Recent progress and remaining challenges of 2D material-based terahertz detectors

Infrared Physics and Technology 102 (2019) 103024

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Recent progress and remaining challenges of 2D material-based terahertz detectors Yingxin Wang, Weidong Wu, Ziran Zhao

T

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Key Laboratory of Particle & Radiation Imaging (Tsinghua University), Ministry of Education, Department of Engineering Physics, Tsinghua University, Beijing 100084, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Terahertz Photodetector 2D materials

Two-dimensional(2D) materials have attracted enormous interests owing to their unique band structures as well as extraordinary electronic and optical properties. Atomic thickness combined with the weak van der Waals interaction between layers provides exceptional advantages in wafer-scale production and integration. The versatility of 2D materials makes it possible to achieve ultra-fast and ultra-sensitive terahertz (THz) detection at room temperature. Therefore, since graphene was discovered in 2004, 2D material-based THz detectors have rapidly become a hot research topic. Here, a review of the latest progress in THz detection based on the emerging 2D materials is presented. Firstly, the excellent optoelectronic properties of 2D materials, including graphene, transition-metal dichalcogenides (TMDCs) and black phosphorus (BP), are introduced. Then, an insight into different photodetection mechanisms as well as the key factors for the comparison between different detectors are introduced. State-of-art photodetectors classified by different detection mechanisms are summarized systematically. Finally, the remaining challenge and perspective of 2D material-based THz detectors are discussed briefly.

1. Introduction Terahertz (THz) radiation is typically defined as the electromagnetic waves whose frequencies range from 0.1 THz to 10 THz (wavelength from 30 μm to 3 mm, photon energy from 0.4 meV to 40 meV). This frequency range lies in the convergence between the photonics and electronics field, lending THz radiation various unique properties, including no radiation damage to human tissues, penetrability to most dielectric materials and fingerprint identification of innumerable molecules through their characteristic spectroscopic peaks. As a consequence, THz radiation has shown its great potential in an increasingly wide variety of applications, such as homeland security and imaging [1,2], materials diagnostics [3], biology and medical sciences [4,5], and information and communication technology [6]. However, due to the lack of reliable THz sources and detectors working at room temperature, THz region is the most elusive in the electromagnetic wave spectrum, which is known as the “THz gap” [7]. In order to take full advantage of THz technology, recently, nanostructured materials, such as carbon nano-tube, semiconductor nanowire, quantum well and 2D materials, are regarded as the new opportunities for high performance THz detectors [8–11]. Attributed to the remarkable electronic and optical properties of 2D materials,

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optoelectronic devices based on these materials including THz detectors have attracted enormous research interests. Comparing to the traditional semiconductors like Si and GaAs, etc., these 2D materials, including graphene, transition-metal dichalcogenides (TMDCs) and black phosphorus (BP), are more competitive in THz detection. Firstly, owing to the quantum confinement in the vertical direction of 2D plane, sharp peaks appear in the electronic density of states near the edges of conduction and valence bands, leading to stronger interaction between materials and incident photons as well as higher light absorption [12]. Secondly, ultra-broadband photoresponse can be realized by 2D materials because of their various bandgaps [13–16]. The bandgaps of 2D materials range from zero bandgap of graphene (Fig. 1d) to narrow bandgap of TMDCs and BP (∼2 eV, shown in Fig. 1e and f, respectively), corresponding to the spectral response from visible to microwave. Thirdly, there are no dangling bonds on the surface of 2D materials [17]. Because of the natural passivation of the surface, using 2D materials, one can construct vertical heterojunctions without the conventional ’lattice mismatch’ issue. Diverse heterojunctions formed by 2D materials have various optoelectronic properties, providing more options for light detection [18–22]. Fourthly, atomic thickness and weak interaction between layers allow 2D materials easily to be scaled down to produce nano-device. Finally, 2D materials are suitable to monolithic

Corresponding author. E-mail address: [email protected] (Z. Zhao).

https://doi.org/10.1016/j.infrared.2019.103024 Received 22 May 2019; Received in revised form 26 August 2019; Accepted 27 August 2019 Available online 31 August 2019 1350-4495/ © 2019 Published by Elsevier B.V.

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Fig. 1. Lattice structure and band gap of different 2D material monolayer [23] Copyright 2014, Springer Nature. (a, d) Graphene; (b, e). TMDC; (c, f) BP.

great potential material for THz field effect transistor (FET) detector, which is also theoretically confirmed [49]. Owing to the remarkable gapless band structure and exceptional electronic property, the absorption spectrum of graphene covers an ultra-broadband range from visible to terahertz [50–53]. However, doping is unavoidable in graphene, openning an optical bandgap of 2EF, where EF is the Fermi energy [15]. Therefore, there are two photoexcitation modes when graphene absorbs photons: interband transition and intraband transition. For visible and near infrared light with the photon energy (hν , where h and ν are the plank constant and the light frequency, respectively) higher than the bandgap, electrons can be excited from the valence band to the conduction band and this process is named as the interband transition [54]. In these bands, monolayer graphene has a constant light absorption of 2.3% [55]. In the low frequency region, such as mid-infrared and far-infrared, the photon energy (hν ) is lower than 2EF. According to the Pauli blocking effect, the interband transition is forbidden while the intraband transition dominates. Graphene absorbs photons mainly through the free-carrier (Drude) absorption [56] which is described by the complex dynamic conductivity. Meanwhile, the transmission spectrum of graphene is related to the wavelength [57]. In THz band, the coupling of graphene and photons can be enhanced by intraband transition, and thus it is possible to achieve sensitive THz detection.

integration with traditional electronic materials. In addition, optoelectronic properties of 2D materials can be modulated by several means such as the gate voltage, which enable us to tune the transport properties of the channel easily. Consequently, these extraordinary properties have triggered an unprecedented interest in 2D material-based THz detectors in recent years. In this paper, we start with concise introduction to the physical properties of various 2D materials as well as figure-of-merits for THz photodetectors. In the next place, different photodetection mechanisms are discussed and the performance of the current state-of-art THz detectors are summarized. Finally, we put forward the prospects and remaining challenges of THz detectors based on 2D materials. 2. Properties of 2D materials 2.1. Graphene Graphene is a planar allotrope of carbon. The sp2 hybridization forms a hexagonal honeycomb structure with a carbon-carbon distance of 1.42 Å [24] (Fig. 1a). The remaining half full pz orbits are perpendicular to the lattice plane and form a conjugated π bond with each other. In 1947, P.R.Wallace first used tight-binding model to demonstrate the band structure of graphene and showed the unusual semimetallic behavior [25]. The band structure is symmetric around zero energy and the conduction band and valence band intersect at the Dirac point [26] (shown as K or K′ in Fig. 1d), forming the zero bandgap of graphene. The Fermi energy of intrinsic graphene is located at Dirac point. Around the Dirac point, the energy is linear to the momentum and thus carriers act as relativistic massless Dirac fermions. Consequently, they can travel a long distance without scattering [27–29]. Different from monolayer graphene, bilayer graphene or graphene nanoribbons become semiconductors with a small bandgap [30,31]. Besides, the Fermi energy of graphene can be regulated by other means like gate voltage [32] and chemical doping [33]. The transport of relativistic massless Dirac fermions is ballistic on the sub micrometer scale, leading to a long mean free path [34,35]. On this account, graphene has ultra-high carrier mobility [36–38]. For suspended graphene, carrier transport is only limited by the intrinsic sources, including longitudinal acoustic phonon scattering [39], lattice defects [40], ripples [41] and topological defects [42]. The carrier mobility is up to about 230,000 cm2 V−1 s− 1 [43]. When graphene is placed on the substrate, carrier mobility decreases rapidly because of the extrinsic limitations such as impurity scattering [44], interface scattering [45] and the wrinkles and cracks formed in the preparation process [46,47]. On Si/SiO2 substrate, the carrier mobility of graphene is only 40,000 cm2 V−1 s−1 [48]. Due to its high room-temperature mobility and high carrier saturation velocity, graphene is considered as a

2.2. TMDCs The chemical formula of representative TMDCs is MX2, where M is a transition metal of groups 4–10 and X is a chalcogen. Different TMDCs have the similar crystal structure. In monolayer, the structure is in the form of X-M-X, a plane of metal atoms sandwiched between two planes of chalcogen atoms [58] (Fig. 1b). As for the bulk materials, the adjacent layers are linked to each other weakly by van der Waals bonds. Due to the different stacking orders and metal atom coordination, bulk TMDCs have a variety of polytypes, including 1T, 2H, and 3R [59]. Diversity of the metal atoms and its coordination environment leads to various band structures of TMDCs, which make these materials more fascinating [60–62]. For example, PtSe2 is metallic [63] while MoS2 is semiconductive [64]. In addition, there are other attractive properties existing in TMDCs such as charge density wave in TaS2 [65,66]and superconductivity in NbSe2 [67]. In the field of photodetection, the semiconductive TMDCs like MoS2 , MoSe2 , WS2 and WSe2 are more prevailing. Owing to the quantum confinement, the band structures of semiconductor TMDCs depend on the number of layers. The bulk materials are indirect semiconductors having a bandgap of ∼0.8 eV. In contrast, monolayer TMDCs are direct semiconductors with a bandgap of ∼2 eV [68,69] (shown in Fig. 1e). The direct bandgap results in strong 2

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where P is the light power. There are two definitions of the light power P: the incident power and the absorbed power. The incident power is the optical power on the device’s active area, given as Pin = Pout Sa / S0 where Pin, Pout , Sa and S0 denote the incident power, the output power of light source, the device’s active area and the beam spot area, respectively. The absorbed power is the partial incident power that is absorbed by materials through different mechanisms. Unless otherwise mentioned, the responsivity in this paper is calculated by the incident power.

photoluminescence and opens up the opportunities of novel optoelectronic applications [69]. Besides, the band structures of TMDCs can also be tuned by the gate voltage [70], mechanical strain [71] or chemical doping [72], providing diverse methods to engineer the optical response. As semiconductors, TMDCs are suitable for FET due to the large Ion /Ioff ratios (up to 1 × 108 ) and small subthreshold swings (down to ∼60 mV/dec) [73]. Similar to graphene, the carrier mobilities of TMDCs are limited by ripples, phonon scattering, impurity scattering and interface scattering [68,69,74]. The carrier mobility in monolayer MoS2 is ∼100 cm2 V−1 s−1 at room temperature [75]. The main interaction process between TMDCs and photons is the photovoltaic effect [76]. Due to the thickness-related band structure, the absorption spectra of TMDCs change with thickness [77]. At the same time, TMDCs exhibit strong photoconductive effect [78]. The ultrafast response time of photoconductivity (∼ps) leads to great potential for highspeed THz devices [79].

3.2. Response time and cut-off frequency The response time reflects the speed of photodetectors. It includes rise time τ1 and fall time τ2 , which are usually defined as the time required that photocurrent rises(decays) from 10%(90%) to 90%(10%) of the net photocurrent. For most photodetectors, photoresponsivity is related to the light modulation frequency f, response time τ and the photoresponsivity measured when f = 0 (R 0 ), expressed as R0 R (f ) = . Photoresponsivity decreases with the increase of 2

2.3. BP

1 + (2πτf )

BP is the most stable planar allotrope of phosphorus. In the bulk material of BP, adjacent layers are connected by weak van der Waals force with a spacing of 5.5 Å [80]. Due to the interaction between layers, the lattice structure of the multilayer black phosphorus will change slightly. In monolayer BP, each atom is strongly and covalently bonded with the other three adjacent atoms through sp3 hybridization, which forms a puckered honeycomb lattice [81,82] (Fig. 1c). This particular structure defines two in-plane crystalline directions: zigzag direction (ZZ, y direction in Fig. 1c) and armchair direction (AC, x direction in Fig. 1c). Such anisotropy induces the anisotropic electrical, optical and thermal properties in BP. For intrinsic BP, the effective mass of carriers along ZZ is an order of magnitude larger than that along AC [83,84]. Consequently, the carrier mobility and conductance along AC are larger than those along ZZ [85,86]. In addition, the thermal conductivity of black phosphorus along ZZ is larger than that along AC [87], which is the stem of anisotropic thermoelectric characteristics. The remaining orbit of each atom is occupied by a lone pair that is very reactive to oxygen and water, and thus BP degrades rapidly under ambient conditions [88]. This annoying degradation limits the potential of BP for practical applications. BP is a direct semiconductor with a sizable bandgap. The bandgap decreases monotonically with the number of layers [89,90], from ∼2 eV (monolayer) to ∼0.3 eV (bulk). The bandgap of BP (0.3 eV–2 eV) lies between graphene (0–0.3 eV) and TMDCs (1 eV–2 eV), so BP bridges the bandgap between graphene and TMDCs. In addition to the number of layers, the band structure of BP also can be engineered by gate voltage [91], mechanical strain [92,93] or chemical doping [94,95]. Thanks to the high carrier mobility (∼1000 cm2 V−1 s−1) and large Ion / Ioff ratios (∼ 105 ) [96], BP is suitable for THz detection considering about the low dark current and large responsivity [97]. Similar to graphene, there are two interaction processes between BP and photons: interband transition and intraband transition. If the photon energy is larger than the bandgap(hν > Eg), the interband transition is dominant. Because the bandgap of BP lies from visible to near-infrared in the electromagnetic spectrum, BP is suitable for infrared detector through photovoltaic effect [98]. If the photon energy is lower than the bandgap (hν < Eg), the intraband transition dominates the photon absorption process. At the long wavelength, the high carrier mobility enables the large Drude absorption and allows BP for THz detection in the nano transistors with a fast response [97].

D∗ is another important parameter to characterize the sensitivity of photodetectors, especially in the comparison of photodetectors with different geometries in a normalized bandwidth. It can be expressed as D∗ = Sa /NEP = Sa B ·Rv / v N ,where B is the noise bandwidth and Sa, Rv and v N are defined as above. The unit of D∗ is Jones or cm·Hz0.5 ·W−1.

3. Figure-of-merits for photodetectors

4. Types of 2D material-based THz detectors

3.1. Photoresponsivity

THz detector is a device to convert the weak THz signal to other forms that can be easily observed and recorded. Owing to their excellent physical properties, 2D materials can achieve sensitive THz detection through various mechanisms, including bolometric effect,

modulation frequency. When the photoresponsivity decreases to 0.707R 0 (3 dB), the modulation frequency is called the cut-off frequency fc , also known as the 3 dB bandwidth. 3.3. Photoconductive gain(G) The photoconductive gain describes the number of carriers generated by a single incident photon. When the life of photogenerated carriers(τlife ) is longer than the transit time (τtran ), one photogenerated carrier can circulate many times in the channel and lead to photoconductive gain, defined as G = τlife/ τtrans . The transit time depends on the bias voltage (V), carrier mobility( μ ) and channel length (L), given as τtran = L2 /(μV 2) . 3.4. Noise Equivalent Power (NEP) NEP is the minimum radiation power that photodetectors can detect or distinguish from total noise. The expression is NEP = iN / RI or NEP = v N/ RV , where iN , v N are the noise current spectra density and noise voltage spectral density of 1 Hz bandwidth, respectively. The unit of NEP is W/Hz0.5 . The major noise sources of THz detectors are the thermal noise, the Johnson noise, the 1/ f noise and the shot noise. Therefore, the NEP of a THz detector can be expressed as [99,100]:

NEP2 = 4kB T 2Gth +

2 kUDC 4kB TR 2eUDC R2 + + 2 RV fR V2 RV2

(1)

where kB, T , R, k , UDC, f , e and Gth denote the Boltzmann constant, temperature, resistance, dimensionless constant, DC bias voltage, frequency, electron charge and thermal conductivity, respectively. 3.5. Normalized Detectivity (D∗)

Photoresponsivity is defined as the photocurrent Ip or photovoltage Vp generated by per optical power, expressed as RI = Ip/ P or RV = Vp/ P , 3

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Table 1 THz detectors based on 2D materials. D∗

Response Time

Ref

–

–

4 nW/Hz0.5

∼ 1.2 × 107 Jones

50 ps < 1 ms

[101] [102]

5.6 pW/Hz0.5

∼ 3.3 × 109 Jones

< 100 μs

[103]

0.2 fW/Hz0.5

∼ 2.2 × 1012 Jones

< 2.5 ns

[104]

1.1 nW/Hz0.5

∼ 1.9 × 105 Jones

110 ps

[105]

1.7 nW/Hz0.5 –

∼ 2.2 × 106 Jones –

< 50 ms ∼5 s

40 nW/Hz0.5

∼ 1.2 × 106 Jones

< 2 ms

[107] [108]

45 nW/Hz0.5 –

∼ 1 × 106 Jones –

< 2 ms

[102]

30 nW/Hz0.5

∼ 1.5 × 106 Jones

20 ps < 1 ms

[109] [110]

Mechanism

Material

Frenquency

Responsivity

NEP

Bolometer Bolometer

Graphene BP

2 THz ∼ 0.3 THz

∼ 8 nA/W 7.8 V/W –

Bolometer

Graphene

0.3–1.6 THz

Bolometer

Graphene

0.15 THz

PTE

Graphene

2.52 THz

5 × 1010 V/W > 10 V/W

PTE

Graphene

2 THz

4.9 V/W

PTE PTE

Graphene BP

8.4 mV/W 0.15 V/W

PTE

BP

2.52 THz ∼ 0.3 THz ∼ 0.3 THz

PTE Plasma wave rectification

BP Graphene

∼ 0.3 THz ∼ 0.3 THz

0.9 mV/W 0.15 V/W

1.1 V/W

[106]

Plasma wave rectification

Graphene

∼ 0.6 THz

14 V/W

515 pW/Hz0.5

∼ 4.9 × 107 Jones

< 30 μs

[111]

Plasma wave rectification

Graphene

∼ 0.3 THz

1.2 V/W

2 nW/Hz0.5

∼ 2.3 × 107 Jones

< 2.5 ms

[112]

Plasma wave rectification

Graphene

∼ 0.3 THz

30 V/W

163 pW/Hz0.5

∼ 3 × 108 Jones

< 5 μs

[113]

Plasma wave rectification

BP

∼ 0.3 THz

5 V/W

10 nW/Hz0.5

∼ 4.8 × 106 Jones

10 ms

[102]

Plasma wave rectification

Graphene

230–375 GHz

0.25 V/W

80 nW/Hz0.5

∼ 5.8 × 105 Jones

< 1.2 ms

[114]

Plasma wave rectification

Graphene

0.13 THz

20 V/W

0.6 nW/Hz0.5

∼ 1.9 × 108 Jones

–

[115]

Photoconduction

BP

0.15 THz

300 V/W

1 nW/Hz0.5

∼ 1.7 × 106 Jones

4 μs

[116]

Photoconduction

Graphene

400 V/W

∼ 3.5 × 108 Jones – –

[100]

Graphene DLG

0.5 nW/Hz0.5 – –

20 μs

Schottky junction Tunneling

0.15 THz ∼ 0.1 THz

1 ms –

[117] [118]

1 THz

1000 V/W 1.55 A/W

bias (assuming it is a constant current source) is small, the Joule heating can be neglected and the responsivity is expressed as [119]:

photothermoelectric (PTE) effect, plasma wave rectification and so on. In this section, we will discuss the mentioned mechanisms in detail and review THz detectors based on 2D materials briefly. Performance of representative detectors is summarized in Table 1. The D∗ in Table 1 are estimated by the formula of D∗ = Sa /NEP , where Sa and NEP are obtained from the references.

RV =

αηIDC Gth (1 + i (2πf ) τth )

(2)

where α, η and τth are the temperature coefficient of resistance (TCR), the absorption efficiency with respect to an average incident optical intensity and the thermal time constant, respectively. The TCR is to characterize the change of resistance near the operating temperature T0 , defined as:

4.1. Bolometers Thermal detectors are commonly used to observe the THz radiation. In this type of detectors, the radiation energy is converted into heat and then induces a temperature increase that is measured by a thermometer. A THz thermal detector gathers a radiation absorber, a thermometer, a thermal link and a heat sink. The radiation absorber absorbs and stores the energy of THz radiation. The thermometer converts the stored energy into an electrical signal that can be recorded. The heat sink is used to keep the temperature of the absorber stable when there is no radiation. According to the mechanisms of the thermometer, THz thermal detectors can be divided into several types. In 2D materials, the most common mechanism is the bolometric effect.

α=

1 dR R dT

T = T0

(3)

The response time of a bolometer mainly depends on the thermal time constant τth , expressed as τth = Cth/ Gth , where Cth denotes the heat capacity. When the effect of Joule heating (P bias ) cannot be neglected, the thermal conductivity becomes Geff = Gth (1 + L) and the time constant becomes τeff = τth/(1 + L) where L = −αRth P bias is the electro-thermal feedback [119]. Thus increasing bias cannot increase the responsivity and decrease the response time at the same time. In addition, TCR only describes the variation of resistance near T0 . If the bias is too high, the electro-thermal feedback could result in destruction of the bolometer. There are some important parameters to design a highly sensitive bolometer: (1) an absorber with a high absorption efficiency; (2) a large TCR; (3) a small thermal conductivity; (4) a small thermal time constant. In 2013, Martin Mittendorff et al. reported an antenna-coupled graphene bolometer operated at room temperture [101] (Fig. 2a). This device realized broadband photodetection (Fig. 2d) from 8.3 μm to 151 μm with a response time as short as 50 ps. The THz photoresponsivity was up to a few nA/W. Riccardo Degl’Innocenti et al. demonstrated an array THz detector based on graphene loaded plasmonic antenna arrays [120] (Fig. 2b). Each cell of the array is arranged in series and operated as a bolometer. The maximum responsivity is ∼ 2 mA/W, which represents a progress in the field of graphene-based integrated THz detectors. In addition to graphene, other 2D materials are also appreciate for THz bolometers. For BP, the thermal conductivity along ZZ direction is much larger than that along AC direction. Besides, the interaction between the out-of-plane phonon and the

4.1.1. THz bolometric detectors The bolometric effect exploits the resistivity change of a temperature-sensitive material, which is induced by the heating effect of the incident radiation. Under the external light illumination, the radiation absorber absorbs the light energy and then the thermometer temperature increases, causing a change in the thermometer resistivity. Under an external bias, the current across the thermometer increases or decreases, resulting in an electrical signal that is related to the light flux. It should be noted that there are two mechanisms inducing the resistivity change under electromagnetic radiation illumination: (1) a change in carrier mobility owing to the change of temperature; (2) a change in the number of carriers that has contribution to the current. The former is the bolometric effect while the latter is considered as the photovoltaic effect or photoconductive effect. As the THz photon energy (∼4 meV at 1 THz) is far less than the bandgaps of most materials, the photovoltaic effect or photoconductive effect generally does not exist. For an ideal bolometer, the thermal balance equation mainly involves four parts: the power stored in the bolometer, the power from the absorption of the incident light, Joule heating induced by the bias and the power flowing from the thermometer to the heat sink. If the 4

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Fig. 2. Different types of THz bolometers. (a), (d) Device structure and broadband spectral response of graphene room temperature bolometer. Reproduced with permission [101]. Rights managed by AIP Publishing; (b) structure of a graphene THz bolometer array [120]; (c) BP THz bolometer [102]; (e) hBN-BP-hBN heterojunction THz bolometer. Reproduced with permission [121]. Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

lattice, which governs the electron-lattice cooling, is weak and thus enlarges the thermal resistance [102]. Therefore, in BP, the ZZ direction is more suitable for bolometers than other directions. Leonardo Viti et al. demonstrated the first BP THz bolometer [102]. The source (S)drain (D) channel was defined along the AC direction (Fig. 2c). The S and D electrodes were patterned as a bow-tie antenna to enhance the absorption of THz radiation. At room temperature, the detector showed a highest responsivity up to 7.8 V/W and a response time less than 1 ms. However, BP degrades rapidly under ambient environment because of the reactions with oxygen and water, which limits the application of BP. In order to overcome this obstacle, a hexagonal boron nitride (hBN)-BPhBN heterojunction was reported in 2016 [121] (Fig. 2e). Under the protection of hBN, the device had an excellent environmental stability, enabling BP more suitable for practical applications. The 2D materials act as the thermometer and absorber at the same time in the aforementioned bolometers. Unfortunately, the absorptivities of graphene and BP are less than 20% through Drude absorption in the THz region and the TCR is just ∼ 10−2 /K at room teperature [122,123]. It is somewhat difficult to employ these materials to develop bolometers with high sensitivities at room temperature. However, in graphene, the light absorption can cause a large change in the electron temperature because of the weak electron-phonon coupling and low electron heat capacity. This property makes graphene very suitable for the hot electron bolometer [124], which is much more sensitive.

Geph is the thermal conductance due to the electron-phonon interaction, expressed as 4ΣATe3 [129], where Σ is the electron-phonon coupling constant and A is the device area. Gdiff is the thermal conductance caused by the electron diffusion from the material to the heat sink. The electron diffusion follows the Wiedemann-Franz relationship [130]: Gdiff = 12L0 Te/ R where L0 is the Lorenz constant. Gphoton , the thermal conductance deriving from the thermal radiation that has a very close connection to the Johnson noise, is expressed as Gphoton ≈ kB B where B ≪ kB T / h is the noise bandwidth [131]. Considering that G = dPth/ dT where Pth is the heat flow power and the thermal radiation has a very close connection to the Johnson noise [132], Johnson noise can be used to measure the electron temperature [133]. Besides, the effects of disorder, also known as supercollision, can speed up the phonon-cooling rate in graphene [134,135]. When TL < TBG , where TBG is the BlochGruneisen temperature, the supercollision is not significant, thus Geph ∼ Te3 . When TL > TBG , the supercollision dominates the electronphonon interaction and induces Geph ∼ Te2 . Because the thermal resistance increases rapidly with the decreasing temperature, the sensitivity of the bolometer is enhanced significantly. In addition to the responsivity, the response time of the hot electron bolometer also dramatically decreases because the electron heat capacity is much smaller than the lattice heat capacity. W. Miao et al. reported a THz hot electron bolometer based on epitaxial graphene [103]. A logarithmic antenna was coupled to enhance the absorption of THz radiation (Fig. 3a). This device was operated at 3 K and the NEP was as low as 5.6 pW/Hz0.5 . However, the bolometer performance is limited by the electron diffusion (Fig. 3c), which can be reduced by some methods. In the pristine graphene, although the hot electron effect increases the thermal resistance significantly, the resistivity still hardly depends on temperature. There are several methods to overcome this drawback, including creating a tunable band gap by top and bottom gates in double-layer graphene [137], enhancing the localization through introducing defects [124], using the superconducting contact pads [138] and inducing a bandgap by fabricating graphene quantum dots [104]. In 2016, Abdel El Fatimy et al. demonstrated a graphene quantum dot THz bolometer [104] (inset of Fig. 3b). Due to the quantum confinement, the variation of resistance with temperature is extremely high at low temperature (Fig. 3b). In a 30 nm dot, the ∂R/ ∂T is higher than 430 MΩ/K at 6 K. The D∗ was up to 2.2 × 1012 Jones, which is orders of magnitude higher than other graphene hot electron bolometers. Moreover, these quantum dot bolometers show a good performance at liquid nitrogen temperature. Due to the high responsivity, the extraordinarily low NEP and short response time, graphene quantum dot bolometers are of great potential. The performance of the detector

4.1.2. THz hot electron bolometers The hot electrons are usually adopted to describe the non-equilibrium electrons in semiconducting and metallic materials. The hot electrons can be generated by the incident radiation through the light absorption process that is divided into two steps. Firstly, the radiation is absorbed by the electrons in materials. Secondly, the energy transfers from electrons to the lattice through the electron-phonon interaction and the lattice temperature rises. If the electron-phonon interaction time (τep ) is much longer than the collision time between electrons (τee ), the electron temperature (Te ) will remain at a level higher than the lattice temperature(TL ) for a while after illumination [125], thus generating hot electrons. At low temperatures, the thermal resistance increases significantly due to the hot electron effect [126], which greatly enhances the bolometer performance [127]. As shown in Table 1, the D∗ of hot electron bolometers are typically several orders higher than the THz detectors based on other mechanisms. In graphene, the thermal conductance of a hot electron bolometer can be expressed as [128]:

Gth = Geph + Gdiff + Gphoton

(4) 5

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Fig. 3. (a), (c) The device structure of a graphene hot electron bolometer and the influence of electron diffusion on the detector performance [103] Copyright 2018, Springer Science Business Media, LLC, part of Springer Nature. The star in (c) is the experimental NEP and the solid line is the simulated data, including electron-phonon scattering, electron diffusion and microwave photon emission. The dotted line is the simulated data that excludes the contribution of electron diffusion; (b), (d) a graphene quantum dot THz detector. (b) Resistance of the quantum dot varying with the temperature, and the insert is the structure of the quantum dot detector [104] Copyright 2016, Springer Nature; (d) broadband response of the quantum dot detector and the insert shows the NEP [136].

respectively. Therefore, a temperature gradient induced by the incident radiation will generate a photovoltage. Assuming xL is the left end coordinate of the material and xR is the right end coordinate along x direction, the photovoltage can be expressed as:

decreases with the increase of quantum dot size. For 150 nm quantum dots, the responsivity decreases to 1 × 108 V/W at 2.5 K, two orders of magnitude lower than that of 30 nm dots. In addition, this type of hot electron bolometer demonstrates an ultra-broadband photodetection from THz to the ultraviolet(Fig. 3d) [136]. The response of a bolometer is little dependent on the photon energy, so bolometers can achieve broadband photodetection. Because the heat capacity of 2D materials is smaller than that of bulk materials, the 2D material-based bolometer has a fast response. However, the bolometer with a high responsivity usually has to be operated at a low temperature. Besides, the bolometer will be saturated under high radiation power [139], which is an obstacle for practical applications.

ΔV = −

(5)

where S and ΔV are the Seebeck coefficient and the potential difference, respectively. If kB T ≪ EF , according to the Mott formula, S is given as [140]:

S = −L0 eT

1 ∂σ σ ∂ε

=− ε = EF

π 2kB2 T 1 ∂σ 3e σ ∂ε

ε = EF

∇T ·S (T , EF ) dx

(7)

where ∇T is the temperature gradient. As one can see, the PTE responsivity is only related to the Sebeck coefficient and the temperature gradient, so the device can be operated at zero bias. Obviously, only if the upper integral is not zero will the PTE effect contribute to the photoresponse. This requires the asymmetry of the temperature distribution or the Seebeck coefficient along the channel that can be achieved by means of forming p-n junction [141], forming interface junction [142], using gate-controll [143], using asymmetric antenna [108] and using asymmetric electrode materials [105]. Due to the weak electron-phonon interaction, the hot carrier effect plays an important role in the photo response and carrier transport in 2D materials even at room temperature [144,145]. Under light excitation, graphene absorbs energy by interband or intraband transitions and then generates hot carriers in a short time(∼ fs) through strong electron-electron interactions [146]. The hot carriers can hold a temperature Te higher than TL for hundreds of picoseconds due to the high energy of optical phonons (∼200 meV) [147]. The final equilibrium between the hot carriers and lattice is induced by the interaction between electrons and acoustic phonons in a nanosecond timescale [148,149]. The carrier heat capacity is much smaller than the lattice heat capacity, which leads to a larger temperature gradient in the channel and thus enhances the PTE effect. Owing to the hot carrier effect, one can assume all of the absorbed radiation energy transfers to the carriers. According to the Wiedemann-Franz relationship: κ = L0 Te σ , where κ denotes the thermal conductivity, the photoresponsivity calculated by the absorbed energy can be expressed as:

Similar to the bolometric effect, the PTE effect is also produced by the radiation heating. In contrast, the PTE effect is rooted in the Seebeck effect but not the resistance variation with the temperature. Considering a conductor or a semiconductor with a temperature difference between its two ends, the carries in the hot region have more energy and greater velocities than those in the cold region. Consequently, the carriers will diffuse from the hot region to the cold region, resulting in a potential difference between the two ends. This phenomenon is known as the Seebeck effect. The magnitude depends on the Seebeck coefficient whose definition is the potential developed by per unit temperature difference, expressed as:

d ΔV dT

xR

L

4.2. PTE detectors

S=

∫x

(6)

ΔV S 2 = =− ΔP κ σEF

where σ and ε denote the conductivity and electron energy, 6

(8)

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Fig. 4. Different types of PTE detectors. (a) Up: diagram of the asymmetric electrodes. Down: the profiles across the device of Fermi level and the photoresponse [105] Copyright 2014, Springer Nature; (b) Up: diagram of asymmetric contact. Down: the optimization of Si lenses, data was normalized. Reproduced with permission from [106] Copyright 2015, American Chemical Society; (c) Asymmetric distribution heat generated by the illumination positions [107]; (d), (e) the electrode orientation (d) and asymmetric antennas (e) of BP PTE detectors [108] Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; (f) polarization dependent THz response in BP. The black and red line were photoresponses measured at 1.55 μm and 120 μm, respectively. The yellow shape was the antenna structure [109]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the source and gate respectively. The drain is a metal line leading to a small pad. Owing to the asymmetric antenna, the channel between gate and source absorbed more radiation than that between the gate and drain. Consequently, there was an asymmetric thermal distribution between the source and drain electrodes, resulting in a PTE response. The maximum responsivity was up to 0.15 V/W and the NEP was 40 nW/Hz0.5 . At low temperatures, the BP detectors exhibited much better performance due to the increasing of carrier mobility and the decreasing of thermal noise [121]. In 2017, Edward Leong et al. reported a polarization sensitive THz detector based on BP [109]. The irregular shape of the sample yielded the asymmetry and resulted in the PTE response. Due to the anisotropic crystal structure, the photoresponse was very sensitive to the polarization angle. The maximum photoresponse was achieved when the light was polarized along the AC direction, shown in Fig. 4f. As a thermal effect, the PTE effect can achieve low-power-consumption, ultra-broadband and room temperature photodetection. Besides, the enhancement of the hot carrier effect and the small heat capacity are also considerable in 2D material-based optoelectronic devices. However, the requirement of asymmetry is somewhat annoying. In microdevices, the methods to achieve the asymmetry mostly require complex fabrication process or the extra power consumption. Moreover, the magnitude of photovoltage is usually small and thus the PTE detectors need high-quality Ohmic contact, which is a limitation for high-responsivity detectors.

In addition to graphene, similar phenomena were also observed in other 2D materials at room temperature [150]. For example, Youwei Zhang et al. measured the phonon temperature through Raman spectroscopy in monolayer MoS2 [151]. The actual PTE voltage was one order magnitude higher than the theoretical value under red laser illumination, which could be attributed to the hot carrier effect. Xinghan Cai et al. reported a highly sensitive graphene PTE THz detector with asymmetric electrodes fabricated by a standard doubleangle evaporation technique in 2014 [105]. Due to the dissimilar metal contacts, the Fermi energy and Seebeck coefficient are asymmetric across the device channel (Fig. 4a), which lead to a photovoltage under radiation. Meanwhile, gate voltage may also tune the Fermi energy of graphene, thus influencing the photoresponse. The response time is mainly determined by the thermal time constant of graphene, which is ∼100 ps. In 2015, Jiayue Tong et al. demonstrated a half-edge contacted graphene PTE detector with asymmetric electrodes [106] (Fig. 4b). The device was enhanced by a double-patch antenna and an on-chip silicon lens. These optimization increased the detector performance by four orders of magnitude (Fig. 4b), reaching a maximum responsivity of 4.9 V/W and a typical D∗ of ∼ 2.2 × 106 Jones. In addition to asymmetric electrodes, the asymmetric thermal distribution can also produce PTE response. Xiangquan Deng et al. reported a positiondependent photovoltage in a detector consisting of a large area monolayer graphene stripe with two Au electrodes [107] (Fig. 4c). The antisymmetric photovoltage was a function of THz illumination position and the maximum responsivity (8.4 mV/W) was obtained when the THz illumination was focused on a single junction of metal electrode and graphene. Meanwhile, the responsivity was exponential to the channel length and the response time was proportional to the channel length. These results indicated that the THz response was attributed to the asymmetric thermal distribution and consistent with the PTE effect. In addition to graphene, other 2D materials can also be used for PTE detectors, such as BP. As mentioned above, the PTE effect is related to the temperature gradient, which means that the material with a smaller thermal conductivity would be more proper for PTE detectors. Therefore, the electrode orientation of BP in a PTE detector should be along the AC direction. Leonardo Viti et al. demonstrated the first BP PTE THz detector in 2015 [102] (Fig. 4d, e). The device used SiO2 as the gate and two halves of a planar bow-tie antenna were connected to

4.3. Plasma wave rectification THz detectors Mikhail Dyakonov and Michael Shur first proposed the plasma wave rectification mechanism in the 1990s [49,152,153]. When the frequency of impinging radiation is much smaller than the electron-electron scattering rate, the electron gas in the channel behaves like the acoustic wave and follows the laws of hydrodynamics, forming the plasma waves. If the boundary condition of a FET device satisfies the source short circuit and the drain open circuit, the plasma wave excited by an ac electromagnetic field will generate a DC signal between the source and drain, which enables sensitive THz detection. In practical devices, this boundary condition is achieved by the asymmetric antenna, as shown in Fig. 5a and Fig. 5c. It should be noted that the high 7

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Fig. 5. Plasma wave rectification THz detectors and imaging results. (a) Logarithmic asymmetric antennacoupled graphene FET [110] Copyright 2012, Springer Nature; (b) buried grating device structure [112] Rights managed by AIP Publishing; (c) antenna-coupled graphene FET [113] Copyright 2017 Elsevier Ltd. All rights reserved; (d) imaging by BP detector [102]; (e) imaging by graphene detector [113] Copyright 2017 Elsevier Ltd. All rights reserved; (f) Array THz detector based on MoSe2 [156].

for graphene by using graphene nano-strips or double-gated transistors, so that the conductivity is more related to the gate voltage. (iii) optimizing the antenna structure. L. Vicarelli et al. reported the first THz FET detector based on the monolayer and bilayer graphene that were exfoliated on Si/SiO2 [110] (Fig. 5a). In order to satisfy the boundary condition, the ends of the logarithmic antenna were gate and source respectively while the drain was a metal line leading to a small pad. At room temperature, the maximum responsivity was up to 0.15 V/W and the minimum NEP was 30 nW/Hz0.5 . In particular, the experimental results showed that bilayer graphene was more suitable for plasma wave ratification detectors because the number of carriers could be controlled by the gate voltage more easily. Davide Spirito et al. further studied the influence of the gate configuration in a bilayer graphene FET detector [112]. The device exhibited better performance by employing wide-gate geometries or buried gate configurations (Fig. 5b), where a responsivity of 1.2 V/W was achieved. The first plasma wave THz detector based on BP was reported in 2016 [102]. The electrode orientation was 45-degree angle to the AC direction. A large area fast imaging was achieved (Fig. 5d). In 2017, Qin Hua et al. demonstrated a graphene FET coupled with a three-terminal antennas based on the SiC substrate. The boundary condition was satisfied by the orientation of gate antenna [113] (Fig. 5c). The detector achieved a maximum responsivity more than 30 V/W and a D∗ up to 3 × 108 Jones near 0.33 THz. THz transmission imaging of a fresh leaf was also demonstrated, shown in Fig. 5e. In addition to a single high-performance detector, FET devices can be easily combined with the traditional MEMS processes to fabricate detector arrays. In 2016, H. Liu et al. reported a THz detector array based on the large area monolayer MoSe2 (Fig. 5f) [156]. The high-quality monolayer MoSe2 was grown by CVD method and then an array detector was fabricated by UV lithography and plasma etching. The responsivity near 0.3 THz was up to 30 mV/W, and the minimum NEP was about 6.6 × 10−6 W/Hz0.5 . It is noteworthy that the asymmetric antenna structure will allow the coexistence of PTE effect and plasma wave rectification. Both effects have the similar dependence of gate voltage. Therefore, a detailed analysis is necessary to disentangle the contributions of these two effects. Audrey Zak et al. reported the PTE signal in a CVD graphene FET detector [111] (Fig. 6a). When Vg < VCNP , where VCNP is the gate voltage at charge neutrality point, there is an obvious competition between the

mobility is crucial to take full advantage of plasma wave rectification. In a FET, the velocity of the plasma wave is expressed as [154]:

s=

e (Vg − Vth ) m∗

(9)

m∗

denote the gate voltage, the threshold voltage and where Vg, Vth and the effective mass of electron, respectively. One can get the intrinsic frequency of the plasma wave as:

ωN = (2N − 1) ω0 ω0 =

π 2L

(10)

e (Vg − Vth ) m∗

(11)

where L and ω0 are the gate length and the mode frequency of the plasma wave, respectively. Obviously, the velocity of the plasma wave has no relation to the transit time of carriers, and thus the device can be operated at a high frequency that is beyond the FET cut-off frequency. The quality factor of the plasma wave can be expressed as:

Q = ωp τp

(12)

where ωp is the plasma wave frequency and τp is the plasma wave lifetime. When Q ≫ 1, the plasma wave is under-damped and realizes the resonant detection. The response is large while the detection bandwidth is narrow and most time it has to work at a low temperature. When Q ≪ 1, the plasma wave is over-damped, which leads to broadband detection. The response is relatively weak while the detection bandwidth, primarily dependent on the antenna bandwidth, is wide and it can be operated at room temperature. In the over-damped case, the photovoltage ΔU is expressed as [155]:

ΔU =

Ua2 ∂ (lnσ ) · ×Λ 4 ∂Vg

(13)

where Ua is the potential induced by the incident THz wave and Λ is the antenna factor. The photoresponse is proportional to the radiation power. In addition, the photoresponsivity is mainly limited by two factors. One is the antenna structure and the other is the variation of conductivity controlled by the gate voltage. There are several ways to optimize the performance of the plasma wave rectification detectors [112]: (i) reducing the residual carriers in materials, which is mainly determined by the impurities and lattice defects; (ii) opening a bandgap 8

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Fig. 6. (a) and (b) The competition between PTE and plasma wave rectification in CVD graphene (a) and epitaxial graphene devices (b) [114], respectively. The blue, red and black line represent the measured, theoretical plasma wave, theoretical PTE responses, respectively. (a) Reproduced with permission from [111] Copyright 2014, American Chemical Society. (b) Rights managed by AIP Publishing; (c) and (d) The PTE (c) and plasma wave signals(d) in hBN-graphene-hBN heterojunction [115]. The peak in (c) at low temperature corresponds to the additional rectification induced by the p-n junction, and τ in (d) is the electron-electron interaction time. Rights managed by AIP Publishing.

Fig. 7. THz detectors based on the hot carrier-assisted photoconduction, the hot carrier-assisted Schottky junction and the tunneling effect. (a)–(c) The hot carrier-assisted photoconductivity effect [100] 2018 Elsevier Ltd. All rights reserved. (a) Graphene THz detector structure; (b) The temperature distribution and the potential distribution under illumination; (c) The comparison of the resistance change versus the light power between the experimental results (circles) and the theoretical results (squares). (d), (e) The hot carrier-assisted Schottky diode [117]. (d) The device structure of the graphene Schottky diode; (e) The hot carrierassisted quasi-Fermi level that reduced the potential barrier. (f), (g) Double graphene layered heterostructure [118]; (f) Device structure of heterostructure; (g) Fermi level diagram under the bias and gate voltage.

9

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current is as followed:

PTE effect and plasma wave rectification, leading to a weak sensitivity. When Vg > VCNP , the PTE signal can be neglected and a maximum THz responsivity of 15 V/W was achieved. Besides, F. Bianco et al. observed a similar competition phenomenon in 2015 [114] (Fig. 6b). Plasma wave rectification was counterbalanced by the PTE effect, resulting in a significant reduction of photoresponsivity. In 2018, D.A. Bandurin et al. revealed the role of different mechanisms in a graphene FET device encapsulated between two slabs of hBN [115] (shown in Fig. 6,c d). It was found that the doping of graphene, induced by the metal electrode and the gate voltage, was the main factor to decide the major mechanism. For a uniformly n-doped graphene, the plasma wave rectification dominated while the PTE effect was minimized. Otherwise, if the graphene in the channel was p-doped while the graphene-metal interface was n-doped due to the metal electrode, the PTE effect became significant. In addition, at low temperature, there was an additional rectification mechanism induced by the p-n junction, which could further improve the sensitivity of the FET THz photodetectors.

qV

I = Is ⎛e k B T − 1⎞ ⎝ ⎠ ⎜

⎟

(14)

where V is the bias voltage and Is is the saturation current. The saturation current is expressed as:

Is = ACT 2 e

−

qφB kB T

(15)

where A, C and φB denote the area of the junction, the effective Richardson constant and the potential barrier of the Schottky junction, respectively. In graphene, the quasi-fermi level generated by hot carriers reduces the potential barrier significantly (Fig. 7e) and increases the saturation current exponentially. Therefore, the influence of the external bias can amplify the voltage response, and thus using the Schottky junction can realize sensitive detection of THz radiation. A responsivity up to 2000 V/W was obtained under an appropriate bias. However, due to the high resistance of Schottky junction, its response time is mainly limited by the electrical time constant.

4.4. Hot carrier-assisted and graphene heterostructure THz detectors

4.4.3. Double graphene layered heterostructure THz detectors V. Ryzhii et al. theoretically substantiated that the resonant tunneling effect in double graphene layered heterostructures can be used for THz emission and detection [159]. The double graphene layered heterostructure consists of a narrow potential barrier like BN and WS2 sandwiched by two layers of graphene (Fig. 7f). The voltage(Vtop − bottom ) applied between the source and drain combined with gate voltage Vg modulates the chemical potentials and formed a p-i-n structure [160] (Fig. 7g). When the energies of Dirac points of two layers of graphene are exactly the same, the system is in resonance. Conservation of energy and momentum is satisfied during the tunneling, which increases and maximizes the probability of resonant tunneling between two layers of graphene, resulting in the negative differential conductance. When there is a band-offset(Δ ) between two Dirac points (ℏω), the TM polarized photons with energy ℏω ∼ Δ can mediate the electron tunneling in two layers of graphene, thus generating a photocurrent. This process is referred as the photon assisted resonant tunneling. Besides, the work of Vasko et al. showed not only the resonant tunneling but also nonresonant tunneling could generate a photocurrent in double graphene layered heterostructures [161]. Deepika Yadav et al. reported the first experimental observation of the double graphene layered heterostructure THz detector [118]. The potential barrier sandwiched by two monolayer graphene was ∼3 nm thickness hBN. Modulated by Vtop − bottom and Vg , the detector generated ∼15 pA tunneling current under 1 μW radiation, leading to a responsivity of about 1.55 A/W .

4.4.1. Hot carrier-assisted photoconductive detectors In 2017, Wang Lin et al. observed an abnormal strong photoresponse similar to bolometric effect in a BP THz detector [116]. The resistance change was over 10 Ω under 15 mW radiation. Considering that the TCR of BP is only ∼ 10−2 /K , the resistance change was several orders of magnitude larger than that caused by bolometric effect. Thus the bolometric effect could be excluded. Meanwhile, the response time was on the same order of magnitude as the thermal time constant, implying that the photoresponse arose from a kind of thermal effect. Wang Lin et al. presented a new mechanism called as hot carrier-assisted photoconductive effect to explain these phenomena. The basic process is described as follows [100,116,157]. When 2D materials absorb the incident radiation, the carrier temperature rises and hot carriers are generated. The carrier temperature distribution between two electrodes is non-uniform because the electrodes serve as heat sink and thus the carriers are heated more efficiently in the channel. As a consequence, the thermal electric potential as a function of position is also non-uniform in the channel (Fig. 7b). The potential gradient drives the holes (electrons) to move from the high (low) potential to the low (high) potential. Assuming that the material’s Seebeck coefficient is positive, the potential at the channel is higher than that at the electrode. The electrons are dislodged from the metal-material interface to the channel, forming an unbalanced carrier distribution, which accelerates the recombination rate and reduces the number of free carriers. This phenomenon increases the resistance of 2D materials and seems like the photoconductive effect. The response time is consistent with the thermal time constant because the hot carrierassisted photoconduction is based on the thermoelectric effect. It should be noted that in contract to PTE, this kind of detectors does not require any asymmetry in devices but requires a bias. A similar phenomenon was also observed in CVD graphene [157]. The detector was symmetric (Fig. 7a), and the resistance variation induced by the THz radiation was two orders of magnitude higher than that caused by bolometric effect due to the low TCR of graphene(only ∼ 10−3 /K ), as shown in Fig. 7c. The response time was less than 1 ms and on the same order of magnitude as the thermal time constant. Owing to this mechanism, the responsivity could reach 100 V/W while the minimum NEP was only 100 pW/Hz0.5 and the D∗ was ∼ 3.5 × 108 Jones under a suitable bias. In addition, the performance could be improved by using a narrow metallic slit as the gate [158].

5. Conclusion and perspective In this paper, we concisely introduced the recent progress in the field of 2D material-based THz detectors. The diversity and excellent physical properties of 2D materials result in the various detectors, which can be classified in terms of detection mechanisms including the bolometer, the PTE detectors, the plasma wave rectification detectors, the hot carrier-assisted photoconduction detectors, the hot carrier-assisted Schottky junction detectors and the detectors based on the tunneling effect in double graphene layered heterostructures. Due to these attractive devices, 2D materials are promising for THz detection. More importantly, 2D materials are still awaiting full exploitation. On the one hand, the pristine characteristics of 2D materials have not been completely understood and thus there exists many new photodetection mechanisms that are well worth the further exploration. For example, the plasmon ratchet effect, appearing in the spatially periodic graphene structure, has been theoretically proven as an efficient way for THz detection [162,163] and need the experimental confirmation. The electromagnetically induced transparency in graphene, related to the particular selection rules between Landau levels, has been

4.4.2. Hot carrier-assisted Schottky junction detector In 2018, Mina Amirmazlaghani et al. reported a THz detector based on Graphene-Si Schottky junction [117] (Fig. 7d). Schottky junction refers to the interface between metal and semiconductor that has rectifying characteristics. The relationship between the voltage and 10

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performed to realize THz signal detection [164]. The mechanical resonators, employing the outstanding mechanical properties but not the exceptional electronic and photonic properties of 2D materials, are also suitable for THz detection [165]. On the other hand, the improvement of methodology inspires new types of THz detectors. Photoconductive THz detectors, generally thought impossible due to the very low energy of THz photons, has been developed by using the graphene nanomeshes [166]. The three-dimensional microporous graphene, composed of randomly oriented and interconnected monolayer graphene sheets, is confirmed as an efficient way to enhance the response of THz bolometeric and PTE detectors. Besides, the heterostructures open up the opportunity for novel THz detectors. hBN-graphene-hBN structure not only intensifies the plasma rectification detectors [115,167] but also can be utilized in the graphene ballistic rectifier [168]. Obviously, with the maturity of methodology and new insights into photodetection mechanisms, the great potential of 2D materials will be fully exploited in the near future. However, at present, in order to realize a high performance THz detector, more efforts are necessary to optimize the fabrication techniques. For one thing, most of sensitive THz detectors employ layered materials or heterostructures fabricated by mechanical exfoliation. However, this process is not only inefficient but also complex in consideration of heterojunction fabrications. Therefore, controllable synthesis of large area and high quality 2D materials is the key for the practical application like THz detector arrays. For the other thing, some 2D materials degrade rapidly under ambient environment due to the large surface and volume ratio, which reduces the device performance seriously. Hence, mature surface passivation or encapsulation technology is also meaningful, especially for the BP THz detectors. In conclusion, although the extraordinary physical properties taken together with the advantages of integrality with silicon technologies and CMOS compatibility have lend 2D materials a bright future in THz detection, there is still a long way for the practical applications.

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