MoS2 Thin Films Grown by Sulfurization of DC Sputtered Mo Thin Films on Si/SiO2 and C-Plane Sapphire Substrates

Here we report the growth of molybdenum disulfide (MoS2) films with different thicknesses on silicon dioxide/silicon (SiO2/Si) and c-plane sapphire substrates by sulfurization of direct current (DC) sputtered Mo precursor films in a chemical vapor deposition furnace with sulfur powder at 900°C. The structural, morphological, optical, and electrical properties of the films on different substrates were investigated through a series of characterization in detail. X-ray diffraction (XRD) results showed that the grown films on sapphire substrates had better crystallization and a well-stacked layered structure than the films on SiO2/Si substrates. The frequency difference between the characteristic modes E2g1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ E_{2g}^{1} $$\end{document} and A1g of hexagonal phase MoS2 was determined as ~ 26 cm−1 which is consistent with the typical value of bulk MoS2. Energy-dispersive x-ray (EDX) spectra exhibited that the films were near-stoichiometric. A small shift towards the lower binding energies in the Mo 3d5/2 peak positions was observed due to the valency of Mo below +4 depending on the compositional ratios of the films in x-ray photoelectron spectroscopy (XPS) spectra. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) analysis indicated that the films had smooth surfaces and a well-packed crystal structure. However, when the thickness of the films deposited on sapphire substrates increased, the strain between the sapphire substrate and MoS2 film caused the formation of the micro-domes in the film. In addition, the films exhibited high absorption and reflection properties in the near-infrared (NIR) and mid-infrared (MIR) regions in Fourier transform infrared (FTIR) analysis. Therefore, it is considered that the films can be used for photodetector applications in these regions and infrared shielding coating applications.

Here we report the growth of molybdenum disulfide (MoS 2 ) films with different thicknesses on silicon dioxide/silicon (SiO 2 /Si) and c-plane sapphire substrates by sulfurization of direct current (DC) sputtered Mo precursor films in a chemical vapor deposition furnace with sulfur powder at 900°C. The structural, morphological, optical, and electrical properties of the films on different substrates were investigated through a series of characterization in detail. X-ray diffraction (XRD) results showed that the grown films on sapphire substrates had better crystallization and a well-stacked layered structure than the films on SiO 2 /Si substrates. The frequency difference between the characteristic modes E 1 2g and A 1g of hexagonal phase MoS 2 was determined as $ 26 cm À1 which is consistent with the typical value of bulk MoS 2 . Energy-dispersive x-ray (EDX) spectra exhibited that the films were near-stoichiometric. A small shift towards the lower binding energies in the Mo 3d 5/2 peak positions was observed due to the valency of Mo below +4 depending on the compositional ratios of the films in xray photoelectron spectroscopy (XPS) spectra. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) analysis indicated that the films had smooth surfaces and a well-packed crystal structure. However, when the thickness of the films deposited on sapphire substrates increased, the strain between the sapphire substrate and MoS 2 film caused the formation of the micro-domes in the film. In addition, the films exhibited high absorption and reflection properties in the near-infrared (NIR) and mid-infrared (MIR) regions in Fourier transform infrared (FTIR) analysis. Therefore, it is considered that the films can be used for photodetector applications in these regions and infrared shielding coating applications.

INTRODUCTION
Molybdenum disulfide (MoS 2 ), which is a member of transition metal dichalcogenides (TMDCs; MX 2 , where M = molybdenum (Mo) or W and X = sulfur (S) or selenium (Se)) family, has recently gained great attention due to its unique electrical, mechanical, optical and magnetic properties. [1][2][3][4][5] It is a semiconductor material that has a layered structur,e and while the atoms within each layer in MoS 2 are strongly covalently bonded, the adjacent layers are held together by weak van der Waals interactions. 6,7 These weak van der Waals interactions enable exfoliation of MoS 2 to individual atomically thin layers. 8 Single and multilayer thin films of MoS 2 have interesting properties such as thickness-dependent band gap, 9 high carrier mobility, 10 large surface-to-volume-ratio, 11 strong spin-valley coupling, 12 chemical stability, and high mechanical flexibility. 13,14 The monolayer of MoS 2 shows a direct energy band gap ($ 1.9 eV), whereas the multilayer/bulk of MoS 2 shows an indirect band gap ($ 1.3 eV). 15 These properties make MoS 2 a promising material in future nanoscale electronic and optoelectronic device applications such as photocatalysis, 16 photodetectors, 17 biosensors, 18 gas sensors, 19 phototransistors, 20 field-effect transistors (FETs), 21 solar cells 22 and light-emitting diodes (LEDs). 23 There have been many various attempts including top-down and bottom-up methods such as mechanical exfoliation, 24 chemical exfoliation, 25 hydrothermal synthesis, 26 physical vapor deposition (PVD), 27 and chemical vapor deposition (CVD) 28 to produce MoS 2 thin films. Initially, researchers have intensely focused on the exfoliation method for obtaining monolayers of MoS 2 films and it is still the most commonly used method for the growth of MoS 2 films. However, it has been recently thought that the exfoliation method is not suitable for their large-scale production 29,30 because exfoliation results in the formation of MoS 2 flakes with random thicknesses at random distributions. 31,32 Therefore, they are considered to be limited in terms of controlling the thickness and thus the number of MoS 2 layers. In this context, the synthesis of uniform large-area MoS 2 layers by controlling the film thickness is necessary for the practical use of this material in electronic and optoelectronic applications in the industry. CVD is one of the most promising methods to produce continuous MoS 2 films over large areas as an alternative to exfoliation methods. Another advantage of CVD is that it can be avoided from the interfacial contamination during the layer by layer transfer process. 6,33,34 Various approaches such as high-temperature sulfurization of sputtered Mo films 35 or molybdenum trioxide (MoO 3 ), 28 molybdenum pentachloride (MoCl 5 ), 36 thermolysis of ammonium tetrathiomolybdate ((NH 4 ) 2 MoS 4 ) 37 and chemical reaction of molybdenum hexacarbonyl (Mo(CO) 6 ) and diethyl sulfide ((C 2 H 5 ) 2 S) 38 can be used for the synthesis of MoS 2 films by using CVD. However, the controllable growth of MoS 2 films over large areas by CVD is still a big challenge and there are shortcomings in the fundamental understanding of the CVD growth process. For this reason, the optimization studies on the growth of MoS 2 films by CVD are still underway.
Our previous study has presented the production of MoS 2 films from direct current (DC)-sputtered Mo films using a two-zoned CVD system. 39 In this process, S powder and Mo films were placed in different zones having different temperature values. The films produced with this method had a highly oriented polycrystalline structure on silicon dioxide/ silicon (SiO 2 /Si) and sapphire substrates, but they were non-stoichiometric. Therefore, the carrier mobility values of the films were relatively low. To improve the film characteristics, in this study, a one-zoned CVD system was utilized to produce MoS 2 thin films on SiO 2 /Si and sapphire substrates. In this context, structural, morphological, optical, and electrical characteristics of the grown films were reported in detail as a function of thickness. To comprehensively identify the structural and morphological properties and determine S/Mo ratios of the MoS 2 thin films with different thicknesses, the films were characterized by x-ray diffraction (XRD), Raman spectroscopy, secondary ion mass spectroscopy (SIMS), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive x-ray (EDX), and x-ray photoelectron spectroscopy (XPS). The optical properties of the films were presented in both ultraviolet-visible (UV-Vis) and infrared (IR) regions. In addition, their electron-transport properties were investigated by room temperature Hall effect measurements. We believe that the growth of the MoS 2 films by this approach will provide an opportunity for the growth of largearea single and multilayer films of MoS 2 . Therefore, we aim to focus on reducing the thickness of the Mo precursor films depending on the deposition time and produce thinner MoS 2 films over large areas by this approach in future studies. Moreover, we consider that the characterization results of the films obtained by using a lot of different techniques in our study will contribute to the literature as important scientific outputs and the grown MoS 2 films in our study can be used for the development of near-infrared (NIR) and mid-infrared (MIR) electro-optical devices and IR shielding coating applications due to their high absorption and reflection properties in the NIR and MIR regions.

EXPERIMENTAL
MoS 2 films were prepared on SiO 2 /Si and c-plane sapphire substrates by sulfurization of DC-sputtered Mo precursor films. Prior to the deposition process, the substrates were ultrasonically cleaned in acetone and ethanol for 5 min, respectively. Afterwards, they were blow-dried by a high-purity nitrogen jet. The deposition of the precursor films was conducted in a three-target co-sputtering system at room temperature. The schematic diagram of the co-sputtering system and the deposition process of Mo precursor films were given in Fig. 1.
A Mo target with a purity of 99.95% was used to deposit the Mo precursor films. After the base pressure of the sputtering system was reached to $ 1.33 9 10 À7 kPa, the deposition process was started at a working pressure of 6.67 9 10 À4 kPa in argon (Ar) atmosphere for all deposition processes. The sputtering power was 60 W and the deposition times were determined as 12 min and 17 min to obtain the films with the different thicknesses both on SiO 2 /Si and c-plane sapphire substrates. The deposition times for the precursor films and the corresponding thickness values obtained by profilometer measurement are presented in Table I.
All deposited Mo precursor films in this work were treated to the same sulfurization process. Figure 2 depicts the schematic diagram of the sulfurization furnace and temperature-time profile of the sulfurization process. Firstly, the precursor films and 0.5 g of elemental sulfur (S) powder (99.999%) were placed into a quartz boat which was loaded into the quartz tube furnace. The quartz tube furnace was pumped down to a base pressure of $ 10 À3 mbar and flushed with Ar gas flow before the sulfurization process.
The precursor films were heated to 900°C at a ramping rate of $ 20°C/min and maintained at this temperature for 90 min. The working pressure was kept at 3.4 mbar during the entire sulfurization process. Finally, the samples were cooled down to room temperature at a rate of 10°C/min. The sulfurized films of Mo precursors with the thicknesses of 95 nm, 135 nm, 100 nm, and 132 nm deposited on SiO 2 /Si and c-plane sapphire substrates were named as Si1, Si2, Sa1, and Sa2 regarding the name of the substrates, respectively.
The crystal structure of MoS 2 films was analyzed by using an APD 2000 PRO x-ray diffractometer equipped with a Cu-Ka source (k = 1.54052 Å ). Raman scattering measurements were carried out by a Nanofinder HE (LOTIS TII) confocal Raman spectrometer at room temperature. For the excitation, a 532 nm continuous-wave solid-state laser was used. Raman spectra were obtained with a spectral resolution better than 3.0 cm À1 . The composition and the morphology of the films were investigated by a Hitachi S-4800 (Japan) scanning electron microscope arranged with an EDX spectrometer. The surface morphology of the films was also examined by AFM analysis by using a Solver Nano tool (NT-MDT) in the semi-contact mode with a scanning probe of 10 nm tip radius. XPS studies were performed by using an Omicron XPS setup with a Mg Ka x-ray source (1253.6 eV) operating at 10 kV. High-resolution scans were obtained at 20 eV. The peak positions were calibrated based on the C1s peak at 284.6 eV. The elemental depth profile of the films was acquired by SIMS using a Hiden Analytical SIMS Workstation. A primary beam of O 2+ ions with an energy of 3.850 keV and a beam current of 400 nA were used for the analysis. UV-Vis spectroscopy (Perkin Elmer Lambda 2S) was used to collect the optical transmittance and reflectance spectra of the films. Fourier transform infrared (FTIR) spectra were obtained by Bruker Vertex 80 FTIR spectrometer in the spectral area between 400 cm À1 and 7500 cm À1 at room temperature. The number of scans was fixed to 32 with a spectral resolution of 4 cm À1 . The electrical properties of the films including carrier mobility, carrier density, and resistivity were characterized by the van der Pauw technique at room temperature using LakeShore Hall-effect measurement system. The thickness values of the Mo precursor films and the sputtering depths from SIMS craters were determined by a stylus type profilometer (Veeco, Dektak 150).

RESULTS AND DISCUSSION
The XRD patterns of MoS 2 thin films with different thicknesses prepared on SiO 2 /Si and cplane sapphire substrates are given in Fig. 3. All the films exhibited two diffraction peaks at 2h = 32.82°and 2h = 58.78°corresponding to (100) and (110) planes of hexagonal MoS 2 , respectively (JPDS:37-1492).
For the sample Si1, another observed peak at 2h = 61.97°was attributed to the (008) plane of hexagonal MoS 2 . The peaks corresponding to the (400) and (113) crystal planes of Si and c-plane sapphire coming from substrates appeared at 2h = 69.57°and 2h = 42.37°for the samples Si1, Si2, and Sa1, respectively. The diffraction peaks corresponding to the (002) crystal plane of hexagonal MoS 2 were obtained for the films grown on sapphire substrates. With the appearance of the (002) diffraction peak at 2h = 14.37°for the films grown on c-plane sapphire substrates, it can be concluded that the MoS 2 films on c-plane sapphire substrates had better crystallization and wellstacked layered structure than those of the films on SiO 2 /Si substrates. As a result, the XRD patterns revealed that the grown films in this study were  predominantly oriented along the c-axis which represents favorable crystal growth orientation of the hexagonal MoS 2 . Figure 4a shows the Raman spectra of MoS 2 films. The characteristic modes E 1 2g (in-plane vibration of Mo and S atoms) and A 1g (out-of-plane vibration of S atoms) of hexagonal phase MoS 2 were observed at around 383 cm À1 and 409 cm À1 with a difference of $ 26 cm À1 Raman shift (frequency difference, D = A 1g À E 1 2g ) which is consistent with the typical value of bulk MoS 2 , respectively. 9,40 The full width at half maximum (FWHM) values and the intensity of the characteristic peaks may give an impression about the crystal quality of the films. 37 The FWHM value for the sample Sa2 (7 cm À1 ) was slightly higher than that of the others (6 cm À1 ). This was attributed to a decrease in the crystallinity of the sample Sa2 due to the formation of micro-domes on the surface of the film, as will be discussed in AFM and SEM analysis later. On the other hand, the intensities of the peaks were higher for the films grown on the c-plane sapphire substrates than those of the films grown on SiO 2 /Si substrates and increased with the increase in the thicknesses of the films. The expanded Raman spectra in the range of 370-420 cm À1 and 400-480 cm À1 are given in Fig. 4b, c, f, respectively. The small peaks of E 1g , 2LA(M), and A 2u modes were also detected at 286 cm À1 , 453 cm À1 , and 463/ 466 cm À1 in the spectra of the films. 3,35,[40][41][42] The MoS 2 films with different thicknesses prepared on different substrates did not show remarkable differences in Raman peak positions.
The elemental composition of the films was analyzed by EDX. Figure 5 shows EDX spectra of the films with insets indicating of atomic percentage % and atomic ratios of S and Mo.
The S/Mo ratios of the samples Si1, Si2, Sa1, and Sa2 were determined as 2.30, 1.98, 1.92, and 1.90, respectively. These results showed that the composition value of the sample Si2 was the nearest to the stoichiometric value 2 obtained from the S/Mo ratio for the MoS 2 compound and by the increase in the thicknesses of the Mo precursor films, binding of S element to Mo element decreased during the sulfurization. It has been clearly seen that the increase in the chalcogen vapor pressure with the decrease in the volume of the sulfurization atmosphere thanks to the use of the single-zoned CVD furnace caused an increase in the incorporation of S atoms into the MoS 2 matrix. This caused the composition values to come much closer to the stoichiometric value 2 in this study. As seen in Fig. 5, for all samples, EDX spectra exhibited a peak corresponding to the platinum (Pt) element which is a consequence of the fact that the samples were sprayed with platinum before SEM and EDX analysis. Furthermore, the Si, aluminum (Al), and oxygen (O) signals coming from SiO 2 /Si and sapphire (Al 2 O 3 ) substrates were also detected in the spectra. Even though the Si, Al, Pt, carbon (C), and O signals originating from the substrates and contaminations in the films appeared in EDX spectra, the Mo and S signals were the strongest in the spectra.
XPS measurements were carried out to obtain information regarding the chemical states of the films. High-resolution XPS spectra for Mo 3d and S 2p core levels of the films grown on SiO 2 /Si and sapphire substrates were shown in Fig. 6. The samples Si1 and Si2 exhibited Mo binding energies of 229.6 eV, 229.3 eV corresponding to Mo 3d 5/2 and 232.4 eV, 232.1 eV corresponding to Mo 3d 3/2 orbitals of Mo +4 states, respectively (Fig. 6a, b). In the case of the samples Sa1 and Sa2, it was seen that the characteristic peaks corresponding to the Mo 3d 5/2 and Mo 3d 3/2 doublet were located at $ 229.2 eV and $ 231.8 eV, respectively ( Fig. 6c and d). The S binding energies for all samples were observed at around 161.8 eV and 163.6 eV corresponding to S 2p 3/2 and S 2p 1/2 orbitals of divalent sulfide ions (S 2À ), respectively (Fig. 6e, f, g and h). In addition, a peak appeared at around 226 eV for all samples, which corresponds to S 2 s binding energy. The binding energy values of Mo 3d, S2s, and S2p states in the films were presented in Table II.
The XPS results showed that there was a small shift towards the lower binding energies in the Mo 3d 5/2 peak positions depending on the compositional ratios of the films discussed in the EDX analysis above. This small shift was attributed to the valency of Mo below +4 due to the formation of the films in MoS 2-x structure in our study. Similar results related to the shift in Mo 3d 5/2 peak positions were obtained by Zeng and et al. and Baker et al. 43,44 The linear relationship between the MoS 2-x stoichiometry and Mo 3d 5/2 binding energy can be seen in Fig. 7.
Two-dimensional (2D) 10 9 10 lm 2 AFM images of the films are given in Fig. 8. Roughness parameters and average grain size values of the films obtained by AFM analysis are given in Table III. From the AFM images, it was observed that all the grown films had uniform surfaces except for the sample Sa2. The MoS 2 film of the sample Sa1 on the sapphire substrate was the smoothest one with a root mean square roughness (R rms ) value of 5.95 nm among the samples.
On the other hand, sample Sa2 exhibited the roughest morphology due to the micro-domes on its surface. Although the c-plane sapphire substrate has the same hexagonal symmetry as MoS 2 , there is a large coefficient of thermal expansion mismatch between them. 45,46 This mismatch causes an increase in the strain between the sapphire  substrate and MoS 2 film and may result in the formation of the micro-domes in the film. Therefore, it may be thought that the micro-domes become prominent depending on the film thickness. Consequently, the sample Sa2 has hills and valleys on its surface due to these micro domes as seen in both AFM and SEM analysis and the R rms value obtained from AFM analysis for the sample Sa2 was significantly higher due to these micro-domes than the other samples. When it comes to the films grown on SiO 2 /Si substrates, as seen from AFM images, such micro-domes did not form on the surfaces of the films due to the smaller coefficient of thermal expansion mismatch between SiO 2 /Si substrate and MoS 2 film. The surface morphologies of the films were also evaluated in terms of other surface roughness parameters average roughness (R a ), surface skewness (R sk ), and surface kurtosis (R ku ). As it is known, the R a parameter is less sensitive to large deviations from the mean line than the R rms parameter and it can be considered as a general description of the height variations on the surfaces. It was seen that the R a values were smaller than the R rms values but exhibited similar decrease or increase trends as R rms values. Another roughness parameter R sk describes the symmetry/asymmetry of the distribution of the surface heights about the mean line.
The sample Si1 exhibited a negative R sk value of -0.82 which means the valleys are more predominant than peaks on the surface. Conversely, the samples Si2, Sa1, and Sa2 showed positive skewness values of 3.41, 2.61, and 1.46, respectively. The other roughness parameter is the R ku which describes the level of the flatness or peakedness of the distribution above and below the mean line. R ku values were determined to be less than 3 for sample Si1 and more than 3 for the other samples. This can be considered an indication that the surface of sample Si1 was bumpy and had platykurtic distributions, and the surfaces of the other samples were spiky and had leptokurtic distributions. Figure 9 shows the histograms of the grain size distributions obtained by AFM analysis in the films. It was seen that the grain size distributions in the films were accumulated at around the submicronsized grains for the samples.
The average grain size was found to be 0.55 lm, 0.27 lm, 0.23 lm, and 0.20 lm for the samples Si1, Si2, Sa1, and Sa2, respectively. Figure 10 shows the SEM surface and crosssection images of the MoS 2 films. The surface images of the films exhibited that the films had a smooth and well-packed crystal structure with submicron-sized grains and without any cracks or voids.
No micro-defects such as pinholes were observed on the surface of the films. On the other hand, it was observed that the nanometer-sized grains grown in lateral were uniformly distributed over the surface of the films. The compact microstructure of the films can be also seen from the cross-section SEM images. The formation of highly compact MoS 2 films can be attributed to the grain growth in lateral. The thicknesses of the films were 157 nm, 224 nm, 173 nm, and 212 nm for the samples Si1, Si2, Sa1, and Sa2, according to the cross-section SEM images, respectively. In addition, as seen in Fig. 10d, randomly distributed flower-like micro-domes were observed on the surface of the film for sample Sa2. In order to see the distribution of these micro-domes on the surfaces better, the SEM images of sample Sa2 with lower magnifications are given in Fig. 11. Similar micro-domes on the MoS 2 films were reported by Abouelkhair and et al.. 47 The formation of these micro-domes was attributed to the coefficient of thermal expansion mismatch between the sapphire substrate and the MoS 2 film. 45,46 Because of the large surface area of these micro-domes, these kinds of films may be considered remarkable in terms of sensor applications. The increase in the thickness values of the films may have caused the formation of these micro-domes in the films. SIMS depth profiling was performed to clarify the distributions of the Mo and S elements throughout the film thickness direction. Figure 12 presents the SIMS depth profiles of the MoS 2 films. All samples exhibited very uniform distributions of the constituent elements of Mo and S within the whole thickness of the films. In addition, the profiles of Mo and S elements had almost identical trends for all  the samples. On the other hand, it was observed that the levels of the Si and Al elements which come from the SiO 2 /Si and sapphire substrates increased in the region near to the substrates. In the SIMS depth profiles, the converted thickness values using the etching rate were matched well with the thickness values observed in the SEM images. Transmittance spectra and the plots of absorption coefficient (a) and (ahm) 2 vs. photon energy (hm) of the MoS 2 films grown on sapphire substrates were given in Fig. 13. It was observed that the films had average optical transmittance values up to 40% and 44% within the wavelength of 700-1100 nm for the Sa1 and Sa2 samples, respectively, while they had no transmittance down to 500 nm (Fig. 13a).
It is worthy to note that the MoS 2 films are transmissive in the NIR region up to the wavelength of 1100 nm in terms of their possible electrooptical applications in this region. In addition, the films showed clear electronic transitions at around 621 nm (2.01 eV) and 678 nm (1.86 eV); 624 nm (2.0 eV), and 674 nm (1.84 eV) for the samples Sa1 and S2a, respectively. These transitions denoted as A and B in Fig. 13a were attributed to the excitonic transitions at the K point of the Brillouin zone in bulk MoS 2 which are the direct-gap transitions between the maxima of split valance bands and the minimum of the conduction band. 1,29,48,49 Figure 13b and c depicts the plots of the a and (ahm) 1/2 vs. hm of the films.  The absorption coefficients of the films were calculated using the following relation: where t and T are the thickness and the transmittance of the films, respectively. Absorption coefficients of the films were found to be over 10 5 cm À1 in the energy region above 2.5 eV. However, there is an obvious decrease in the value of the absorption coefficient below 2 eV due to the excitonic transitions and band gap transition. The optical band gaps of the films were calculated by the Tauc method: where h is the Planck constant, m is photon frequency, A is a constant and Eg is the band gap. n equals 2 for indirect allowed transitions. The optical band gaps of the films were estimated as 1.44 eV and 1.56 eV by extrapolating of the linear part to the energy axis of the plot of the (ahm) 1/2 vs. hm for the samples Sa1 and Sa2, respectively. It was seen that the band gap values of the MoS 2 films were larger than that of bulk MoS 2 . FTIR spectroscopy was also used to determine the optical properties of the films in NIR and MIR spectral regions at room temperature. Figure 14a presents the FTIR transmittance spectra of the samples Sa1 and Sa2 in the range of 350-7500 cm À1 .
The spectra were also given in the range of 1200-1700 cm À1 to be clearly seen the absorptions of the films in Fig. 14b. The grown films exhibited an average transparency of 60% in the range of 1600 cm À1 to 7500 cm À1 because the sapphire substrate has opacity at wavenumbers below 1600 cm À1 . The absorption at 1427 cm À1 was attributed to the Mo-S vibration of MoS 2 . 50 To evaluate the optical response in NIR and MIR regions of the MoS 2 films grown on SiO 2 /Si substrates, FTIR transmittance spectra data in the range of 350-7500 cm À1 and FTIR reflectance and absorbance spectra data in the range of 350-4500 cm À1 were also collected and given in Fig. 15.
The observed absorption peak at around 467 cm À1 in the expanded transmittance spectra of the films was attributed to the Mo-S vibration of MoS 2 (Fig. 15b). 51 The strong absorptions located at 614 cm À1 and 1103 cm À1 were attributed to Si-Si stretch vibration of the Si substrate and Si-O stretch vibration originating from interstitial O impurities, respectively. 52 On the other hand, it was seen that the MoS 2 films grown on SiO 2 /Si had a strong broadband absorption in the range of 850-2850 cm À1 (3.5-11.8 lm) (Fig. 15c). Similar broadband absorption in the NIR and MIR regions was obtained by Goswami and et al. for the bulk MoS 2 films. 24 As expected, the absorption of the films increased with the film thickness. Owing to this strong absorption of the films in these regions, it is considered that they can be used for IR detection applications. 53 The films showed an average IR reflection of 87% in the range of 350-7000 cm À1 (27.7-1.5 lm). Therefore, the films can be used as IR shielding coating for several applications in this spectral region due to their high reflectance properties. Moreover, in the reflectance and absorbance spectra of the films, vibrational peaks centered at around 467 cm À1 , 611/614 cm À1 , and 1091/ 1093 cm À1 were attributed to the Mo-S vibration, Si-Si stretch vibration of Si substrate, and Si-O-Si stretch vibration originating from SiO 2 /Si substrates, respectively.
To investigate the electrical transport properties of the MoS 2 thin films, Hall effect measurements were conducted at room temperature at B = 0.4 T and the resistivity, carrier concentration, Hall mobility, and Hall coefficient were calculated by the Van der Pauw method. Calculated electrical parameters were listed in Table IV. The films showed both n-type and p-type conductivity. This electrical transport behavior of the films 54, 55 The formation of the oxygen impurities behaving like acceptors in the films during the sulfurization process might be another reason for the transformation of the n-type conductivity to ptype. The carrier concentrations of the films prepared on sapphire substrates were higher than those of the films prepared on SiO 2 /Si substrates. However, the lower mobility values were obtained for the films prepared on sapphire substrates. In addition, it was seen that the mobility values of the films obtained in this study were higher than those of the films produced by two-zoned CVD in our previous study. 39

CONCLUSION
In summary, the MoS 2 films with different thicknesses were prepared by sulfurization of Mo precursor films deposited by DC magnetron sputtering on SiO 2 /Si and c-plane sapphire substrates. The sulfurization process was carried out in a singlezoned CVD furnace with sulfur powder at 900°C. After sulfurization, the structural, morphological, optical, and electrical properties of the films were analyzed in detail. The appearance of a (002) diffraction peak in the XRD patterns of samples Sa1 and Sa2 showed that the films grown on sapphire substrates exhibited better crystallization and a well-stacked layered structure than the films on SiO 2 /Si substrates. The formation of the  hexagonal phase of bulk MoS 2 was also confirmed by the characteristic modes E 1 2g and A 1g with a frequency difference of $ 26 cm À1 via Raman spectroscopy analysis. EDX analysis showed that the compositions of the films were near-stoichiometric. However, a small shift was observed to the lower binding energies in the Mo 3d 5/2 peak positions due to the valency of Mo below +4 depending on the compositional ratios of the films in XPS spectra. From the AFM and SEM analysis, it was seen that the films had uniform surfaces and a well-packed crystal structure. However, the strain due to thermal expansion coefficient mismatch between the sapphire substrate and MoS 2 film resulted in the formation of the micro-domes on the surface of the film for the sample Sa2. This was associated with larger film thickness of the sample Sa2 than the sample Sa1. Since the formation of such microdomes was not observed in the films grown on SiO 2 / Si substrates, it has been concluded that the strain in the films grown on sapphire substrates is of great importance on the morphological properties of the films and should be certainly taken into consideration in the process of the MoS 2 films grown by this method. Because of the high absorption and  reflection properties of the films grown on SiO 2 /Si and sapphire substrates in the NIR and MIR regions, it was suggested that they can be used for NIR and MIR photodetectors and also for IR shielding coating applications. Additionally, it can be concluded that the decrease in the volume of the sulfurization atmosphere and thus the increase in the chalcogen vapor pressure strongly influenced the compositions of the films and brought them closer to an ideal value of 2 and increased the mobility values of the films. Moreover, the formation of the micro-domes on the surfaces of the films grown on sapphire substrate with an increase in the film thicknesses in this study was another difference and therefore the films may be considered remarkable in terms of sensor applications.

ACKNOWLEDGMENTS
This work is supported by the Presidency of Turkey, Presidency of Strategy and Budget, and TUBITAK under the 2016K121220 and 115F280 project numbers, respectively, and by the Belarusian State Programme for Research « Physical material science, new materials and technologies » .