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Various performance aspects and the photocurrent generation mechanisms of photosensitive devices based on two-dimensional transition metal dichalcogenides are reviewed. The work begins with discussions on light absorption and charge separation mechanisms in TMD-based photodetectors. Then, performances of simple metal-TMDs-metal detectors are discussed in terms of responsivity and response time. Thereafter, novel photosensitive devices involving TMDs, either incorporating other materials or novel photosensitive device structure designs, are summarized, and basic ideas behind these devices to improve the photodetection performance are elaborated. Lastly, two valley degree of freedom related photoresponses, circular polarized photogalvanic effect and valley Hall effect, are presented. The review ends with prospectives of future challenges and opportunities in developing TMDs based photodetection devices.
© 2016 Optical Society of America
Corrections
17 June 2016: Corrections were made to the title.
1. Introduction
The conversion of absorbed photons into electric signal and conversion of injected electric current into light emission are the two faces of a coin that governs the major principles of optoelectronic applications. The former mainly leads to light detection related application such as video imaging, biomedical imaging, night vision, gas sensing and motion detection; while the latter mainly corresponds to the light emission application such as light emission diode (LED) and lasers. Although all these optoelectronics devices have already been commercialized and accompanied our everyday life for decades, the need for better performance optoelectronic devices keeps growing. In terms of photodetection which is the major scope of this review, ultimate performance photodetection platforms in terms of operation speed, quantum efficiency, spectrum range, flexibility, transparency and CMOS integrability are highly desired.
The emergency of two dimensional (2D) materials [1] with distinct physical, chemical and mechanical properties promise their great potentials for photodetection [2–5]. Compared to three dimensional semiconductors, two dimensional materials offer additional advantages in context of photodetection because of their transparency, mechanical flexibility, electronically highly tunable and easy processing. These advantages great facilitate applications in flexible optoelectronics and large scale CMOS integration into optoelectronics networks. A variety of prototype photodetection devices based on 2D materials were demonstrated during the last decade. These effort started with graphene, a pioneer of 2D family, showing many fantastic capabilities, such as broadband detection from ultraviolet (UV) to Terahertz (THz), ultrafast response time [6–8] and tunable optical properties via electrostatic doping [9, 10]. Many of these unique capabilities have already sparked enormous interests for commercialization. Comparing with graphene based photodetectors, 2D transitional metal dichalcogenides (2D-TMDs) processes sizable energy gap, allows lower dark currents and higher responsivity, but the operation speed and response spectrum range is limited. Although the current performance of TMDs based photodetectors cannot compete with the silicon based technology for conventional applications, the large excitonic effect and novel valley degree of freedom of TMDs enables unconventional applications that is not possible with conventional materials and (semi)metallic 2D counterpart such as graphene.
Despite its not until the recent few years the research of TMDs revives, several reviews have been available on optoelectronics and nanophotonics of 2D-TMDs [2–5] (TMDs for short below), in this work we mainly focus on the current state-of-the-art of phototectors and scanning photocurrent studies of photoresponse based on 2H-phase TMDs. First, the various performance aspects and photocurrent generation mechanism of typical metal-TMDs-metal photodetectors made by single species of TMDs are compared and summarized. Second, we discuss various approaches and their mechanisms in improving the performance over single species TMDs based photodetectors. Then, two valley degree of freedom related photoresponses of TMDs: circular polarized galvanic effect and valley Hall effect, are discussed. At last, we conclude with the major challenges and future opportunities in this field of research.
2. Performance of TMDs based photosensing device
The photodetection and many other optoelectronics applications relies on the light absorption and subsequent conversion of absorbed photons into an electric current. From this point of view, the optical absorption spectrum of TMDs and the electric current generation mechanism from absorbed photons are the two key components toward understanding the photoresponse of the TMDs.
2.1 Spectrum response of TMDs and photocurrent generation mechanisms
2H phase TMDs consist of a single layer of transition metal atoms sandwiched between two layers of chalcogen atoms in the trigonal prismatic structure as shown in Fig. 1(a), this lattice structure have out-of-plane mirror symmetry and broken in-plane inversion symmetry. Bulk TMDs are indirect-bandgap semiconductors with conduction band minimum and valence band maximum located at the Q and the Γ points, while in the monolayer limit, direct-bandgap semiconductors with gaps located at the K and the K’ points is formed [11, 12], this results in strong photoluminescence in monolayer TMDs [13, 14]. The light absorption capability of monolayer TMDs is dominated by direct transition between valence and conduction band states (band-to-band transition) around the K and K’ points, these transitions are strongly modified by the electron hole interaction due to reduced dielectric screening effect relative to the bulk. The exciton binding energy in monolayer TMDs ranges from 0.5 to 0.9 eV according to theoretical calculations [15–17] and optical/scanning tunneling spectroscopy measurement [18–22]. Higher order excitonic quasiparticles such as trions and biexcitons are also reported in monolayer TMDs [23–26]. These high order excitonic states can survive at room temperature in monolayer TMDs due to relatively large binding energy comparing to conventional quasi-2D semiconductors. These have important consequences for optical absorption spectrum of monolayer TMDs as significant oscillation strength is transferred from band-to-band transition to resonant absorption of excitonic states. In addition to that, the strong excitonic effect also leads to relatively short radiative lifetime [27] which may put the fundamental limit of the operation speed of a photo sensing device [28].
Fig. 1 (a). Crystal structure of TMD. (b). The honeycomb lattice structure and first Brillouin zone with high-symmetry points. (c).The optical selection rules in MoS2, the left circular polarized light/right circular polarized light can selected to excite carriers in K/K' valley respectively. (d). The absorption spectra and PL spectra of one-layer and two-layer MoS2, the spectra are displaced along the vertical axis for clarity.[From Mak.K.F. et. al, Phys Rev Lett 105.136805(2010)] (e). Schematic representation of photocurrent generation in biased TMD devices. (f). Schematic representation of photocurrent generation due to photothermalelectric effect.
The interaction between light and monolayer TMDs is very prominent, comparable to or even stronger than that of graphene in visible range. Figure 1(d) shows the absorption spectrum of a monolayer MoS2 [14]: below the A-exciton resonance, the absorbance is relatively low and negligible. The absorbance increases to 10% at photon energy in resonance with A/B excitons and keep climbing with photon energy due to band nesting effect of C band [29, 30], at 3 eV it can reach 30% [31]. A summary of basic electronic properties of different TMDs including the band masses, optical gaps, exciton binding energy conduction/valence band spin-orbital splitting can be found in a recent review [4]. The direct bandgaps in monolayer TMDs fall into visible and near infrared wavelength range, it not only depends on the TMDs species and number of layers but also can be continuously tuned with binary/ternary TMDs alloys [32–34].
Although strongly correlated to each other, the absorption spectrum not necessarily coincides with the photo response spectrum of a photo detection device as the current generation also relies on the efficient separation of charges that are created by absorption of photons. Two major charge separation mechanisms: photovoltaic effect (PV), which is based on the separation of photo generated charges by built-in electric fields at junctions; and photo-thermalelectric effect (PTE), which is based on the charge separation by thermalelectric power and electron temperature difference induced by the photoexciation, are both reported in TMDs based photodetectors [35–38], it was found that the relative contribution of PV and PTE can be tuned by the junction configurations and illumination photon energies [37, 38]. Thus both the excitation photo energy and the junction configuration are critical to the function mechanisms. With excitation in resonance with the excitonic state, despite the advances on the mechanism studies about charge separation, key issues for photodetectors because of extremely strong excitonic effects, such as the dissociation process of various excitonic states, are still not clear. Fundamental questions regarding these issues such as the exciton dissociation time and dissociation mechanism remain to be elucidated. On the other hand, the nature of the TMDs and metal contact, whether it’s Ohm contact and the height of the Schottky barriers, also strongly affect the charge separation and thus the response time and responsivity of the photodectors. It’s found the low Schottky barrier device exhibits a very high photo gain and but very slow response speed. In contrast, the high Schottky-contact barrier device display a fast response time, but the photo gain and specific detectivity decrease by several orders of magnitude [39].
2.2 Photo responsivity and response time
The reported photo responsivity and response time of TMDs based photodetectors spread over very large parameter ranges. Table 1 has summarized the major device performance parameters of metal-TMDs-metal structure photodectors reported so far. As shown in the table, the responsivities of different photodetectors range from 22 µA/W [40] to up to 880 A/W [41], while the response time ranges from few µs [42] to second [41] are reported in different devices by different groups. A rough “law” that can be drawn from Table 1 is that a device shows large responsivity turns to be slow in operation speed and vice versa. For example, photoconductive exfoliated monolayer-MoS2 detector exhibits high external responsivity of 880 A/W, but operates at very narrow bandwidth of 0.1 Hz [41]; while another fast device made from CVD grown monolayer MoS2 exhibits few ms response time, but the responsivity is only 3 mA/W [43]. Doping and trapping of photogenerated carriers by the impurity states has been identified as the main mechanism that change the device conductance in TMDs [42, 44]. This directly explains the sample dependent response time and responsivity reported in the literatures. Actually it’s not a surprise the performance of TMDs based photodectors are so different as reported from different groups or even from different works of the same group, considering that different TMDs materials (exfoliated or artificially grown), fabrication processes and electrode materials are used. It simply implies the performance of TMDs summarized in Table 1 should have considerable contribution from extrinsic effects related to impurity and defects states.
Table 1. Comparison of Device Performance of TMD based Photodetectors
In terms of response speed, ultimate performance such as “ultrafast response” down to few ps [28] and persistent photoconductivity (PPC) over minutes [59] are both reported in TMDs. In the following, we'd like to discuss these two apparently conflicting circumstances in more details.
The standard method to measure the ultrafast transient response of photodetector is two-pulse photovoltage correlation (TPPC) [7]. This technique is performed by Wang et al. on a monolayer MoS2 photodector [28]. The as measured TPPC result (|ΔVc| as function of the time delay Δt between the two pulses) as shown in Fig. 2a exhibits transient photocurrent response with two distinct timescales: a fast component on the order of 3-5 ps, which is attributed by the Wang et al. to the short lifetime of the photoexcited carriers due to fast electron capture process by defects via Auger scattering (Fig. 2(b) II); a slow component on the order of 80-110 ps, which is attributed to carrier trap process by slow defects (Fig. 2(b) III). The fast timescale implies the ultrafast TMD photodetector can achieve current modulation bandwidths in 200-300 GHz range, but the modulation depth is limited by the relative contribution to the total photocurrent of this fast component. We note that the fast response time measured by Wang et al is not due to an improvement in the device. The captured electrons and holes could still be thermal excited and result in the very slow response after the transient response. The TPPC measurement can only resolve the fast response component of the photocurrent, the slow recovering part, ranging from tens of µs to several minutes as summarized in Table 1, is not magnified in a TPPC, but it may contribute to the major responsivity of the devices due to the accumulated response over relatively long time period after the transient response.
Fig. 2 (a). TPPC (Vc) as a function of the time delay (Δt) between two pulses show a transient photocurrent response with two distinct timescales (i) a fast time scale of ~4.3ps and(ii) a slow timescale of ~105ps. [From Wang, H. N. et al., Nature Commu 6:8831(2015).] (b). The three temporal regions of ultrafast carrier dynamics of MoS2, (I) carriers thermalization and cooling process within 500 fs. (II) electrons and holes captured by fast defects within 1-2 ps (III) electrons captured by slow defects on a 60-70 ps timescale [From Wang, H. N. et al., Nano Lett. 15, 339−345(2015).] (c).The photoresponse of the MoS2 device can be classified into 5 stages. In addition to the photoresponse due to band-band transition (stage 2 and 4), the device exhibits a slow increase in the photocurrent under illumination (stage 3) and the PPC effect (stage 5). [From Wu, Yueh-Chun et al., Scientific Reports 5:11472(2015).] (d). The trapped electrons and holes are thermally excited to defects and result in PPC effect.
Persistent photoconductivity (PPC) refers to sustained conductivity after photo illumination is terminated (Fig. 2(c)). From a general point of view, as photoresponse always takes finite time depending on the photocarrier dynamics and transport in the device, remaining conductivity should be universal in any photoresponse including ultrafast graphene based photodetector with several ps response time. Here we emphasize the PPC refers to sustained conductivity over relatively long time scale (on the order of seconds) after photo illumination. As the PPC effect last over long timescale, it may significantly modifies the transport properties, thus ideally the transport phenomena can be controlled through the history of the photon irradiation through PPC effect in optoelectronic devices.
Yueh-chun Wu et al. studied the PPC dependence on the temperature, the photon dose, the excitation photon energy and substrate effect in monolayer MoS2 field effect transistors systematically and conclude the effect can be attributed to random localized potential fluctuations (RLPF) in the devices [59]. The random localized potential fluctuation arise either from intrinsic disorders or extrinsic charge impurities which can localize the photogenated electron holes to local minima, thus hinder the recombination of photoexcited electron-hole pairs. After the photo illumination is off, the thermal excitation of carriers to higher energy states above the mobility edge produces photocurrent and results in the PPC effect, as shown in Fig. 2(d). Another two mechanisms: large lattice relaxation (LLR) and the microscopic carrier (MB) can be ruled out in photoresponse of TMDs as these two mechanisms are inconsistent with the experimentally observed temperature/excitation photon energy dependence and stretched exponential decay of the PPC effect respectively. Although the MB effect is not significant in photo response of pure TMD device, it is account for the PPC observed in various TMD based hybrid structures, such as MoS2/graphene [60, 61], quantum-dot/graphene [62], MoS2/Black Phosphorus [63], MoS2/Si [64] and chlorophyll/graphene heterostructures [65] as described in the following session.
2.3 Novel photosensitive device based on TMDs
So far, all discussions are based on photo sensing device built from individual TMDs species and concerns simple metal-TMDs-metal structure, which is a starting prototype device to study photocurrent response and understand the basic light-electric conversion mechanisms. However, frankly speaking, the performance of such device is not impressive comparing to traditional semiconductors and graphene.
Enormous efforts have been invested to enhance the performance of TMDs based photosensing devices by incorporating other materials especially two dimensional materials and designing novel device structure to enhance certain performance such as responsivity, spectrum range and response time. These new designs mainly fall into two categories, in the first category, novel device structures is designed to enhance certain performance: such as PN junctions is built to enhances the separation between electron and hole more efficiently; in the second category, heterostructures between TMD and other 2D materials or 3D semiconductors is employed to integrating advantage of different material systems.
PN junctions can facilitate separation of photoexcited electron-hole pairs and thus achieving better quantum efficiency. PN junctions on TMDs can be realized using either electrostatic doping [47, 66] or chemical doping [67, 68], as shown in Fig. 3(a). The responsivities of the photodetectors with a PN junction vary from several mA/W to several A/W depends on the working conditions of devices. A photoresponsivity of 5.07A/W with EQE of 1200% on 500 nm excitation photon energy was reported in MoS2 PN junction under a bias voltage of 1.5 V and a gate voltage of 0 V [67]. Plasmonic structure [69, 70], dielectric cavity [71], quantum dots [72, 73] or organic dye molecules [74] were incorporated into TMD devices to adjust the spectrum response of TMD and enhance the responsivity at given spectrum range, as shown in Fig. 3(b). The highest responsivity of 1.6*104 A/W was reported for photodectors based on the hybrid of MoS2 and graphene quantum dots [73].
Fig. 3 (a). CPGE in bilayer WSe2 with excitation photon energy (1.17eV) that is below the indirect bandgap. [From Yuan, H. T. et al., Nat Nanotechnol 9, 851-857 (2014).] (b). The σ + and σ- excitations which are on-resonance with A exciton—a hole in the upper VB and electron in the CB lead to photocurrent generation in K and -K valleys, respectively. (c). CPGE current is generated due to interband transitions with excitation photon energy on-resonance with excitonic transition (1.96eV) and disappears with off-resonance excitations. [From Eginligil, M. et al., Nat Commun. 6 (2015).] (d). The valley-dependent optical selection rules and the electrons at the K and K′ valleys that possess opposite Berry curvatures (left) and a photoinduced AHE driven by a net valley polarization (middle) and an image of the Hall bar device (right). (e). The source drain bias dependence of the Hall voltage for the monolayer device. [From Mak, K. F. et al., Science 344, 1489-1492 (2014).]
TMD has also been integrated into 3D semiconductors such as silicon to form a heterojunction in order to adjust the spectrum response (Fig. 3(d)) [64, 75–77]. A heterojunction with n-type MoS2 and P-type Si was reported to have a maximum responsivity of 300 mA/W and response speed of 3 µs with spectral response ranging from 350 to 1100nm [77]. While hybridding with 3D materials has the advantage of compatibility with current matured fabrication technologies, hybridding TMDs with other 2D materials, such as graphene [30, 61, 78–80], black phosphorus [63, 81] or another kind of TMDs [80, 82–85] provide more fantastic possibilities. Graphene-TMDs structures are the most studied structure of this kind and provides multiple advantages: first, graphene can be employed as contact material of TMDs, graphene has better optical transparency and minimal effect on band structure of TMDs compared to metal contact; second, the Dirac point of graphene usually lie in the bandgaps of TMDs, thus result in more efficient extraction of both electrons and holes from TMDs; third, a graphene-TMDs heterojunction will also benefit from the enhancement of light interaction with TMDs, the reported EQE of graphene-TMD heterojuctions is around 10%~55%; at last, the barrier height for graphene-TMD can be tuned by tuning the electric gating of Fermi level in graphene, and thus control the separation and transport of the excited electrons and holes with an external field [79]. Beyond graphene, incorporating TMD with BP or another kind of TMD will form different type of heterojunctions depending on the band alignment of the two materials. This allows for bandgap engineering of heterostructures, and the convenient electrostatically tuning of the carrier densities and band alignments of two-dimensional semiconductors also offers an alternative tuning knob for band engineering beyond switching materials. Such example is demonstrated by contacting layers of p-type WSe2 and n-type MoS2 to form a PN heterojunction (Fig. 3(c)) [82, 83]. Besides responsivity enhancement, the spectrum response can also be extent using these heterostructures. For example, an infrared (1.55 um) photodetector was reported using a type-II MoTe2/MoS2 van der waals heterostructures [85].
3. Valley related photoresponse
The broken inversion symmetry in monolayer TMDs yields an effective magnetic field (Berry curvatures Ω) in momentum space with opposite sign at the K and K’s valley [86, 87]. The berry curvature together with orbital magnetic moments gives rise to unique valley–dependent optical properties in monolayer TMDs. Direct addressing valley degree of freedom in layered TMDs with circular polarized light enables the opportunity to develop valley based information technology with TMDs and finally evokes the enormous research interests in TMDs [82, 86, 88, 89]. This is eight years after TMDs was first exfoliated into monolayer [1] and 3 year after monolayer MoS2 was found to be a direct bandgap semiconductor [13, 14]. The valley polarized photoluminescence after left/right circular polarized optical excitation is a direct way to test the valley degree of freedom in TMDs. On the other hand, helicity resolved photocurrent measurement of circular polarized photogalvanic effect(CPGE) and valley Hall effect (VHE) provide spin-valley related signature of photoresponse that can be measured experimentally.
3.1 Circular polarized photogalvanic effect
Benefiting from strongly couple spin-valley degree of freedom, CPGE has been demonstrated to generate spin-coupled valley polarization in TMDs, which enables additional degree of control for quantum-confined spintronics/valleytronics devices. CPGE in TMDs was first demonstrated by Yuan et al. in bilayer WSe2 with excitation photon energy (1.17eV) that is below the indirect bandgap. In this scheme, the current is generated due to intraband absorption at Λ valley within the conduction band (CB) as shown in Fig. 4(b). Under obliquely illumination with nonzero θ, the obtained photocurrent exhibits strong dependence on light circular polarization. The helicity dependent photocurrent arise from the asymmetric optical excitation of the splitting bands, the dependence of the photocurrent on light helicity indicates direct relationship between light helicity and spin or orbital angular momentum near WSe2 valleys. Furthermore, as the CPGE phenomenon is highly sensitive to degree of inversion asymmetry, applying a perpendicular electric field provides a convenient control of the CPGE current in bilayer WSe2 [90]. Later on, Eginligil et al. demonstrates the CPGE effect in inversion symmetry breaking chemical vapor deposition grown monolayer MoS2 in spin-valley coupled K valleys. In this work, CPGE current is generated due to interband transitions with excitation photon energy on-resonance with excitonic transition (1.96 eV, as shown in Fig. 4(c)), this CPGE effect disappears with off-resonance excitations. In both work, besides CPGE, linear polarized photogalvanic effect is also observed.
Fig. 4 (a). The Schematic of a PN junction in monolayer WSe2 device controlled by two local gates. [From Baugher, Britton W. H. et al., Nat Nanotechnol 9, 262-267 (2014).] (b). The Schematic of a MoS2-GQDs heterostructure phototransistor [From Chen, C. Y. et al., Scientific Reports 5:11830 (2015).] (c).The Schematic of a MoS2/WSe2 heterostructure. [From Lee, Chul-Ho et al., Nat Nanotechnol 9,676-681 (2014).] (d). The Geometry of a MoS2/Si heterojunction diode. [From Lopez-Sanchez, Oriol et al., ACS Nano 8, 3042-3048, (2014).]
3.2 Valley Hall effect
Although CPGE provides a signature of unique spin-valley degree of freedom of TMDs through photocurrent measurement, direct demonstration of so-called valley Hall effect gives more clear and direct signature of valley degree of freedom in TMDs. Mak et al. [91] experimentally observed the valley Hall effect (VHE) in a monolayer of MoS2. This is done in a hall bar device as shown in Fig. 4(d), when the hall bar is illuminated with circular polarized light, the carriers are excited into a specific valley depending on the excitation helicity of the light. This is because in a monolayer hall bar device with inversion symmetry broken, carriers in two valleys experience effective magnetic fields with opposite signs causing by the berry curvature. The opposite magnetic field drives electrons in different valleys to move in opposite directions perpendicular to the drift current as it biased in a hall bar device and gives the VHE.The tiny Hall voltage is observed by high frequency modulation between left/right circular polarization and detecting the VHE voltage modulation with a lock-in amplifier. In a bilayer, as inversion symmetry is recovered, VHE effect disappears, but it revokes with an perpendicular electric field added to break the inversion symmetry in the bilayer and makes the VHE effect controllable with a top gate, this is also observed experimentally in a recent work [92, 93].
4. Prospectives
Although TMDs has been demonstrated great potential for high performance photosensing with the possibility to incorporate fantastic spin-valley coupled degree of freedoms, many challenges remain to be overcome and new opportunities remain to be explored. With a lot of previous efforts devoted to TMDs based photodetection, our understanding of the intrinsic photoresponse of TMDs are still limited, further advance in understanding the function mechanisms relies on ultrafast dynamics studies of photoexcited carrier [94–111], especially excitonic quasiparticles in TMDs [26, 112–126], contact between metal/hybrid materials and TMDs [127–132] and better control of defects and impurity levels in material growth [50, 133–137] and device fabrication for studying the intrinsic photo response of TMDs. In terms of device performance, TMDs suffer from certain limitations such as limited spectrum response, and operation bandwidth et al., this will reply on innovation of the device structures and hybriding with other materials systems, especially van der waals hetrostructures with other 2D materials with different functionalities. Initial efforts toward spin- and valley-polarized light emission and detection carrying valley degree of information has been practiced [138, 139]. Incorporating the coupled valley-spin degree of freedom into TMDs based optoelectronic is an attractive endeavor and it can possibility renovate future information technology.
Acknowledgments
The authors would like to acknowledge the funding support from the following: the National Basic Research Program of China (973 Grant Nos. 2012CB921300, 2014CB920900), the National Natural Science Foundation of China (NSFC Grant Nos. 11274015), the Recruitment Program of Global Experts, Beijing Natural Science Foundation (Grant No. 4142024) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No.20120001110066).
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