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Photo Sensor Based on 2D Materials
C H A P T E R
13 Photo Sensor Based on 2D Materials Dattatray J. Late1, Anha Bhat1,2 and Chandra Sekhar Rout3 1
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. HomiBhabha Road, Pune, India 2Department of Metallurgical and Materials Engineering, National Institute of Technology, Srinagar, India 3Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Bangalore, India
13.1 INTRODUCTION In the frontiers of device electronics are emerging applications in the fields of medical diagnosis, optical communications, radiation and smoke detection, flame detection, barcode detection, process control, environmental sensing, motion sensing, thermography, night vision, and astronomy. The most important development over the past decade includes the integration of optics into electronic systems, which has paved the way for multifunctionality and enhanced performance. One such development is the harnessing of light into electric signals with enhanced speed, efficiency, and flexibility over the range of wavelengths packaged as photodetectors. A photo detection process can operate through various ways like phototconductive effect [1], photo-thermoelectric effect (PTE) [2], and photovoltaic effect [3] fulfilling the consensus of absorbing the photons to generate an electric response. When a photon with sufficient energy enters the junction, it strikes an atom to release an electron from the atomic structure. This mechanism is known as the inner photoelectric effect. During this mechanism, free electron and hole pairs are created.
Fundamentals and Sensing Applications of 2D Materials DOI: https://doi.org/10.1016/B978-0-08-102577-2.00013-0
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Copyright © 2019 Elsevier Ltd. All rights reserved.
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These free electron and hole pairs are either combined or they remain free. Movement of free electrons and hole pairs from the depletion layer due to external electric field produces a photocurrent. The photocurrent is proportional to the amount of light entering the intrinsic region. The chapter deals with different two-dimensional (2D) material architectures ranging from transition-metal dichalcogenides (TMDCs) to black phosphorous (BP) and graphene/TMDC heterostructures with a general comparison of photoresponse for all of them. The heterostructures have proven to be quite efficient material as photodetectors and sensors due to their optimized properties and good charge trapping efficiency.
13.2 CHARACTERISTICS OF PHOTOSENSORS BASED ON THE 2D MATERIALS Bulk silicon has an indirect bandgap of 1.1 eV, which limits its photon absorption capacity to the visible and near infrared (IR) region of the electromagnetic spectrum, and is a major factor contributing to its low efficiency as a photodetector. Many alternatives such as 3D structures of Gallium Arsenide (GaAs) have been proposed to detect IR wavelengths, but there are industrializing drawbacks such as packaging, an increase in the size of the device, and many additive fabrication steps. With the advent of graphene and few-layered TMDCs, alternative materials have been found to provide higher absorption efficiency in the visible range with a wide operational wavelength [4 6]. Features including increased absorption efficiency and transparency that are attributed to single atomic scale thickness make these materials a potential player in cutting-edge applications of photovoltaics, photodetection, and photonics. TMDs show out-of-plane quantum confinement effects and vertical confinement where the reduced thickness constricts the excitons and hence increases the absorption efficiency [7]. A characteristic feature of 2D semiconductor crystals are the Van Hove singularities leading to sharp peaks in the density of states at a particular energy attributed to the d orbital localization of electronic bands. In the case of 2D semiconductors, these singularities fall in the vicinity close to the conduction and valence band edges. Thereby a photon with energy close to the bandgap has an increased probability to excite an electron-hole pair, which makes them receptive to the incident light. This property, along with remarkable elastic modulus .10% has extended their applications to flexible sensors by tuning the optical properties on the basis of strain engineering [8,9]. Based on the different mechanisms of photons to electrical signal conversion, the photo-sensing effect can be achieved by the following three effects: photovoltaic effect, photoconductive effect, and PTE.
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13.3 PHOTOVOLTAIC EFFECT
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13.3 PHOTOVOLTAIC EFFECT In the photovoltaic effect the electromagnetic radiation energy is converted into electric energy in certain semiconductor materials. The photovoltaic-based photodetector contains p-/n-type (PN) photodiodes formed by two semiconductors with opposite doping type. When light of a suitable wavelength is incident on these cells, energy from the photon is transferred to an atom of the semiconducting material in the p-n junction and Schottky barrier photodiodes are formed at the interface between a semiconductor and a metal. The built-in difference in electric field is necessary for the photovoltaic effect and is created either by local chemical doping [10] and electrostatic control through gates [11]. The configuration of photodetectors based on this approach is usually in the cadre of PN diodes used at zero bias, which is in coherence with photovoltaic mode or under reverse bias, which is the photoconductive mode. In the former, the dark current is the lowest, which is good for detectivity, but at the same time due to no internal gain, absolute responsivity is usually smaller than photoconducting. However the speed of a photodiode as shown in Fig. 13.1 is increased due to the reduction of the junction capacitance in case of reverse bias [13,14].
FIGURE 13.1 (A) p-n junction band alignment, where absorption of a photon with
Eph . Ebg generates an electron-hole pair, which is then separated and accelerated by builtin electric field at the junction. (B) Ids 2 Vds, which results in a short-circuit current Isc and an open-circuit voltage Voc. With maximum power generation denoted by Pel max in the fourth quadrant [12]. Source: Printed with permission.
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13.4 PHOTOCONDUCTIVE EFFECT In a photoconductive effect after electromagnetic radiation absorption the electrical conductivity of nonmetallic solids is increased due to the generation of additional free electron carriers. A typical photoconductor consists of a semiconductor as a channel with two ohmic contacts affixed to opposite ends of the channel, which serves as source/drain electrodes [15]. In the absence of light is a zero current between two electrodes called the dark current. The electromagnetic radiation having higher energy than the bandgap is able to generate electronholes pairs which can be separated by applying a voltage. Such free electrons and holes move due to voltage drift to their respective majority sides (toward electrodes). Due to this a depletion layer is created on either side of the junction. The drifted electrons and holes increase the conductivity of the device. The current as depicted in Fig. 13.2 is generated due to the photoconductive effect and is dependent on the intensity of the electromagnetic radiation, length of the transistor channel, source drain voltage, and the charge carrier mobility [16,17].
13.5 PHOTO-THERMOELECTRIC EFFECT In PTE a light-induced heating leads to a temperature gradient through a semiconductor channel. There is a temperature difference between the two ends of the semiconductor channel. Due to the Seebeck effect, as shown in Fig. 13.3, the temperature difference gets converted into a voltage difference ΔV whose magnitude is linearly proportional to the temperature gradient [16,18].
13.6 MOLYBEDNUM DISULFIDE-BASED PHOTOSENSING DEVICES The abundant availability of molybdenum disulfide (MoS2) has made it one of the most widely studied TMDCs. Bulk MoS2 is subjected to various methods like chemical exfoliation [18], ultrasonic treatment, and mechanical exfoliation [19,20] in order to yield a monolayer. For impurity-free studies, researchers would prefer to grow crystals by processes like chemical vapor deposition [21]. This method of homegrown crystals also allows for the induction of dopants to engineer the bandgaps [22]. Phototransistors of MoS2 have been reported with a photoresponsivity of about 7.5 3 1023A W21. A monolayer phototransistor
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FIGURE 13.2 (A) Band alignment with small current Idark under external bias for a semiconductor channel contacted with two metals (M) without illumination. (B) Band alignment under illumination with photons of energy (Eph) higher than the bandgap (Ebg). (C) Ids 2 Vg traces in the dark and under illumination. Illumination results in an increase in the conductivity (vertical shift) and a positive photocurrent across the entire gate voltage range. (D) Ids 2 Vds curves in the dark (black line) and under illumination (red line), which results in an increase of the conductivity and a positive photocurrent under illumination [16]. Source: Printed with permission.
fabrication has been reported which involved the deposition of MoS2 on Si/SiO2. The height of monolayer MoS2 measured by AFM is B0.8 nm, and the photoluminescence of a single-layer MoS2 sheet was observed at room temperature using the 488 nm laser, which showed a dominant PL peak at 676 nm. The dominant peak arises from the direct intraband recombination of the photogenerated electron-hole pairs in the singlelayer MoS2, while the weak peak at B623 nm is considered to be the
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FIGURE 13.3 (A) Schematic of a field-effect transistor whose metal contact is locally illuminated by a laser. The circuit is open and a thermoelectric voltage ΔVPTE develops across the contacts. (B) Thermal circuit of the device. (C) Ids 2 Vds characteristics in the dark (black line) and under illumination (red line) of a device whose photo-response is dominated by the photo-thermoelectric effect [16]. Source: Printed with permission.
contribution of the energy split (Fig. 13.4) of valence band spin-orbital coupling of MoS2 [20,23,24]. The device showed unprecedented characteristics of incident-light control, efficient photoresponsivity, and photoswitchability, which allows the fabrication of cheap and affordable optoelectronic devices. Further, the study in this device stated that the photocurrent was dependent on the optical power. When the device was subjected to energy above the excitation wavelength of 670 nm, the photocurrent was lower than after the expulsion of dark drain current. However, there was a significant increase in the photocurrent when the wavelength was lowered from 670 nm, satisfying the basic principle of incident photon energy being greater than the energy gap, which is about 1.83 eV. The electrons having large incident photon energy (hν . 1.83 eV) can generate more photoelectrons contributing to the increased photocurrent. Moreover this device also showed good stability and efficient photoswitchability. The switching behavior was observed when drain current shot up to high value under illumination and resumed to lower values under dark, which is an off state [25]. In other cases of chemical vapor deposition (CVD)-grown MoS2, a photosensor reported by Lopez et al. [25] was made of triangular structures of MoS2 grown on Si/SiO2 substrates by mild sulfurization of MoO3. Electron beam lithography was used to fabricate the metal contacts of reportedly 4 nm thick Ti and 100 nm thick Au at 10 7 Torr
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FIGURE 13.4 (A) Drain current (Ids) as a function of excitation wavelength of the illumination source at constant optical power. (Inset shows single-layer MoS2 AFM image and optical image of fabricated device). (B) Output characteristics of phototransistor at different illuminating optical powers. (C) Dependence of photocurrent on optical power at different Vds. (D) Photoswitching characteristics of single-layer MoS2 phototransistor at different optical power and drain voltage [20]. Source: Printed with permission.
pressure. The Raman spectrum obtained at a laser excitation of 488 nm revealed a PL spectrum with maximum intensity at 670 nm corresponding to 1.86 eV photon energy. The first order Raman modes are observed due to D3h point group symmetry which is the characteristic of noncentrosymmetric monolayer MoS2. The PL and Raman spectra were obtained in the region between cathode and anode confirming the presence of monolayer MoS2 and its direct bandgap. The sensor response was observed when two wavelengths of the light along the order of 488 and 514.5 nm were used to probe the photosensitivity. The laser excitation and bias voltage is observed between 0 and 90 s, which is complemented further with current versus time plot as the readings are accounted for every 200 ms. The applied bias voltage was increased from 1.5 to 2.0 V around tB52 s, barely producing a flicker in the dark current whereas for 5 s starting at t 5 14 s and t 5 64 s, the excitation laser illuminated the device. As shown in Fig. 26 noting the response to excitation in both cases, the photocurrent response was found to be around 50% larger in response to the bias voltage at 2.0 V (63 s , t , 68 s) as compared to V 5 1.5 V (14 s , t , 19 s) [25] (Fig. 13.5).
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FIGURE 13.5 (A) Bias voltage and laser excitation. (B) Total current as a function of time for the CVD-grown monolayered MoS2 device. (C) Effect on photocurrent. IV plots of the device under darkness and two different wavelengths (514.5 and 488 nm) [25]. Source: Printed with permission.
FIGURE 13.6 Schematic of MoSe2 phototransistor fabrication by polymer transistor [27]. Source: Printed with permission.
13.7 MOLYBDENUM DISELENIDE-BASED PHOTOSENSOR The monolayer MoSe2 has properties like direct bandgap (EgB1.6 eV) and large exciton binding energy similar to that of MoS2, which has made it one of the most sought-after choices for investigating electrical and optical properties. The argument is further strengthened by the observance of strong photoluminescence in MoSe2 [26]. It is expected to show a higher photoresponse in the solar spectrum range and has high antiphotocorrosion stability. A phototransistor was fabricated by depositing 50 nm titanium (Ti) electrodes by electron-beam evaporation on silicon substrates. As shown in Fig. 13.6 MoSe2 was transferred onto the Ti electrodes using polydimethylsiloxane (PDMS) transfer technique [27].
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FIGURE 13.7 (A) Drain-source voltage dependence of the drain current at different values of the gate voltage of an MoSe2-based phototransistor. (B) Gate voltage dependence of the drain current at different values of the drain-source voltage of an MoSe2 phototransistor [27]. Source: Printed with permission.
The higher saturation current is observed in the phototransistor structure compared to multilayered MoSe2. The variation of the drain current (Id) with drain-source voltage (Vds) at varied values of the gate voltage is shown in Fig. 13.6(B) while linear dependence was observed at low drain-source voltages along with transfer characteristics for a range of drain-source voltages. This shows the ohmic nature of the contacts. The phototransistor showed n-type behavior, accumulating the electrons at the interface between the gate oxide and MoSe2. The threshold voltage (Vth) and the current on/off ratio at Vds 5 10 V were observed to be about 225 V and 3 3 104 while the high threshold voltage is unsuitable for low power consumption applications. High κ-dielectrics material if used instead of SiO2 as the gate oxides will reduce the threshold voltage (Fig. 13.7) and enhance the current on/off ratio [27 29].
13.8 TUNGSTEN DISULFIDE-BASED PHOTOSENSOR Different observations of the photocurrent response of CVD grown WS2 on quartz substrate has been reported. The spectral responsivity was proportional to the wavelength of the excitation source with the limits for lower excitation (λ 5 647 nm; E 5 1.91 eV) and was found to be 2.0 μ A W21 while for the higher energy (λ 5 568 nm; E 5 2.71 eV) it was higher value of 21.2 μ A W21. The sensitivity with different excitation intensities was found to support the observation that the photocurrent, IP, is nonlinearly related to the power of the laser excitation [30]. Such a photosensitive material has good sensitivity at low intensities and higher sensitivity at higher intensities. The photocurrent
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measurements were carried out at room temperature using a vacuum chamber at 1 3 10 6 Torr pressure, and were coupled to a micro-Raman spectrometer. In all the photocurrent measurements, the photocurrent produced by switching the laser beam on and off is the time-varying component. The plots below show photocurrent versus applied voltage plots and the three different illumination intensities were at powers of 0.65, 3.25, and 6.25 mW. These I V plots show a linear increase of photocurrent with applied voltage. A large photocurrent was observed by the spectral responsivity of the WS2 device as a function of the wavelength of the laser excitation. The photocurrent was monitored as a function of time with periodic laser beam illumination revealing the strong dependence on the wavelength and on the illumination power. The fast measured response resulted in 5.3 ms for response and recovery times with good responsivity, and stability as compared to other TMDs, demonstrating that WS2 is a good option for optoelectronics [31]. In the case of WS2, as shown in Fig. 13.8, Zheng et al. reported a unique
FIGURE 13.8 Device structure and photoresponse of photodetector. (A) Schematic representation of the photodetector consisting of a WS2 film and the Ti/Au contacts on quartz, and the laser sources were applied perpendicularly to the film. (B) Current-voltage plot obtained without illumination; the inset displays the microscope image of a pair of the Ti/Au electrodes. (C) I V curves of the device obtained under different light intensities from 12.1 to 395 μ W cm 2 at a wavelength of 365 nm. (D) The corresponding responsivity vs. voltage plots acquired under various light intensities [32]. Source: Printed with permission.
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FIGURE 13.9 (A) Optical image of a BP photodetector. The ring-shaped structure shows the photocurrent collector where yellow lines are Ti/Au electrodes and the area enclosed by a white line is the BP flake. (B) Corresponding photocurrent microscopy image of the device with illumination at 1500 nm. (C) Polarization dependence of photoresponsivity with illumination from 400 to 1700 nm [34]. Source: Printed with permission.
photoelectrical conversion property with a high responsivity of 53.3 A W 1 and a high detectivity of 1.22 3 1011 Jones at 365 nm as well as a technique for large-scale growth of WS2 film, which can be transferred to be developed for such applications as photosensors, solar cells, and photo-electrochemical cells.
13.9 BLACK PHOSPHOROUS-BASED PHOTOSENSOR BP is the anisotropic material that exhibits direct bandgap of about B1.8 eV in single layer which is tunable with different thicknesses covering the visible to mid-IR spectral range, as compared to bandgap of B0.3 eV in its bulk form [33]. Photodetector-based on BP layer (Fig. 13.9) shows high responsivity of about 103 A W21 at 300 K and 7 3 106 A W21 at 20 K in the near-IR region (900 nm). The photogenerated carriers are effectively collected due to good ohmic contact to BP [34]. BP was also integrated with silicon waveguide to realize high responsivity photodetection. The photovoltaic current dominates the photocurrent at low doping where the intrinsic responsivity reaches 135 mA W21 for a thickness of about 11.5 nm and 657 mA W21 for a thickness of about 100 nm at room temperature and having high response bandwidth exceeding 3 GHz [35,36].
13.10 2D HETEROSTRUCTURES The heterostructures of 2D materials aim to develop the structures that have optimized emergent properties due to the in-proportion growth of constituent. Graphene/MoS2 structures aim at harnessing the high carrier mobility of graphene and photon absorption capability of
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MoS2. A high photo gain greater than 108 and a photoresponsivity value higher than 107 A W21 is exhibited in the photodetector based on this heterostructure. The high photo gain is due to a recirculation of electrons in graphene due to the enhanced lifetime of charge trapping of holes due to MoS2. The electric field of an external gate modulates the amplitude and polarity of the photocurrent in the gated vertical heterostructures with the maximum external quantum efficiency (EQE) of 55% and internal quantum efficiency up to 85% [37]. The p-type WSe2 and n-type MoS2 heterostructures have been reported to have tuned the photocurrent by the gate voltages leading to an interlayer tunneling recombination of majority carriers [38]. Similarly, MoS2-graphene-WSe2 heterostructures have the photoexcited electrons and holes which, due to the built-in electric field, are effectively separated. The depletion region of the p-n junction enables photodetection over a broad range with high sensitivity. Due to the photon energy being larger than the bandgaps of the TMDs materials in the visible range, the abundant photogenerated free. In case of IR wavelength, the photon energy is comparatively smaller than the bandgap of TMDs, which leads to the forbidden interband absorption of both monolayer MoS2 and WSe2, leaving graphene alone as a photon-absorbing medium. This has implications in relatively smaller photoresponse of the heterostructure in the IR range. The device exhibits unprecedented performance with photoresponsivity of 104 A W21 at 400 nm, and the specific detectivity shows up to 1015 Jones and 1011 Jones in the visible and near-IR region respectively, as carriers are produced by each constituent, resulting in a considerably higher photoresponse [39,40] (Fig. 13.10).
13.11 RECENT DEVELOPMENT AND APPLICATIONS The photosensor devices are marked by certain parameters like responsivity, EQE, internal quantum efficiency, time response, and wavelength and noise equivalent. The photodetectors based on different 2D materials like MoS2, MoSe2, WS2 even though grown on the same substrate show huge variability in responsivity. The photodetectors based on MoS2 present comparatively larger responsivity and response times with respect to MoSe2 or WS2 devices. One common factor about being receptive to environmental changes is what makes them a potent option for light-sensitive gas detectors. The semiconducting di- and tri-chalcogenides have been observed to not show photoresponse at telecommunication wavelengths which paved the way to investigate the BP and which demonstrates sizable responsivity (about 0.1 A W21) and response speed (f3dB B3 GHz) under λ 5 1550 nm excitation. BP has developed as a promising candidate for fast and broadband detection in
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FIGURE 13.10 (A) Schematic of the photodetector based on graphene/MoS2 heterostructure along with the graphs showing photoresponsivity (left) and photogain (right) for the graphene/MoS2 photodetectors. (B) Graphene-MoS2-graphene heterostructures laserilluminated device and its I V characteristics. Inset represents the schematic illustration of the device. (C) Schematic diagram of a van der Waals-stacked MoS2/WSe2 heterojunction device with lateral metal contacts and the corresponding graph shows the measured and simulated photocurrent at Vds 5 0 V as a function of gate voltages with photocurrent map as inset of the device. (D) Cross-section model of MoS2-graphene-WSe2 heterostructurebased photodetector with photoresponsivity R (left) and specific detectivity D (right) for wavelengths ranging from 400 to 2400 nm measured in ambient air (bottom). Inset is the optical image the device [41]. Source: Printed with permission.
IR region which could also be exploited for light energy harvesting in the IR part of the spectrum. It is evident that photodetectors based on semiconducting layered materials display a large (about 10 orders of magnitude) variation in their responsivity. The limiting dark current and improved device performance have been the novel features of design variations of TMD-based photodetectors. In the case of 2D heterostructures, for the visible range, the photon energy is larger than the bandgaps of the individual TMDs materials and sufficient amount
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of photogenerated free carriers are produced by each layered materials, resulting in a considerably higher photoresponse. Besides these, the indium, gallium, and tin chalcogenides are fast catching up as Ga, In, and Sn compounds show responsivities that are comparable or larger than the one measured from TMDC-based photo-FETs. They can also be used as UV detectors as the operation can be extended to UV region. The superior performance of 2D materials are due to their emergent properties, which leads to ultrafast response time, generation of carriers, and better control on switchability. All of these factors have made them a better choice for optoelectronics and photonic devices.
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13 Photo Sensor Based on 2D Materials Dattatray J. Late1, Anha Bhat1,2 and Chandra Sekhar Rout3 1
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. HomiBhabha Road, Pune, India 2Department of Metallurgical and Materials Engineering, National Institute of Technology, Srinagar, India 3Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Bangalore, India
13.1 INTRODUCTION In the frontiers of device electronics are emerging applications in the fields of medical diagnosis, optical communications, radiation and smoke detection, flame detection, barcode detection, process control, environmental sensing, motion sensing, thermography, night vision, and astronomy. The most important development over the past decade includes the integration of optics into electronic systems, which has paved the way for multifunctionality and enhanced performance. One such development is the harnessing of light into electric signals with enhanced speed, efficiency, and flexibility over the range of wavelengths packaged as photodetectors. A photo detection process can operate through various ways like phototconductive effect [1], photo-thermoelectric effect (PTE) [2], and photovoltaic effect [3] fulfilling the consensus of absorbing the photons to generate an electric response. When a photon with sufficient energy enters the junction, it strikes an atom to release an electron from the atomic structure. This mechanism is known as the inner photoelectric effect. During this mechanism, free electron and hole pairs are created.
Fundamentals and Sensing Applications of 2D Materials DOI: https://doi.org/10.1016/B978-0-08-102577-2.00013-0
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Copyright © 2019 Elsevier Ltd. All rights reserved.
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These free electron and hole pairs are either combined or they remain free. Movement of free electrons and hole pairs from the depletion layer due to external electric field produces a photocurrent. The photocurrent is proportional to the amount of light entering the intrinsic region. The chapter deals with different two-dimensional (2D) material architectures ranging from transition-metal dichalcogenides (TMDCs) to black phosphorous (BP) and graphene/TMDC heterostructures with a general comparison of photoresponse for all of them. The heterostructures have proven to be quite efficient material as photodetectors and sensors due to their optimized properties and good charge trapping efficiency.
13.2 CHARACTERISTICS OF PHOTOSENSORS BASED ON THE 2D MATERIALS Bulk silicon has an indirect bandgap of 1.1 eV, which limits its photon absorption capacity to the visible and near infrared (IR) region of the electromagnetic spectrum, and is a major factor contributing to its low efficiency as a photodetector. Many alternatives such as 3D structures of Gallium Arsenide (GaAs) have been proposed to detect IR wavelengths, but there are industrializing drawbacks such as packaging, an increase in the size of the device, and many additive fabrication steps. With the advent of graphene and few-layered TMDCs, alternative materials have been found to provide higher absorption efficiency in the visible range with a wide operational wavelength [4 6]. Features including increased absorption efficiency and transparency that are attributed to single atomic scale thickness make these materials a potential player in cutting-edge applications of photovoltaics, photodetection, and photonics. TMDs show out-of-plane quantum confinement effects and vertical confinement where the reduced thickness constricts the excitons and hence increases the absorption efficiency [7]. A characteristic feature of 2D semiconductor crystals are the Van Hove singularities leading to sharp peaks in the density of states at a particular energy attributed to the d orbital localization of electronic bands. In the case of 2D semiconductors, these singularities fall in the vicinity close to the conduction and valence band edges. Thereby a photon with energy close to the bandgap has an increased probability to excite an electron-hole pair, which makes them receptive to the incident light. This property, along with remarkable elastic modulus .10% has extended their applications to flexible sensors by tuning the optical properties on the basis of strain engineering [8,9]. Based on the different mechanisms of photons to electrical signal conversion, the photo-sensing effect can be achieved by the following three effects: photovoltaic effect, photoconductive effect, and PTE.
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13.3 PHOTOVOLTAIC EFFECT
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13.3 PHOTOVOLTAIC EFFECT In the photovoltaic effect the electromagnetic radiation energy is converted into electric energy in certain semiconductor materials. The photovoltaic-based photodetector contains p-/n-type (PN) photodiodes formed by two semiconductors with opposite doping type. When light of a suitable wavelength is incident on these cells, energy from the photon is transferred to an atom of the semiconducting material in the p-n junction and Schottky barrier photodiodes are formed at the interface between a semiconductor and a metal. The built-in difference in electric field is necessary for the photovoltaic effect and is created either by local chemical doping [10] and electrostatic control through gates [11]. The configuration of photodetectors based on this approach is usually in the cadre of PN diodes used at zero bias, which is in coherence with photovoltaic mode or under reverse bias, which is the photoconductive mode. In the former, the dark current is the lowest, which is good for detectivity, but at the same time due to no internal gain, absolute responsivity is usually smaller than photoconducting. However the speed of a photodiode as shown in Fig. 13.1 is increased due to the reduction of the junction capacitance in case of reverse bias [13,14].
FIGURE 13.1 (A) p-n junction band alignment, where absorption of a photon with
Eph . Ebg generates an electron-hole pair, which is then separated and accelerated by builtin electric field at the junction. (B) Ids 2 Vds, which results in a short-circuit current Isc and an open-circuit voltage Voc. With maximum power generation denoted by Pel max in the fourth quadrant [12]. Source: Printed with permission.
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13.4 PHOTOCONDUCTIVE EFFECT In a photoconductive effect after electromagnetic radiation absorption the electrical conductivity of nonmetallic solids is increased due to the generation of additional free electron carriers. A typical photoconductor consists of a semiconductor as a channel with two ohmic contacts affixed to opposite ends of the channel, which serves as source/drain electrodes [15]. In the absence of light is a zero current between two electrodes called the dark current. The electromagnetic radiation having higher energy than the bandgap is able to generate electronholes pairs which can be separated by applying a voltage. Such free electrons and holes move due to voltage drift to their respective majority sides (toward electrodes). Due to this a depletion layer is created on either side of the junction. The drifted electrons and holes increase the conductivity of the device. The current as depicted in Fig. 13.2 is generated due to the photoconductive effect and is dependent on the intensity of the electromagnetic radiation, length of the transistor channel, source drain voltage, and the charge carrier mobility [16,17].
13.5 PHOTO-THERMOELECTRIC EFFECT In PTE a light-induced heating leads to a temperature gradient through a semiconductor channel. There is a temperature difference between the two ends of the semiconductor channel. Due to the Seebeck effect, as shown in Fig. 13.3, the temperature difference gets converted into a voltage difference ΔV whose magnitude is linearly proportional to the temperature gradient [16,18].
13.6 MOLYBEDNUM DISULFIDE-BASED PHOTOSENSING DEVICES The abundant availability of molybdenum disulfide (MoS2) has made it one of the most widely studied TMDCs. Bulk MoS2 is subjected to various methods like chemical exfoliation [18], ultrasonic treatment, and mechanical exfoliation [19,20] in order to yield a monolayer. For impurity-free studies, researchers would prefer to grow crystals by processes like chemical vapor deposition [21]. This method of homegrown crystals also allows for the induction of dopants to engineer the bandgaps [22]. Phototransistors of MoS2 have been reported with a photoresponsivity of about 7.5 3 1023A W21. A monolayer phototransistor
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FIGURE 13.2 (A) Band alignment with small current Idark under external bias for a semiconductor channel contacted with two metals (M) without illumination. (B) Band alignment under illumination with photons of energy (Eph) higher than the bandgap (Ebg). (C) Ids 2 Vg traces in the dark and under illumination. Illumination results in an increase in the conductivity (vertical shift) and a positive photocurrent across the entire gate voltage range. (D) Ids 2 Vds curves in the dark (black line) and under illumination (red line), which results in an increase of the conductivity and a positive photocurrent under illumination [16]. Source: Printed with permission.
fabrication has been reported which involved the deposition of MoS2 on Si/SiO2. The height of monolayer MoS2 measured by AFM is B0.8 nm, and the photoluminescence of a single-layer MoS2 sheet was observed at room temperature using the 488 nm laser, which showed a dominant PL peak at 676 nm. The dominant peak arises from the direct intraband recombination of the photogenerated electron-hole pairs in the singlelayer MoS2, while the weak peak at B623 nm is considered to be the
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FIGURE 13.3 (A) Schematic of a field-effect transistor whose metal contact is locally illuminated by a laser. The circuit is open and a thermoelectric voltage ΔVPTE develops across the contacts. (B) Thermal circuit of the device. (C) Ids 2 Vds characteristics in the dark (black line) and under illumination (red line) of a device whose photo-response is dominated by the photo-thermoelectric effect [16]. Source: Printed with permission.
contribution of the energy split (Fig. 13.4) of valence band spin-orbital coupling of MoS2 [20,23,24]. The device showed unprecedented characteristics of incident-light control, efficient photoresponsivity, and photoswitchability, which allows the fabrication of cheap and affordable optoelectronic devices. Further, the study in this device stated that the photocurrent was dependent on the optical power. When the device was subjected to energy above the excitation wavelength of 670 nm, the photocurrent was lower than after the expulsion of dark drain current. However, there was a significant increase in the photocurrent when the wavelength was lowered from 670 nm, satisfying the basic principle of incident photon energy being greater than the energy gap, which is about 1.83 eV. The electrons having large incident photon energy (hν . 1.83 eV) can generate more photoelectrons contributing to the increased photocurrent. Moreover this device also showed good stability and efficient photoswitchability. The switching behavior was observed when drain current shot up to high value under illumination and resumed to lower values under dark, which is an off state [25]. In other cases of chemical vapor deposition (CVD)-grown MoS2, a photosensor reported by Lopez et al. [25] was made of triangular structures of MoS2 grown on Si/SiO2 substrates by mild sulfurization of MoO3. Electron beam lithography was used to fabricate the metal contacts of reportedly 4 nm thick Ti and 100 nm thick Au at 10 7 Torr
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FIGURE 13.4 (A) Drain current (Ids) as a function of excitation wavelength of the illumination source at constant optical power. (Inset shows single-layer MoS2 AFM image and optical image of fabricated device). (B) Output characteristics of phototransistor at different illuminating optical powers. (C) Dependence of photocurrent on optical power at different Vds. (D) Photoswitching characteristics of single-layer MoS2 phototransistor at different optical power and drain voltage [20]. Source: Printed with permission.
pressure. The Raman spectrum obtained at a laser excitation of 488 nm revealed a PL spectrum with maximum intensity at 670 nm corresponding to 1.86 eV photon energy. The first order Raman modes are observed due to D3h point group symmetry which is the characteristic of noncentrosymmetric monolayer MoS2. The PL and Raman spectra were obtained in the region between cathode and anode confirming the presence of monolayer MoS2 and its direct bandgap. The sensor response was observed when two wavelengths of the light along the order of 488 and 514.5 nm were used to probe the photosensitivity. The laser excitation and bias voltage is observed between 0 and 90 s, which is complemented further with current versus time plot as the readings are accounted for every 200 ms. The applied bias voltage was increased from 1.5 to 2.0 V around tB52 s, barely producing a flicker in the dark current whereas for 5 s starting at t 5 14 s and t 5 64 s, the excitation laser illuminated the device. As shown in Fig. 26 noting the response to excitation in both cases, the photocurrent response was found to be around 50% larger in response to the bias voltage at 2.0 V (63 s , t , 68 s) as compared to V 5 1.5 V (14 s , t , 19 s) [25] (Fig. 13.5).
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FIGURE 13.5 (A) Bias voltage and laser excitation. (B) Total current as a function of time for the CVD-grown monolayered MoS2 device. (C) Effect on photocurrent. IV plots of the device under darkness and two different wavelengths (514.5 and 488 nm) [25]. Source: Printed with permission.
FIGURE 13.6 Schematic of MoSe2 phototransistor fabrication by polymer transistor [27]. Source: Printed with permission.
13.7 MOLYBDENUM DISELENIDE-BASED PHOTOSENSOR The monolayer MoSe2 has properties like direct bandgap (EgB1.6 eV) and large exciton binding energy similar to that of MoS2, which has made it one of the most sought-after choices for investigating electrical and optical properties. The argument is further strengthened by the observance of strong photoluminescence in MoSe2 [26]. It is expected to show a higher photoresponse in the solar spectrum range and has high antiphotocorrosion stability. A phototransistor was fabricated by depositing 50 nm titanium (Ti) electrodes by electron-beam evaporation on silicon substrates. As shown in Fig. 13.6 MoSe2 was transferred onto the Ti electrodes using polydimethylsiloxane (PDMS) transfer technique [27].
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FIGURE 13.7 (A) Drain-source voltage dependence of the drain current at different values of the gate voltage of an MoSe2-based phototransistor. (B) Gate voltage dependence of the drain current at different values of the drain-source voltage of an MoSe2 phototransistor [27]. Source: Printed with permission.
The higher saturation current is observed in the phototransistor structure compared to multilayered MoSe2. The variation of the drain current (Id) with drain-source voltage (Vds) at varied values of the gate voltage is shown in Fig. 13.6(B) while linear dependence was observed at low drain-source voltages along with transfer characteristics for a range of drain-source voltages. This shows the ohmic nature of the contacts. The phototransistor showed n-type behavior, accumulating the electrons at the interface between the gate oxide and MoSe2. The threshold voltage (Vth) and the current on/off ratio at Vds 5 10 V were observed to be about 225 V and 3 3 104 while the high threshold voltage is unsuitable for low power consumption applications. High κ-dielectrics material if used instead of SiO2 as the gate oxides will reduce the threshold voltage (Fig. 13.7) and enhance the current on/off ratio [27 29].
13.8 TUNGSTEN DISULFIDE-BASED PHOTOSENSOR Different observations of the photocurrent response of CVD grown WS2 on quartz substrate has been reported. The spectral responsivity was proportional to the wavelength of the excitation source with the limits for lower excitation (λ 5 647 nm; E 5 1.91 eV) and was found to be 2.0 μ A W21 while for the higher energy (λ 5 568 nm; E 5 2.71 eV) it was higher value of 21.2 μ A W21. The sensitivity with different excitation intensities was found to support the observation that the photocurrent, IP, is nonlinearly related to the power of the laser excitation [30]. Such a photosensitive material has good sensitivity at low intensities and higher sensitivity at higher intensities. The photocurrent
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measurements were carried out at room temperature using a vacuum chamber at 1 3 10 6 Torr pressure, and were coupled to a micro-Raman spectrometer. In all the photocurrent measurements, the photocurrent produced by switching the laser beam on and off is the time-varying component. The plots below show photocurrent versus applied voltage plots and the three different illumination intensities were at powers of 0.65, 3.25, and 6.25 mW. These I V plots show a linear increase of photocurrent with applied voltage. A large photocurrent was observed by the spectral responsivity of the WS2 device as a function of the wavelength of the laser excitation. The photocurrent was monitored as a function of time with periodic laser beam illumination revealing the strong dependence on the wavelength and on the illumination power. The fast measured response resulted in 5.3 ms for response and recovery times with good responsivity, and stability as compared to other TMDs, demonstrating that WS2 is a good option for optoelectronics [31]. In the case of WS2, as shown in Fig. 13.8, Zheng et al. reported a unique
FIGURE 13.8 Device structure and photoresponse of photodetector. (A) Schematic representation of the photodetector consisting of a WS2 film and the Ti/Au contacts on quartz, and the laser sources were applied perpendicularly to the film. (B) Current-voltage plot obtained without illumination; the inset displays the microscope image of a pair of the Ti/Au electrodes. (C) I V curves of the device obtained under different light intensities from 12.1 to 395 μ W cm 2 at a wavelength of 365 nm. (D) The corresponding responsivity vs. voltage plots acquired under various light intensities [32]. Source: Printed with permission.
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FIGURE 13.9 (A) Optical image of a BP photodetector. The ring-shaped structure shows the photocurrent collector where yellow lines are Ti/Au electrodes and the area enclosed by a white line is the BP flake. (B) Corresponding photocurrent microscopy image of the device with illumination at 1500 nm. (C) Polarization dependence of photoresponsivity with illumination from 400 to 1700 nm [34]. Source: Printed with permission.
photoelectrical conversion property with a high responsivity of 53.3 A W 1 and a high detectivity of 1.22 3 1011 Jones at 365 nm as well as a technique for large-scale growth of WS2 film, which can be transferred to be developed for such applications as photosensors, solar cells, and photo-electrochemical cells.
13.9 BLACK PHOSPHOROUS-BASED PHOTOSENSOR BP is the anisotropic material that exhibits direct bandgap of about B1.8 eV in single layer which is tunable with different thicknesses covering the visible to mid-IR spectral range, as compared to bandgap of B0.3 eV in its bulk form [33]. Photodetector-based on BP layer (Fig. 13.9) shows high responsivity of about 103 A W21 at 300 K and 7 3 106 A W21 at 20 K in the near-IR region (900 nm). The photogenerated carriers are effectively collected due to good ohmic contact to BP [34]. BP was also integrated with silicon waveguide to realize high responsivity photodetection. The photovoltaic current dominates the photocurrent at low doping where the intrinsic responsivity reaches 135 mA W21 for a thickness of about 11.5 nm and 657 mA W21 for a thickness of about 100 nm at room temperature and having high response bandwidth exceeding 3 GHz [35,36].
13.10 2D HETEROSTRUCTURES The heterostructures of 2D materials aim to develop the structures that have optimized emergent properties due to the in-proportion growth of constituent. Graphene/MoS2 structures aim at harnessing the high carrier mobility of graphene and photon absorption capability of
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MoS2. A high photo gain greater than 108 and a photoresponsivity value higher than 107 A W21 is exhibited in the photodetector based on this heterostructure. The high photo gain is due to a recirculation of electrons in graphene due to the enhanced lifetime of charge trapping of holes due to MoS2. The electric field of an external gate modulates the amplitude and polarity of the photocurrent in the gated vertical heterostructures with the maximum external quantum efficiency (EQE) of 55% and internal quantum efficiency up to 85% [37]. The p-type WSe2 and n-type MoS2 heterostructures have been reported to have tuned the photocurrent by the gate voltages leading to an interlayer tunneling recombination of majority carriers [38]. Similarly, MoS2-graphene-WSe2 heterostructures have the photoexcited electrons and holes which, due to the built-in electric field, are effectively separated. The depletion region of the p-n junction enables photodetection over a broad range with high sensitivity. Due to the photon energy being larger than the bandgaps of the TMDs materials in the visible range, the abundant photogenerated free. In case of IR wavelength, the photon energy is comparatively smaller than the bandgap of TMDs, which leads to the forbidden interband absorption of both monolayer MoS2 and WSe2, leaving graphene alone as a photon-absorbing medium. This has implications in relatively smaller photoresponse of the heterostructure in the IR range. The device exhibits unprecedented performance with photoresponsivity of 104 A W21 at 400 nm, and the specific detectivity shows up to 1015 Jones and 1011 Jones in the visible and near-IR region respectively, as carriers are produced by each constituent, resulting in a considerably higher photoresponse [39,40] (Fig. 13.10).
13.11 RECENT DEVELOPMENT AND APPLICATIONS The photosensor devices are marked by certain parameters like responsivity, EQE, internal quantum efficiency, time response, and wavelength and noise equivalent. The photodetectors based on different 2D materials like MoS2, MoSe2, WS2 even though grown on the same substrate show huge variability in responsivity. The photodetectors based on MoS2 present comparatively larger responsivity and response times with respect to MoSe2 or WS2 devices. One common factor about being receptive to environmental changes is what makes them a potent option for light-sensitive gas detectors. The semiconducting di- and tri-chalcogenides have been observed to not show photoresponse at telecommunication wavelengths which paved the way to investigate the BP and which demonstrates sizable responsivity (about 0.1 A W21) and response speed (f3dB B3 GHz) under λ 5 1550 nm excitation. BP has developed as a promising candidate for fast and broadband detection in
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FIGURE 13.10 (A) Schematic of the photodetector based on graphene/MoS2 heterostructure along with the graphs showing photoresponsivity (left) and photogain (right) for the graphene/MoS2 photodetectors. (B) Graphene-MoS2-graphene heterostructures laserilluminated device and its I V characteristics. Inset represents the schematic illustration of the device. (C) Schematic diagram of a van der Waals-stacked MoS2/WSe2 heterojunction device with lateral metal contacts and the corresponding graph shows the measured and simulated photocurrent at Vds 5 0 V as a function of gate voltages with photocurrent map as inset of the device. (D) Cross-section model of MoS2-graphene-WSe2 heterostructurebased photodetector with photoresponsivity R (left) and specific detectivity D (right) for wavelengths ranging from 400 to 2400 nm measured in ambient air (bottom). Inset is the optical image the device [41]. Source: Printed with permission.
IR region which could also be exploited for light energy harvesting in the IR part of the spectrum. It is evident that photodetectors based on semiconducting layered materials display a large (about 10 orders of magnitude) variation in their responsivity. The limiting dark current and improved device performance have been the novel features of design variations of TMD-based photodetectors. In the case of 2D heterostructures, for the visible range, the photon energy is larger than the bandgaps of the individual TMDs materials and sufficient amount
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of photogenerated free carriers are produced by each layered materials, resulting in a considerably higher photoresponse. Besides these, the indium, gallium, and tin chalcogenides are fast catching up as Ga, In, and Sn compounds show responsivities that are comparable or larger than the one measured from TMDC-based photo-FETs. They can also be used as UV detectors as the operation can be extended to UV region. The superior performance of 2D materials are due to their emergent properties, which leads to ultrafast response time, generation of carriers, and better control on switchability. All of these factors have made them a better choice for optoelectronics and photonic devices.
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