Current-induced enhancement of photo-response in graphene THz radiation detectors K. Indykiewicz1C. Bray2C. Consejo2F. Teppe2S. Danilov3S.D. Ganichev3 4and A. Yurgens5a

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Current-induced enhancement of photo-response in graphene THz radiation detectors

K. Indykiewicz,1C. Bray,2C. Consejo,2F. Teppe,2S. Danilov,3S.D. Ganichev,3, 4 and

A. Yurgens5, a)

1)Faculty of Electronics, Photonics and Microsystems, Wroc law

University of Science and Technology, 50-372 Wroc law,

Poland

2)CNRS/Laboratoire Charles Coulomb (L2C), 34095 Montpellier,

France

3)University of Regensburg, Faculty of Physics, D-93053 Regensburg,

Germany

4)CENTERA Laboratories, Institute of High Pressure Physics,

Polish Academy of Sciences PL-01-142 Warsaw, Poland

5)Chalmers University of Technology, SE-412 96 G¨oteborg,

Sweden

(Dated: 7 October 2022)

Thermoelectric readout in a graphene THz radiation detector requires a p-n junction

across the graphene channel. Even without an intentional p-n junction, two latent

junctions can exist in the vicinity of the electrodes/antennas through the proximity

to metal. In a symmetrical structure, these junctions are connected back-to-back and

therefore counterbalance each other with regard to rectiﬁcation of the ac signal. Be-

cause of the Peltier eﬀect, a small dc current results in additional heating in one- and

cooling in another p-n junction thereby breaking the symmetry. The p-n junctions

then no longer cancel, resulting in a greatly enhanced rectiﬁed signal. This allows

to simplify the design and eﬀectively control the sensitivity of the THz-radiation

detectors.

a)yurgens-at-chalmers.se

arXiv:2210.02839v1 [cond-mat.mes-hall] 6 Oct 2022

The graphene-based Terahertz (THz) detectors can be fast and sensitive devices in a

wide frequency range.1,2 There are several readout mechanisms in graphene detectors such

as bolometric,3thermoelectric (TEP),4ballistic,5based on noise thermometry,6ratchet

eﬀects,7,8 and electron-plasma waves,9,10 also called Dyakonov-Shur (D-S) mechanism.11,12

Detectors with the TEP readout mechanism are simple, do not require electrical bias and

therefore have no 1/fnoise, allow for scalable fabrication using CVD graphene, and have un-

demanding electrical contacts. High eﬃciency of such detectors stems from a large radiation-

induced increase of the electronic temperature Tebecause of a weak electron-phonon (e-ph)

coupling in graphene13,14 and a large value of the Seebeck coeﬃcient (S∼Te/3µV/K).15,16

Ap-n junction across the graphene channel must be formed to fully realize the TEP

readout in a graphene-based radiation detector (see Fig. 1a). It can be done either chemically

or electrostatically, by using a split top gate.17 Without p-n junctions, the TEP signal is

usually insigniﬁcant.1,2

However, there can be latent p-n (or p-p’ or n-n’) junctions in the vicinity of the elec-

trodes/antennas through the proximity to metal.18–20 These junctions do not normally con-

tribute to rectiﬁcation21 of the ac current induced by THz radiation because the junctions

(diodes) are connected back to back, i.e., symmetrically in the opposite directions (see

Fig. 1b). Here, we show experimentally and by numerical simulations that a small dc cur-

rent breaks the symmetry and the ac current gets rectiﬁed, which considerably increases the

signal. This allows for an eﬀective control of sensitivity of the THz-radiation detector.

We fabricated the devices from a chemically-vapor-deposited (CVD) graphene grown on a

2” large copper foil 25- or 60 µm thick in the commercial cold-wall CVD system (AIXTRON

Black Magic II). Pure Ar and H2were used as a buﬀer- and nucleation-controlling gases,

respectively. The precursor gas was CH4diluted in Ar (5%). The nominal temperature was

regulated by using a thermocouple in contact with the graphitic heater. Many patches of two-

and three layer graphene were seen in the majority of samples. The resulting charge-carrier

mobility µof such a graphene transferred to ordinary oﬃce lamination foil (EVA/PET) was

nonetheless surprisingly high, reaching 9000 cm2/(Vs).22,23

The THz detectors were fabricated in many ways, with graphene both under- and on

top of metal electrodes/antennas. We chose also diﬀerent metals for the electrodes, Au,

Pt, Pd, which were expected to have diﬀerent proximity-doping eﬀects on graphene.18 The

CVD graphene was either transferred to SiO2/Si substrate by using the PMMA- or paraﬃn-

FIG. 1. (a) The model geometry of a symmetric graphene detector. Graphene is outlined by the

dashed line. The arrows mark two latent p-n junctions in the vicinity of the electrodes (log-periodic

antenna in this case). (b) Schematic doping proﬁle in a device. The regions under the electrodes

are assumed to be ndoped because of the proximity to the metal. The latent p-n junctions (diodes)

are connected back to back (the inset). (c) Schematic cross section of graphene channel with two

metal electrodes. Red arrows show a current ﬂow and its distribution (crowding). (d) a lumped-

element representation of the device, where C,R, and G= 1/r are the capacitance, graphene

resistance, and contact conductance per unit length, respectively. Note the similarity with the

classical transmission line (see, e.g.,24 or Wikipedia), allowing for a straightforward estimation of

the current-crowding length λj= 1/√RG ∼1−5µm. In the self-gating scenario, Ris a function

of the local voltage drop Vlacross the contact resistance r,R=µ−1(C2V2

l+c2

00e2

0)−0.5, which

introduces a signiﬁcant non-linearity at high bias. Here, c00 is the residual charge density and e0

is electron charge.

assisted technique,25 or simply glued to a substrate by an epoxy-based adhesive. Bow-

tie or log-periodic antennas were lithographically patterned to have a better coupling to

THz radiation (see Fig. 1a). However, the antennas appeared to only play a minor role

in the frequency range of our measurements because of a relatively high graphene-to-metal

contact resistance resulting in a signiﬁcant impedance mismatch. This leaves spacey room

for uncomplicated improvement of the detectors in the future, promising a much better

performance than demonstrated in this work.

For optical excitation, we used Gunn diodes and pulsed THz laser26,27 optically pumped

by a transversely excited atmospheric-pressure CO2laser.28 The Gunn diode provided a

linearly polarized radiation with the frequency of 94 GHz and estimated incident power

from 1 to 10 mW. The radiation was modulated by an optical chopper at the frequency of

37 Hz, allowing measurements of photoresponse with the standard lock-in technique. The

THz power delivered to the samples in the cryostat through the optical windows is somewhat

diﬃcult to reliably estimate because of the multiple reﬂections from the metal walls of the

cryostat resulting in light interference and a complex pattern of maxima and minima of the

light intensity.

The THz laser provides single pulses of monochromatic radiation with the pulse duration

in the order of 100 ns, repetition rate of 1 Hz, and peak power in the order of hundreds

of kW. The peak power was monitored with the THz photon-drag detectors.29 The laser

operated at the frequencies f= 0.61, 1.07, 2.02, and 3.31 THz. The photoresponse to the

THz pulses was measured with a digital oscilloscope as a voltage drop across 50-Ω load

resistor.

Fig. 2a shows the response signal versus dc current demonstrating initially linear increase

of the signal, which then have a tendency to saturation- and even decrease at the maximum

current. The sign of the signal changes with the direction of dc current. In the samples

with dissimilar metals on both ends of the graphene channel, there was usually an oﬀset in

the vertical direction common to all curves, which meant that the signal at low temperature

was signiﬁcant even at zero dc current.

The signal decreases with temperature (see Fig. 2b); the shape of this decreasing function

is sample dependent. In Fig. 2b, the signal changes gradually and survives up to room

temperature. However, in several other samples, the signal decayed to zero at 150 −200 K.

A couple of devices showed a very abrupt change of the signal that vanished at already

∼40 K (see Supplementary material). The mechanism behind this temperature dependence

is unclear and requires further experiments.

FIG. 2. Output response signal to 94-GHz radiation versus dc current (a) and the temperature

dependence of the signal at the two ﬁxed dc currents marked by the vertical dashed lines (b).

Since the response signal in our devices is due to the thermoelectric eﬀects and involves

electron heating, the decay of the signal with temperature should be largely attributed to

increased cooling of hot electrons. The electrons are cooled by interactions with phonons.

These interactions are generally weak because the population of optical phonons is expo-

nentially small at low temperature. The cooling eﬃciency through the acoustical phonons is

impeded because of the momenta mismatch, but can be somewhat improved when involving

scattering by impurities (supercollisions).13 However, there can be many other modes involv-

ing the out-of-plane direction in a multilayer graphene, e.g., the shear mode at 31 cm−1.30,31

Many double-layer patches and these phonons can in principle be an eﬀective channel for

cooling of the electrons.

The heating of electrons can be regarded by simply considering graphene as a conducting

layer with a Drude-like frequency-dependent conductivity σ(ω) = σ0(1−iωτ)−1. The heating

eﬀects are described by the real part of the conductivity, P(ω)∼v2Re(σ(ω)) ∼v2σ0/(ωτ)2,

for ωτ ≥1. Here, Pis the Joule heating power, vis the ac-voltage amplitude in graphene,

σ0is the dc conductivity, τis the scattering time, ω= 2πf, and fis the frequency.

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Current-inducedenhancementofphoto-responseingrapheneTHzradiationdetectorsK.Indykiewicz,1C.Bray,2C.Consejo,2F.Teppe,2S.Danilov,3S.D.Ganichev,3,4andA.Yurgens5,a)1)FacultyofElectronics,PhotonicsandMicrosystems,WroclawUniversityofScienceandTechnology,50-372Wroclaw,Poland2)CNRS/LaboratoireCharlesCoulomb(...

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Current-induced enhancement of photo-response in graphene THz radiation detectors K. Indykiewicz1C. Bray2C. Consejo2F. Teppe2S. Danilov3S.D. Ganichev3 4and A. Yurgens5a.pdf

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Current-induced enhancement of photo-response in graphene THz radiation detectors K. Indykiewicz1C. Bray2C. Consejo2F. Teppe2S. Danilov3S.D. Ganichev3 4and A. Yurgens5a

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