**3. Basics of pulsed laser deposition**

PLD falls under the category of physical vapor deposition and is a method used to synthesize materials (generally thin films) in an ultrahigh vacuum environment. The development in the field of laser-assisted film growth can be traced back to 1960, after the successful technical realization of the first laser by Maiman [52]. Following on from there, from just being a growth method for fundamental laboratory research, PLD has moved on to become a technique employed in industries. A typical PLD system mainly consists of a laser (usually an excimer laser), an optical path system consisting of apertures, lenses and reflectors, and a stainless-steel

**29**

**Figure 3.**

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

Research Centre, Indian Institute of Science, Bangalore, India.

growth chamber equipped with gas paths, vacuum pumps, vacuum gauges, and a heating source [37]. The basic principle behind a PLD process is that a high-intensity pulsed laser interacts with the target or the source material (called ablation) and produces a plasma plume of the target material [37]. The formation of plasma involves a sequence of complex phenomena such as collision, localized heating and subsequent ionization of atoms and molecules. Afterwards, the plasma plume expands, travels downstream, condenses on the substrates and finally crystallizes into the desired materials [37]. **Figure 3** shows the PLD setup located in Materials

The major advantage of a PLD system is that the laser can be operated from outside the vacuum chamber. Thus, just by changing the optical paths of the laser beam, a single laser source can be used for multiple deposition systems. All the other components such as the target carousel, substrate holder, heater, vacuum gauges, and so on are mounted in the vacuum chamber. A set of optical components such as apertures and mirrors are used to focus the pulsed laser beam over the target surface. Therefore, a variety of materials (semiconductors, metals and insulators) can be grown by PLD just by optimizing the growth parameters and by incorporation of different gases during the process in a controlled manner. With this, the exact stoichiometry of the target material can be copied down onto the substrates, which is one of the major benefits of PLD over other deposition techniques. Some of

the important parameters associated with PLD have been described below.

*PLD setup in Materials Research Centre, Indian Institute of Science, Bangalore, India.*

*Laser source:* A krypton fluoride (KrF) laser is a type of excimer laser, and with a wavelength of 248 nm, it is a deep UV laser which is commonly used for the growth of various thin films as the absorption spectrum of most of the inorganic materials lies in the range of 200–400 nm. Typically, excimer lasers contain a mixture of two gases: a noble gas such as argon, xenon or krypton; and a halogen such as chlorine or fluorine. Under suitable conditions of stimulation and pressure, an excimer molecule is created, which decays via a stimulated emission and a coherent beam of stimulated radiation is emitted in the UV range. The continuous emission is

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

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

growth chamber equipped with gas paths, vacuum pumps, vacuum gauges, and a heating source [37]. The basic principle behind a PLD process is that a high-intensity pulsed laser interacts with the target or the source material (called ablation) and produces a plasma plume of the target material [37]. The formation of plasma involves a sequence of complex phenomena such as collision, localized heating and subsequent ionization of atoms and molecules. Afterwards, the plasma plume expands, travels downstream, condenses on the substrates and finally crystallizes into the desired materials [37]. **Figure 3** shows the PLD setup located in Materials Research Centre, Indian Institute of Science, Bangalore, India.

The major advantage of a PLD system is that the laser can be operated from outside the vacuum chamber. Thus, just by changing the optical paths of the laser beam, a single laser source can be used for multiple deposition systems. All the other components such as the target carousel, substrate holder, heater, vacuum gauges, and so on are mounted in the vacuum chamber. A set of optical components such as apertures and mirrors are used to focus the pulsed laser beam over the target surface. Therefore, a variety of materials (semiconductors, metals and insulators) can be grown by PLD just by optimizing the growth parameters and by incorporation of different gases during the process in a controlled manner. With this, the exact stoichiometry of the target material can be copied down onto the substrates, which is one of the major benefits of PLD over other deposition techniques. Some of the important parameters associated with PLD have been described below.

*Laser source:* A krypton fluoride (KrF) laser is a type of excimer laser, and with a wavelength of 248 nm, it is a deep UV laser which is commonly used for the growth of various thin films as the absorption spectrum of most of the inorganic materials lies in the range of 200–400 nm. Typically, excimer lasers contain a mixture of two gases: a noble gas such as argon, xenon or krypton; and a halogen such as chlorine or fluorine. Under suitable conditions of stimulation and pressure, an excimer molecule is created, which decays via a stimulated emission and a coherent beam of stimulated radiation is emitted in the UV range. The continuous emission is

*Practical Applications of Laser Ablation*

sapphire substrates using PLD, varying the thickness from 60 monolayers down

In the past few years, TMDCs have elicited tremendous research interest, owing to their novel properties in the 2D form, which has triggered a spark in the growth of TMDCs using PLD [18]. In this part of the chapter, a few reports describing the chronological developments in the area of PLD grown TMDCs have been briefly discussed. One of the earliest works on PLD deposited MoS2 was reported by Zabinski *et al*. [40], where they have prepared PbO-MoS2 thin films for tribological applications. Other early reports on MoS2 thin films by PLD include growth of amorphous MoS2 by McDevitt *et al*. [41] and MoS2 coatings for friction-related studies by Mosleh *et al*. [42]. The major advancements in this area have been achieved in the past few years. In 2010, Fominski *et al*. [43] have experimentally studied the fabrication of MoSex thin films with varying compositions obtained by PLD in vacuum condition and in presence of different rarefied gases. They also developed a processbased mathematical model which played a dominant role on the chemical composition of these thin films. Loh *et al*. [44] in 2014 fabricated MoS2 on different metals such as Ag, Ni, Al, and Cu by PLD. In 2015, a significant step towards the fabrication of transfer-free TMDC-based PDs was demonstrated by Serrao *et al*. [45] in which MoS2 was directly deposited on substrates like sapphire, SiC-6H and GaN. The first significant work towards the wafer-scale growth of TMDCs by PLD was done by Siegel *et al*. [32] who reported growth of centimeter-scale MoS2 thin films of varying thickness (from monolayer to 60 monolayers). Growth of other members of the TMDC family such as WS2 was also investigated simultaneously. In 2015, Loh *et al*. [46] synthesized WS2 thin films on Ag substrates by PLD and reported the thickness dependent Raman and PL spectra. However, the obtained WS2 was having a mixed phase (1T and 2H). In subsequent works, Yao *et al*. [47] obtained the pure 2H phase WS2 directly by PLD on insulating SiO2/Si substrates. The growth of selenides via PLD usually suffers from a large number of Se vacancies. Therefore, a two-step growth method was adopted for production of MoSe2 [48]. This included deposition of MoO3 film via PLD, followed by its selenization. Later on, Mohammed *et al*. [49] achieved 1–8 monolayers of WSe2 via single-step deposition. This was achieved through a hybrid PLD cluster, where a tungsten target was ablated by the laser beam and selenium vapors were synchronously provided from an effusion cell by thermal evaporation. In 2018, a single-step PLD approach was used by Seo *et al*. [50] to deposit WSe2 on Al2O3 and SiO2/Si substrates by using a Se-rich target. Lately, Gao *et al*. [51] demonstrated a two-step synthesis route to fabricate 2D WTe2, which included PLD of amorphous WTe2 target followed by annealing treatment of the thin films in a Te atmosphere. These reports suggest that the technique of PLD can be suitably applied for the successful production of various TMDCs. In the next section, we will discuss in detail about the fundamentals associated with PLD.

PLD falls under the category of physical vapor deposition and is a method used to synthesize materials (generally thin films) in an ultrahigh vacuum environment. The development in the field of laser-assisted film growth can be traced back to 1960, after the successful technical realization of the first laser by Maiman [52]. Following on from there, from just being a growth method for fundamental laboratory research, PLD has moved on to become a technique employed in industries. A typical PLD system mainly consists of a laser (usually an excimer laser), an optical path system consisting of apertures, lenses and reflectors, and a stainless-steel

to a single monolayer, just by tuning the number of laser pulses.

**28**

**3. Basics of pulsed laser deposition**

then converted into a pulse by various discharge mechanisms and a pulse width of ~10–20 nanoseconds (ns) can be achieved.

*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].

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

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

*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

**31**

substrate [57].

of industrial and practical applications.

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

highly energetic species in the dilating plasma plume [58].

be kept in the case of depositions carried out at high pressures.

ultra-smooth thin films can be obtained at optimized deposition rates.

**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

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

and optimized conditions of background pressure at specific temperature. Plasma species with kinetic energies greater than 50 eV can re-sputter the material already deposited on the substrate and this usually leads to a lower deposition rate, modifications in the stoichiometry of the film, and increase the surface roughness. Controlling the background pressure can reduce re-sputtering of the deposited thin films. Increasing the background gas pressure to an optimum value slows down the

*Target-substrate distance:* The target to substrate distance is a useful parameter for reducing the particulate formation since majority of the PLD depositions are carried out in high pressure conditions. If the thin film is deposited in vacuum environment, the target to substrate distance mainly affects the angular spread of the ejected flux. Thus, the effect of target to substrate distance and the background gas pressures is inter-related. The plume length decreases as the ambient gas pressure increases, because of the increased collisions between the plume species and background gas molecules. Therefore, a smaller target to substrate distance should

*Deposition rate:* This mainly depends on the repetition rate or the frequency of laser shots which controls the volume or amount of the plume species reaching the substrate and, thus, controls the thickness of the deposited thin film. Deposition rate also depends on the background gas pressure as described previously and mainly modulates the super-saturation process during deposition, which has an influence on the critical nucleation point and the density of nucleation sites. Also,

*Substrate temperature:* Substrate temperature plays a critical role in terms of the diffusion barrier during the growth process and strongly affects the growth modes. It influences the nucleation process as well as the mobility of the condensed species across the substrate, and therefore, is crucial in deciding the phase boundary in the crystalline thin films during PLD growth. At lower substrate temperatures, the thin film produced may be amorphous or polycrystalline due to the lower nucleation rate, as the thermal energy provided is too small for overcoming the nucleation barrier. When the substrate temperature is too high, the nucleation rate gets limited due to the high rate of atomic exchange between the solid and gaseous species. Thus, an optimum temperature is required for the easy crystallization of thin films as it becomes easier to overcome the nucleation barrier and form nuclei on the

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

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

and optimized conditions of background pressure at specific temperature. Plasma species with kinetic energies greater than 50 eV can re-sputter the material already deposited on the substrate and this usually leads to a lower deposition rate, modifications in the stoichiometry of the film, and increase the surface roughness. Controlling the background pressure can reduce re-sputtering of the deposited thin films. Increasing the background gas pressure to an optimum value slows down the highly energetic species in the dilating plasma plume [58].

*Target-substrate distance:* The target to substrate distance is a useful parameter for reducing the particulate formation since majority of the PLD depositions are carried out in high pressure conditions. If the thin film is deposited in vacuum environment, the target to substrate distance mainly affects the angular spread of the ejected flux. Thus, the effect of target to substrate distance and the background gas pressures is inter-related. The plume length decreases as the ambient gas pressure increases, because of the increased collisions between the plume species and background gas molecules. Therefore, a smaller target to substrate distance should be kept in the case of depositions carried out at high pressures.

*Deposition rate:* This mainly depends on the repetition rate or the frequency of laser shots which controls the volume or amount of the plume species reaching the substrate and, thus, controls the thickness of the deposited thin film. Deposition rate also depends on the background gas pressure as described previously and mainly modulates the super-saturation process during deposition, which has an influence on the critical nucleation point and the density of nucleation sites. Also, ultra-smooth thin films can be obtained at optimized deposition rates.

*Substrate temperature:* Substrate temperature plays a critical role in terms of the diffusion barrier during the growth process and strongly affects the growth modes. It influences the nucleation process as well as the mobility of the condensed species across the substrate, and therefore, is crucial in deciding the phase boundary in the crystalline thin films during PLD growth. At lower substrate temperatures, the thin film produced may be amorphous or polycrystalline due to the lower nucleation rate, as the thermal energy provided is too small for overcoming the nucleation barrier. When the substrate temperature is too high, the nucleation rate gets limited due to the high rate of atomic exchange between the solid and gaseous species. Thus, an optimum temperature is required for the easy crystallization of thin films as it becomes easier to overcome the nucleation barrier and form nuclei on the substrate [57].
