WO2022169423A1

WO2022169423A1 - Terahertz photodetector, method of forming the same, and method for controlling the same

Info

Publication number: WO2022169423A1
Application number: PCT/SG2022/050058
Authority: WO
Inventors: Xuechao Yu; Chongwu WANG; Qijie Wang
Original assignee: Nanyang Technological University
Priority date: 2021-02-08
Filing date: 2022-02-07
Publication date: 2022-08-11

Abstract

According to embodiments of the present invention, a terahertz photodetector is provided. The terahertz photodetector includes a pair of source/drain terminals, a monolayer tungsten ditelluride electrically coupled to the pair of source/drain terminals, and a gate terminal arrangement configured to receive an electrical signal to control an electrical property of the monolayer tungsten ditelluride. According to further embodiments of the present invention, a method of forming a terahertz photodetector and a method for controlling a terahertz photodetector are also provided.

Description

TERAHERTZ PHOTODETECTOR, METHOD OF FORMING THE SAME, AND METHOD FOR CONTROLLING THE SAME

Cross-Reference To Related Application

[0001] This application claims the benefit of priority of Singapore patent application No. 10202101317V, filed 8 February 2021, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] Various embodiments relate to a terahertz photodetector, a method of forming the terahertz photodetector, and a method for controlling the terahertz photodetector.

Background

[0003] The valley degree of freedom of electrons has sparked intense activity with the discovery of graphene and transition metal dichalcogenides (TMDCs). Compared to centrosymmetric graphene, monolayer TMDCs with broken spatial inversion symmetry are quintessential platforms permitting the generation of valley selective electron populations which can be manipulated optically owing to the valley-contrasting Berry curvatures and orbital magnetic moments. The utilisation of valley dependent properties is increasingly arousing a wide range of applications such as the valley Hall effect, circular dichroism light emitting through optical or electrical pumping. However, the optical band gaps of most 2H TMDCs (where 2H refers to 2 layers per H(exagonal) unit cell) (such as molybdenum disulphide (M0S2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2)) are in the infrared-visible spectral range, and it remains a formidable challenge to investigate the valley properties by long wavelength optical excitations such as infrared and terahertz (THz) ranges. This elementary process underlies the essential physics of many phenomena and applications, including optical communication, digital imaging and lightemitting, and so on. [0004] lT’-WTe2 (tungsten ditelluride) (where IT refers to one layer per (Trigonal) unit cell) has attracted much interest because of its remarkable properties both in the bulk and monolayer. Bulk lT’-WTe2 was demonstrated to be a type-II Weyl semimetal exhibiting extreme nonsaturating magnetoresistance and room temperature ferroelectric semimetal. Meanwhile, high- temperature quantum spin Hall (QSH) states and gate-tunable superconductivity were observed in monolayer WTe2 thanks to its nontrivial spin-resolved Berry curvature properties. The spinorbit coupling in monolayer lT’-WTe2 induces an inverted, indirect QSH gap near Q and Q’ valley in momentum space. As a result, optical excitation of the QSH gap with mid-infrared circular polarised laser was proposed and demonstrated to exhibit in-plane circular photogalvanic currents. The QSH gap of monolayer lT’-WTe2 is calculated to be around 30 meV by density functional theory (DFT) with generalised gradient approximation method, indicating that long infrared or THz excitation would be more suitable for studying the QSH states and valley dependent transitions.

[0005] Valley polarisation has been created in known TMDCs using excitation by circularly polarised light and has been detected both electrically and optically, reversely indicating great potential in helicity-sensitive photodetectors utilising these internal degrees of freedom of valleys. Compared to ordinary helicity-sensitive photodetectors achieved through external optical antenna or metasurface structures, valley-dependent helicity photoresponse is intrinsically originated from electrons excited from a particular valley via circularly polarised light with different chirality. Previously reported helicity photodetectors with TMDCs mainly focused on visible/near-infrared spectrum regime due to the band gap limitation, on the other hand, Weyl semimetals such as tantalum arsenide (TaAs), and Td-WTe2 were demonstrated to show helicity-sensitive, broadband photoresponse from visible to 10.6 pm. Anisotropic PtTe2- based (platinum telluride) phototransistor was shown to sustain room temperature photogalvanic effects in the sub-THz and millimeter regime. The interaction between valley excitons and THz photons in two-dimensional platforms that may bring many unprecedented applications, however, has remained elusive. Summary

[0006] The invention is defined in the independent claims. Further embodiments of the invention are defined in the dependent claims.

[0007] According to an embodiment, a terahertz photodetector is provided. The terahertz photodetector may include a pair of source/drain terminals, a monolayer tungsten ditelluride electrically coupled to the pair of source/drain terminals, and a gate terminal arrangement configured to receive an electrical signal to control an electrical property of the monolayer tungsten ditelluride.

[0008] According to an embodiment, a method of forming a terahertz photodetector is provided. The method may include forming a pair of source/drain terminals, electrically coupling a monolayer tungsten ditelluride to the pair of source/drain terminals, and forming a gate terminal arrangement configured to receive an electrical signal to control an electrical property of the monolayer tungsten ditelluride.

[0009] According to an embodiment, a method for controlling a terahertz photodetector is provided. The method may include applying a terahertz optical signal to the terahertz photodetector described herein, wherein the terahertz optical signal is circularly polarised.

Brief Description of the Drawings

[0010] In the drawings, like reference characters generally refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which: [0011] FIG. 1A shows a schematic cross-sectional view of a terahertz photodetector, according to various embodiments.

[0012] FIG. IB shows a flow chart illustrating a method of forming a terahertz photodetector, according to various embodiments.

[0013] FIG. 1C shows a method for controlling a terahertz photodetector, according to various embodiments. [0014] FIG. 2A shows the crystal structure of monolayer IT’- WTe2 (tungsten ditelluride) having a mirror plane M_a and a screw rotation symmetry C2a-

[0015] FIG. 2B shows the angle-dependent Raman spectrum contour maps of a monolayer WTe2 (tungsten ditelluride).

[0016] FIG. 2C shows the polar plots of a monolayer WTe2 (tungsten ditelluride).

[0017] FIG. 2D shows a transmission electron microscope (TEM) image of a monolayer WTe2 (tungsten ditelluride). The scale bar represents 500 nm. The inset shows a fast Fourier transform (FFT) pattern of a monolayer WTe2. The scale bar in the inset represents 2 nm ¹.

[0018] FIG. 2E shows a high-resolution transmission electron microscope (HRTEM) image of a monolayer WTe2 (tungsten ditelluride). The scale bar represents 2 nm.

[0019] FIGS. 2F and 2G show, respectively, the band structure of a monolayer WTe2 (tungsten ditelluride) without and with spin-orbit coupling.

[0020] FIG. 3A shows a schematic experiment setup for detecting terahertz (THz) circular photogalvanic effect in a monolayer WTe2 (tungsten ditelluride) based field effect transistor (FET).

[0021] FIG. 3B shows a schematic illustration of optical selection rules in the opposite Q and Q’ valleys.

[0022] FIG. 4A shows an optical image of a monolayer WTe2 (tungsten ditelluride) based field effect transistor (FET), according to various embodiments. The scale bar represents 10 pm.

[0023] FIG. 4B shows the results for the source-drain current dependence of the gate voltage at a bias voltage of 500 mV.

[0024] FIG. 4C shows the results for the source-drain current dependence of the bias voltage at gate voltages from -10 V to -lO V.

[0025] FIG. 4D shows a schematic of the device architecture of a dual-gated monolayer WTe2 (tungsten ditelluride) FET (field effect transistor), according to various embodiments.

[0026] FIG. 4E shows the laser spectrum of the QCL (quantum cascade laser) THz (terahertz) laser employed in various embodiments.

[0027] FIG. 4F shows the calculated optical resonance spectrum of the THz (terahertz) antenna of various embodiments based on Finite-difference time-domain (FDTD) method. The resonance peak of the THz antenna is tuned to the laser spectrum peak by optimising the size/structure of the antenna. [0028] FIG. 5 shows the simulation result for the optical distribution of a terahertz (THz) antenna of various embodiments.

[0029] FIG. 6A shows results for polarisation-dependent photocurrent along b direction (lb) with THz laser illumination and different gate voltages.

[0030] FIG. 6B shows results for polarisation-dependent photocurrent along a direction (I_a) with THz laser illumination and different gate voltages.

[0031] FIG. 6C shows results for photocurrent along a direction (I_a) as a function of the sourcedrain voltages.

[0032] FIG. 6D shows results for excitation power dependence of the photocurrent (I_a) at a fixed excitation spot. The source-drain voltage and the gate voltage are controlled to be 5 mV and 10 V respectively.

[0033] FIG. 7A shows results for the measured photocurrent as a function of the gate voltage.

[0034] FIG. 7B shows results for the relative photoresponse output as a function of the laser intensity modulation frequency at a bias voltage of IV, and a gate voltage of 10 V under illumination with a power of 220 mW. A response degradation of ~ 3dB is observed at ~ 8kHz.

[0035] FIG. 7C shows the transient reflectance measurement results for monolayer WTe2 with 1550-nm pump at different pump fluences.

[0036] FIG. 7D shows the exponential decay time constants deduced from fitting of the curves in FIG. 7C as a function of pump fluences.

[0037] FIGS. 8A and 8B show the density of Berry curvature dipoles in two directions.

Detailed Description

[0038] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. [0039] Embodiments described in the context of one of the methods or devices are analogously valid for the other methods or devices. Similarly, embodiments described in the context of a method are analogously valid for a device, and vice versa.

[0040] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0041] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0042] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

[0043] Various embodiments may provide one or more atomically thin high performance terahertz (THz) detectors. For example, various embodiments may provide helicity sensitive Terahertz detection in monolayer IT’- WTe2 (tungsten ditelluride).

[0044] lT’-WTe2 refers to WTe2 materials with atomic structure in the IT’ form or phase. For transition metal dichalcogenides (TMDCs), the transition metal is surrounded by chalcogen atoms, and forms a structure like a trigonal prism. Another form is the metallic IT phase in which the transition metal is synchronized by chalcogen atoms octahedrally. A distorted version of IT phase is referred to as IT’ phase, where in this structure, the transition metal atoms are just about octahedrally synchronized by chalcogen atoms but with less symmetry.

[0045] Valley tronics, identified as electronic properties of the energy band extrema in momentum space, has been revived following the emergence of two-dimensional transition metal dichalcogenides (TMDCs) as their valley information can be controlled and probed through the spin angular momentum of light. Previous optical investigations of valleytronics are limited in the visible/near-infrared spectral regime because the band gap of most TMDCs is quite large, and through which the electrons of most TMDCs can be excited. Monolayer WTe2 (tungsten ditelluride) with broken time -reversal symmetry and a narrow band gap provides a fertile platform to study the long wavelength optical or photonic properties in different valleys. The techniques disclosed herein employ circular-polarised Terahertz (THz) laser to selectively excite the valley of monolayer WTe2 and demonstrate that helicity dependent photoresponse (i.e., a response that depends on the helicity of the incident light) is generated via photogalvanic effect (PGE). The photocurrent is observed to be controlled by circular polarisation and external electrical field (as may be observed in FIGS. 6 A - 6C to be described further below) due to the tunable Berry curvature dipole derived from the nontrivial wavefunctions near the inverted gap edge in monolayer WTe2. The results, as will be described further below, provide a venue for controlling, detecting, and processing valleytronics and applications in on-chip THz imaging and quantum information processing.

[0046] Circular-polarisation is one parameter of a laser, and may be achieved by putting 1/2 (half) and 1/4 (quarter) waveplates in the light path of the laser. In various embodiments employing a quantum cascade laser (QCL), circular polarisation of the QCL laser may be achieved by employing a photonic structure on the facet of the laser, where, for example, the photonic structure may be or may include a circular concentric grating (CCG). The CCG may be designed as a second-order grating.

[0047] In the techniques disclosed herein, circular-polarised THz laser (~3.2 THz) is utilised to excite the QSH states in lT’-WTe2 and helicity-dependent photocurrent is generated corresponding to the polarisation of the incident light through circular photogalvanic effect (CPGE). On the other hand, the helicity-dependent photocurrent only occurs in a direction of the crystallographic axis but vanishes in the b direction of the crystallographic axis, which can be attributed to the inversion symmetry broken of monolayer IT’-WTe2. The THz excitation is expected to excite the lowest QSH states and thus induces a nonlinear photoresponse along a direction of the crystallographic axis, which can be tuned by an external gate voltage. The results can be explained by Berry curvature dipole in monolayer IT’-WTe2. Furthermore, the relaxation time is also measured via transient reflection experiments and it is found that Auger recombination is the dominant relaxation pathway in the monolayer IT’- WTe2 based THz detectors. The disclosure herein not only establishes monolayer lT’-WTe2 as a platform for valley engineering under THz laser illumination but also paves the way for implementing chip-scale THz optoelectronic applications such as chiral detection, modulation and imaging devices.

[0048] FIG. 1A shows a schematic cross-sectional view of a terahertz photodetector 100, according to various embodiments. The terahertz photodetector 100 includes a pair of source/drain terminals 102, 104, a monolayer tungsten ditelluride 106 electrically coupled to the pair of source/drain terminals 102, 104, and a gate terminal arrangement 108 configured to receive an electrical signal to control an electrical property of the monolayer tungsten di telluride 106.

[0049] In other words, a terahertz (THz) photodetector 100 to detect or for detecting THz optical signal or light may be provided. Accordingly, the terahertz (THz) photodetector 100 is responsive to a THz optical signal or light. The terahertz photodetector 100 includes one pair of source/drain terminals (or electrodes) 102, 104. The source/drain terminals 102, 104 may be arranged spaced apart from each other. The pair of source/drain terminals 102, 104 include a source terminal (e.g., 102) and a drain terminal (e.g., 104). In the context of various embodiments, the term “source/drain terminal” may refer to a source terminal or a drain terminal. For a transistor, as the source terminal and the drain terminal are generally fabricated such that these terminals are geometrically symmetrical, these terminals may be collectively referred to as source/drain terminals. In various embodiments, a particular source/drain terminal may be a “source” terminal or a “drain” terminal depending on the voltage to be applied to that terminal.

[0050] The THz photodetector 100 further includes a monolayer tungsten di telluride (WTe2) 106 that is arranged to be electrically coupled to the pair of source/drain terminals 102, 104. The monolayer WTe2 106 may be arranged or provided in between the pair of source/drain terminals 102, 104. The monolayer WTe2 106 is (electrically) conductive. The monolayer WTe2 106 or part thereof may be part of or may define an active region (or active regime) of the photodetector 100. In the context of various embodiments, a monolayer WTe2 means a single layer of WTe2.

[0051] The THz photodetector 100 further includes a gate terminal arrangement 108. The gate terminal arrangement 108 may be arranged over the monolayer WTe2 106 or part thereof. An electrical signal (e.g., voltage) may be provided or applied to the gate terminal arrangement 108 to control an electrical property of the monolayer WTe2 106. This may enable, during operation, a current to flow between the pair of source/drain terminals 102, 104 through the monolayer WTe2 106. Therefore, the monolayer WTe2 106 provides a conduction channel for the current flow between the pair of source/drain terminals 102, 104. As a non-limiting example, the electrical signal applied to the gate terminal arrangement 108 may affect the electric field within the monolayer WTe2 106, which in turn, may affect the current flow. The electrical signal applied to the gate terminal arrangement 108 may tune the carrier density in the monolayer WTe2 106. The electrical signal applied to the gate terminal arrangement 108 may also alter the Berry curvature of the monolayer WTe2 106.

[0052] It should be appreciated that the THz photodetector 100 may be a transistor-based (FET- based) device or photodetector.

[0053] In various embodiments, the THz photodetector 100 may receive an incident THz optical signal. The THz optical signal may be received by the monolayer WTe2 106.

[0054] The THz photodetector 100 may further include a boron nitride (BN) layer between the monolayer tungsten ditelluride 106 and the gate terminal arrangement 108. For example, the BN layer may be or may include a thin flake of BN. The BN layer may be or may include hexagonal boron nitride (h-BN). The BN layer may act as a dielectric layer between the monolayer WTe2 106 and the gate terminal arrangement 108, and also as a protective layer for the monolayer WTe2 106. The pair of source/drain terminals 102, 104 and the monolayer WTe2 106 may be arranged on another BN layer (e.g., h-BN), for example, on the second boron nitride layer described below.

[0055] The gate terminal arrangement 108 may include a pair of gate terminals (i.e., two gate terminals). The pair of gate terminals may be arranged over the monolayer WTe2 106 or part thereof.

[0056] For each gate terminal of the pair of gate terminals, the gate terminal may include a body section (which may be a central section), and a plurality of arms extending alternately from opposite sides of the body section. The plurality of arms may have a varying size or dimension (e.g., length, width, etc.).

[0057] The length of each arm of the plurality of arms may be between about 50 pm and 1 mm (i.e., 1000 pm), for example, between about 50 pm and 500 pm, between about 50 pm and 200 pm, between about 200 pm and 1000 pm, between about 500 pm and 1000 pm, between about 300 pm and 800 pm, or between about 400 pm and 600 pm.

[0058] The width of each arm of the plurality of arms may be between about 50 pm and 1 mm (i.e., 1000 pm), for example, between about 50 pm and 500 pm, between about 50 pm and 200 pm, between about 200 pm and 1000 pm, between about 500 pm and 1000 pm, between about 300 pm and 800 pm, or between about 400 pm and 600 pm. [0059] Each arm of the plurality of arms may be curved for the plurality of arms of the pair of gate terminals to partially define (a plurality of) concentric circles. In other words, parts of concentric circles may be formed by the plurality of arms. The pair of gate terminals, with the plurality of arms, may define a THz (optical) antenna configured to concentrate a THz light incident on the THz photodetector 100 on or to or towards the monolayer WTe2 106. The antenna may be arranged over the monolayer WTe2 106.

[0060] The terahertz photodetector 100 may further include a second gate terminal arrangement, wherein the monolayer WTe2 106 may be arranged between the gate terminal arrangement 108 and the second gate terminal arrangement, and wherein the second gate terminal arrangement may be configured to receive a second electrical signal to control the electrical property of the monolayer WTe2 106. In this way, a dual-gated monolayer WTe2 photodetector or FET may be provided. The gate terminal arrangement 108 may be a top gate arrangement, while the second gate terminal arrangement may be a bottom gate arrangement. It should be appreciated that description in relation to the gate terminal arrangement 108 may be applicable also to the second gate terminal arrangement.

[0061] The terahertz photodetector 100 may further include a second boron nitride (BN) layer between the monolayer WTe2 106 and the second gate terminal arrangement. For example, the second BN layer may be or may include a thin flake of BN. The second BN layer may be or may include hexagonal boron nitride (h-BN). The second BN layer may act as a dielectric layer between the monolayer WTe2 106 and the second gate terminal arrangement. The second BN layer may act as a protective layer for the monolayer WTe2 106.

[0062] In various embodiments, the terahertz photodetector 100 is responsive to a circularly polarised terahertz optical signal. The terahertz optical signal has a circular polarisation. In response to receiving the terahertz optical signal incident on the THz photodetector 100, the THz photodetector 100 provides a response corresponding to the characteristics of the circular polarisation of the terahertz optical signal.

[0063] FIG. IB shows a flow chart 140 illustrating a method of forming a terahertz (THz) photodetector, according to various embodiments.

[0064] At 142, a pair of source/drain terminals is formed.

[0065] At 144, a monolayer tungsten di telluride (WTe2) is electrically coupled to the pair of source/drain terminals. [0066] At 146, a gate terminal arrangement is formed, configured to receive an electrical signal to control an electrical property of the monolayer tungsten ditelluride.

[0067] The method may further include arranging or forming a boron nitride (BN) layer between the monolayer WTe2 and the gate terminal arrangement.

[0068] In various embodiments, at 146, a pair of gate terminals may be formed. This may include forming, for each gate terminal of the pair of gate terminals, a body section, and a plurality of arms extending alternately from opposite sides of the body section.

[0069] The plurality of arms may have a varying size.

[0070] Each arm of the plurality of arms may be curved for the plurality of arms of the pair of gate terminals to partially define concentric circles.

[0071] The method may further include forming a second gate terminal arrangement, wherein the monolayer tungsten ditelluride is arranged between the gate terminal arrangement and the second gate terminal arrangement, and wherein the second gate terminal arrangement is configured to receive a second electrical signal to control the electrical property of the monolayer tungsten ditelluride.

[0072] The method may further include arranging or forming a second boron nitride (BN) layer between the monolayer WTe2 and the second gate terminal arrangement.

[0073] FIG. 1C shows a method 150 for controlling a terahertz (THz) photodetector, according to various embodiments. The method 150 includes applying a terahertz optical signal to the terahertz photodetector described herein (e.g., 100, FIG. 1A), wherein the terahertz optical signal is circularly polarised. The terahertz optical signal may be applied to or received by the monolayer tungsten ditelluride (WTe2) of the terahertz photodetector.

[0074] The method may further include subjecting the terahertz photodetector to a cooling process (e.g., using liquid nitrogen).

[0075] It should be appreciated that description in the context of the THz photodetector 100 may correspondingly be applicable in relation to the method of forming a THz photodetector described in the context of the flow chart 140, and the method 150 for controlling a THz photodetector.

[0076] Various embodiments will now be further described by way of the following nonlimiting examples with reference to FIGS. 2 A to 8B. It should be appreciated that a bulk material means a material with macro-level sizes in any dimensions. A bulk lT’-WTe2 has much more than one layer. A monolayer lT’-WTe2 means a single layer lT’-WTe2. In the context of various embodiments, a monolayer lT’-WTe2 may be obtained using two different methods: mechanical exfoliation, and chemical vapour deposition.

[0077] The crystal structure of a monolayer IT’ WTe2, with W (tungsten) atoms 190 and Te atoms 192, is shown in FIG. 2A, which exhibits a mirror symmetry and a two-fold screw rotational symmetries. The combination of the two distinct symmetries induces the inversion symmetry of the IT’ phase. Angle-dependent Raman spectrum is employed to characterise the quality and atomic orientation of a mechanically exfoliated monolayer WTe2. It is clearly shown in the Raman contour map of FIG. 2B that there are four dominant optical vibrational modes, namely B¹⁰l, A³2, A⁷1, and A⁹1, compared with those reported in bulk WTe2 and flakes. With the polarisation angle rotated from 0° to 90°, the intensities of the A2 modes (-162.2 cm ¹) display fourfold symmetry while the Al modes (-79.3 cm ¹) exhibit twofold symmetry. Furthermore, the rotation angle between the Al mode and A2 mode as shown in the polar plots and fit curves of FIG. 2C may be employed to identify the zigzag and armchair directions of the monolayer WTe2, which are simultaneously verified by the TEM and HRTEM images shown in FIGS. 2D and 2E. It is observed in the techniques disclosed herein that a monolayer WTe2 with zigzag edges may be fabricated by mechanical exfoliation in a nitrogen protected glovebox. The clear zigzag edges make it convenient to identify the crystal orientation of the monolayer WTe2.

[0078] Bulk WTe2 is a type-II Weyl semimetal, which features tilted band structures and holds chiral Weyl nodes. However, the inversion symmetry broken in monolayer WTe2 gives rise to strong spin-orbit couplings (SOC). The strong SOC in monolayer WTe2 gives rise to a large energy gap. Furthermore, the broken inversion symmetry gives rise to the Rashba SOC and induces spin splitting of the bands with nonzero Berry curvature, which results in circular photogalvanic effect (CPGE). The low-energy band structure of monolayer WTe2 without SOC as shown in FIG. 2F illustrates a tilted Dirac band structure crossed at the Q and Q’ points (which refer to the symmetric points in the band structure; see also FIG. 3B). However, the SOC lifts the degeneracy at the Dirac point and introduces an inverted QSH band gap as shown in FIG. 2G with the bottom of the conduction band located at the Q and Q’ points and the top of the valence band located at the T point (F points refer to the symmetric points in the band structure; see also FIG. 3B). According to the k-p model-based calculations carried out internally, the band gap is around 20 meV, which is generally consistent with the known value. This narrow band gap enables investigation of the valley dependent photocurrent of monolayer WTe2 with circular-polarised THz laser excitation. The CPGE caused by the spin and optical selection rules between the conduction band and the valence band makes monolayer WTe2 a candidate for helicity dependent photodetector. It should be noted that despite intense development efforts in known quantum wells, the ability to detect the helicity of THz radiation for on-chip photonics has seen limited success to date. The realisation of helicity sensitive THz detector was previously achieved via circular photogalvanic effect (CPGE) originating from the unequal population of excited charge carriers in preferential momentum direction, the polarity and magnitude of photoresponse can be controlled by the chirality of the elliptically polarised optical excitation. This effect, however, is restrained in gyrotropic materials with strong spinorbit coupling or semiconductors with externally broken inversion symmetry.

[0079] The techniques disclosed herein demonstrate helicity sensitive photodetection in the THz regime based on a monolayer WTe2 field effect transistor (FET) with liquid nitrogen cooling. FIG. 3A shows a schematic experiment setup 380 for detecting terahertz (THz) circular photogalvanic effect in a monolayer WTe2 based FET. The setup 380 includes an internally custom-made THz quantum cascade laser (QCL) 381, powered using a power supply 382, that is employed to detect the CPGE induced by the interband transition across the inverted QSH gap. A chopper 383 is arranged at the output of the QCL 381 to modulate the laser light. The modulated light then passes through a lens 384 to focus the modulated light, which subsequently may pass through a half waveplate 385, followed by a quarter waveplate 386, where the polarisation is achieved by adjusting the quarter waveplate 386. The modulated light is then directed to a monolayer WTe2 based FET (not shown) placed on a 3D (sample) stage 387. The 3D stage 387 is electrically coupled to a semiconductor analyzer 388 and a lock- in amplifier 390. The semiconductor analyzer 388 is used to collect electrical signals from the monolayer WTe2 based FET. The chopper 383 is connected to a chopper controller 391, which in turn is connected to the lock-in amplifier 390. The lock-in amplifier 390 is also electrically coupled to an analyzer 392, which may be a computer or processor with a data collector. The lock-in amplifier 390 is used to decrease noise and collect data.

[0080] Analogous to the optical selection rules in common TMDCs, an optical excitation with a particular helicity can only populate one valley as illustrated in FIG. 3B which shows the optical transition processes in WTe2. The solid straight arrows illustrate electron spin up and down. According to the spin-locking effect and momentum conservation law, the transitions labelled with bold crosses are forbidden while the transitions labelled with circular arrows are permitted.

[0081] FIGS. 4A to 4F show the device architecture and electrical characterisations.

[0082] Dual-gated (e.g., having top and bottom gates) monolayer WTe2 FET devices with multiple electrodes have been fabricated via electron beam lithography (EBL), followed by electron beam evaporation of metal electrodes. FIG. 4A shows an optical image of a fabricated device 400a while FIG, 4D shows a schematic of the device architecture 400d corresponding to the device 400a. Each device (or THz photodetector) 400a, 400d includes a source terminal (or electrode) 402a, 402d, a drain terminal (or electrode) 404a, 404d and two gate terminals (or electrodes) 408a, 410a (the gate terminals for device 400d are illustrated as one gate terminal (arrangement) 408d). Nevertheless, it should be appreciated that one gate terminal may be sufficient. Referring to FIG. 4D, an electrical signal, e.g., a voltage, VT, may be applied to the gate terminals 408a, 410a, and the gate terminal 408d.

[0083] Each device 400a, 400d may be encapsulated by a thin layer or flake of boron nitride (BN) (e.g., hexagonal boron nitride, h-BN) 412a, 412d, as may be more clearly seen in FIG. 4D. As may be observed in FIG. 4D, a pair of perpendicular electrodes, i.e., the source and drain terminals/electrodes 402a, 404a, 402d, 404d, may be patterned along the armchair and zigzag directions according to the band structure analysis that the Q and Q’ valleys are only located along the zigzag direction (also denoted as the a direction). Each device 400a, 400d further includes a monolayer WTe2 406a, 406d which may be arranged in between the source and drain terminals/electrodes 402a, 404a, 402d, 404d. The monolayer WTe2 406a, 406d is electrically coupled to the source and drain terminals/electrodes 402a, 404a, 402d, 404d. In FIG. 4A, the dashed lines trace the boundaries of the monolayer WTe2 406a and the h-BN 412a. The two gate terminals 408a, 410a and the gate terminal (arrangement) 408d may be arranged on top of or over the monolayer WTe2 406a, 406d, and, therefore, may be referred to as the top gates. The BN (e.g., h-BN) layer 412a, 412d may act as a protection layer for the monolayer WTe2406a, 406d (which may not be sufficiently stable), and may be simultaneously employed as a dielectric layer of the top gate 408a, 410a, 408d. Further, as may be observed in FIG. 4D, the device 400d further includes another BN layer 414d on a substrate having a silicon oxide (SiO₂) layer 416d. Referring to FIG. 4D, a second gate terminal (arrangement) (not visible in FIG. 4D) may be provided or formed on the SiO2 layer 416d, where a second electrical signal, e.g., another voltage, VB, may be applied to the second gate terminal. The second gate terminal may be as described in relation to the two gate terminals 408a, 410a and the gate terminal (arrangement) 408d. The second gate terminal (arrangement) may be referred to as the bottom gate(s). Having the gate terminals 408a, 410a and the gate terminal (arrangement) 408d, e.g., as the top gate(s), and the second gate terminal (arrangement), e.g., as the bottom gate(s), may provide flexibility or more freedom to tune the carrier density in the WTe₂.

[0084] Using the device 400d of FIG. 4D as a non-limiting example, in operation, the device 400d may receive a terahertz (THz) signal 420d. The THz signal 420d may be a circularly polarised optical signal from a THz laser that may be incident on the photodetector 400d and received by the monolayer WTe₂ 406a, 406d. A first voltage (e.g., top gate voltage), VT, is applied to the gate terminals 408a, 410a, 408d. A second voltage (e.g., bottom gate voltage), VB, is applied to the second gate terminal (arrangement) (e.g. a metal electrode layer) formed on the bottom SiO₂ layer 416d. A current (e.g., source-drain current) may flow between the source and drain terminals/electrodes 402a, 404a, 402d, 404d, through the monolayer WTe₂ 406a, 406d. It should be appreciated that either VT or VB may be applied, or both VT or VB may be applied simultaneously, depending on the requirements for the overall gate voltage.

[0085] In various embodiments, as the active layer, WTe₂ 406a, 406d, has a suitable band gap for THz light absorption, when voltages are applied, the band gap of WTe₂ 406a, 406d is changed so that the photocurrent is changed.

[0086] The electrical characterisations of the FET devices 400a, 400d are illustrated in FIGS. 4B and 4C, which show, respectively, the results for the source-drain current dependence of the gate voltage at a bias voltage of 500 mV, and the results for the source-drain current dependence of the bias voltage at gate voltages from -10 V to 10 V. In FIG. 4C, the dashed arrows indicate the direction of the gate voltages from -10 V (solid line) to 10 V. The bias voltage is applied across the source and drain electrodes 402a, 404a, 402d, 404d. The gate voltage is applied across the thin h-BN layer 412a, 412d. The results indicate that the gate voltage may modulate transporting behaviors similar to known reports. The THz signal 420d is provided by a home-made THz laser, where the lasing peak of the THz laser is located at ~3.2 THz (-106 cm ¹) as shown in the laser spectrum of FIG. 4E.

[0087] To enhance the light intensity in the monolayer WTe2 FET 400a, 400d, a THz antenna 424a (which may be defined by at least part of the gate terminals 408a, 410a as may be observed in FIG. 4A) with arms (e.g., four arms are illustratively represented by 426a) connected to the gate electrodes, 408a, 410a may be designed and defined by e-beam lithography on top of the active regime including the monolayer WTe2 FET 400a. Referring to FIG. 4A, the active regime corresponds to the area within the dashed lines, as may be defined by the source and drain electrodes 402a, 404a, and the width of WTe2 406a, The THz optical antenna 424a may help to concentrate light (e.g., THz laser light 420d, FIG. 4D) in the active area, e.g., the surface of the monolayer WTe2 406a, 406d. The plurality of arms 426a may be curved as the arms 426a are designed as THz antennas. The plurality of arms 426a may define, at least partially, concentric circles. The dimension of the plurality of arms 426a may change (e.g., decrease in one direction) as the arms 426a are designed as THz antennas. Arms 426a of different sizes are provided for different wavelengths of incident light. The plurality of arms 426a may extend, alternately, from opposite sides of a body section of each of the gate terminals 408a, 410a. Effectively, the gate terminals 408a, 410a may include a plurality of arms 426a, where the arms 426a and, effectively, the gate terminals 408a, 410a, may be curved. The gate electrodes 408a, 410a, with the plurality of arms 426a, serve two functions: (i) as an electrical conductor, and (ii) as a THz optical antenna.

[0088] The size of the THz antenna 424a may be determined based on the FDTD results illustrated in FIG. 4F, where the resonance peak is tuned to the QCL THz lasing peak. The effective antenna coupling features the same optical field distribution for the different chirality of the incoming radiation (e.g., 420d, FIG. 4D) as indicated by the simulation result in FIG. 5. In FIG. 5, the dashed arrows indicate the directions of increasing intensity according to the legend provided in FIG. 5. As a result, the monolayer WTe2 FETs 400a, 400d coupled with the THz antenna 424a have the potential for investigations of helicity-dependent photodetection as well as the potential to unravel the interplay between chiral THz photons and electron/spin in particular valleys.

[0089] FIGS. 6 A to 6D show results relating to helicity dependent photocurrent generation in monolayer WTe2 field effect transistor (FET). The measured photocurrents along the two orthogonal directions (I_a and lb) as a function of the polarisation angle are presented in FIGS. 6A and 6B. The experiment setup is shown in FIG. 3 A. It may be observed that only I_a shows a polarisation modulation curve while lb remains relatively constant for a certain gate voltage. The photocurrents along the a and b directions may be attributed to the photo-thermal effect due to the different Seebeck coefficients of WTe2 and the metal electrodes. However, distinct photocurrent dependence may be observed with incident laser polarisation angle along the a direction. For gate voltage, V_g = 10 V, I_a reaches its maximum for right-circularly polarised light illumination and reaches the minimum for left-circularly polarised light illumination. Therefore, the I_a under linear polarisation (polarisation angles at 0°, 90° and so on) may be attributed to the thermal effect along the a direction. It may be observed that the thermal photocurrent along the a direction is much lower than that along the b direction, indicating that the Seebeck coefficient along the a direction is much lower than that along the b direction. This may be elucidated by the band structure of monolayer WTe2 as shown in FIGS. 2F and 2G, where the anisotropic tilt direction of energy dispersion is aligned to the b- axis rather than the a-axis of the WTe2 crystal. It may also be observed that the photocurrents along both a and b directions show clear gate- voltage dependences. The gate- voltage dependent lb may be attributed to the electrical conductivity modulation with the external gate-voltage as shown in FIGS. 4B and 4C. For example, the injection of electrons to the monolayer WTe2 with positive gate voltage induces a higher Seebeck coefficient and thus may enhance the photothermal current. This is due to the Seebeck coefficient, S, being related to the electrical conductivity c according to the Mott formula, S = -

- - , where I B is the Boltzmann

3e <7 dV_g constant, e is the electron charge, T is the temperature, and V_g is the gate voltage applied to the sample. On the other hand, the thermal current contribution along the a direction also shows similar behaviors to that along the b direction. However, the CPGE induced photocurrent exhibits distinct modulation behaviors with the gate voltage. The CPGE induced photocurrent may change its sign under negative gate voltage, as will be described further below.

[0090] The CPGE induced photocurrent exhibits a linear dependence with the increase of the bias voltages, as shown in FIG. 6C. Since the optical illumination is assumed to be uniform, this behavior indicates that the E-field within the monolayer WTe2 FET channel may be influenced by applying a source-drain bias. A maximum photocurrent of ~ 36 nA may be achieved at a source-drain bias voltage of approximately IV, corresponding to a responsivity of about 0.6 A/W with an incident laser power of 220 mW, which is comparable to known THz detectors. Another anomalous observation is the nonlinear laser power dependence as shown in FIG. 6D, where the photocurrent has a threshold at the laser power of about 300 mW. When the laser power increases to higher values, the photocurrent dependence shows a higher slope, which is in contrast to previous reports on two- dimensional Dirac materials. The nonlinear photoresponse may be understood by the nonlinear photocurrent tensor originating from the diverging Berry curvature in monolayer WTe2 as predicated in known reports. These results afford a strategy to optimise the photodetection performance, for example, the responsivity may increase by almost one order of magnitude higher when the laser power reaches 400 mW.

[0091] To further investigate the operation mechanism of monolayer WTe2 FET-based photodetector, the dependence of the CPGE with a vertical gate voltage has been systematically studied. As indicated in FIGS. 6A and 6B, the applied gate voltage may significantly tune the CPGE along the a direction, which is contrary to that along the b direction that is induced by thermal effect. Further, FIGS. 7 A to 7D show the THz detector performances and transient measurement results. The detailed gate voltage dependence of the CPGE with gate voltage increasing from -10 V to 10 V is shown in FIG. 7A. Firstly, the CPGE photocurrent flips its sign as the direction of the gate voltage is reversed. Such electrical displaced mediated switching of the lateral CPGE photocurrent through the out-of-plane displacement field has not been demonstrated in graphene, TMDC or known materials and only exists in monolayer WTe2. For example, graphene-based THz detectors are achieved either by the photothermoelectric effect or the plasma- wave-assisted mechanism induced by the second-order nonlinear response when an oscillating THz field is applied between the gate and source electrodes. On the other hand, the electrical gating of three-dimensional bulk materials only affects the surface or the interface regimes. However, the in-plane CPGE may be modulated by the out-of-plane displacement field due to the in-plane polarity along the b direction, because the monolayer WTe2 lattice features a tilted parallelogram.

[0092] Secondly, it may be observed that the magnitude of the CPGE photocurrent may be widely tuned via the external gate voltage as shown in FIG. 7A. Taking the positive gating regime as an example, the CPGE photocurrent continually increases with the gate voltage increasing from 0 to 10 V while the source-drain voltage and the illumination conditions are kept constant. Together with the previous observation that the CPGE photocurrent changes direction as the displacement field is reversed, it may be deduced that the band structure is significantly changed by the applied displacement field. The band structure of monolayer WTe2 is calculated based on a k-p model as shown in FIGS. 8 A and 8B illustrating, respectively, the ' density distributions of ‘ x in the ^K « direction, and the density distributions of " in the ■>’ direction. In FIG. 8A, the pair of areas 882a and the pair of areas 884a have opposite signs, and, in FIG. 8B, the pair of areas 882b and the pair of areas 884b have opposite signs. It is observed that the band splitting increases with increasing displacement field. Simultaneously, calculation shows that the band splittings are dominantly localised near the band edges of the Q and Q’ valleys. As a result, the CPGE photocurrent may be attributed to the bandgap change and the net Berry curvature dipole with the external gate voltage. With the gate voltage increasing from 0 to 10V, the bandgap decreases from 20 meV to ~8 meV, resulting in an enhancement of the optical absorption of monolayer WTe2. The negative gate voltage exhibits a similar trend, but the direction is opposite as discussed above. Furthermore, the anisotropic curve of the CPGE photocurrent dependent on gate voltage from -10 V to 10 V may be induced by the dielectric environment difference of the top (BN) and bottom gates (SiO2).

[0093] To examine the response speed of the THz photodetector for realistic optical applications, the photoresponse is measured via changing the modulation frequency of the QCL THz laser (e.g., 381, FIG. 3a). The laser powers are calibrated by the output at 10 kHz modulation speed. A 3-dB bandwidth of 8 kHz may be determined as shown in FIG. 7B, which surpasses most commercial THz bolometers. For FIG. 7B, the curve is displaced vertically, so the response starts from 0 dB. To understand the optical response of monolayer WTe2, ultrafast degenerate reflectivity spectroscopy is employed to reveal the carrier dynamics. A Ti: sapphire amplifier with central wavelength 800 nm femtosecond laser is used to produce near pumps below the 1.55 excitation. Ultrafast degenerate differential reflectivity changes, AR/R, with ~ 30 fs time resolution are recorded with tunable pump fluences and shown in FIG. 7C, which indicates negative transient signals. In FIG. 7C, the arrow indicates the direction of increasing pump fluence. The exponential decay time constants may be well fitted from the logarithmic scale plot by a bi-exponential function: AR/R = Aiexp(-t/zi) + A2exp(-t/72). The short time constant, 7i, is around 1.5 ps and increases slightly with the increase of pump fluences, which may be attributed to the movement of photoexcited carriers from the probing window to the band edges during the thermalization processes. The increase of 7i may be normally understood by the relaxation component of carrier-phonon coupling involving carriers in the conduction band, where higher pump fluence may excite more carriers and thus may lead to longer relaxation time. On the other hand, the longer time constant, 72, is the relaxation time for the carriers diffusing out of the excitation volume and may be attributed to the carrier lifetime in monolayer WTe2 near the QHS (Quantum hall state) gap. The increase of 7? with the increase of pump fluence as shown in FIG. 7D may be caused by the increase of photocarriers combined with the effect of the opening of bandgap which may reduce the phase-space available to Auger recombination. Within the measured pump fluences, the carrier life is in the range of 50 to 65 ps, indicating that the photoresponse speed may be further improved to high-speed operation regime by device architecture design and optical setup optimisations.

[0094] As described above, various techniques have demonstrated the valley dependent optical excitations of monolayer WTe2 with THz laser, and helicity sensitive THz photodetection has been achieved. Helicity sensitive THz photodetection is due to the properties of WTe2. The valley dependent photocurrent is generated due to the Berry curvature dipole originating from the broken spatial symmetry of monolayer WTe2. The photocurrent magnitude and direction may be efficiently tuned by an external gate voltage, which is induced by the in-plane polarity arising from the out-of-plane displacement field. The theoretical calculations indicate that the QHS band gap of monolayer WTe2 is changed with the employment of external gate voltage. Furthermore, a 3-dB bandwidth of approximately 8 kHz is achieved, which is limited by the RC constant of the device and the experiment setup according to the ultrafast differential reflection spectroscopy. In combination with the valley properties in the THz regime, monolayer WTe2 is demonstrated to be not only a promising platform to investigate fundamental valley physics in the THz regime but also a useful candidate for a variety of applications including, but not limited to, optical communications, remote sensing, environmental monitoring and surveillance. [0095] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A terahertz photodetector comprising: a pair of source/drain terminals; a monolayer tungsten ditelluride electrically coupled to the pair of source/drain terminals; and a gate terminal arrangement configured to receive an electrical signal to control an electrical property of the monolayer tungsten ditelluride.

2. The terahertz photodetector as claimed in claim 1, further comprising a boron nitride layer between the monolayer tungsten ditelluride and the gate terminal arrangement.

3. The terahertz photodetector as claimed in claim 1 or 2, wherein the gate terminal arrangement comprises a pair of gate terminals.

4. The terahertz photodetector as claimed in claim 3, wherein, for each gate terminal of the pair of gate terminals, the gate terminal comprises: a body section; and a plurality of arms extending alternately from opposite sides of the body section.

5. The terahertz photodetector as claimed in claim 4, wherein the plurality of arms have a varying size.

6. The terahertz photodetector as claimed in claim 4 or 5, wherein each arm of the plurality of arms is curved for the plurality of arms of the pair of gate terminals to partially define concentric circles.

7. The terahertz photodetector as claimed in any one of claims 1 to 6, further comprising a second gate terminal arrangement, wherein the monolayer tungsten ditelluride is arranged between the gate terminal arrangement and the second gate terminal arrangement, and wherein the second gate terminal arrangement is configured to receive a second electrical signal to control the electrical property of the monolayer tungsten ditelluride.

8. The terahertz photodetector as claimed in claim 7, further comprising a second boron nitride layer between the monolayer tungsten ditelluride and the second gate terminal arrangement.

9. The terahertz photodetector as claimed in any one of claims 1 to 8, wherein the terahertz photodetector is responsive to a circularly polarised terahertz optical signal.

10. A method of forming a terahertz photodetector comprising: forming a pair of source/drain terminals; electrically coupling a monolayer tungsten ditelluride to the pair of source/drain terminals; and forming a gate terminal arrangement configured to receive an electrical signal to control an electrical property of the monolayer tungsten ditelluride.

11. The method as claimed in claim 10, further comprising arranging a boron nitride layer between the monolayer tungsten ditelluride and the gate terminal arrangement.

12. The method as claimed in claim 10 or 11, wherein forming the gate terminal arrangement comprises forming a pair of gate terminals.

13. The method as claimed in claim 12, wherein forming the pair of gate terminals comprises forming, for each gate terminal of the pair of gate terminals: a body section; and a plurality of arms extending alternately from opposite sides of the body section.

14. The method as claimed in claim 13, wherein the plurality of arms have a varying size.

15. The method as claimed in claim 13 or 14, wherein each arm of the plurality of arms is curved for the plurality of arms of the pair of gate terminals to partially define concentric circles.

16. The method as claimed in any one of claims 10 to 15, further comprising forming a second gate terminal arrangement, wherein the monolayer tungsten ditelluride is arranged between the gate terminal arrangement and the second gate terminal arrangement, and wherein the second gate terminal arrangement is configured to receive a second electrical signal to control the electrical property of the monolayer tungsten ditelluride.

17. The method as claimed in claim 16, further comprising arranging a second boron nitride layer between the monolayer tungsten ditelluride and the second gate terminal arrangement.

18. A method for controlling a terahertz photodetector comprising: applying a terahertz optical signal to the terahertz photodetector as claimed in any one of claims 1 to 9, wherein the terahertz optical signal is circularly polarised.

19. The method as claimed in claim 18, further comprising subjecting the terahertz photodetector to a cooling process.