Wafer scale millimeter-wave integrated circuits based on epitaxial graphene in high data rate communication

Abstract

In recent years, the demand for high data rate wireless communications has increased dramatically, which requires larger bandwidth to sustain multi-user accessibility and quality of services. This can be achieved at millimeter wave frequencies. Graphene is a promising material for the development of millimeter-wave electronics because of its outstanding electron transport properties. Up to now, due to the lack of high quality material and process technology, the operating frequency of demonstrated circuits has been far below the potential of graphene. Here, we present monolithic integrated circuits based on epitaxial graphene operating at unprecedented high frequencies (80–100 GHz). The demonstrated circuits are capable of encoding/decoding of multi-gigabit-per-second information into/from the amplitude or phase of the carrier signal. The developed fabrication process is scalable to large wafer sizes.

Full text

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Scientific RepoRts | 7:41828 | DOI: 10.1038/srep41828
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Wafer scale millimeter-wave
integrated circuits based on
epitaxial graphene in high
data rate communication
Omid Habibpour1, Zhongxia Simon He1, Wlodek Strupinski2, Niklas Rorsman1 &
Herbert Zirath1
In recent years, the demand for high data rate wireless communications has increased dramatically,
which requires larger bandwidth to sustain multi-user accessibility and quality of services. This can
be achieved at millimeter wave frequencies. Graphene is a promising material for the development of
millimeter-wave electronics because of its outstanding electron transport properties. Up to now, due
to the lack of high quality material and process technology, the operating frequency of demonstrated
circuits has been far below the potential of graphene. Here, we present monolithic integrated
circuits based on epitaxial graphene operating at unprecedented high frequencies (80–100 GHz). The
demonstrated circuits are capable of encoding/decoding of multi-gigabit-per-second information into/
from the amplitude or phase of the carrier signal. The developed fabrication process is scalable to large
wafer sizes.
To meet the fast growing demand for telecommunication services, developing high-data-rate communication
links in the range of multi-gigabit per second (Gbps) is necessary. e high speed data links can be implemented
using either wireless or ber optic technologies. Wireless technology, particularly in urban areas, has several
advantages over ber optics such as mobility, universal deployment, short installation time and cost eectiveness.
However, to achieve data rates comparable to that of the ber optics, there is a need to develop wireless systems
with a very large bandwidth (∼ 10 GHz). is may be achieved by operating at millimeter wave (mm-wave) fre-
quencies (30–300 GHz)1. Even though mm-wave covers a broad range of frequencies, only a certain part of the
spectrum is suitable for wireless transmission. is is because atmospheric absorption is only relatively small
in these so-called atmospheric windows. e mm-wave atmospheric windows are centered at 35, 90, 140 and
220 GHz2. ere is therefore a special interest in the 90 GHz band since it simultaneously oers a low loss medium
and a large band width (up to 30 GHz). Hence, development of electronic circuits operating in this band is a huge
step forward for the realization of multi-Gbps wireless links.
In this regard, graphene is a promising material for the development of mm-wave electronics due to its excel-
lent electron transport properties. Recently, there is a rapid progress in the development of graphene eld eect
transistors (G-FETs). G-FETs with intrinsic current-gain cuto frequencies (fT) of 400 GHz and maximum oscil-
lation frequency (fMAX) of 100 GHz have been demonstrated3,4. In addition, many G-FET based circuits including
frequency multipliers5–8, mixers6,9–12, ampliers13–16 and power detectors17–20 have been presented. Most of the
demonstrated circuits so far are not integrated circuits (ICs) requiring external circuitries for operation. ICs allow
for high frequency and complex circuits but at the cost of laborious fabrication process. At mm-wave frequencies,
broadband circuits can practically only be realized in IC technology. Up to now, there are only few demonstra-
tions of graphene based ICs performing complex wireless communication functions such as signal modulation
and demodulation (encoding/decoding information into/from a carrier signal)21–24. e operating frequency of
the presented ICs is mainly restricted to a few GHz which is far below the potential of graphene. In addition, the
demonstrated data rate is limited to tens of megabits per seconds (Mbps) which is too low for high data rate com-
munications. To compete with existing technologies in high frequency applications, graphene lms should exhibit
1Chalmers University of Technology, Gothenburg 41296, Sweden. 2Institute of Electronic Materials Technology,
Wolczynska 133, 01-919 Warsaw, Poland. Correspondence and requests for materials should be addressed to O.H.
(email: omid.habibpour@chalmers.se)
Received: 22 August 2016
Accepted: 28 December 2016
Published: 01 February 2017
OPEN

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a high carrier mobility as well as a low sheet resistance. ese properties can be found in hydrogen intercalated
epitaxial graphene on silicon carbide (SiC) substrate25. Even though processing ICs on SiC substrate is very chal-
lenging, it paves the way for the realization of graphene based high speed data communications.
Here, we present results on monolithic mm-wave IC (MMIC) based on epitaxial graphene in high data rate
applications in the 90 GHz band. e developed process is scalable up to full wafer sizes. Currently the limiting
factor is not the wafer size but the uniformity of available epitaxial graphene resulting in about 70% yield. e
fabricated MMIC has dierent circuits elements capable of receiving and retrieving information embedded in
the amplitude and phase of the carrier signal at the rate of 4 Gbps. Furthermore, the developed circuits are highly
linear allowing to generate modulated signal up to the rate of 8 Gbps at 90 GHz band with a bit-error-rate (BER)
below 10−5. e operating frequency is about 20 times higher and the achieved data rate is more than 200 times
better than the previously reported graphene based IC24. is work elevates graphene based radio frequency (RF)
ICs’ performance to the level that start competing with the existing matured technologies.
Results
Epitaxial graphene. Typically, the term “epitaxial graphene” refers to graphene grown on SiC which is in
fact not done by epitaxy but by sublimation of silicon. In this study graphene is grown by traditional Chemical
Vapor Deposition (CVD) epitaxy using carbon precursor or more accurately by Vapor Phase Epitaxy (VPE)25.
is method enables the growth of carbon layers directly on SiC surface on both silicon (Si) and carbon (C)
polarities with the precision of synthesizing a pre-dened number of carbon layers. e CVD graphene has been
studied for both Si-terminated and C-terminated SiC, however, more attention is directed to Si-face growth due
to its higher accuracy. To reduce the substrate eect on the synthesized graphene, hydrogen atoms are interca-
lated and consequently quasi-free-standing (QFS) graphene26 is formed. QFS-graphene exhibits much higher and
temperature independent carrier mobility, which is desirable for high-speed electronics. e carrier mobility and
sheet resistance in our material are 3500–6500 cm2/V and 150–250 Ω /Vs respectively. More details of graphene
growth and its properties can be found in the method and Supplementary information sections.
Graphene based high data rate wireless communication landscape. Figure1a shows a prospective
application for graphene based high speed electronics. It is a point-to-point high capacity wireless link operating
Figure 1. Graphene based high data rate communication landscape. (a) A perspective view of high data rate
link based on graphene transmitter (Tx) and receiver (Rx). (b) Fabricated chip on a 70-μ m thick SiC (chip size:
15 × 15 mm2) consists of frequency mixer ICs (le, circuit size: 1.35 × 1.1 mm2) and integrated power detector
ICs (right, circuit size: 1.35 × 0.7 mm2).

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at 90 GHz atmospheric window. Simplied block diagrams of a typical transmitter and receiver are also shown.
e basic function of a transmitter is to modulate the data into a high frequency sine wave carrier signal that
can be radiated by an antenna. e information can be mapped into either amplitude, phase or frequency of the
carrier signal by a modulator. To use the RF spectrum more eciently the carrier of the modulated signal need to
be converted to higher frequencies. is can be done by frequency mixers. To provide frequency conversion, an
external sinusoidal source called local oscillator (LO) is needed. Generally, it is dicult to generate such a signal
with sucient power and signal stability at mm-wave frequencies. erefore, there is need for a frequency mul-
tiplier, which generates an output signal whose output frequency is a multiple of its input frequency. Finally, an
amplier can be used to increase the output power of the transmitter for compensating dierent losses between
the transmitter and the receiver. e function of a receiver is to recover the data by more or less reversing the
function of the transmitter components. Hence, the receiver consists of essentially the same RF components.
To achieve complete wireless communication systems based on graphene it is needed to realize all the above
mentioned components with graphene technology. Transistors are versatile elements and by choosing proper
operating points and passive components, all needed functionalities are attainable.
erefore, G-FETs are the only needed graphene based active elements for these circuits. In addition, it is
desirable to have whole transmitter and receiver chain on a single chip which facilitates the very broadband
performance needed for the high data rate applications. Hence, developing G-FET based MMICs is a key require-
ment to obtain graphene based high speed wireless systems.
Circuit performance depends on both device and circuit technology. Device (G-FET) technology has recently
shown a rapid progress. However, to have G-FET based ampliers operating at 90 GHz band, G-FETs with fMAX
more than 200 GHz is needed. erefore, more development in G-FET technology is needed to realize G-FET
based ampliers at this frequency band. However, for other applications where the G-FETs operate in other
modes, the existing G-FETs technology based on hydrogen intercalated graphene is sucient. is is because
this type of graphene exhibits a high carrier mobility with a very low sheet resistance. It results in a reduction
of channel resistance and consequently achieving small time constants in G-FETs needed for higher frequency
operations. Developing microstrip MMIC process on SiC substrates is a laborious task especially at high frequen-
cies where the parasitic elements and unwanted propagation modes can signicantly deteriorate the circuit per-
formance. In addition, graphene compatibility is extremely important and because of that many existing circuit
fabrication technologies cannot be directly applied for the development of graphene based MMICs. ese process
have several steps that can potentially degrade graphene. One way to avoid this problem is to fabricated the pas-
sive elements rst and then transfer graphene and perform G-FETs fabrication24. However, this method cannot
be used for epitaxial graphene since the graphene already exists on the SiC substrate. Based on a Gallium Nitride
HEMT MMIC process27, we have developed a unique fabrication process technology that preserve graphene
property aer more than 30 process steps. Graphene is very susceptible to plasma processes and therefore it
should be encapsulated. is can be performed by utilising a thick (90 nm) layer of Al2O3 deposited by the atomic
layer deposition (ALD) method28. A thick layer of ALD Al2O3 exhibits a poor thermal conductance and a high
tensile stress. Since SiC via hole etching is performed at very high powers, the generated heat causes the Al2O3
layer to crack. is completely damages G-FETs as well. To avoid it, a thin layer (15 nm) of ALD Al2O3 is depos-
ited and it is followed by 100 nm SiN sputtered at a very low density plasma. is process does not damage the
graphene. In addition, the new encapsulated layer can completely protect graphene from the remaining plasma
steps. Furthermore, the new layer has a better thermal conductance and a lower stress and can survive the via hole
etching process. In this process the SiC substrate is thinned down to 70-μ m providing a platform to develop cir-
cuits up to 250 GHz. By using our device and circuit technologies we are able to realize needed RF modules except
ampliers at 90 GHz band. RF signal in mm-wave can be generated by using G-FET based frequency multipliers.
However, since the output power of these frequency multipliers are very low5–8 ampliers are needed to achieve
practical LO sources. e fabricated MMIC and the corresponding circuits are shown in Fig. 1b. In this gure, the
le circuit is a single-ended resistive mixer that can be used for the signal modulation and demodulation as well
as frequency conversions. e passive part of the mixer consists of RF, LO and IF lters as well as an integrated
bias tee to bias the gate of the transistor. In addition, a matching network is used for matching the LO port. e RF
signal is applied to the drain of the G-FET and the IF signal is extracted from the same terminal. e right circuit
is an integrated power detector that can be used for signal demodulation. e same passive elements are used.
e RF signal is applied to the gate of the G-FET and the rectied signal is extracted from the drain terminal. e
I-V characteristic and drain-source resistance of the fabricated 250-nm G-FETs are plotted in Fig.2 showing that
the channel resistance is almost constant around zero drain bias over a wide range of gate voltage. is property
allows the realization of very linear mixers and power detectors, needed for high data rate communications.
MMIC performance in high data rate applications. e most fundamental digital modulation tech-
niques are binary amplitude-shi keying (BASK) and binary phase-shi keying (BPSK) where binary informa-
tion is coded in the amplitude and phase of the carrier signal respectively (Fig.3a). A mixer can be used for
modulation and demodulation of the BASK and BPSK signals as shown in Fig.3b. In this gure b(t) and m(t)
are the binary data and modulated signal respectively. For BASK the binary data should be represented by uni-
polar voltages (0, + V) while for BPSK it is bipolar voltages (− V, + V). BPSK modulation is less susceptible to
the noise than BASK modulation, since the amplitude of the carrier is more aected by the noise than its phase.
e signal quality in high speed communication systems are mainly evaluated and analysed by a special type of
diagram, called an eye diagram. e eye diagram is formed by overlapping the trace of a certain number of bits
in time domain. Due to the bandwidth limitation and device instability, the transitions do not line up on top of
each other completely and an eye-shaped pattern emerges29 (Fig.3c). Much information can be obtained from
the eye diagram such as the amount of noise that can be tolerated, the amount of signal distortion and sensitivity
to the timing error. Dierences in amplitude and timing of bits can cause the eye opening to shrink and degrade

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data transmission quality. A BPSK modulated signal at 90 GHz is used to assess the performance of the G-FET
demodulator MMIC. e input signal is modulated at the rate of 1, 2 and 4 Gbps. e eye diagrams of the demod-
ulated data are shown in Fig.3d. As expected, the opening of the eye diagram shrinks with increasing data rate.
is is because at higher data rates the data quality is more aected by distortion and bit timing error (jitter). e
signal-to-noise ratio (SNR) and noise margin of a 1 Gbps demodulated signal are 11 dB and 17 mV respectively.
For 2 and 4 Gbps signals, the SNR and noise margin are reduced to 6–7 dB and 5 mV. In addition, the jitter is
estimated to be 160–180 ps. is limits the data rate to about 5 Gbps at which the eyes become completely closed.
More details of the mixer performance can be found in the Supplementary information. As mentioned before,
a BASK signal is more susceptible to noise. In addition, it has a lower power eciency since it does not have a
constant envelope. However, it oers a possibility of having a simple demodulator circuit. e circuit is based on
a power detector and a low pass signal (Fig.1b right). is topology does not need a LO signal which makes it to
be more compact and cost eective. G-FETs can be used as a power detector in mm-waves and THz region for
signal rectication and detection.
To evaluate the performance of the power detector based demodulator MMIC, a BASK signal at 90 GHz with
modulation rates of 1, 2 and 4 Gbps is used. Due to the band width limitation of our arbitrary waveform generator,
the BASK signals have triangular-shaped pulses (Supplementary information). e eye diagrams of demodulated
signals are presented in Fig.3e. As can be seen, the relative amplitude variation is larger compared to BPSK espe-
cially at high data rates. In this case, the noise margin is about 1–2 mV. e extracted SNR are approximately 8, 6
and 5 dB respectively and the jitter is 160–180 ps. erefore, the maximum data rate that can be demodulated is
below 5 Gbps.
e performance of the demonstrated G-FET based demodulators and this work is shown in Table1. In the
Supplementary information, details of the power detector performance are presented.
e MMIC mixer circuit can be used for signal modulation as well. As shown in Fig.3b, a BPSK signal is
generated by mixing a bipolar base band signal (± V) and a carrier signal. e spectrum of the generated BPSK
signal at the rate of 1, 2 and 4 Gbps are plotted in Fig.4a, b and c respectively. e carrier frequency is 92 GHz and
the generated signal exhibits a SNR above 20 dB. As expected for the BPSK signal, the null-to-null bandwidth is
about twice the data rate. In addition, due to the signal leakage, the spectrum also contains the LO signal. is can
be mitigated by utilizing more complex modulator circuits29,30. In order to use the available RF bandwidth more
eciently it is needed to utilize more complex modulation schemes. In these modulation schemes more infor-
mation can be embedded into the amplitude and phase of the carrier signal. e in-phase (I) and quadrature (Q)
components of transmitted symbols can be visualized as points on a constellation diagram (Fig.4d). Quadrature
phase-shi keying (QPSK) and 16-quadrature amplitude modulation (16-QAM) have 4 and 16 symbols rep-
resenting 2 and 4 bits per symbol respectively. Generally, it is dicult to carry out these types of modulations
directly at mm-wave frequencies. Hence, the signal modulation is performed at low frequencies and by using
a mixer, the carrier frequency is up-converted to the desired frequencies. To keep the modulation constellation
intact, it is necessary to have a highly linear mixer. As shown in Fig.2b, the channel resistance of the G-FET is
almost constant over a wide range of gate voltages. is suggests that G-FET based mixer will exhibit a high lin-
earity. To evaluate the performance of the mixer, QPSK and 16-QAM modulated signals at 4 GHz are used. e
8 Gbps QPSK modulated signal is up-converted to 88 GHz by using the MMIC mixer. e spectrum of the signal
Figure 2. G-FET DC characteristic. (a) Output characteristics of the G-FET with Vgs ranging from − 1 to 1 V
with step of 0.25 V. (b) Drain-source resistance versus drain bias.

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is plotted in Fig.4e. e data is retrieved from the lower sideband and the corresponding constellation is shown in
Fig4g. From this diagram a BER of 10−6 can be estimated. e experiment is repeated by 16-QAM signals modu-
lated at the rate of 4, 8 and 16 Gbps. e reconstructed constellation diagrams are plotted in Fig.4h, i and j and the
extracted BERs are about 10−6, 10−5 and 10−4 respectively. It can be seen that the achieved constellation diagrams
are not distorted meaning that the mixer is highly linear. Fig4f shows the spectrum of the 16-QAM signal mod-
ulated at the rate of 8 Gbps. It is seen that the required bandwidth is about half of the QPSK modulation with the
same data rate as expected. Table2 summarizes the performance of multi-Gbps modulators in other technologies
(at 70–90 GHz) as well as our results.
Discussion
To achieve high frequency devices base on graphene having a high carrier mobility is not sucient. Graphene
lms should have high carrier density as well in order to obtain low sheet resistance levels needed for high fre-
quency applications. ese properties exist in hydrogen intercalated epitaxial graphene and thereby it is more
benecial for RF devices and circuits. e development of G-FET MMICs that can perform multi-Gbps signal
modulation and demodulation at mm-wave makes a start on the realization of complete high data rate transceiv-
ers based on graphene. Prior to this work, the operating frequency and data rate of the demonstrated graphene
Figure 3. Signal modulation and eye diagram. (a) Binary amplitude and phase digital modulation. (b) BASK
and BPSK signal modulation and demodulation by using a mixer. (c) Formation of an eye diagram by using
two consecutive bits. (d,e) Eye diagrams formed by demodulation of 90 GHz BPSK (d) and BASK (e) signals
respectively at the rate of 1, 2 and 4 Gbps using G-FET MMICs.
Reference Gate length (μm)
Carrier mobility
(cm2/Vs) Sheet resistance (Ω/Vs) Operation Mode Data r ate Operation Frequency (GHz)
18 1 900–1000 700–800 Direct detection 20 Mbps 5
is work 0.25 3500–6500 150–250 Direct detection 4 Gbps 90
24 0.9 1000–3000 1000–2000 Heterodyne 20 Mbps 5
is work 0.25 3500–6500 150–250 Heterodyne 4 Gbps 90
Table 1. Comparison of G-FET based demodulators performance.

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based ICs are far below the performance of the existing technologies. is is mainly due to the lack of high quality
process technology and material. For high data rate communication, the linearity of the RF components plays a
key role. e fabricated MMIC mixer is highly linear allowing to generate and retrieve multi-Gbps data at 90 GHz
atmospheric window. is work demonstrates G-FETs’ great potential for high data rate communication systems.
To achieve complete wireless communication systems based on graphene, more development in G-FET technol-
ogy is needed to realize ampliers at mm-wave band.
Methods
Growth of epitaxial graphene. Epitaxial graphene is grown at 1600 °C and 30 mbar pressure by CV tech-
nique on (15 × 15) mm2 nominally on-axis 4H-SiC (0001) or 6H-SiC (0001), Si-face semi-insulating chemo-me-
chanically polished substrates. Graphene layers are grown under an argon laminar ow at hot-wall Aixtron
VP508 and Aixtron G5 reactors where graphene growth on 4″ SiC substrates is under development. e process
Figure 4. Spectrum and constellation of modulated signals. (a–c) Spectrum of BPSK modulated signal
at the rate of 1, 2 and 4 Gbps respectively. (d) Constellation diagram for BASK, BPSK, QPSK and 16-QAM
modulations. (e,f) Spectrum of up-converted 8 Gbps QPSK and 16-QAM signals. (g) Constellation diagram for
8 Gbps QPSK signal. (h,i,j) Constellation diagram for 4, 8 and 16 Gbps 16-QAM signals respectively. e green
crosses are the expected values and the purple dots are the measured points.
Reference Frequency (GHz) Modulation Data rate (Gbps) Technology
31 70–80 16 QAM 10
100 nm
GaAs
p-HEMT
32 70–80 QPSK 18 130 nm SiG
BiCMOS
33 83–88 BPSK/QPSK 2.5 65 nm
CMOS
34 92–95 16 QAM 6
100 nm
GaAs
p-HEMT
is work 80–90 16 QAM 8250 nm
G-FET
Table 2. Performance comparison of the G-FET MMIC modulator with existing technologies at 70–90 GHz
(BER < 10−5).

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depends critically on the creation of the ow conditions in the reactor that control Si sublimation rate and enable
mass transport of hydrocarbon to the SiC surface. Reynolds number (Re) measures the ratio of inertial forces to
viscous forces and consequently quanties the relative importance of these two forces in a given gas ow. Tuning
the value of the Re number allows to form a thick enough Ar boundary layer to prevent Si sublimation. is lets
the diusion of hydrocarbon to the SiC surface and growth of epitaxial graphene. e intercalation of hydrogen
is obtained in situ at the temperature of 1100 °C and 900 mbar pressure. In our samples the presence of hydrogen
atoms is maintained up to 700 °C, high enough to meet the requirements of high-speed electronics and high-tem-
perature sensing.
MMIC fabrication. e fabrication process has about 30 dierent steps including 3 ebeam lithography
and 13 photolithography steps. Ebeam-lithography is used for the fabrication of G-FETs since ebeam resists are
more compatible with graphene. For G-FET passivation a combination of ALD Al2O3 (15 nm) and sputtered SiN
(100 nm) layers are utilized. e rest of the fabrication process are based on photolithography. For forming thin
lm resistors, tantalum Nitride (TaN) is sputtered. e metal-insulator-metal (MIM) capacitors are based on SiN.
Aer the fabrication of MIM capacitors, dielectric via etching is performed to reach to the G-FET metal pads for
the deposition of the large pads and transmission lines. Gold electroplating is done to reduce the metal resistivity
and form air-bridges. en a thick layer of resist is used to protect the surface and the chip is glowed to a carrier
wafer for the back side processing. e substrate is thinned down to 70 μ m and polished by a CMP process. For
via hole formation, a thick layer of nickel is plated as a hard mask and high power ICP is used to etch via holes.
e nickel mask is wet striped and a seed layer is sputtered on the back side of the chip. Finally, gold is plated for
the metallization of the ground plane and via holes.
MMIC measurement. An Agilent signal generator and an OML S10MS W-band source module are used to
provide the LO signal. e digital modulated signals at baseband and intermediate frequency are generated by a
Keysight M8195A 65-Gsps sampling rate 8-bit arbitrary waveform generator. e millimeter wave output is meas-
ured by a Lecroy Labmaster 10-100Zi real time oscilloscope which has 100 GHz input bandwidth and 240 Gsps
sampling rate. High data rate BASK (on o keying) signal is generated by an in-house fabricated W-band fre-
quency multiplier module based on InP DHBT MMIC. e data captured by the real time oscilloscope is pro-
cessed using Matlab code.
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Acknowledgements
is work is supported in part by the European Union’s Horizon 2020 research and innovation program under
grant agreement No. 696656 and in part by the Knut and Alice Wallenberg foundation.
Author Contributions
H.Z. designed the circuits; O.H. and N.R. developed the fabrication process and O.H. fabricated MMICs; W.S.
synthesised the epitaxial graphene; Z.S.H. and O.H. performed MMIC measurement and analysed the data;
O.H. wrote the manuscript. All authors discussed the results and commented on the manuscript.
Additional Information
Supplementary information accompanies this paper at http://www.nature.com/srep
Competing nancial interests: e authors declare no competing nancial interests.
How to cite this article: Habibpour, O. et al. Wafer scale millimeter-wave integrated circuits based on epitaxial
graphene in high data rate communication. Sci. Rep. 7, 41828; doi: 10.1038/srep41828 (2017).
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