# Terahertz rectennas on flexible substrates based on one-dimensional metal-
insulator-graphene diodes

Andreas Hemmetter  Advanced Microelectronic Center Aachen (AMICA), AMO GmbH,
Otto-Blumenthal-Str. 25, 52074 Aachen, Germany  Xinxin Yang  Department of
Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296
Gothenburg, Sweden  Zhenxing Wang  Advanced Microelectronic Center Aachen
(AMICA), AMO GmbH, Otto-Blumenthal-Str. 25, 52074 Aachen, Germany<EMAIL_ADDRESS>Martin Otto  Advanced Microelectronic Center Aachen (AMICA), AMO GmbH, Otto-
Blumenthal-Str. 25, 52074 Aachen, Germany  Burkay Uzlu  Advanced
Microelectronic Center Aachen (AMICA), AMO GmbH, Otto-Blumenthal-Str. 25,
52074 Aachen, Germany  Marcel Andree  Institute for High-Frequency and
Communication Technology, University of Wuppertal, 42119 Wuppertal, Germany
Ullrich Pfeiffer  Institute for High-Frequency and Communication Technology,
University of Wuppertal, 42119 Wuppertal, Germany  Andrei Vorobiev  Department
of Microtechnology and Nanoscience, Chalmers University of Technology,
SE-41296 Gothenburg, Sweden  Jan Stake  Department of Microtechnology and
Nanoscience, Chalmers University of Technology, SE-41296 Gothenburg, Sweden
Max C. Lemme  Advanced Microelectronic Center Aachen (AMICA), AMO GmbH, Otto-
Blumenthal-Str. 25, 52074 Aachen, Germany  Daniel Neumaier  Chair of Smart
Sensor Systems, University of Wuppertal, 42119 Wuppertal, Germany

###### Abstract

Flexible energy harvesting devices fabricated in scalable thin-film processes
are important components in the field of wearable electronics and the Internet
of Things. We present a flexible rectenna based on a one-dimensional junction
metal-insulator-graphene diode, which offers low-noise power detection at
terahertz (THz) frequencies. The rectennas are fabricated on a flexible
polyimide film in a scalable process by photolithography using graphene grown
by chemical vapor deposition. A one-dimensional junction area reduces the
junction capacitance and enables operation in the D-band (110 - 170 GHz). The
rectenna on polyimide shows a maximum voltage responsivity of 80 V/W at 167
GHz in free space measurements and minimum noise equivalent power of 80
pW/$\sqrt{\text{Hz}}$.

Chair of Electronic Devices, Faculty of Electrical Engineering and Information
Technology, RWTH Aachen University, Otto-Blumenthal-Str. 2, 52074 Aachen,
Germany Chair of Electronic Devices, Faculty of Electrical Engineering and
Information Technology, RWTH Aachen University, Otto-Blumenthal-Str. 2, 52074
Aachen, Germany Chair of Electronic Devices, Faculty of Electrical Engineering
and Information Technology, RWTH Aachen University, Otto-Blumenthal-Str. 2,
52074 Aachen, Germany Advanced Microelectronic Center Aachen (AMICA), AMO
GmbH, Otto-Blumenthal-Str. 25, 52074 Aachen, Germany

##

Figure 1: Rectenna structure and fabrication. a) Cross-section of a 2D MIG
diode compared to a b) 1D MIG diode with equivalent circuit. c) Key
fabrication steps for the 1D MIG diode. d) Micrograph of a rectenna on
polyimide. e) Photograph of the sample on polyimide after peeling it off the
Si carrier substrate. f) Close-up of a 1D MIG diode at the antenna feedpoint.

Terahertz (THz) radiation is a region of the electromagnetic spectrum with
frequencies between 0.1 and 10 THz 1, 2, 3. Miniaturized THz sources and
detectors enable a variety of applications, such as communications,
surveillance screening, material analysis, biomedical diagnostics and personal
healthcare tracking 1, 2, 4, 5. The emergence of wearable electronics and
networks of small, independent sensors for Internet of Things (IoT)
applications is driving research both in low power electronic circuits and in
energy harvesting on the device or chip level. Miniaturized THz power
detectors may become crucial components that can function as energy harvesting
devices, in particular on flexible thin-film substrates, where they can
overcome the form factor limitations of silicon (Si) electronic chips and can
be fabricated in scalable roll-to-roll processes. Thus, they have the
potential to power decentralized sensor networks, passive readout circuits or
integrated mobile devices without the need for batteries or an external power
supply 6.

Rectennas \- antenna-coupled diodes - are versatile two-terminal devices that
directly rectify a detected signal. Their zero-bias operation makes them
suitable for energy harvesting applications 7, 8. This principle has been well
established in the microwave region since 1966 9, due to the availability of
Schottky diodes with sufficiently short response times 10, 11, 12, 13.

Increased cutoff frequencies in the THz range 14 have been achieved by metal-
insulator-metal (MIM) diodes, which utilize tunneling and thermionic majority
carrier conduction. However, MIM diodes generally show inferior DC performance
compared to conventional p-n junction or Schottky diodes 15, 16. Metal-
insulator-graphene (MIG) diodes (Figure 1a), where the cathode metal of a MIM
diode is replaced by graphene, combine excellent DC performance with high
cutoff frequencies 15, 17, 18, 19. The high charge carrier mobility and
flexibility of graphene 19 allow such devices to be used in flexible THz
rectennas, an application space that has been inaccessible due to the
rigidity, bias and fabrication requirements of conventional semiconductor-
based detectors. We present rectennas based on edge-contacted MIG diodes
(Figure 1b), fabricated in a scalable thin-film compatible process, enabling
high-throughput fabrication. The results point towards the possibility to
integrate rectenna arrays as power supplies in flexible, wearable and
conformal devices, for example wearable biomedical or distributed
environmental sensors.

##

The operation frequency of a diode can be estimated using:

$f_{c}=\frac{1}{2\pi R_{s}C_{b}}$ (1)

where $R_{s}$ is the access resistance and $C_{b}$ is the barrier capacitance
from the diode’s equivalent circuit seen in Figure 1b 19, 20. The low access
resistance of conventional MIM diodes allows them to reach very high cutoff
frequencies in the THz range, however with rather poor rectification
performance 15. MIG diodes, which have a similar layout as MIM diodes (Figure
1a), offer better rectification, but at the expense of a larger access
resistance and therefore lower operation speed, as one metal layer is replaced
by graphene. Furthermore, the current through a two-dimensional (2D) MIG diode
is emitted perpendicularly to the graphene plane across a small ”van-der-Waals
gap” and is therefore affected by surface species at the graphene-insulator
interface. This can increase the junction resistance, hindering an efficient
coupling between antenna and diode18.

By forming a one-dimensional (1D) diode junction only to the graphene edge, as
shown in the schematic cross section in Figure 1b, the junction capacitance
can be significantly reduced, enabling operation frequencies up to the THz
range 21. The effective junction area is then given by $A=w\cdot t$, where $w$
is the channel width and $t$ is the thickness of graphene, about 0.3 nm 22.
This leads simultaneously to a reduced junction capacitance ($C_{b}$),
improved current injection at the insulator-graphene and graphene-metal
interfaces and reduced operating voltage due to the electric field enhancement
at the 1D edge 21.

##

The rectennas consist of edge-contacted metal-insulator-graphene diodes at the
feedpoint of broadband bowtie antennas. The rectennas have been fabricated on
THz-transparent polyimide (PI) films on silicon handling substrates using
standard microprocessing techniques such as contact photolithography and
reactive ion etching. Key fabrication steps are shown in Figure 1c. The PI has
first been covered with an aluminum oxide (Al2O3) layer through plasma-
enhanced atomic layer deposition (ALD). CVD-grown graphene has been
transferred in a wet process 23 onto the substrate and encapsulated with 20 nm
Al2O3 in a thermal ALD process. A photolithographically defined mask has been
used for reactive ion etching (RIE) through the Al2O3 and the graphene. The
resist remains on the chip at this point and acts as a lift-off mask for the
subsequent atomic layer deposition of 5 nm TiO2 insulator, 20 nm sputtered
titanium (Ti) anode metal 18, 19 and 13 nm nickel (Ni). The self-aligned
nature of the process prevents contamination of the graphene edge from
photoresist residues or misalignment of graphene and metal layers, which would
increase the diodes’ capacitance and resistance. This process results in a
one-dimensional MIG junction, because only the graphene edge is in electrical
contact with the TiO2 barrier (Figure 1b), as opposed to the two-dimensional
contact in conventional MIG stacks (Figure 1a). A 25 nm thick Ni contact to
the graphene has been fabricated in a similar way, i.e. by reactive ion
etching through the Al2O3/graphene/Al2O3 stack and subsequent metal
deposition. Here, the graphene edge is in direct contact with the Ni and forms
an ohmic contact 19, 24, 25. At this point, graphene still covers the entire
chip apart from directly below both metal contacts. The remaining graphene has
been patterned into 6 $\mu m$ long and 50 $\mu m$ wide channels, before 100 nm
thick aluminum (Al) has been deposited as antennas, transmission lines and
contact pads.

A key feature of the presented metal-insulator-graphene rectenna is that it is
compatible with conventional thin-film technologies, i.e. it does not involve
high temperature processing steps $>$ 300∘C and uses only $\mu$m-scale
lithography. A micrograph of a fabricated rectenna is shown in Figure 1d. The
polyimide film is removed from the Si carrier substrate after the device
fabrication for electrical measurements. Figure 1e shows a flexible PI
substrate with several THz rectennas. A close-up of the finished diode on a
flexible substrate is shown in Figure 1f.

The transmission behavior of the antennas was designed for broadband
absorption to match the measurement setup tailored to the D band from 110 to
170 GHz. The parameter space was modeled using the electromagnetic finite
element solver HFSS with a lumped port ($Z_{a}$ = 377 $\Omega$) at the
feedpoint. The chosen bowtie antenna design with an opening angle of 30∘ has a
large bandwidth of 43% at 157 GHz (defined as a voltage standing wave ratio
VSWR $\leq$ 2), enabling free space measurements over the D band from 110 to
170 GHz. A pair of 1 mm long metal traces lead from the diode contacts to
larger contact pads for biasing and signal readout. The transmission line
serves as a low-pass filter that rejects the received THz signal and only
allows the rectified DC component to pass.

##

###

Figure 2: DC characteristics of a diode on a polyimide film. a) Linear (blue)
and logarithmic (red) $I$-$V$ curve and b) responsivity $\gamma_{DC}$.

Direct current (DC) characteristics of the rectennas have been measured under
ambient conditions. Current-voltage ($I$-$V$) measurements, where the bias
voltage $V_{DC}$ has been swept from -2 to +2 V, are shown linearly and
logarithmically in Figure 2a. The device shows a non-linear $I$-$V$
characteristic and reaches maximum currents of $I_{DC}=0.8$ mA at a forward
bias voltage of $V_{DC}=2$ V and $I_{DC}=0.1$ mA at $V_{DC}=-2$ V. Typically,
such currents are normalized by the device dimensions in order to compare them
to the state of the art. Here, only the graphene edge emits charge carriers
across the barrier. Taking into account the graphene thickness of
approximately 0.3 nm and the device width of 50 µm, the resulting maximum
current density at the graphene edge exceeds $5\times 10^{6}$ A/cm2 in the
forward direction, consistent with previously reported 1D-MIG diodes by Wang
et al. 19.

The $I$-$V$ curve allows calculating the responsivity $\gamma_{DC}$ of the
diode, one of the decisive figures of merit for RF applications. The
responsivity is a measure for the rectification efficiency of the diode 18:

$\gamma_{dc}=\frac{\text{d}^{2}I/\text{d}V^{2}}{\text{d}I/\text{d}V}$ (2)

As can be seen from its expanded units
($\frac{1}{V}=\frac{A}{VA}=\frac{A}{W}$), a diode’s responsivity is related to
the rectified current at a given input power. The flexible diode reaches a
maximum responsivity of 8 V-1 (Figure 2b) in the vicinity of $V_{DC}=0$ V.

###

Figure 3: THz measurement of a flexible rectenna. a) Schematic of the free-
space measurement setup and b) equivalent circuit of the system. c) Bias
current dependency of the voltage responsivity. d) Frequency dependency of the
rectified voltage under illumination with THz radiation.

Measurements at THz frequencies between 110 and 170 GHz were conducted in free
space under ambient conditions. A sine-wave modulated THz signal (333 Hz) was
transmitted by a horn antenna, collimated and prefocused by two identical PTFE
(Teflon) lenses and finally focused onto the back of the sample by a hyper-
hemispherical Si lens. The setup is shown schematically in Figure 3a. After
peeling off the PI film from the Si substrate, the flexible chip was mounted
onto a high-resistivity Si lens in the beam path. Microprobes contacted the
sample behind the beam path. A Keithley 2604B source meter provided the DC
input bias as a current through an external resistor and simultaneously
measured the DC voltage at the diode. The THz source was composed of a VDI
WR6.5GX extender driven by an Agilent 83650A signal generator with 30 dB sine
wave modulation at 333 Hz. The rectified voltage response was measured using
an SR830 lock-in amplifier. For calibration, the available output power of the
extender $P_{ava}$ across the D band was measured using an Erickson
calorimetric power meter directly at the waveguide flange of the extender on
which the horn antenna was mounted. To describe the diode’s behavior during
THz measurement we formulate a small-signal equivalent circuit, shown in
Figure 3b.

A key THz detector characteristic is the optical voltage responsivity
$\gamma_{THz}$, which is defined in Eq. 3 as the DC output voltage per THz
power and is given in units of [V/W]:

$\gamma_{THz}=\frac{2\sqrt{2}\Delta V}{P_{ava}}=\beta_{THz}R$ (3)

Here, $\Delta V$ is the rectified DC voltage measured by the lock-in amplifier
26. The optical current responsivity $\beta_{THz}$ in [A/W] can be calculated
from the voltage responsivity and the total diode resistance $R$ at the
respective operating point. The pre-factors originate in the peak-to-peak and
root mean square (rms) amplitudes of the lock-in amplifier 26. The bias-
dependent THz response was measured at 167 GHz with a THz output power
$P_{ava}=1~{}\text{mW}$. The maximum optical voltage responsivity
$\gamma_{THz}=83.7~{}\text{V/W}$ was reached at an applied bias current of 20
µA and a measured DC voltage of 1.47 V (Figure 3c). This corresponds to an
optical current responsivity $\beta_{THz}=1.13~{}\text{mA/W}$ at a total
device resistance $R=74~{}\text{k$\Omega$}$.

It is noticeable that the optical responsivity under THz measurements
increases with applied bias, while the DC responsivity shows a peak close to
zero bias. This can be explained by the non-ideal impedance matching between
diode and antenna. As a higher bias is applied, the junction resistance of the
diode decreases, thus better matching its impedance to that of the antenna,
and increasing the power delivered to the diode.

The rectified voltage over the D band from 110 to 170 GHz is shown in Figure
3d. During the frequency sweep the bias current was fixed at -10 µA and the
rectified voltage was recorded. The voltage $V_{THz}$ does not show any
noticeable frequency dependency, but fluctuations, possibly due to
interferences in the polyimide film. The Teflon and silicon lenses used to
focus the THz beam contribute an absorption loss of around 3 dB, and a further
unknown part of the beam is reflected or scattered at various interfaces
without contributing to the incident power. Even though the substrate
materials are transparent to radiation in the THz regime, the mismatch in
refractive indices introduces undesirable frequency dependent reflections that
reduce the RF intensity at the rectenna. As such, the measured value is a
lower bound of the optical responsivity.

Figure 4: Noise measurements. a) Voltage noise spectrum from 500 Hz to 250
kHz, measured on-chip. b) Bias dependence of the voltage noise at 500 Hz (top)
and 250 kHz (bottom).

A test device with ground-signal-ground pads and an identical diode to the one
in the rectenna has been fabricated to measure the voltage noise $V_{noise}$
directly on-chip. Figure 4a shows the noise at different bias levels at
frequencies between 500 Hz and 250 kHz. The bias dependence of the voltage
noise is shown in Figure 4b. The noise equivalent power
($\text{NEP}={V_{noise}}/{\gamma_{THz}}$) of the detector can be calculated
from the voltage noise $V_{noise}$ and the responsivity $\gamma_{THz}$. At 250
kHz (500 Hz), the minimum NEP is 81.0 pW/$\sqrt{\text{Hz}}$ (3.5
nW/$\sqrt{\text{Hz}}$) at a bias voltage of 1 V and 4.3 nW/$\sqrt{\text{Hz}}$
(44.1 nW/$\sqrt{\text{Hz}}$) at zero bias.

|  detector + antenna |  $f$ [GHz] |  $\gamma_{THz}$ [$\frac{V}{W}$] | NEP [$\frac{\text{pW}}{\sqrt{\text{Hz}}}$] | diode | flexible | ref.
---|---|---|---|---|---|---|---
|  MIM + bowtie∗ | 28300 | $0.2$ |  | ✓ |  | 27
|  traveling wave MIM + bowtie∗ | 28300 | $0.3$ |  | ✓ |  | 28
|  GFET + bowtie | 487 | $2$ | 3000 |  | ✓ | 26
|  CNT Schottky | 540 | $2.6$ | 20000 |  |  | 29
|  photothermoelectric | 2519 | $10$ | 1100 |  |  | 30
|  MSM + log spiral∗ | 300 | $10.8$ | 100 | ✓ |  | 31
|  flexible 1D-MIG + bowtie | 167 | $83.7$ | 81 (3500) | ✓ | ✓ |
|  unipolar nanodiode + bowtie | 1500 | $300$ | 330 | ✓ |  | 32
|  photothermoelectric∗ | 2519 | $715$ | 16 |  |  | 30
|  ballistic diode + bowtie∗ | 685 | $764$ | 34 | ✓ |  | 33

* responsivity calculated from incident or absorbed rather than emitted power

Table 1: Comparison to other THz detectors from literature. The rectenna
discussed in this work is highlighted in grey. The hexagon in the first column
indicates if the detector is based on graphene. The detectors marked with an
asterisk use the actual incident power on the device, rather than the emitted
power of the source. The values reported for our detector come from the total
emitted power $P_{ava}$ from the source and are thus likely underestimated.
The NEP of the device from this work was measured at 250 kHz. The NEP at a
frequency of 500 Hz is provided in parenthesis as a comparison.

Table 1 compares our rectenna on a flexible substrate to other THz detectors
from literature. The rectenna shows a higher responsivity than MIM or metal-
semiconductor-metal (MSM) based rectennas, despite being fabricated on a
flexible substrate. It also outperforms the only other reported flexible THz
detector by Yang et al. based on a graphene field-effect transistor 26 and
most other THz detectors based on the absorption effects. It should be noted
that many references derive values for the optical responsivity from the
incident or absorbed power on the device area. Since our reported optical
responsivity uses the total emitted power $P_{ava}$ from the source (not
scaled to the device area), it represents a lower bound that is mainly limited
by the impedance mismatch between diode and antenna and absorption and
scattering in the beam path.

##

In summary, we have demonstrated detection of THz signals through an edge-
contacted graphene-based rectenna in free space measurements. The flexible
rectenna, fabricated using scalable, thin-film compatible processes and CVD-
grown graphene, reaches a responsivity of 83.7 V/W at 167 GHz with a NEP of
81.0 pW/$\sqrt{\text{Hz}}$, enabling low-cost energy harvesting at the chip
level in flexible electronics.

##  1 Author Information

###  1.1 ORCID

  * •

Andreas Hemmetter: 0000-0002-0932-2183

  * •

Xinxin Yang: 0000-0003-4464-6922

  * •

Zhenxing Wang: 0000-0002-2103-7692

  * •

Martin Otto: 0000-0001-6704-3468

  * •

Burkay Uzlu: 0000-0001-6776-8901

  * •

Marcel Andree: 0000-0003-3645-0831

  * •

Ullrich Pfeiffer: 0000-0002-6753-7879

  * •

Andrey Vorobiev: 0000-0003-2882-3191

  * •

Jan Stake: 0000-0002-8204-7894

  * •

Daniel Neumaier: 0000-0002-7394-9159

  * •

Max C. Lemme: 0000-0003-4552-2411

###  1.2 Author contributions

The experiments have been conceived by A.H., Z.W. ,M.L. and D.N. A. H.
designed the devices, and performed the electrical characterization. A.H.,
M.O. and B.U. fabricated the devices. X. Y. performed the electrical and
optical characterization. M. A. performed the noise measurement. All authors
contributed to discussions and the analysis and interpretation of the results.
All authors have given approval to the final version of the manuscript.

###  1.3 Notes

The authors declare no competing financial interest.

This research was financially supported by the EU projects Graphene Flagship
(Contract No. 881603) and WiPLASH (Contract No. 863337) and the DFG projects
HiPeDi (Contract No. WA4139/1.1) and GLECS-2 (Contract No. NE1633/3).

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