**3.4 Black phosphorous photodetectors**

Phosphorous, in its elemental nature, can exist in many forms. One such form of phosphorous is called black phosphorous (BP). With a formation energy of −395 KJmol−1− black phosphorous is a thermodynamically stable form of phosphorous at room temperature. Black phosphorous is similar to graphite in its appearance, properties and structure. Black phosphorous sheets have a puckered geometry [98]. Black phosphorous was first successfully exfoliated in 2014 and has received a lot of attention since then [99, 100]. In its monolayer form, the phosphorous atoms form covalent bonds with three adjacent atoms, which results in a wrinkled honeycomb structure. The corresponding layers are held together by van der Waals forces [101]. Unlike graphene, black phosphorous is a semiconductor with a direct bandgap. Due to its strong anisotropic interaction with electrons and photons, black phosphorous is a strong candidate for electronic and optoelectronic applications.

The bandgap's direct nature in black phosphorous makes it easy for the carriers to transit to excited states, as there are negligible chances of phonon scattering [102]. The photoelectric characteristics of a black phosphorous FET were studied by Buscema et al. [103]. **Figure 4** shows the model of the device. The device operates at a wavelength ranging from visble to NIR part of the spectrum. The device shows an On/Off ratio of 10 along with an electron mobility of 0.5 cm2 V−1 s−1. Wavelength ranging from visible to NIR results in a photocurrent generation in the proposed device. The responsivity exhibits a typical increase with a decreasing wavelength and attains a maximum value of 4.8 mAW−1. Chen et al. used a sandwich of hBN-BP-hBN to demonstrate a photodetector with a widely tuneable infrared wavelength range [104]. The device shows an absorption of 3% at a wavelength of 3.4 μm, and the absorption of the device was observed to decrease with increasing wavelength. Furthermore, it was observed that the light absorption decreases with an increase in vertical electrical bias. Due to the vertical bias, the bandgap shrinks, giving rise to an increase in carrier concentration. The high carrier concentration results in decreased photo-carrier lifetime and degraded performance of the device. The hBN layer aims to prevent the black phosphorous from oxidation and provide a clean interface.

One of the primitive methods to improve the performance of TMDC photodetectors is to use doping. Accordingly, Keng et al. demonstrated an n-type and p-type black phosphorous photodetectors [105]. The concentration of the dopants was found to be dependent on the thickness of the black phosphorous layer. The device shows a responsivity of 1.4 × 104 AW−1 for a device with a black phosphorous thickness of 10 nm [105].

Using a transparent substrate opens up the possibility of novel device designs. Miao et al. have fabricated a photodetector based on multilayer black phosphorous on polyimide film substrate [106]. The device shows a responsivity of 53 AW−1. It is observed that when the device is illuminated by infrared light, enhanced scattering of the carriers with the phonons occurs, which eventually degrades the carrier mobility and the performance of the device. However, such behavior is not observed when a SiO2/Si substrate is used instead of polyimide film.

### **3.5 Photodetectors based on 2D-heterostructures**

The ever-growing evolution and development of 2D materials have led to the formulation of 2D van der Waals heterostructures. Based on these heterostructures, several photodetectors have been reported recently. Apart from their high degree of integration, these devices exhibit excellent performance. The electronic structure and properties induced between these 2D heterostructures' layers show promising characteristics as far as electronic and optoelectronic applications are concerned. 2D heterostructures/heterojunctions are essential building blocks of modern

**119**

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

teristics for sub-picosecond applications.

values of photoconductive gain and responsivity.

electronic devices [107]. The band structures of the constituent 2D materials of these heterostructures undergo considerable changes due to electrostatic interactions. Xue et al. have fabricated a MoS2/WS2 vertical heterostructures based photodetector [108]. Mo, S and WO3 were used to prepare the MoS2/WS2 heterojunction. The device shows a high rectification along with a considerable responsivity of 2.3 AW−1. The characteristics of the device are evaluated at an illuminatring light of 450 nm. The interfacial built-in electric field prompts the separation of the photogenerated carriers [109]. On transferring the heterojunction to the polydimethylsiloxane (PMDS) substrate, a decrease in photocurrent is observed due to trapping states between the heterojunction and the substrate [47]. Duan et al. demonstrated a heterojunction diode based on WSe2/MoS2 heterojunction [110]. The heterojunction was obtained by transferring the exfoliate MoS2 to a physical vapor deposition (PVD) grown WSe2 monolayer. A significant rectification ratio, along with high external quantum efficiency (EQE), was observed at an operating wavelength of 514 nm. It is noteworthy to mention that the EQE of the device is much higher than what is achieved in a lateral doped WSe2 p-n homojunction [111]. Such a behavior is a consequence of the much better charge separation at the vertically stacked junction interface. Peng et al. have also reported a heterojunction between MoS2 and WSe2 [112]. The MoS2/WSe2 heterojunction is obtained by mechanical exfoliation and transfer methods. A high charge transfer of 99% from WSe2 to MoS2 is observed in a very short time of 470 fs [112]. The device shows promising charac-

Apart from the semiconducting materials based heterostructures, graphene has also been utilized for heterostructures formation. Graphene may not be suitable for photodetector applications independently due to its zero bandgap and high light transmittance. Yu et al. formulated a photodetector based on MoTe2/graphene heterostructures [113] as shown in **Figure 5**. MoTe2 multilayer serves as a light active material in the said heterostructure, and graphene monolayer serves as an efficient transport path for photo-excited carriers. The heterostructure shows better performance as compared to individual graphene and MoTe2 based devices. MoTe2/graphene photodetectors work on the principle of photogating effect. Due to this photogating effect, electrons are trapped in localized states of MoTe2 and holes are shifted towards the graphene layer. The high carrier mobility of graphene allows for a quick extraction of the holes injected into the graphene layer. This results in an enhanced photocurrent in the device. The device shows exceptional

Britnell et al. demonstrated a photodetector based on the heterostructures of a few-layer TMDCs and graphene [114]. The device's performance depends on the encapsulation of one or more layers of TMDC sheets with graphene. The device has a sandwich structure wherein the TMDC photoactive layer is encapsulated between the top and bottom graphene electrodes. Because of the transparency of graphene, the illuminating light can reach efficiently to the TMDC layer. An appreciable photocurrent is observed when the illuminating light impinges on the overlapped regions of graphene and TMDC. The direction of the photocurrent aligns with the direction of the built-in electric field resulting from the gate voltage. This allows to modulate the photocurrent through gate voltage. Due to graphene, the extraction of charges is swift, thus reducing the recombination rate of photo-excited carriers. A similar structure is reported by Duan et al. as well [115]. The device consists of a vertical sandwich of graphene-MoS2-graphene heterojunction. Similar to the device reported by Britnell et al. [114], the top and bottom layers of graphene act as electrodes, whereas the middle MoS2 layer acts as the barrier layer. Upon illuminating the MoS2 layer, the electron–hole pairs get separated asymmetric potentials at the graphene/MoS2 interface, which leads to

**Figure 4.** *Model of the few layer black phosphorous photodetector [103].*

#### *Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

black phosphorous from oxidation and provide a clean interface.

when a SiO2/Si substrate is used instead of polyimide film.

**3.5 Photodetectors based on 2D-heterostructures**

*Model of the few layer black phosphorous photodetector [103].*

shows a responsivity of 1.4 × 104

ness of 10 nm [105].

ratio of 10 along with an electron mobility of 0.5 cm2

photoelectric characteristics of a black phosphorous FET were studied by Buscema et al. [103]. **Figure 4** shows the model of the device. The device operates at a wavelength ranging from visble to NIR part of the spectrum. The device shows an On/Off

from visible to NIR results in a photocurrent generation in the proposed device. The responsivity exhibits a typical increase with a decreasing wavelength and attains a maximum value of 4.8 mAW−1. Chen et al. used a sandwich of hBN-BP-hBN to demonstrate a photodetector with a widely tuneable infrared wavelength range [104]. The device shows an absorption of 3% at a wavelength of 3.4 μm, and the absorption of the device was observed to decrease with increasing wavelength. Furthermore, it was observed that the light absorption decreases with an increase in vertical electrical bias. Due to the vertical bias, the bandgap shrinks, giving rise to an increase in carrier concentration. The high carrier concentration results in decreased photo-carrier lifetime and degraded performance of the device. The hBN layer aims to prevent the

One of the primitive methods to improve the performance of TMDC photodetectors is to use doping. Accordingly, Keng et al. demonstrated an n-type and p-type black phosphorous photodetectors [105]. The concentration of the dopants was found to be dependent on the thickness of the black phosphorous layer. The device

Using a transparent substrate opens up the possibility of novel device designs. Miao et al. have fabricated a photodetector based on multilayer black phosphorous on polyimide film substrate [106]. The device shows a responsivity of 53 AW−1. It is observed that when the device is illuminated by infrared light, enhanced scattering of the carriers with the phonons occurs, which eventually degrades the carrier mobility and the performance of the device. However, such behavior is not observed

The ever-growing evolution and development of 2D materials have led to the formulation of 2D van der Waals heterostructures. Based on these heterostructures, several photodetectors have been reported recently. Apart from their high degree of integration, these devices exhibit excellent performance. The electronic structure and properties induced between these 2D heterostructures' layers show promising characteristics as far as electronic and optoelectronic applications are concerned. 2D heterostructures/heterojunctions are essential building blocks of modern

V−1 s−1. Wavelength ranging

AW−1 for a device with a black phosphorous thick-

**118**

**Figure 4.**

electronic devices [107]. The band structures of the constituent 2D materials of these heterostructures undergo considerable changes due to electrostatic interactions. Xue et al. have fabricated a MoS2/WS2 vertical heterostructures based photodetector [108]. Mo, S and WO3 were used to prepare the MoS2/WS2 heterojunction. The device shows a high rectification along with a considerable responsivity of 2.3 AW−1. The characteristics of the device are evaluated at an illuminatring light of 450 nm. The interfacial built-in electric field prompts the separation of the photogenerated carriers [109]. On transferring the heterojunction to the polydimethylsiloxane (PMDS) substrate, a decrease in photocurrent is observed due to trapping states between the heterojunction and the substrate [47]. Duan et al. demonstrated a heterojunction diode based on WSe2/MoS2 heterojunction [110]. The heterojunction was obtained by transferring the exfoliate MoS2 to a physical vapor deposition (PVD) grown WSe2 monolayer. A significant rectification ratio, along with high external quantum efficiency (EQE), was observed at an operating wavelength of 514 nm. It is noteworthy to mention that the EQE of the device is much higher than what is achieved in a lateral doped WSe2 p-n homojunction [111]. Such a behavior is a consequence of the much better charge separation at the vertically stacked junction interface. Peng et al. have also reported a heterojunction between MoS2 and WSe2 [112]. The MoS2/WSe2 heterojunction is obtained by mechanical exfoliation and transfer methods. A high charge transfer of 99% from WSe2 to MoS2 is observed in a very short time of 470 fs [112]. The device shows promising characteristics for sub-picosecond applications.

Apart from the semiconducting materials based heterostructures, graphene has also been utilized for heterostructures formation. Graphene may not be suitable for photodetector applications independently due to its zero bandgap and high light transmittance. Yu et al. formulated a photodetector based on MoTe2/graphene heterostructures [113] as shown in **Figure 5**. MoTe2 multilayer serves as a light active material in the said heterostructure, and graphene monolayer serves as an efficient transport path for photo-excited carriers. The heterostructure shows better performance as compared to individual graphene and MoTe2 based devices. MoTe2/graphene photodetectors work on the principle of photogating effect. Due to this photogating effect, electrons are trapped in localized states of MoTe2 and holes are shifted towards the graphene layer. The high carrier mobility of graphene allows for a quick extraction of the holes injected into the graphene layer. This results in an enhanced photocurrent in the device. The device shows exceptional values of photoconductive gain and responsivity.

Britnell et al. demonstrated a photodetector based on the heterostructures of a few-layer TMDCs and graphene [114]. The device's performance depends on the encapsulation of one or more layers of TMDC sheets with graphene. The device has a sandwich structure wherein the TMDC photoactive layer is encapsulated between the top and bottom graphene electrodes. Because of the transparency of graphene, the illuminating light can reach efficiently to the TMDC layer. An appreciable photocurrent is observed when the illuminating light impinges on the overlapped regions of graphene and TMDC. The direction of the photocurrent aligns with the direction of the built-in electric field resulting from the gate voltage. This allows to modulate the photocurrent through gate voltage. Due to graphene, the extraction of charges is swift, thus reducing the recombination rate of photo-excited carriers. A similar structure is reported by Duan et al. as well [115]. The device consists of a vertical sandwich of graphene-MoS2-graphene heterojunction. Similar to the device reported by Britnell et al. [114], the top and bottom layers of graphene act as electrodes, whereas the middle MoS2 layer acts as the barrier layer. Upon illuminating the MoS2 layer, the electron–hole pairs get separated asymmetric potentials at the graphene/MoS2 interface, which leads to

an appreciable photocurrent [115]. A graphene-WSe2-graphene heterostructure based photodetector is reported by Massicotte et al. [116]. The heterostructure is packaged with hBN layers. The device exhibits an ultra-fast response time of 5.5 ps.

Apart from graphene, another novel material called silicene has received a lot of attention in recent years. Silicene, regarded as the 'silicon version of graphene,' also has a hexagonal structure [117–119]. Silicene is the single-layer version of graphene, having the constituent Si atoms arranged in a hexagonal form via covalent bonds [22, 23]. Silicene shares many properties of graphene, like zero bandgap, high mobility of carriers and the presence of a Dirac cone in its band structure. Apart from these excellent electronic properties, one advantage of silicene over graphene is its expected integration with the present state of the art Si-based technology. Kharadi et al. have proposed a photodetector based on silicene/MoS2

**Figure 5.** *Photodetector based on MoTe2/graphene heterostructure [113].*

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(4.76 × 1010 Jones).

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

Single-Layer MoS2 Phototransistor [46]

Ultra-Sensitive Monolayer MoS2 Photodetector [47]

High-detectivity multilayer MoS2 phototransistors [49]

High-gain CVD-grown MoS2 monolayer phototransistor [51]

High photosensitivity few-layered MoSe2 back-gated field-effect phototransistor [72]

Multilayer WS2 Nanoflakes Photo responsive

WSe2 Monolayer Phototransistor [84]

ReSe2 nanosheet transistor [93]

Few-layer Black Phosphorus FET [103]

Silicene/MoS2 heterostructure [22]

**Table 1.**

High Photo responsive Few-layered WSe2 Transistor[85]

Few Layer WS2 Phototransistor [81]

FET [82]

**Device Type Wavelength Responsivity Response Speed NEP/Detectivity**

532 nm 2.2 × 103

633 nm 5.7 AW−1 in

650 nm 1.8 × 105

650 nm 5.66 × 105

*Characteristics of photodetectors and phototransistors based on different 2D materials.*

Vacuum 884 AW−1 in NH3 Environment

670 nm 7.5 mAW−1 50 ms —

Up to NIR 100 mAW−1 — —

532 nm 97.1 AW−1 ~10 ms —

457-647 nm 92 μAW−1 5 ms —

532 nm 7 AW−1 10 μs —

633 nm 95 AW−1 ~ 10 ms —

Visible-NIR 48 mAW−1 — —

 AW−1 in Vacuum 780 AW−1 in Air

680 nm 800 mAW−1 0.6 s 1.5 × 1015 WHz−1/2

— —

< 20 ms —

AW−1 < 23 ms 1014 Jones

AW−1 — 4.76 × 1010 Jones

heterostructure [22]. The model of the device is shown in **Figure 6**. Due to the high mobility of carriers in silicene, it is used as a high-velocity transport path for the photo-excited carriers. Illuminating the device's active region with a light of 650 nm results in the electron–hole pair generation. The electron–hole pairs are separated at the silicene/MoS2 heterostructure interface due to the built-in electric field generated by a combined effect of charge transfer between silicene and MoS2 and the gate voltage. Apart from an appreciable photoconductive gain of 2.5 × 1011, the device

**Table 1**. presents the characteristics of the optoelectronic devices based on different 2D materials. In general it can be seen that the light sensitive devices based on 2D materials have shown a steady increase in the performance over the years. Depending on the bandgap of the material used, the photosensitive device can be

used in different wavelength regions of the electromagnetic spectrum.

AW−1) and detectivity

exhibits considerable values of responsivity (5.66 × 105

**Figure 6.** *Model of Si/MoS2 heterostructure based photodetector [22].*


*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

of 5.5 ps.

an appreciable photocurrent [115]. A graphene-WSe2-graphene heterostructure based photodetector is reported by Massicotte et al. [116]. The heterostructure is packaged with hBN layers. The device exhibits an ultra-fast response time

Apart from graphene, another novel material called silicene has received a lot of attention in recent years. Silicene, regarded as the 'silicon version of graphene,' also has a hexagonal structure [117–119]. Silicene is the single-layer version of graphene, having the constituent Si atoms arranged in a hexagonal form via covalent bonds [22, 23]. Silicene shares many properties of graphene, like zero bandgap, high mobility of carriers and the presence of a Dirac cone in its band structure. Apart from these excellent electronic properties, one advantage of silicene over graphene is its expected integration with the present state of the art Si-based technology. Kharadi et al. have proposed a photodetector based on silicene/MoS2

**120**

**Figure 6.**

**Figure 5.**

*Photodetector based on MoTe2/graphene heterostructure [113].*

*Model of Si/MoS2 heterostructure based photodetector [22].*

#### **Table 1.**

*Characteristics of photodetectors and phototransistors based on different 2D materials.*

heterostructure [22]. The model of the device is shown in **Figure 6**. Due to the high mobility of carriers in silicene, it is used as a high-velocity transport path for the photo-excited carriers. Illuminating the device's active region with a light of 650 nm results in the electron–hole pair generation. The electron–hole pairs are separated at the silicene/MoS2 heterostructure interface due to the built-in electric field generated by a combined effect of charge transfer between silicene and MoS2 and the gate voltage. Apart from an appreciable photoconductive gain of 2.5 × 1011, the device exhibits considerable values of responsivity (5.66 × 105 AW−1) and detectivity (4.76 × 1010 Jones).

**Table 1**. presents the characteristics of the optoelectronic devices based on different 2D materials. In general it can be seen that the light sensitive devices based on 2D materials have shown a steady increase in the performance over the years. Depending on the bandgap of the material used, the photosensitive device can be used in different wavelength regions of the electromagnetic spectrum.
