The terahertz band, which is loosely defined as the electromagnetic frequency ranged from 0.1 THz to 10 THz, has found many unique properties, such as highly penetrable to non-metallic/polar materials, non-invasive due to its low energy, and especially commensurate with spectroscopic region of rotational and vibration modes of many biomedical molecules. Therefore, THz techniques have recently received considerable interest in the the study of biodetection,^[1] stand-off spectroscopy for security screening,^[2–4] space-based observatory,^[5,6] and many other fields. Despite the vast amount of promising applications, generation and detection at THz band is limited by the development of microwave and infrared technology.

To convert THz light into an electrical signal, a series of methods have been delineated within the capability of satisfying peculiar circumstances. Typically, the direct detection of the thermal radiation within the so-called thermal detectors in the forms of the bolometer,^[7] pyroelectric detector,^[8] and Golay cell have been historically rendered as classical schemes with continuous achievements thanks to the progress of material science and nanotechnology. Principally, for a thermal-based detector, the incident radiation is absorbed to change the material temperature. The resultant change in some physical property is used to generate electrical output. In pyroelectric detectors, a change in the internal electrical polarization is measured, whereas in the case of bolometer a change in the electrical resistance is measured. However, deep cryogenic-cooling or even refrigeration cooling at temperature below 4.2 K is needed in the state-of-art thermal-based detector, suffering from a slow response time (at about ), which impedes the widespread application for practical usages.

Field-effect transistors (FETs) host a two-dimensional electron gas with high electron mobility and have proven to be potential candidates for room temperature applications due to their high sensitivity, low noise and simplicity of integration.^[9,10] A field effect transistor with nanometer length gate can support collective electron oscillation-plasma wave, which is similar with other electronic-based devices, producing nonlinear rectification of THz field. The appearance of van der Waals materials, such as graphene and transition metal dichalcogenides (TMDs) offers new opportunities as groundbreaking materials with extraordinary optoelectronic properties that allow unprecedented integration of two-dimensional channel with ultrahigh mobility with the aim of both plasma waves and photothermoelectric THz detection at room temperature.^[11] A recent study has shown that cooperative behavior in TMDs caused by the condensate of strong correlated interaction can be controlled either electrically or optically, offering substantiate routes to build suitable principle for THz detection. Unlike the plasma wave effect, charge density waves, as an alternate collective order of electronic reconstruction caused by the periodic lattice distortion under strong electron–phonon coupling, can carry a charge current in a collective fashion. Therefore, the complete control of such many-body state will revolutionize photon absorption and its energy conversion. Among such materials, the 2H–TaSe₂ could be a canonical system due to its clear phase diagram in the absence of compete from superconducting transition and Mott state.^[12] This material exhibits normal metallic state at room temperature, and a second-order near commensurate ordered phase at 122 K, with the capability of overcoming the bottleneck of higher temperature operation in thermal detectors. In this work, we show on multilayer transistors, with 2H–TaSe₂ as the active channel, displaying a rapid response time and relative high responsivity at THz band. The results of the proposed charge density wave transistors can be further optimized to meet the practical applications of high-performance photodetectors and represent an important guide to explore many-body states for the next generation of condensed matter research and device application.

In this experiment, multilayer 2H–TaSe₂ were mechanically exfoliated from bulk counterpart and transferred by using a scotch-tape method onto the Si/SiO₂ wafer, which is covered by a d_ox = 285 nm thick thermally grown oxide layer. The channels were defined by oxygen plasma etching followed by ultraviolet (UV) lithography and electron-beam evaporation process with Cr/Au stack (8-nm Cr and 65-nm Au) to form the drain and source electrodes after lift-off. The optical microscopic and atomic force microscope (AFM) images of the fabricated TaSe₂ field effect transistor (FETs) is shown in Figs. 1(a) and 1(b), respectively, the thickness of 2H–TaSe₂ was measured to be 60 nm. The source and drain electrodes composed of antenna have a channel width of , and the antenna is designed to concentrate the center terahertz field at the center when incident terahertz radiation. Figure 1(c) displays the Raman spectrum of a sample excited by a 532-nm laser line in ambient environment. Here, it is obviously seen that three peaks featured at 144.722 cm⁻¹, 207.892 cm⁻¹, and 233.611 cm⁻¹, which correspond to the in-plane E_1g, E_2g, and A_1g vibrations of 2H–TaSe₂, following well with other work.^[13]

Fig. 1.

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	Fig. 1. (a) The image of the device under the microscope. (b) AFM image of the FET device. (c) The Raman spectrum of the 2H–TaSe2 FET. (d) The schematic cross section and equivalent circuit of the detector.

A cross-sectional view of the 2H–TaSe₂ FET and its electrical connections is shown in Fig. 1(d). Then, both electrical and photoresponse characteristics are studied within flowchart, as shown in Fig. 2(a). The I–V characteristics of our device were measured first with a Keithley 4200 semiconductor parameter analyzer. Due to the metallic nature of normal state, a perfect I–V characteristic in the swept voltage range from −0.2 V to 0.2 V displays the linear behavior, and the resistance TaSe₂ channel is deemed to be 160 Ω. All experiments were carried out at room temperature under atmosphere environment. The THz beam was outer-coupled via rectangular waveguide connected to the WR 9.0 VDI (Virginia Diode Inc.) tripler, which is based upon the microwave signal generator (Agilent E8257D) to produce the fundamental frequency tunable from 20 GHz to 40 GHz, so that the frequency can be multiplied up to 120 GHz. The THz beam is guided toward the device through an aperture and an off-axis parabolic mirror in the form of parallel output within the beam spot area S_b = 8 cm². Figure 2(c) shows the photocurrent (I_ph) response versus incident power P_in under 40-GHz electromagnetic radiation, which exhibits excellent linear relationship whenever the device is under zero bias or under finite bias voltage. Therefore, the CDW detector studied here can be rendered as a power detector, the response of which is depending unambiguously on the electric field intensity . Furthermore, the bolometric mechanism which is widely recognized with the scheme in similar with those depends on the meal–insulator phase transition in Mott insulator, e.g., VO_x, can be ruled out here. This consensus is supported well by the opposite trend between experimental results and the routinely defined bolometric effect. Typically, for 2H–TaSe₂, the metallic behavior of the normal state dominates at room temperature, which predicts the increased resistance under heating by photon absorption, and ultimately leads to the bolometric effect. Whereas, the channel resistance of the detector under impulse electromagnetic excitation in Fig. 2(b) is counter-intuitively since the resistance is dropped when the device is under electromagnetic radiation, strongly hints the non-bolometric origin. Thus, the number of carriers is increased in the channel, meaning that the unfreezing of the localized carriers at the metal–TaSe₂ interface. Such phenomenon has been confirmed well by the power-dependent properties of response at all bias range, meaning the good linear behavior of power absorption for the non-equilibrium carrier diffusion. In the case of terahertz radiation and zero bias, it is shown in Fig. 2(d) that the photocurrent is relatively stable even when the modulation frequency is between 0 kHz and 20 kHz, which is sufficiently fast for real-time imaging due to the fast renormalization of CDW transition.

Fig. 2.

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	Fig. 2. (a) Detection of terahertz radiation using a home-built experimental setup. (b) I–V curve of the device. (c) The photoresponse versus incident power Pin under 40-GHz electromagnetic radiation. (d) The relationship between photoresponse and modulation frequency under 0.12-THz terahertz radiation at zero bias.

To elucidate more clearly the performance of CDW device, the voltage/current responsivity is calculated with the following relationship:^{[14] [14] [14]} where r is the resistance, is the incident power onto the device, here S_A (=0.18 mm²) and S_b (=8 cm²) are the total area of the device including the area of antenna and bonding pad, etc., and the beam spot area determined by optical path, respectively, P_out is the output power of signal generation (30 mW for 40 GHz and 6 mW for 120 GHz). Figure 3(a) displays the bias-dependence of the photoconductive responsivity for the device. The small image on the upper left is the spectrum response of the detector, and we chose the peak with a higher frequency during the test. When the device is at zero bias, the responsivity is approximately 1.5 V/W, whereas the signal is enlarged by an order of magnitude within only 80-mV bias applied across the source and drain. Therefore, this large inherent gain may originate from the enhanced injection of non-equilibrium carrier facilitated by the broken of inversion symmetry under electric bias. The incident power is estimated on the basis of the total antenna aperture including contact pads, being obviously much larger than the active area of the detector, signifying that the present responsivity R_V up to 27 V/W is only a lower limit. Table 1 compares the responsivity of the fabricated device with that of other detectors, reflecting that the detectorʼs responsivity is relatively high, but can still be further enhanced.

Fig. 3.

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	Fig. 3. (a) The spectrum response, responsivity, and PNEP of the device under 120-GHz radiation. (b) The rise time of the terahertz detector when applied an 80-mV bias. (c) The Ids–Vgs curve and the structure of the 2H–TaSe2 photodetector. (d) IPhoto varies with the gate voltage, when the bias between the source and the drain is fixed at 10 mV.

Table 1.

Table 1.

Table 1. Performance parameters of some terahertz detectors. . Description Responsivity Frequency/THz Bilayergraphene field-effect transistora 30 V/W 0.33 1T–TaS2 photodetectorb 1 A/W 0.3 BP-based FETc 0.15 V/W 0.29 hBN-BP-hBNNanodetectord 2 V/W 0.26–0.37 2H–TaSe2 FET 27 V/W 0.12 a Ref. [15] b Ref. [16] c Ref. [17] d Ref. [18]

Table 1. Performance parameters of some terahertz detectors. .

To fully assess the sensitivity of the detector, noise equivalent power (P_NEP) extracted from the ratio V_N/R_V is another important quality factor. It has been assumed that the main contribution of the noise figure is the thermal Johnson–Nyquist noise (N_th), and V_N is described by the following equation:^{[14] [14] [14]} where k_B is the Boltzmann constant, T = 300 K, r is the resistance of the device. The P_NEP of this CDW detector achieves a minimum level of 0.1 nW/Hz^0.5,^[19] being superior to the other graphene-based ones.

In addition to the sensitivity, the response time is also crucial to practical applications. Therefore, the time resolved photocurrent was also measured, as shown in Fig. 3(b) under 0.12-THz radiation with fast on/off modulation at 1000 Hz. As can be seen from Fig. 3(b), the response time which is defined as the signal growing from 10% up to 90% on rising edge of the signal or analogously on the falling edge,^[20] is doomed to be (at all bias range) showing the excellent stability and fast operability of the device. The phototransistor based on 2H–TaSe₂ herein is within metal–semiconductor–metal planer structure. When photons are impinging on the semiconductor channel, photocarriers can be generated following the electromagnetic process, due to the nonequilibrium diffusion from metal to TaSe₂ channel assisted by the electrical bias. Therefore, unilateral flow of photocurrent will be generated along with the fast and reversible process of electromagnetic interaction.

Furthermore, it has been reported previously that top-gated configurations are critical for suppressing Coulomb scattering in TMDs (e.g., MoS₂) channels, which can be useful for low-power operation, as well as the feasibility for tuning the electronic and optoelectronics properties.^[21–23] Especially, recent studies have revealed the essential role of electrostatic doping in tuning the strong correlation interaction in TMDs due to their two-dimensional nature, opening up the possibility in uncovering intriguing phenomena of condensed matter physics and device applications. To better understand the role of gating effect, we have also tried to prepare the top gate configuration in order to tuning the absorption and conductivity of 2H–TaSe₂ within a few additional processing steps. On the basis of above-mentioned MSM structure, a 30-nm Al₂O₃ layer was deposited as a dielectric layer on 2H–TaSe₂ through atomic layer deposition (ALD) at about 300 °C, and the gate fingers are aligned by electron–beam lithography. It can be found from Fig. 3(c) that the source–drain current I_SD changes from to about , when the gate voltage varies from −1 V to 1 V under only 10-mV V_DS bias. Figure 3(d) displays the gate-dependent photocurrent under 0.04-THz radiation, which shows decreased tendency under positive gate voltage following well the I_d–V_g character in Fig. 3(c). This gate tunability of photocurrent experiences a significant departure from the previous graphene counterpart, where either polarity inversion or extreme value of photoresponse exists at the threshold bias point. The polarity inversion, or in other words, the change of direction of photocurrent is attributable to the photothermoelectric or plasma wave rectification process following the plasma theory proposed. Whereas, both of these phenomena are absent in CDW FET, and only decreased trend can be found when approaching to the threshold voltage. Therefore, the photocurrent appear in CDWFET is beyond the driving force of plasma wave or the photothermoelectric effect. While it is still reasonable to see that the free electron absorption of normal states dominant and it drops obviously under positive gate voltage, leading to the decreased energy conversion efficiency. In the binary transition metal chalcogenide compound, when the electrons and the crystal lattice are periodically modulated, a charge density wave is generated, which significantly changes the electrical and thermal properties of the compound. The interior correlation between CDW state and normal state change significantly the transport property of a single device, which departs obviously from the Fermi liquid behavior and ultimately exhibiting significant difference in gate voltage dependence of photocurrent.

In summary, we have achieved a room temperature photodetector with a fast response in a 2H–TaSe₂-based terahertz system. By integrating with a logarithmic antenna, the responsivity exceeds 27 V/W and the response time can reach . The reported device has room-temperature capability, fast response, relative high responsivity, and shows great potential to compete with other established detector technologies, reflecting the unique properties of two-dimensional TMDs in optical and electrical devices, in flexible transistors, flexible LED devices, sensors, optoelectronic devices and energy storage, and thus have great application prospects.^[24]