemical reactions. On the other hand,
graphene is increasingly being considered an extremely useful material, which can help in the
creation of efficient electro-optical modulators [4–13]. Graphene shows tunability of its
optical conductivity by gating over a broad range of wavelengths, from mid-infrared to nearinfrared [14]. Other significant advantages of graphene include its chemical inertness,
mechanical stability, durability and ease of integration into optical devices. The simplicity of
graphene monolayer fabrication, the possibility of direct assessment defect densities using
Raman spectrometry, high transfer precision and integration with arbitrary structures, all pave
the way toward graphene based low loss electro-optical modulators. In this letter we
demonstrate a planar electro-optical modulator where light modulation is achieved using a
single layer of graphene. The modulator itself is a simple heterostructure, consisting of a thin
layer of metal (copper in our case), a hafnium oxide and graphene. It possesses a small
modulation volume ~5 μm3 (potentially as small as λ3/10), works at low gating voltages (~1-3
V), has low power consumption (~1 nW), low insertion losses (<10%) and shows broadband
light modulation down to a wavelength of λ = 900 nm. Our devices are CMOS compatible
and could find a range of applications in the optoelectronic industry. Therefore, to the best of
our knowledge, this is the first time where the operating parameters listed above have been
achieved in a free space modulator with a solid dielectric (light modulation at near-infrared
and visible wavelengths was only previously achieved with the help of an ionic liquid spacer,
see [15] and [16]).
Figure 1(a) shows a schematic view of the free space graphene broadband electro-optical
modulator. The modulator is essentially a parallel plate capacitor where the light modulation
is performed by electrical control of graphene optical conductivity via applied bias. The
graphene modulator consists of a quartz substrate, a reflective copper mirror, a quarter
wavelength thick high-k HfO2 dielectric layer (the thickness d was chosen to be λ / (4n) ,
where λ is the desired working wavelength of the modulator and n ≈2 is the refractive index
of HfO2 at telecoms wavelengths) and a high quality, defect-free, mechanically exfoliated
graphene monolayer. Figure 1(b) provides a microscope image of a fabricated modulator.
Devices were fabricated as follows. The quartz substrate was cleaned in acetone and
isopropanol (IPA) to ensure a clean surface and complete removal of dust particles, which
could cause additional optical losses due to Rayleigh scattering. Prior to photolithography a
chromium adhesion layer and the copper layer were deposited on the substrate using a
Moorfield electron beam evaporator. The primary advantages of using copper as the bottom
electrode are the good quality flat surface, high optical and electrical conductivity and low
cost. As copper is a highly reactive material the hafnium oxide dielectric layer with quarter
wavelength thickness was deposited on top of it immediately in order to protect it and
increase electromagnetic fields at graphene location. The copper layer serves two main
functions in the operation of the electro-optical modulator: it forms a reflective metal mirror
and also works as a back gate electrode for graphene electrostatic doping. Graphene flakes
were obtained from graphite by an exfoliation technique onto a SiO2/Si substrate. Flake
thickness and quality were assessed by optical microscopy and Raman spectroscopy. A
Vol. 25, No. 9 | 1 May 2017 | OPTICS EXPRESS 10257
polymethyl methacrylate (PMMA) layer was spin-coated onto the Si-SiO2-graphene structure
in order to perform a wet transfer of the flake to the HfO2 dielectric. The device was
subsequently cleaned using acetone, IPA and water in order to fully remove the PMMA and
any residue. The electrical contacts to the graphene surface were made by laser-writer optical
lithography, followed by chromium (2 nm thickness) and gold (30 nm) electron beam
deposition and lift-off. During the electrical measurements it was found that the graphene
flakes were p-doped, which could be explained by hydrocarbon contamination and water
adsorption.
Fig. 1. Free space graphene-based electro-optical modulator. (a) Schematic of the device. The
modulator heterostructure consists of a quartz substrate, reflective copper mirror, subwavelength high-k HfO2 dielectric layer and a graphene sheet. (b) Microscope image of a
graphene electro-optical modulator designed to operate at λ = 1 µm.
2. Experimental demonstration of graphene based light modulators
Optical characterization of the graphene based free space modulators was performed using a
Bruker Vertex 80 Fourier transform infrared (FTIR) spectrometer with a Hyperion 3000
microscope, operating in reflection mode. Figure 2 highlights the main results of this work,
where the solid state dielectric has been employed to achieve significant modulation using
small bias voltages. Modulators with small operational voltages have already been reported
for supercapacitors, where the modulation effects were achieved with the help of an ionic
liquid electrolyte. For conventional solid state dielectrics much higher voltages of around 50150 V are usually necessary to observe modulation effects at near infrared wavelengths. By
taking advantage of the electrochemical supercapacitor effect observed in the HfO2 dielectric
we were able to produce solid state modulators working at small operation voltages. The
modulation mechanism can be explained in the following way. The change in reflectance of
the devices is produced by the change in the graphene conductivity due to electrostatic
doping. Interference between the incident electromagnetic wave and the wave reflected by the
bottom copper mirror maximizes the optical field on the graphene layer, increasing the
contrast. Figure 2(a) shows the results from a modulator in which the thickness of copper is
35 nm while the thickness of HfO2 is d = 156 nm. The relative reflectance R(Vg)/R(0) is the
reflectance of the modulator with an applied gate voltage Vg relative to the reflectance of the
unbiased modulator. A modulation depth of 3% was obtained at the telecom wavelength of
1500 nm with an applied bias of 2 V. In fact, the device produces significant modulation
across a large wavelength range (from λ = 1200 nm to >2000 nm). By reducing the thickness
of HfO2, the operational wavelength range of the modulator can be blue shifted. As an
example, Fig. 2(b) demonstrates light modulation in the wavelength range from 900 nm to
>2000 nm for a second device, this time with a smaller thickness of HfO2 (d = 100 nm).It is
worth noting that simple capacitance model for graphene gating [1] suggests that much larger
voltages are required in order to observe the measured modulation depths due to optical Pauli
Vol. 25, No. 9 | 1 May 2017 | OPTICS EXPRESS 10258
blocking (for the studied thicknesses of HfO2). Indeed, the Fermi energy of the graphene ε F
that has central neutrality point at a gating voltage of Vg = VCNP can be estimated at zero
temperature as:
ε F = ν F sign(V )
εV
4de
,
(1)
where V = Vg − VCNP , vF is the graphene Fermi velocity, ε is the permittivity of dielectric
and e is the electron charge. Taking d = 100nm and ε = 18 for HfO2 at low frequencies, we
obtain the gate voltage V required to achieve significant modulation at λ = 1000 nm due to
optical Pauli blocking as V = (π c / λν F ) 2 × (4de / ε ) ≈ 30V , which is an order of magnitude
larger than observed experimentally. To elucidate the reason for the discrepancy between
theory and experiment we measured the temperature dependence of the gating characteristics.
Figure 3 shows the electrostatic gate effect on the graphene electrical resistance at low and
high temperatures. These measurements were performed in a Linkam temperature controlled,
liquid nitrogen cryostat at reduced pressure.
Fig. 2. Light modulation by the graphene-based heterostructures. (a) Spectral dependence of
the relative reflection with a HfO2 thickness d = 156 nm and gate voltages of −1 V (the blue
curve) and −2 V(the red curve). (b) Relative reflection spectra for d = 100 nm, at gating
voltages of −1 V (the blue curve), −2 V (the red curve), −3 V(the green curve). (c) and (d)
Numerical simulations based on Fresnel theory, corresponding to the spectra presented in (a)
and (b) respectively.
Vol. 25, No. 9 | 1 May 2017 | OPTICS EXPRESS 10259
Figure 3(a) shows the graphene resistance as a function of gate voltage. Sweeps were
made at a rate of 3 V/min, while limiting the voltage range to keep the gate current below 10
nA. The broadening of the R(Vg) peak at low temperatures indicates that the low values of
modulation voltage are most probably connected with ionic conductivity of HfO2. The width
of the R(Vg) peak increased by a factor of three when the temperature was lowered from 80°C
to −80°C, which implies that the effective capacitance of the device decreased by the same
order.
The decrease of leakage current with the decrease in temperature, shown in Fig. 3(b), also
indicates that it is the ionic conductivity of HfO2 that is responsible for the supercapacitance
effect observed in our devices. At room (and higher) temperatures the ionic conductivity of
HfO2 is relatively large and the electrostatic doping of graphene is governed by a double layer
near the graphene sheet. This results in the supercapacitance effect and hence the low
modulation voltages. However, at lower temperatures the ionic conductivity drops and gating
is governed by the “normal” capacitance of the structure. Further modelling also supports the
presence of a supercapacitance effect. Figure 2(c) and (d) show the results of Fresnel
modelling for our heterostructures. The optical conductivity of graphene was taken in the
following form [17]:
σ=
ω − 2μ
e2 1 1
i
(ω + 2μ )2
i 8k BT
μ
[ + arctan(
)−
ln(
)+
ln(2cosh
)], (2)
4 2 π
2 k BT
2π (ω − 2μ )2 + 4k B 2T 2 π  ω + iγ
2k BT
where μ is the chemical potential of graphene, T is the temperature, k B is the Boltzmann
constant and γ is the electron collision rate. The last term in the expression represents the
Drude contribution to graphene’s optical conductivity and is negligible at near-infrared and
visible frequencies. The optical constants of copper and HfO2 thin films were extracted by
spectroscopic ellipsometry from test samples deposited on a glass substrate as explained in
Ref [18].
Fig. 3. Electrical properties of a modulator at different temperatures. (a) Graphene sheet
resistance R as a function of gate voltage at temperatures of −80°C, 0°C and 80°C. (b) Leakage
currents through HfO2 dielectric during these resistance measurements.
It is worth noting that graphene layer can effectively safeguard the excellent plasmonic
properties of the underlying copper layer [18] and keep the electrical properties of the
dielectric separator intact. The model provides a good qualitative fit to the measured
reflection spectra; compare Fig. 2(a) and (b) with Figs. 2(c) and (d) respectively. However, in
order to achieve this fit we had to assume that the chemical potential of graphene changes
with gating much faster than is predicted by a simple capacitance model. For example, in
order to achieve a chemical potential of 0.5 eV in a heterostructure with d = 100 nm one
would need to apply a gate voltage of 18 V, whereas the experimental data suggest that this
Vol. 25, No. 9 | 1 May 2017 | OPTICS EXPRESS 10260
value is reached at Vg = −2V, see Fig. 2(b) and (d). This again indicates the presence of a
supercapacitance effect, where the effective thickness of the dielectric separating the charges
is an order of magnitude smaller than the actual physical thickness. A detailed study of the
HfO2 supercapacitance effect will be reported elsewhere.
3. Summary
To summarize, we have fabricated, characterized and tested simple, efficient and low-loss
graphene-based electro-optical modulators working at near infrared wavelengths. A
modulation depth of 3% was achieved at λ = 1.5 µm (and 1.5% at λ = 1 µm), with gate
voltages of just a few volts applied to the graphene heterostructure. The unexpected
supercapacitance effect observed in the high-k HfO2 gate dielectric allowed us to create
extremely simple free space, CMOS compatible, telecom modulators with significant electrooptical modulation effect, low power consumption and small modulation volume. This
supercapacitance effect could be useful for other optoelectronic applications.
Funding
European Commission (EC) under the Graphene Flagship Core 1 (contract no. CNECT-ICT604391).