Large area MoSe2 and MoSe2/Bi2Se3 films on sapphire (0001) for near-infrared photodetection

The fabrication of heterojunction-based photodetectors (PDs) is well known for the enhancement of PDs performances, tunable nature of photoconductivity, and broadband application. Herein, the PDs based on MoSe2 and MoSe2/Bi2Se3 heterojunction on sapphire (0001) substrates were deposited using a r.f. magnetron sputtering system. The high-resolution x-ray diffraction and Raman spectroscopy characterizations disclosed the growth of the 2-H phase of MoSe2 and the rhombohedral phase of Bi2Se3 thin films on sapphire (0001). The chemical and electronic states of deposited films were studied using x-ray photoelectron spectroscopy and revealed the stoichiometry growth of MoSe2. We have fabricated metal-semiconductor–metal type PD devices on MoSe2 and MoSe2/Bi2Se3 heterojunction and the photo-response measurements were performed at external voltages of 0.1–5 V under near-infrared (1064 nm) light illumination. The bare MoSe2 PD device shows positive photoconductivity behavior whereas MoSe2/Bi2Se3 heterojunction PD exhibits negative photoconductivity. It was found that the responsivity of MoSe2 and MoSe2/Bi2Se3 heterojunction PDs is ~ 1.39 A W−1 and ~ 5.7 A W−1, respectively. The enhancement of photoresponse of MoSe2/Bi2Se3 PD nearly four-fold compared to bare MoSe2 PD shows the importance of heterojunction structures for futuristics optoelectronic applications.


Introduction
Transition metal dichalcogenides (TMDs), such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 , have attracted a lot of interest recently as one of the most significant members of the two-dimensional (2D) materials family due to their exceptional electrical, and optical characteristics [1][2][3][4].It has been demonstrated that MoS 2 , MoSe 2 and WS 2 can absorb up to 5%-10% of incident sunlight at thicknesses below 1 nm [5].The bulk band gap of MoS 2 and MoSe 2 was reported of ∼ 1.3 and ~1.1 eV (indirect) whereas the monolayer MoS 2 and MoSe 2 have direct band nature with bandgap of ∼ 1.9 and ~1.66 eV, respectively [6,7].The large bandgap and higher carrier lifetimes of TMDs like MoS 2 and MoSe 2 make them attractive candidates for high-sensitivity photodetectors (PDs) [8][9][10][11][12][13][14][15].Advantage of MoSe 2 is related to well matched bulk band gap with Si which can be used for various optoelectronic applications under near-infrared (NIR) region.A few works have been reported for PDs based on mono/multilayer MoSe 2 for NIR regions.Recently, Polumati et al, fabricated a MoSe 2 /Mxene/cellulose paperbased photodetector device synthesized by a three-step hydrothermal method and exhibited responsivity of 9.82 mA/W under NIR light illumination [16].Ko et al, synthesized few-layer flake MoSe 2 on SiO 2 /Si substrate by mechanical exfoliation method, fabriacted back-gated phototransistor and found peak responsivity of 238 A/ W under NIR excitation [17].Selamneni et al, fabrication of large area MoSe 2 nanoflowers on cellulose paper using hydrothermal synthesis deposition and found responsivity of 9.73 mA W −1 at NIR light illumination [18].
Jana et al, fabricated MoSe 2 nanoflakes/ZnO nanorods (NR) NIR-based heterostructure device by using the liquid-phase exfoliation method and they have found responsivity of 0.21 A/W under NIR light illumination [19].The stacked-layered MoSe 2 /Si heterojunction was fabricated by the pulsed laser deposition method, and the responsivity was reported to be ∼ 12.8 mA/W under NIR light illumination [20].These reported work clearly demonstrated that the MoSe 2 is well-suited material for PD applications in the NIR region [16][17][18][19][20].
Among various approaches, one of the approach to enhance the photoresponse of PDs is to develop the heterojunctions among various semiconducting materials [21][22][23].Most of the heterojunction and hybrid structures on MoSe 2 have fabricated using conventional semiconductors and organic materials for PD applications in NIR region [16][17][18][19][20].In the quest for new materials, group V-VI binary chalcogenide materials such as Bi 2 Se 3 , Sb 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 have recently been attracted due to their good optical and electrical characteristics [24,25].Among these, Bi 2 Se 3 is one of the well-studied topological insulator (TI) materials that exhibit insulation in bulk while the surface shows a conducting nature [25].Also, Bi 2 Se 3 being a layered material, there is mild van der Waals force between the quintuple layers (QLs) and strong covalent bonding inside each QL, the van der Waals layered growth of Bi 2 Se 3 is advantageous for making the buffer layer [26][27][28].Recently, Bi 2 Se 3 has been integrated with the wide-band gap semiconductor materials to enhance the PDs characteristics [29,30].However, there is limited work on the growth and their applications in fabrication of PDs using Bi 2 Se 3 with MoSe 2 material [31].
High-quality MoSe 2 and Bi 2 Se 3 layers and thin films have been produced using various methods such as liquid exfoliation, scotch tape, physical vapor deposition, hydrothermal, etc [17,19,31].The exfoliation techniques have limitations due to the long process time, repeatability issue, and inability to produce large-area coverage.In contrast, the sputtering technique provides many advantages such as ease of handling, repeatability, and the ability to deposit large areas of thin films [29,30].In this study, we have deposited MoSe 2 film and MoSe 2 /Bi 2 Se 3 heterojunction on sapphire (0001) substrates using r.f.magnetron sputtering system.The sapphire (0001) substrates were preferred over conventional low bandgap Si and Ge substrates as sapphire possesses larger bandgap (∼10 eV), far from NIR region.We have found a high NIR photoresponsivity of ∼ 5.7 A W −1 on MoSe 2 /Bi 2 Se 3 heterojunction PD device as compared to the responsivity of ∼ 1.39 A W −1 on bare MoSe 2 PD device.Interestingly, the MoSe 2 /Bi 2 Se 3 heterojunction PD showed the negative photoconductivity (NPC) behavior whereas sole MoSe 2 PD revealed the positive photoconductivity under NIR (1064 nm) illumination.

Materials synthesis
We have deposited MoSe 2 thin film on bare sapphire (0001) substrate [sample S1] and Bi 2 Se 3 coated sapphire (0001) substrate [sample S2] at 400 °C using a magnetron sputtering system, having a base vacuum of ∼ 2 × 10 −7 mbar.First, we cleaned the single-side polished sapphire substrate with acetone and isopropanol in an ultrasonicator for several minutes followed by drying with N 2 gas.For deposition of MoSe 2 , the stoichiometry MoSe 2 (purity: 99.99%) target was sputtered by applying a forward r.f.power of 100 W in the presence of ultrahigh-pure Ar (99.9999%) gas flow of 20 sccm (working pressure: ∼5.0 × 10 −3 mbar).In the case of 40 nm thick Bi 2 Se 3 buffer layer deposition, the working pressure and forward r.f.power was kept at ∼ 3.3 × 10 −3 mbar and 10 W, respectively.The deposition rate of sputtered film has been deduced by stylus profilometer on various films deposited under different conditions.To achieve good stoichiometry of sputtered films, a post-selenization process was performed in a tubular furnace at 300 °C for 60 minutes in presence of continuous Ar flow.

Materials characterization
Raman spectroscopy in backscattering geometry with an Ar + laser (532 nm) source was employed to characterize the structural properties of MoSe 2 and MoSe 2 /Bi 2 Se 3 thin films on sapphire (0001) substrates.A Cu K α1 x-ray source with a wavelength of 0.15406 nm was used in the high-resolution x-ray diffraction (HR-XRD) to characterize the crystalline properties of thin films.Atomic force microscopy (AFM) in tapping mode and field emission scanning electron microscopy (FESEM) in plan-view were used to study the surface morphology.Thermofisher K-Alpha x-ray photoelectron spectroscopy (XPS) having x-ray source [Al K α :1486.6 eV] was used to analyze the electronic and chemical composition of MoSe 2 and MoSe 2 /Bi 2 Se 3 thin films on sapphire substrates.

Photo-response performance measurement
We have deposited Cr/Au metal electrodes to fabricate metal-semiconductor-metal (MSM)-type PD devices on MoSe 2 (S1) and MoSe 2 /Bi 2 Se 3 (S2) heterojunction.The metal electrode was comprised of a sequential deposition of a ∼ 20 nm Cr adhesion layer followed by ∼ 80 nm Au coating using the thermal evaporation technique.The active area for both PD devices were kept same ∼ 0.01 mm 2 and having resistance in the range of ∼0.5-2 kΩ.To measure the current-voltage characteristics of the devices under NIR (1064 nm) laser illumination with external bias voltage, we employed a Keithley 2450 source meter.We also conducted investigations on the transient photoresponse by using 20-second ON-OFF cycles at different external voltages ranging from 0.1 to 5 V at a fixed laser power of 125 mW.We have also performed spectral response measurement with a Xenon lamp having wavelength in range of 400-1200 nm at applied bias of 5 V.

Results and discussion
Figure 1(a) represents the Raman spectrum of MoSe 2 thin film deposited on sapphire (0001) substrate [sample S1] at 400 °C using a magnetron sputtering system.Raman characteristic peaks of sample S1 were obtained at 169.6, 236.2, and 287.7 cm −1 corresponding to the E 1g (in-plane), A 1g (out-of-plane), and E 1 2g (in-plane) modes for 2H phase of the MoSe 2 [32,33].Remaining two Raman peaks centered at 445.5 and 577.5 cm −1 are related to the sapphire substrate.The Raman spectrum of MoSe 2 thin film on Bi 2 Se 3 /sapphire (0001) substrate [sample S2] is shown in figure 1(b).For sample S2, the three Raman active modes were observed for Bi 2 Se 3 which were present at 76.1.129.8 and 176.4 cm −1 corresponding to A 1 1g , E 2 g and A 2 1g optical phonon modes, respectively [34].The three Raman peaks at 169.2, 237.3 and 287.6 cm −1 are assigned to A 1 1g , E 1 1g , and E 1 2g vibrational Raman modes, respectively related to the 2H-phase of MoSe 2 , and the remaining Raman peaks are indexed to sapphire substrate, similar to sample S1 [32,33].
Further, the crystalline properties of MoSe 2 film deposited on bare and Bi 2 Se 3 coated sapphire (0001) substrates were characterized with HR-XRD.The 2theta-omega scan of samples S1 and S2 is shown in figures 1(c) and (d), respectively.The XRD peaks of sample S1 were found at 12.54, 22.56, and 29.08°position that could be indexed to the (002), (104), and (010) lattice planes of MoSe 2 and related to its hexagonal crystal The surface morphology of sputtered MoSe 2 thin films was characterized by FESEM in plan view mode.Figure 2(a) displays the large area FESEM image of sample S1 in which it can be seen the worm-type MoSe 2 structure.Further, the high magnification FESEM image (figure 2(b)) clearly showed the worm type with platelets and statistical analysis revealed the lateral size of ∼ 100±10 nm.The long and relatively wide worm-type surface morphology of the sample S2 is also seen with the increased lateral size of ∼130±10 nm (figure 2(c)).The platelets of MoSe 2 is quite visible in high magnification image presented in figure 2(d).Further, we have also performed AFM characterization using Si-tips of radii curvature of ∼ 10 nm in tapping mode on these samples and sample S1 reveals the worm-type morphology [figure 3(a)).The lateral sizes of worm-type structure by AFM image seems wider compared to FESEM and it is likely due to the tip effect in lateral directions.However, the AFM images are taken to estimate the surface roughness and thickness of the film as z-direction measurement is independent to the AFM tip shape, size and geometry.The rms surface roughness of the S1 sample is obtained to be ∼ 8.76 nm for a scan area of 2 μm × 2 μm.It can be noted that the bare clean sapphire substrate used in this study has rms surface roughness of ∼ 0.6 nm for a scan area of 2 μm × 2 μm.Further, thickness of MoSe 2 film on sapphire was measured using line profiles across the various pits on the sample S1. Figure 3(b) shows that the height of the film from the sapphire surface was found to be ∼ 40 nm, complements with our calibration for growth rate.In case of MoSe 2 on Bi 2 Se 3 coated sapphire, the longer worm-type morphology was also seen in the AFM image (figure 3(c)).The rms surface roughness of sample S2 was found to be ∼14.6 nm for an AFM scan area of 2 μm × 2 μm (figure 3(c)), slightly rougher than the S1 sample, likely due to the large size of the wormtype structure of sample S2.These observations disclosed that the Bi 2 Se 3 buffer layer promotes the growth of longer and wider worm-type MoSe 2 morphology on sapphire (0001) substrate.[32].The Se/Mo ratio was obtained using a tabulated sensitivity factor and it turned out to be ∼1.94,close to the ideal ratio i.e. 2 [31].
The core level XPS spectra of MoSe   energy (blue shift) as compared to elemental Bi 4f 5/2 and 4f 7/2 peaks at 161.9 and 156.6 eV, respectively [33][34][35].These peaks are further deconvoluted into two more peaks at positions 164.2 and 159.0 eV corresponding to Bi +5 oxidation states of the Bi 4 f state.One peak at 162.4 eV corresponds to the Bi +3 .The oxidation states of Bi 2 Se 3 appear likely due to the ex situ XPS measurements after deposition of the film [35][36][37][38].The fitted corelevel Se 3d spectrum shows two peaks with binding energies of 54.7 and 53.9 eV that could be distributed to Se 3d 3/2 and Se 3d 5/2 , respectively and it could be assigned to the valence state of Se(2) in MoSe 2 and Bi 2 Se 3 compounds [32,35,36].
Further, we have fabricated MSM PDs on samples S1 and S2 and the schematic of the devices is presented in figures 5(a) and (b), respectively.The spectral photoresponse was measured for both the PD devices at fixed bias volatge of 5 V in the wavelength range of 400-1200 nm as presented in figure 5(c).The high photoresponse was obsereved in the wavelength region of 1000-1100 nm revealing its application in NIR photodetection [15].The We have performed voltage-dependent time-resolved photoresponse characteristics at a fixed laser power of 125 mW in the NIR (1064 nm) region as shown in figures 7(a) and (c) for devices S1 and S2, respectively.In the device S1, only the MoSe 2 material absorbs NIR light and generates electron-hole pairs which are separated by the externally applied electric field and contribute to the photocurrent.On the other hand, in device S2, both materials absorb NIR light to produce photocurrent.It was observed that photocurrent value increases with an increase in the external applied bias voltage from 0.1 to 5 V at a fixed maximum laser power of 125 mW by NIR light illumination and it shows NPC behavior.Similar behavior is also obtained for the multilayer graphene/ InSe heterojunction PD device compared to sole InSe PD [39].In the case of MoS 2 /GaN/Si PD device, Singh et al reported the change in the polarity of current by changing the wavelength of the illumination light from ultra-violet (positive) to NIR (negative photocurrent) [40].
For quantitative analysis, the following equations (1-4) have been used to calculate the performance parameters of a photodetector such as a responsivity, specific detectivity, noise equivalent power (NEP) and external quantum efficiency (EQE).
where, I l is the light current, I dark is the dark current, P in is the input optical power density, A is the active area of the device, e is the elementary charge, h is Planck's constant, and c is the speed of light in vacuum, and λ is wavelength of incident light [41].
The performance parameters R and D are plotted in figures 7(b) and (d) with applied bias under illumination of 1064 nm light for devices S1 and S2, respectively.The responsivity and detectivity of device S1 were found to be ~1.39A W −1 and 2.18 × 10 8 Jones at 5 V in PPC behavior, respectively.In the case of device S2, the responsivity and detectivity have been increased and it was found to be ~5.7 A W −1 and 2.91 × 10 8 Jones at 5 V, respectively.The values of NEP and EQE calculated at a bias voltage of 5 V and maximum laser power of 125 mW are 4.72 × 10 −11 W × Hz −1/2 and 162% for device S1 and 3.43 × 10 −11 W × Hz −1/2 and 664% for device S2, respectively.It is clearly seen that the photoresponse of the both S1 and S2 devices increased with an increase in applied bias voltage at fixed laser power.When the external bias increased, subsequently, the electric field across the device also enhanced.Consequently, it increases the charge collection efficiency on the electrodes by minimizing the e-h pair recombination.The value of response (decay) time of devices S1 and S2 was estimated to be 2.02 (3.15) sec and 1.12 (2.74) sec, respectively.The nearly four-fold increase in the photoresponsivity of S2 sample is likely related to the built-in potential at the interface of heterojunction.We have compared the photoresponse characteristics of PDs in NIR region and our value is comparable to the reported works, except [17], as shown in table 1 [17,19,[42][43][44][45][46][47][48][49][50].
In PD sample S1, consisting of a photodetector based solely on MoSe 2 , the PPC arises due to the applied electric field enhancement with increasing voltage.When a voltage is applied across the MoSe 2 material, an electric field is established.This electric field accelerates the photogenerated charge carriers (electrons and holes) towards the metal electrodes, reducing the transit time in the material.The accelerated charge carriers reach the electrodes without undergoing significant recombination, leading to an increase in conductivity.Thus, the observed PPC is attributed to the efficient drift of photogenerated charge carriers to the electrodes under the influence of the applied electric field.In contrast, sample S2, which utilizes a heterostructure of MoSe 2 and Bi 2 Se 3 , exhibits NPC, where the photocurrent decreases with increasing applied voltage.The NPC observed in S2 can be attributed to the interfacial defects at the junction of MoSe 2 /Bi 2 Se 3 or defects induced by selenium vacancies in Bi 2 Se 3 [51].Despite efforts to fill selenium vacancies through post-selenization, the selenium vacancies in the top MoSe 2 layer are filled however a significant population of vacancies remains in the bottom Bi 2 Se 3 layer.Selenium vacancies in Bi 2 Se 3 create localized defect states within the band gap, acting as trap sites  for photoexcited electrons.These trapped electrons are unable to contribute to conductivity and may undergo recombination with holes, thereby reducing the overall conductivity of the device.As the applied voltage increases, the electric field may intensify the trapping of photoexcited electrons at selenium vacancy sites, leading to a more pronounced decrease in conductivity and the observed NPC effect [51].Singh et al also reported that the selenium-deficient Cu 2 Se-based thermoelectric material exihibited NPC behavior [52].The NPC has been also reported for the Bi 2 Te 3 based topological insulator in which the resistance of the topological surface states suddenly increases when the film is illuminated [53].As Bi 2 Se 3 is well known excellent thermoelectric and topological insulator materials, the effet of large Seebeck coefficient and topological surface states on the photoconductivity behavior can not be ignored and it required further detailed theoretical and computational studies [39,40,[51][52][53].

Conclusion
We have deposited large-area MoSe 2 thin film and MoSe 2 /Bi 2 Se 3 heterojunction onto sapphire (0001) substrates using the magnetron sputtering technique followed by post-selenization process.

Figure 4 (
Figure 4(a) shows the XPS survey scan for samples S1 and S2, which confirms the presence of Mo, Bi and Se elements in deposited samples.The Bi element signal was only seen in sample S2 due to the worm-type structures of MoSe 2 thin films on Bi 2 Se 3 /sapphire (0001).Figures 4(b), and (c) show the core level XPS spctra of pristine MoSe 2 film deposited on sapphire substrate [sample S1].The XPS spectra of Mo 3d was deconvoluted into three major peaks at binding energy of 228.2, 231.4,and 228.9 eV corresponding to the two electronic states of Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2 spin-orbit coupled peaks, that signals originates from 2H phase of MoSe 2 and remaining single peak could be assigned to Mo-Se bond (figure 4(b)).In figure 4(c), the Se 3d spectra show two peaks with binding energies of 53.8 and 54.7 eV corresponding to the divalent Se ions Se 3d 5/2 and 3d 3/2 states, respectively, which is consistence with valence state of Se −2[32].The Se/Mo ratio was obtained using a tabulated sensitivity factor and it turned out to be ∼1.94,close to the ideal ratio i.e. 2[31].The core level XPS spectra of MoSe 2 /Bi 2 Se 3 heterostrucutre [sample S2] is shown in figures 4(d)-(f).Figure 4(d) shows two prominent peaks at binding energy of 227.8 and 231.0 eV corresponding to Mo +4 3d 5/2 and Mo +4 3d 3/2 spin-orbit coupled peaks, respectively.These XPS peaks were found slightly shifted towards lower binding energy (red shift) value compared to pristine MoSe 2 indicating the coupling interface between Bi 2 Se 3 and MoSe 2 .Remaining one peak at 226.6 eV could be indexed for Mo-Se bond.The core level spectra for Bi 4 f states in the binding energy range of 155-168 eV is shown in figure 4(e) which shows the two dominant peaks located at binding energy values of 163.2 and 157.9 eV corresponding to Bi 4f 5/2 and 4f 7/2 electronic states, respectively.The binding energy difference of 5.3 eV between spin-orbit coupled peaks indicates the formation of the Bi 2 Se 3 compound [31].These two spin-orbit coupled peaks are found to shift slightly to higher binding

2 /
Figure 4(a) shows the XPS survey scan for samples S1 and S2, which confirms the presence of Mo, Bi and Se elements in deposited samples.The Bi element signal was only seen in sample S2 due to the worm-type structures of MoSe 2 thin films on Bi 2 Se 3 /sapphire (0001).Figures 4(b), and (c) show the core level XPS spctra of pristine MoSe 2 film deposited on sapphire substrate [sample S1].The XPS spectra of Mo 3d was deconvoluted into three major peaks at binding energy of 228.2, 231.4,and 228.9 eV corresponding to the two electronic states of Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2 spin-orbit coupled peaks, that signals originates from 2H phase of MoSe 2 and remaining single peak could be assigned to Mo-Se bond (figure 4(b)).In figure 4(c), the Se 3d spectra show two peaks with binding energies of 53.8 and 54.7 eV corresponding to the divalent Se ions Se 3d 5/2 and 3d 3/2 states, respectively, which is consistence with valence state of Se −2[32].The Se/Mo ratio was obtained using a tabulated sensitivity factor and it turned out to be ∼1.94,close to the ideal ratio i.e. 2[31].The core level XPS spectra of MoSe 2 /Bi 2 Se 3 heterostrucutre [sample S2] is shown in figures 4(d)-(f).Figure 4(d) shows two prominent peaks at binding energy of 227.8 and 231.0 eV corresponding to Mo +4 3d 5/2 and Mo +4 3d 3/2 spin-orbit coupled peaks, respectively.These XPS peaks were found slightly shifted towards lower binding energy (red shift) value compared to pristine MoSe 2 indicating the coupling interface between Bi 2 Se 3 and MoSe 2 .Remaining one peak at 226.6 eV could be indexed for Mo-Se bond.The core level spectra for Bi 4 f states in the binding energy range of 155-168 eV is shown in figure 4(e) which shows the two dominant peaks located at binding energy values of 163.2 and 157.9 eV corresponding to Bi 4f 5/2 and 4f 7/2 electronic states, respectively.The binding energy difference of 5.3 eV between spin-orbit coupled peaks indicates the formation of the Bi 2 Se 3 compound [31].These two spin-orbit coupled peaks are found to shift slightly to higher binding

Figure 4 (
Figure 4(a) shows the XPS survey scan for samples S1 and S2, which confirms the presence of Mo, Bi and Se elements in deposited samples.The Bi element signal was only seen in sample S2 due to the worm-type structures of MoSe 2 thin films on Bi 2 Se 3 /sapphire (0001).Figures 4(b), and (c) show the core level XPS spctra of pristine MoSe 2 film deposited on sapphire substrate [sample S1].The XPS spectra of Mo 3d was deconvoluted into three major peaks at binding energy of 228.2, 231.4,and 228.9 eV corresponding to the two electronic states of Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2 spin-orbit coupled peaks, that signals originates from 2H phase of MoSe 2 and remaining single peak could be assigned to Mo-Se bond (figure 4(b)).In figure 4(c), the Se 3d spectra show two peaks with binding energies of 53.8 and 54.7 eV corresponding to the divalent Se ions Se 3d 5/2 and 3d 3/2 states, respectively, which is consistence with valence state of Se −2[32].The Se/Mo ratio was obtained using a tabulated sensitivity factor and it turned out to be ∼1.94,close to the ideal ratio i.e. 2[31].The core level XPS spectra of MoSe 2 /Bi 2 Se 3 heterostrucutre [sample S2] is shown in figures 4(d)-(f).Figure 4(d) shows two prominent peaks at binding energy of 227.8 and 231.0 eV corresponding to Mo +4 3d 5/2 and Mo +4 3d 3/2 spin-orbit coupled peaks, respectively.These XPS peaks were found slightly shifted towards lower binding energy (red shift) value compared to pristine MoSe 2 indicating the coupling interface between Bi 2 Se 3 and MoSe 2 .Remaining one peak at 226.6 eV could be indexed for Mo-Se bond.The core level spectra for Bi 4 f states in the binding energy range of 155-168 eV is shown in figure 4(e) which shows the two dominant peaks located at binding energy values of 163.2 and 157.9 eV corresponding to Bi 4f 5/2 and 4f 7/2 electronic states, respectively.The binding energy difference of 5.3 eV between spin-orbit coupled peaks indicates the formation of the Bi 2 Se 3 compound [31].These two spin-orbit coupled peaks are found to shift slightly to higher binding

Figure 3 .
Figure 3. (a) AFM morphology in tapping mode of MoSe 2 on sapphire (0001) substrate, (b) Height profile along line PQ, RS and UV as shown in (a) for sample S1.(c) AFM image of MoSe 2 on Bi 2 Se 3 /sapphire (0001) substrate.

Figure 5 .
Figure 5. Schematic of MSM-based PD devices for (a) device S1, (b) device S2.(c) Spectral response of devices S1 and S2 at bias voltage of 5 V under ligh illumination of 400-1200 nm.

Figure 7 .
Figure 7. External bias-dependent characteristics of the PD devices excited by NIR (1064 nm) light illumination for (a) S1 and (c) S2.The responsivity (left) and detectivity (right) evaluations under different applied bias voltages of 0.1 to 5 V of the PD devices :(b) S1 and (d) S2.

Table 1 .
Comparison table of responsivity of the NIR photodetector devices fabricated using MoSe 2 , selenium based compounds and MoS 2 .
Raman and HR-XRD studies confirmed the formation of 2H-MoSe 2 and rhombohedral Bi 2 Se 3 thin films with distinct crystalline phases.The worm-type morpohlogy of MoSe 2 was obtained and XPS study disclosed the nearly stoichiometric MoSe 2 film.The MSM based photodetectors were fabricated on these films and showed excellent responsivity of ~1.39 and ~5.7 A/W for MoSe 2 and MoSe 2 /Bi 2 Se 3 -based devices under NIR illumination, respectively.The enhanced photoresponsivity for MoSe 2 /Bi 2 Se 3 -based PD device is related to the built-in potential at the interface of heterojunction.These results suggest that the MoSe 2 /Bi 2 Se 3 heterojunction-based photodetector has the potential for use in future optoelectronic applications for NIR photodetection.