**4. Recent advancements in the PLD growth of TMDCs-based PDs**

In general, the PLD-grown TMDCs-based PDs exhibit device performance that is comparable with the PDs based on traditional bulk semiconductors. Additionally, PLD is also beneficial for scalable production up to the wafer-scale. Therefore, growth of these TMDCs through PLD for applications in photodetection shows a tremendous potential to translate the fundamental laboratory research to realization of industrial and practical applications.

MoS2 is probably the most studied material among various TMDCs and was probably the first member to be fabricated by PLD. One of the earliest investigations on the photodetection studies of MoS2-based PDs was done by Alkis *et al*. [59], in which the authors have fabricated MoS2 nanocrystallites through PLD in deionized water and have demonstrated ultraviolet photodetection using the thin films of the obtained MoS2 nanocrystallites. Mostly, the PLD fabricated PDs based on the TMDCs are in the form of thin films. The earliest MoS2 thin film-based PD grown by PLD can be traced back to 2014, when Late *et al*. [60] synthesized wafer-scale MoS2 thin films on flexible

*Practical Applications of Laser Ablation*

~10–20 nanoseconds (ns) can be achieved.

then converted into a pulse by various discharge mechanisms and a pulse width of

*Laser fluence:* The laser fluence or laser energy density is defined as the laser output energy per unit area and is a very important parameter which decides the proper ablation of the target where the laser beam interacts with the target. A minimum threshold laser fluence is required to carry out the proper ablation process, otherwise, only evaporation takes place. The plume formation depends upon the target conditions such as its density, porosity, morphology, and compositional impurities as well as the laser conditions such as laser pulse duration and laser pulse width. If the laser fluence is much above the threshold value, crystallographic defects and damage can occur in the deposited thin film because of the bombardment by the ablation particles possessing high kinetic energy. Also, it can lead to macroscopic particles ejection during the process of ablation, particulate formation on thin films as well as back-sputtering of species from the deposited thin film. Various mechanisms have been proposed for the formation of particulates and

several methods have been devised to minimize these effects [53, 54].

and condenses in the form of a thin film [53, 57].

mass transfer of the material from the target onto the substrate.

*Laser-target interactions:* The three main processes taking place during the laser-target interaction are: (i) the laser beam interacts with the surface of the target and gets absorbed into surface layer; (ii) the removal of atomic species from the material is done by vaporization of the surface region in a non-equilibrium state; (iii) afterwards, rapid vaporization further produces a recoil pressure, which leads to the expulsion of the molten pool and produces the plasma plume, and the formed plasma is a collection of electrons, neutral atoms, ions, etc. Therefore, the absorption process is highly dependent on the target properties as well as the laser characteristics. Also, this absorption process is different for metals, insulators and semiconductors [55, 56]. When the laser beam interacts with the target, the photoenergy gets converted into electronic excitations immediately, and the energy relaxation through lattice takes place in ~1 picosecond (ps). Next, the photoenergy is transformed into heat diffusion (over a few microseconds (μs)), which results in the melting of the solid surface (in tens of ns). During the laser-target interactions, the localized temperature of the target reaches up to 10,000 OC or even higher, leading to the evaporation of the target material. At this point of time, the plume formation takes place (in the range of few μs). The plasma plume consists of atoms, electrons, ions and particulates of varying sizes, ranging from nanometer (nm) to micrometer (μm). This plasma reaches the substrate and undergoes re-solidification

In most of the cases, melting of a material depends on the rate of thermal conduction via lattice, which can be well described by the Fick's laws of diffusion. If the heated volume of the material is smaller than or equivalent to the thickness of ablated layer per laser pulse, then congruent melting will take place. Hence, PLD offers the advantage of congruent melting and vaporization. The amount of heated volume depends on the time of the laser-target interaction, i.e. the pulse duration. For a pulse duration of ~10 ps, heat diffusion will not play a role in the melting and vaporization of the material, whereas, above ~20 ps, conventional heat diffusion dependent ablation occurs [57]. Therefore, the use of a pulsed laser with a pulse duration of a few ns is more likely to provide congruent ablation. This allows the PLD process to preserve the anion-cation stoichiometry of the target material during the

*Ambient growth pressure:* The background pressure during deposition is a very important and critical parameter that plays a significant role in the plume collisions and plasma dynamics. Keeping the right background pressure is of utmost importance in order to obtain controlled stoichiometric products during the PLD growth. A specific phase and composition of a material can be achieved under controlled

**30**

#### *Practical Applications of Laser Ablation*

kapton substrates. The devices showed a good photoresponse towards UV light, with stable response and recovery in self-powered mode (**Figure 4a** and **b**). The origination of this self-powered behavior might be from the unintentional inhomogeneities in the contact electrodes [61, 62]. However, the observed response was very weak in the zero-biased mode. This work demonstrated that layered TMDCs can be promising candidates to be used as flexible devices in future photonic applications. In the meantime, researchers across the world started to explore the synthesis of other TMDCs by PLD. In 2015, Yao *et al*. [47] deposited multilayered WS2 by PLD and performed detailed and systematic investigations on its photodetection properties (**Figure 4c**–**f**). The synthesized device exhibited a broadband and reproducible photoresponse with good stability. The photoresponse in ambient conditions reached 0.51 AW−1, which was several orders higher than the CVD-grown WS2 thin films. In vacuum conditions, the responsivity was found to be enhanced to a value of 0.7 AW−1. The lower responsivity in ambient conditions has been explained on the basis of oxygen molecules adsorbed on the surface of WS2 which trap conduction electrons, and form O2 − . These species act as recombination centres for the photogenerated carriers. Thus, a greater

#### **Figure 4.**

*(a) MoS2 thin film deposited on flexible kapton substrate and (b) temporal response of the device in zero-bias mode. Figures have been reproduced with permission from Ref. [60]. (c) Cross-sectional schematic view of the WS2-based photoresistor, (d) temporal response of the device in air, (e) I-V curves of the WS2-based device in vacuum and in air, and (f) schematic of the photodetection mechanism based on adsorption and desorption of O2 molecules. Figures have been reproduced with permission from Ref. [47].*

**33**

responsivities and slower response speeds.

*Pulsed Laser Deposition of Transition Metal Dichalcogenides-Based Heterostructures…*

number of adsorbed O2 molecules are present in ambient air environment, which hampers the responsivity of the device. Furthermore, the device maintained a stable and reproducible photoswitching even after one-month of storage in air, indicating

The progress in the PLD growth of TMDCs has been quite significant, however, the crystal quality normally remains inferior when compared to the bulk natural crystals. This opens a window to further improve the device quality as well as the performance of PLD-fabricated PDs. One such work has been reported recently, where Wang *et al*. [63] have attained a dramatic improvement in the quality of the PLD-synthesized WS2 photoresistors by using a post-synthesis annealing procedure. With increase in the post-deposition annealing temperature from 310 to 610°C, the device performance parameters of the WS2-based PD (annealed at 610°C) enhanced by 2–3 orders of magnitude when compared to the devices annealed at lower temperatures. Annealing treatment usually provides a sufficient amount of energy and time to the atoms and molecules, for the structural reconstruction to annihilate crystal defects, and thus, has been adopted as a universal post-fabrication technique for improving the quality of the products. In another work, Yao *et al*. [64] have synthesized a hybrid WS2/Bi2Te3/SiO2/Si-based PD. The purpose of the insertion of the Bi2Te3 layer in between WS2 and SiO2 was to passivate the interface. The device demonstrated a stable, reproducible and broadband photoresponse (370 to

1550 nm). Moreover, the device showed a high photoresponsivity of 30.7 AW−1 and a pronounced specific detectivity of 2.3 × 1011 Jones with a rise time of 20 ms. The performance of the detector has been attributed to the surface passivation of SiO2 by the Bi2Te3 interfacial layer. SiO2 surface possess a lot of unscreened dangling bonds. When WS2 is directly deposited on SiO2, these bonds can introduce a large density of defects at the bottom of the WS2 layer, which will act as recombination and scattering centers for the photogenerated charge carriers. With the introduction of Bi2Te3 layer, these dangling bonds are greatly suppressed, and this results in the growth of WS2 film with high crystalline quality, which eventually enhances the PD's performance. Spectral range of a PD is equally important when compared with the other figures of merit and altering the effective wavelength range of TMDC-based PDs is extremely important for specific applications. It has been shown that by introducing defect states in the forbidden gap of a semiconductor, the detection range can be dramatically extended, and sub-band gap detection can be accomplished. Xie *et al*. [65] have demonstrated ultra-broadband MoS2-based PDs through PLD by forming sulfur vacancies in MoS2. The S/Mo atomic ratio was modified from 1.89 to 1.94 by controlling the number of laser pulses from 1200 to 300, resulting in a dramatic increase in the band gap of the semiconductor. Consequently, the S-deficient MoS2-based PD exhibited an unprecedented ultra-broadband detection range from 445 to 2717 nm. However, theoretical calculations have been done which indicated that the Mo vacancies in MoS2 possess a higher capability for narrowing the band gap. Therefore, in a subsequent work, Xie *et al*. [66] have synthesized a series of Mo-deficient MoS2-based PDs by moderating the target to substrate distance during the PLD growth. As a result of these modulations, the effective spectral range of a MoS2.17 PD spanned all over from 445 nm to 9536 nm. Although the detection range can be extended up to mid infrared (MIR) with the introduction of the defect states, however, it is accompanied by certain challenges that hinders the usability of this method in practical devices. These include the control over the creation of these defects, which is still an unresolved problem. Furthermore, the electronic properties of the charge carriers are severely hampered owing to the increased scattering effects from these defect states. Thus, such devices often suffer from meager

*DOI: http://dx.doi.org/10.5772/intechopen.94236*

the robustness of the device.

*Pulsed Laser Deposition of Transition Metal Dichalcogenides-Based Heterostructures… DOI: http://dx.doi.org/10.5772/intechopen.94236*

number of adsorbed O2 molecules are present in ambient air environment, which hampers the responsivity of the device. Furthermore, the device maintained a stable and reproducible photoswitching even after one-month of storage in air, indicating the robustness of the device.

The progress in the PLD growth of TMDCs has been quite significant, however, the crystal quality normally remains inferior when compared to the bulk natural crystals. This opens a window to further improve the device quality as well as the performance of PLD-fabricated PDs. One such work has been reported recently, where Wang *et al*. [63] have attained a dramatic improvement in the quality of the PLD-synthesized WS2 photoresistors by using a post-synthesis annealing procedure. With increase in the post-deposition annealing temperature from 310 to 610°C, the device performance parameters of the WS2-based PD (annealed at 610°C) enhanced by 2–3 orders of magnitude when compared to the devices annealed at lower temperatures. Annealing treatment usually provides a sufficient amount of energy and time to the atoms and molecules, for the structural reconstruction to annihilate crystal defects, and thus, has been adopted as a universal post-fabrication technique for improving the quality of the products. In another work, Yao *et al*. [64] have synthesized a hybrid WS2/Bi2Te3/SiO2/Si-based PD. The purpose of the insertion of the Bi2Te3 layer in between WS2 and SiO2 was to passivate the interface. The device demonstrated a stable, reproducible and broadband photoresponse (370 to 1550 nm). Moreover, the device showed a high photoresponsivity of 30.7 AW−1 and a pronounced specific detectivity of 2.3 × 1011 Jones with a rise time of 20 ms. The performance of the detector has been attributed to the surface passivation of SiO2 by the Bi2Te3 interfacial layer. SiO2 surface possess a lot of unscreened dangling bonds. When WS2 is directly deposited on SiO2, these bonds can introduce a large density of defects at the bottom of the WS2 layer, which will act as recombination and scattering centers for the photogenerated charge carriers. With the introduction of Bi2Te3 layer, these dangling bonds are greatly suppressed, and this results in the growth of WS2 film with high crystalline quality, which eventually enhances the PD's performance.

Spectral range of a PD is equally important when compared with the other figures of merit and altering the effective wavelength range of TMDC-based PDs is extremely important for specific applications. It has been shown that by introducing defect states in the forbidden gap of a semiconductor, the detection range can be dramatically extended, and sub-band gap detection can be accomplished. Xie *et al*. [65] have demonstrated ultra-broadband MoS2-based PDs through PLD by forming sulfur vacancies in MoS2. The S/Mo atomic ratio was modified from 1.89 to 1.94 by controlling the number of laser pulses from 1200 to 300, resulting in a dramatic increase in the band gap of the semiconductor. Consequently, the S-deficient MoS2-based PD exhibited an unprecedented ultra-broadband detection range from 445 to 2717 nm. However, theoretical calculations have been done which indicated that the Mo vacancies in MoS2 possess a higher capability for narrowing the band gap. Therefore, in a subsequent work, Xie *et al*. [66] have synthesized a series of Mo-deficient MoS2-based PDs by moderating the target to substrate distance during the PLD growth. As a result of these modulations, the effective spectral range of a MoS2.17 PD spanned all over from 445 nm to 9536 nm. Although the detection range can be extended up to mid infrared (MIR) with the introduction of the defect states, however, it is accompanied by certain challenges that hinders the usability of this method in practical devices. These include the control over the creation of these defects, which is still an unresolved problem. Furthermore, the electronic properties of the charge carriers are severely hampered owing to the increased scattering effects from these defect states. Thus, such devices often suffer from meager responsivities and slower response speeds.

*Practical Applications of Laser Ablation*

kapton substrates. The devices showed a good photoresponse towards UV light, with stable response and recovery in self-powered mode (**Figure 4a** and **b**). The origination of this self-powered behavior might be from the unintentional inhomogeneities in the contact electrodes [61, 62]. However, the observed response was very weak in the zero-biased mode. This work demonstrated that layered TMDCs can be promising candidates to be used as flexible devices in future photonic applications. In the meantime, researchers across the world started to explore the synthesis of other TMDCs by PLD. In 2015, Yao *et al*. [47] deposited multilayered WS2 by PLD and performed detailed and systematic investigations on its photodetection properties (**Figure 4c**–**f**). The synthesized device exhibited a broadband and reproducible photoresponse with good stability. The photoresponse in ambient conditions reached 0.51 AW−1, which was several orders higher than the CVD-grown WS2 thin films. In vacuum conditions, the responsivity was found to be enhanced to a value of 0.7 AW−1. The lower responsivity in ambient conditions has been explained on the basis of oxygen molecules adsorbed on the surface of WS2 which trap conduction electrons, and form O2

species act as recombination centres for the photogenerated carriers. Thus, a greater

*(a) MoS2 thin film deposited on flexible kapton substrate and (b) temporal response of the device in zero-bias mode. Figures have been reproduced with permission from Ref. [60]. (c) Cross-sectional schematic view of the WS2-based photoresistor, (d) temporal response of the device in air, (e) I-V curves of the WS2-based device in vacuum and in air, and (f) schematic of the photodetection mechanism based on adsorption and desorption of* 

*O2 molecules. Figures have been reproduced with permission from Ref. [47].*

− . These

**32**

**Figure 4.**

Recently, Jiao *et al*. [67] have synthesized high-quality and wafer-scale 2D layered MoS2 thin films by PLD. The device exhibited competitive device performance to the commercial Si and Ge-based PDs. The value of the responsivity was recorded to be 1.96 AW−1 for single layer MoS2-based device, under 300 nm light illumination (**Figure 5**). The PD shows a broadband photoresponse ranging from UV to NIR, with a fast response of 96 ms. This enhancement in the performance was attributed to the variation in the Schottky barrier at the Au/MoS2 interface.

The above discussed PLD-grown TMDC-based PDs are based on the metalsemiconductor-metal type device configuration, and suffer from relatively lower photoresponsivity, low on/off ratios, narrowband detection and slower detection speed. Hence, strategies are being developed to overcome these limitations. PDs having transparent electrodes such as indium tin oxide (ITO) and graphene, instead of the conventional metal contacts and PDs based on heterojunctions of two or more materials have many advantages such as low value of dark current, higher on/off current ratios, broadband detection range, and higher responsivities due to favorable band alignments.

One such work was carried out in 2016, when Zheng *et al*. [68] successfully prepared centimeter-scale and highly-crystalline WSe2 thin films on polyimide substrates by the technique of PLD and have fabricated high-performance PDs based on these WSe2 thin films. They obtained a broadband spectral response, ranging from 370 to 1064 nm. Moreover, a reproducible photoresponsivity approaching up to 0.92 AW−1, an EQE of 180% and a fast response speed of 0.9 s have also been achieved. The PD also exhibited excellent air durability and mechanical flexibility. The enhanced performance has been attributed to the good Ohmic contacts WSe2 forms with ITO, because of a low mismatch between the work functions of the two materials. Due to the Ohmic contacts, the carriers can be efficiently injected through the ITO electrodes under an applied bias, which will result in generation of a high photocurrent. Ohmic contacts lead to photodetection mechanisms based on the intrinsic properties of the photosensitive material under light irradiation.

Using a similar approach of integration of ITO electrodes on the device, Kumar *et al*. [69] have reported a UV PD which utilizes few layered MoS2 deposited by PLD. The device shows a high responsivity of 3 × 104 AW−1 and detectivity of 1.81 × 1014 Jones, at a nominal voltage of 2 V with fast response time of 32 ms. This performance is better than most of the reported devices based on 2D layered materials. The PD exhibited a very low value of dark current (~10−10 A) which is the reason behind such an excellent device performance. This may be

#### **Figure 5.**

*(a) Schematic of the interdigital patterned gold electrodes to form a metal-semiconductor-metal type contact, and (b) the wavelength dependence of responsivity (300 to 900 nm) for the device. Figures have been reproduced with permission from Ref. [67].*

**35**

**Figure 6.**

*reproduced with permission from Ref. [75].*

*Pulsed Laser Deposition of Transition Metal Dichalcogenides-Based Heterostructures…*

nitrogen gas, leading to lower number of sulfur vacancies.

because of suitable band alignment with the ITO electrodes as well as the deposition of high-quality films as the deposition was carried out in the presence of

Till now, all the above reported devices require some external bias for obtaining significant photodetection. Over the years, PDs which consume no external power have attracted a lot of attention because in the current scenario of energy crisis, a lot of research has been focused on energy producing and energy storage devices [70–74]. Such self-powered PDs depend on the interfacial built-in potential which enhances the effective separation of photogenerated carriers. This built-in electric potential also suppresses the dark current, which is another benefit for such PDs. Therefore, these self-powered devices have a great prospect

In 2015, Yao *et al*. [75] designed a Bi/WS2/Si heterojunction-based PD by depositing polycrystalline WS2 and Bi thin films on a p-type Si substrate by PLD (**Figure 6**). The PD exhibited a decent responsivity of 0.42 AW−1 and a high detectivity of 1.36 × 1013 Jones with ultrahigh sensitivity. It was observed that the performance of the Bi/Si heterointerface was enhanced by the insertion of the WS2 film. The enhanced device performance has been attributed to the effective passivation of the junction, and enhanced light absorption. Moreover, due to the favorable band alignment, WS2 acts as a selective carrier blocker, which further enhances the

Recently, Singh *et al*. [4] demonstrated an MoS2/AlN/Si-based PD grown by PLD, thus combining the excellent and unique properties of MoS2 with the matured technologies of Si and III-nitride semiconductors. Moreover, due to a large

*(a) Transient behavior of the Bi/Si and Bi/WS2/Si PDs under zero bias. (b) Corresponding single on-off cycle. Schematic of the energy band diagrams of the (c) Bi/Si and (d) Bi/WS2/Si heterointerfaces. Figures have been* 

*DOI: http://dx.doi.org/10.5772/intechopen.94236*

for the next-generation PDs.

device performance.

#### *Pulsed Laser Deposition of Transition Metal Dichalcogenides-Based Heterostructures… DOI: http://dx.doi.org/10.5772/intechopen.94236*

because of suitable band alignment with the ITO electrodes as well as the deposition of high-quality films as the deposition was carried out in the presence of nitrogen gas, leading to lower number of sulfur vacancies.

Till now, all the above reported devices require some external bias for obtaining significant photodetection. Over the years, PDs which consume no external power have attracted a lot of attention because in the current scenario of energy crisis, a lot of research has been focused on energy producing and energy storage devices [70–74]. Such self-powered PDs depend on the interfacial built-in potential which enhances the effective separation of photogenerated carriers. This built-in electric potential also suppresses the dark current, which is another benefit for such PDs. Therefore, these self-powered devices have a great prospect for the next-generation PDs.

In 2015, Yao *et al*. [75] designed a Bi/WS2/Si heterojunction-based PD by depositing polycrystalline WS2 and Bi thin films on a p-type Si substrate by PLD (**Figure 6**). The PD exhibited a decent responsivity of 0.42 AW−1 and a high detectivity of 1.36 × 1013 Jones with ultrahigh sensitivity. It was observed that the performance of the Bi/Si heterointerface was enhanced by the insertion of the WS2 film. The enhanced device performance has been attributed to the effective passivation of the junction, and enhanced light absorption. Moreover, due to the favorable band alignment, WS2 acts as a selective carrier blocker, which further enhances the device performance.

Recently, Singh *et al*. [4] demonstrated an MoS2/AlN/Si-based PD grown by PLD, thus combining the excellent and unique properties of MoS2 with the matured technologies of Si and III-nitride semiconductors. Moreover, due to a large

#### **Figure 6.**

*Practical Applications of Laser Ablation*

favorable band alignments.

material under light irradiation.

*reproduced with permission from Ref. [67].*

Recently, Jiao *et al*. [67] have synthesized high-quality and wafer-scale 2D layered MoS2 thin films by PLD. The device exhibited competitive device performance to the commercial Si and Ge-based PDs. The value of the responsivity was recorded to be 1.96 AW−1 for single layer MoS2-based device, under 300 nm light illumination (**Figure 5**). The PD shows a broadband photoresponse ranging from UV to NIR, with a fast response of 96 ms. This enhancement in the performance was attributed

The above discussed PLD-grown TMDC-based PDs are based on the metalsemiconductor-metal type device configuration, and suffer from relatively lower photoresponsivity, low on/off ratios, narrowband detection and slower detection speed. Hence, strategies are being developed to overcome these limitations. PDs having transparent electrodes such as indium tin oxide (ITO) and graphene, instead of the conventional metal contacts and PDs based on heterojunctions of two or more materials have many advantages such as low value of dark current, higher on/off current ratios, broadband detection range, and higher responsivities due to

One such work was carried out in 2016, when Zheng *et al*. [68] successfully prepared centimeter-scale and highly-crystalline WSe2 thin films on polyimide substrates by the technique of PLD and have fabricated high-performance PDs based on these WSe2 thin films. They obtained a broadband spectral response, ranging from 370 to 1064 nm. Moreover, a reproducible photoresponsivity approaching up to 0.92 AW−1, an EQE of 180% and a fast response speed of 0.9 s have also been achieved. The PD also exhibited excellent air durability and mechanical flexibility. The enhanced performance has been attributed to the good Ohmic contacts WSe2 forms with ITO, because of a low mismatch between the work functions of the two materials. Due to the Ohmic contacts, the carriers can be efficiently injected through the ITO electrodes under an applied bias, which will result in generation of a high photocurrent. Ohmic contacts lead to photodetection mechanisms based on the intrinsic properties of the photosensitive

Using a similar approach of integration of ITO electrodes on the device, Kumar *et al*. [69] have reported a UV PD which utilizes few layered MoS2 depos-

ity of 1.81 × 1014 Jones, at a nominal voltage of 2 V with fast response time of 32 ms. This performance is better than most of the reported devices based on 2D layered materials. The PD exhibited a very low value of dark current (~10−10 A) which is the reason behind such an excellent device performance. This may be

*(a) Schematic of the interdigital patterned gold electrodes to form a metal-semiconductor-metal type contact, and (b) the wavelength dependence of responsivity (300 to 900 nm) for the device. Figures have been* 

AW−1 and detectiv-

ited by PLD. The device shows a high responsivity of 3 × 104

to the variation in the Schottky barrier at the Au/MoS2 interface.

**34**

**Figure 5.**

*(a) Transient behavior of the Bi/Si and Bi/WS2/Si PDs under zero bias. (b) Corresponding single on-off cycle. Schematic of the energy band diagrams of the (c) Bi/Si and (d) Bi/WS2/Si heterointerfaces. Figures have been reproduced with permission from Ref. [75].*

**Figure 7.** *(a) Spectral response of the MoS2/AlN/Si-based PD. (b) Schematic of the deep defect states-modulated carrier transport in MoS2/AlN/Si-based device. Figures have been reproduced with permission from Ref. [4].*

difference in the work functions of these materials, the band bending of the heterojunction at the interfaces resulted in self-powered behavior. The vertical transport of the PD exhibited an exceptional broadband photoresponse (300–1100 nm) in the self-powered mode. The device shows a responsivity of 9.93 AW−1 under zerobias condition with ultrafast response speeds (response/recovery times - 12.5/14.9 μs). The photoresponse of MoS2/Si has also been given, to show the importance of inserting the AlN layer. The MoS2/Si PD exhibits a responsivity of 1.88 AW−1 (~5 times less) in self-powered mode. The authors have shown that the native oxygen defects are present throughout the AlN layer, and this has been confirmed with the help of X-ray photoelectron spectroscopy and transmission electron microscopy. These oxygen impurities form deep donor states in AlN and modulate the transport of the charge carriers, and this leads to the enhanced performance of the MoS2/AlN/ Si-based device (**Figure 7**).

### **5. Summary**

The past few years have undoubtedly witnessed tremendous advances in the PLD growth of TMDCs and their applications in the field of photodetection. In this chapter, the basic properties of TMDCs and the common growth techniques employed for their fabrication have been reviewed briefly, followed by a detailed and elaborated discussion about PLD and the important parameters associated with it. Finally, a progressive investigation about the PDs based on TMDCs fabricated through PLD has been discussed. These extensive achievements in the field of photodetection have unquestionably established PLD as one of the most competitive and reliable methods for fabricating industrialscale and high-quality TMDCs. PLD, therefore, certainly has a lot of potential to contribute in the development of the next-generation TMDCs-based PDs in the future.

## **6. Looking into the future**

Based on the analysis of the previous reports in this area, an outlook regarding the future of PLD-synthesized TMDCs and the related follow-up research work has been summarized below.

**37**

*Pulsed Laser Deposition of Transition Metal Dichalcogenides-Based Heterostructures…*

• The research on the PLD-fabricated TMDCs-based PDs is still in its nascent stage and therefore, there is still a room for improvement in the crystal-quality of the PLD-grown thin films, by selecting appropriate substrates and by further tuning and optimizing the various unexplored growth parameters, such as annealing temperature and time, cooling ramp rate, geometry of the laser spot, surface morphology of the source targets, laser frequency, and so

• Apart from a few reports, most of the research regarding PLD growth of

• The use of transparent 2D semiconductors such as graphene or graphene derivatives and semi-metallic phase of TMDCs, can be used in place of the conventional metal electrodes, as they maximize the area of light absorption

• Till date, researchers and scientists across the world have mostly exploited heterojunctions of TMDCs in their thin film forms. Heterointerfaces based on one dimensional (1D) nanostructures may provide new routes for the development of high-performance devices. The nanowire-based heterostructures of the TMDCs with growth along these nanowires or a core-shell structure will enable a much higher surface to volume ratio and therefore, a larger active interface. This will lead to enhancement in the photoresponse and superior

As a concluding statement, PLD has been proven to be a promising synthesis technique for TMDCs for applications in photodetection, and these PDs have shown outstanding performance that can compete with those of the commercially available PDs. The fabrication through PLD is cost-effective and scalable, and hence, PLD is a perfect tool for fabrication of practical devices for optoelectronic applications at an

D.K.S. is thankful to Council of Scientific and Industrial Research, Government of India, New Delhi for providing senior research fellowship. S.B.K. acknowledges

of III-nitrides, which would lead to better device performance.

along with having outstanding electronic properties.

optoelectronic performance.

industrial scale production.

INSA senior scientist fellowship.

**Acknowledgements**

TMDCs for photodetection application is based on the use of a single photosensitive material. Therefore, promising results are expected on the exploration of the heterojunctions of these layered materials with established bulk semiconductors like III-nitrides, which have shown great results in this field. Moreover, TMDCs can serve as excellent substrates for high quality and epitaxial growth

*DOI: http://dx.doi.org/10.5772/intechopen.94236*

on [18].

*Pulsed Laser Deposition of Transition Metal Dichalcogenides-Based Heterostructures… DOI: http://dx.doi.org/10.5772/intechopen.94236*


As a concluding statement, PLD has been proven to be a promising synthesis technique for TMDCs for applications in photodetection, and these PDs have shown outstanding performance that can compete with those of the commercially available PDs. The fabrication through PLD is cost-effective and scalable, and hence, PLD is a perfect tool for fabrication of practical devices for optoelectronic applications at an industrial scale production.
