Microstructural and optical properties investigation of variable thickness of Tin Telluride thin films

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Microstructural and optical properties investigation of variable thickness of Tin Telluride thin films Praveen Tanwar ConceptualizationinvestigationWriting an initial draft , Amrish K. Panwar Writing & Review and editingAnalysisSupervision , Sukhvir Singh ResourceVisualizationInvestigationSupervision , A.K. Srivatava SupervisionAnalysis PII: DOI: Reference:

S0040-6090(19)30735-7 https://doi.org/10.1016/j.tsf.2019.137708 TSF 137708

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Thin Solid Films

Received date: Revised date: Accepted date:

7 December 2018 20 November 2019 20 November 2019

Please cite this article as: Praveen Tanwar ConceptualizationinvestigationWriting an initial draft , Amrish K. Panwar Writing & Review and editingAnalysisSupervision , Sukhvir Singh ResourceVisualizationInvestig A.K. Srivatava SupervisionAnalysis , Microstructural and optical properties investigation of variable thickness of Tin Telluride thin films, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137708

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Highlights  Cubic crystal structure with nano size grain observed by microstructural study  Uniform distributions of tin and tellurium isotopes investigated by mass spectroscopy.  Optical properties of tin-telluride thin film indicate a shift towards near infrared region.  Observed molecular vibrations and phonon scattering beneficial in infrared applications.

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Microstructural and optical properties investigation of variable thickness of Tin Telluride thin films Praveen Tanwar1,2, Amrish K. Panwar1*, Sukhvir Singh2 and A. K. Srivatava2,3 1

Department of Applied Physics, Delhi Technological University, Shahbad Daulatpur, Main Bawana Road, Delhi-110042

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Indian Reference Materials, CSIR – National Physical Laboratory, India Dr. K. S. Krishnan Road, New Delhi – 110012 Telephone: +91-011-45609308, FAX: - 91-11-45609310

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CSIR - Advanced Materials and Processes Research Institute, Bhopal – 462064, India *

Corresponding Author: [email protected], [email protected]

Abstract A series of Tin Telluride (SnTe) thin films of varied thicknesses are deposited on glass substrates at room temperature using thermal evaporation technique. The optical and microstructural properties of SnTe thin films of thicknesses 33 nm to 275 nm are reported. High-resolution x-ray diffraction patterns of SnTe thin films revealed the polycrystalline nature with [200] orientation having cubic structure. The microstructural and morphological structures of these films were examined using high-resolution transmission electron microscopy and scanning electron microscopy, respectively. The distribution of isotopes of various elements in the thin film along with lateral and longitudina l directions was determined by depth profile measurement using the time of flight - secondary ion mass spectroscopy technique. Fourier transform infrared spectroscopy spectra reveal the molecular vibrations, narrow bandgap property of material and suitability of materials in infrared applications. Longitudinal – optical phonon scattering due to the [222] orientation was observed in the micro-Raman spectra at room temperature which corresponds to a peak in the range 120–130 Raman shift/cm-1. Hence, the change in optical and microstructural properties at the nano-regime resulted in a shift towards the near-infrared region with an increase in the thickness of the thin films. Keywords: - Tin Telluride, Thin films, Fourier – transform infrared spectroscopy, Transmission electron microscopy, Raman spectroscopy. 2

1.0 Introduction In the last five decades, the IV-VI group compounds have created an important platform for semi-metals and narrow bandgap semiconductors with high-temperature phases in various crystal structures transforming into other structural phases on lowering the temperature [1-5]. Many IVVI compound semiconductors and quantum wells of these materials have a broad range of applications such as thermoelectric devices, infrared (IR) lasers, and detectors. Among these lead chalcogenides like lead telluride (PbTe), lead selenide (PbSe) and tellurium doped tin selenide (PbTe1-xSex) has been most extensively studied by researchers among all of these IV-VI materials. IV-VI compound semiconductors in the form of thin film or bulk are highly focused on scientific research and fundamental applications which mostly used in electronics, optoelectronics, solar cells, IR and near-infrared detectors and emission [6,7]. For low band-gap detectors, thermal imaging, free-space optical communications, long wavelength range laser diodes, chemical process control and environmental monitoring of atmospheric pollution, the IVVI lead salts and III–V Sb based compound are the main choices for Mid-Infrared (MIR) emission. But in the MIR region III–V Sb based compounds (3–5 m) have some limitations [812]. Tin Telluride (SnTe) is an excellent example of wide attention in the area of thermoelectric properties and IR applications of bulk material. However, a little attention towards the optical properties of SnTe thin films have been paid and was not explored earlier. Among all the IV-VI compound semiconductors, SnTe has a direct bandgap of 0.18 eV in bulk form at room temperature and mostly used for MIR photo-detectors and thermoelectric heat converters [13-20]. Since, SnTe is always Tin (Sn) deficit, and act as p-type material; extra Tellurium (Te) atom creates two holes in the valance band. Hence, SnTe behaves as heavily doped degenerate p-type semiconductors. SnTe has a NaCl type (B1) cubic structure , and it undergoes structural transformation to the orthorhombic structure under pressure. P-type SnTe compound crystallizes in a rocksalt-structure with excess tellurium (50.44 at. % Te at 806 °C), which causes cation vacancies and very high concentration of hole (1020–1021 cm−3) produced in the bulk compound. Due to the NaCl like structure, SnTe is an excellent material for the study of the effects of anharmonicity on the 3

dispersion curves. The spin-orbit interactions in SnTe may produce qualitative effects that can convert SnTe from a semimetal to a semiconductor [20-23]. Studies of SnTe are complicated due to their strong absorption through the visible and IR spectral region [24-25]. The band structure of SnTe has been widely manifested and recognized to have a valance band with two maxima. These two maxima are reflected upon many characteristics of SnTe as the electrical and optical properties. Due to the quantum size effect mechanism, the emission spectra in SnTe nanocrystals can cover the MIR region and far-infrared region. Here, the complicated fact is that the separation between the light-hole and heavy-hole band edges in SnTe is estimated to be in the range of ∼0.3 to ∼0.4 eV [14, 15, 26-35], which is larger than those of PbTe or PbSe. However, the as-grown SnTe compound is often found to be highly p-type because they naturally form the high concentration of Sn vacancies (p > 1020cm−3 ). Hence, the bulk conduction dominates over the surface contribution and making it very challenging to probe the surface states by transport studies [16, 36-43]. Therefore, one way to enhance the surface contribution is to increase the surface area-to-volume ratio by synthesizing nanometer-scale structures. In our previous work, the structural, optical, paramagnetic, electrical and electronic properties in bulk SnTe compound grown by vertical direction solidification (VDS) technique were explored [17]. In the present study, we report the microstructural and optical behaviour of a series of SnTe thin films deposited on the glass substrate at room temperature with varied thicknesses using the thermal evaporation technique. The approach behind this work is to build interest in the Sn-Te chalcogenide thin films for the development of IR optical components. 2.0 Experimental and Characterization Techniques SnTe bulk compound was grown by the VDS technique using high purity (99.999% pure) Sn and Te elements. These pure elements were taken in stoichiometric proportions in a quartz ampoule. The ampoule was then evacuated to a pressure of 10−3 Pa and sealed. The sealed quartz ampoule was loaded in a temperature-controlled vertical muffle furnace and heated up to 1079 K for three hours in the isothermal zone for the formation of a single-phase SnTe compound. SnTe thin films were deposited on to NaCl and glass substrates at room temperature by using thermal evaporation technique (Model VT-ACG-03) under a high vacuum of ∼ 10−3 Pa using a small piece of the as-grown bulk compound as a source material [17]. The liquid nitrogen trap was used to avoid any contamination during the thin film deposition. The thickness of these thin 4

films were kept at 275 nm, 145 nm, 55 nm and 33 nm and measured by using XP-200 Plus, Stylus Profilometer. SnTe thin films were removed from the NaCl crystal using water as solvent to dissolve the NaCl crystal. The floated and separated thin film of SnTe was then placed on copper (Cu) grid. Hence, the SnTe thin film on Cu grid was made dry under table lamp heat for one hour. Finally, it was used for the microstructural studies under Transmission electron microscope (TEM).

High-resolution x-ray diffraction (HRXRD) measurements on as-deposited thin films were carried out using a PAN analytical x-ray diffractometer (X'Pert PRO MRD) with CuK radiation. To minimize the substrate effect in the diffraction pattern, glancing incident angle xray diffraction geometry was used to record the diffraction pattern of SnTe thin film with grazing angle set up at 2. Microstructural features were recorded using FEI make high-resolution transmission electron microscope (HRTEM, Tecnai G2 STWIN F30) operated at an accelerating voltage of 300 kV. Surface morphology measurements were carried out by using Zeiss make scanning electron microscope (SEM, EVO MA10) with an operating voltage of 10 kV. The time of flight - secondary ion mass spectroscopy (TOF-SIMS) characterization technique helps in the analysis of isotopes of elements present in the material. Determined the depth profile measurement by using TOF-SIMS (GmBH, make, ION TOF-SIMS 5) spectrometer equipped with a reflection time-of-flight mass analyzer operated at 25 keV with bismuth source for impurity level determination. Molecular vibrations and detection of impurity elements in the material were recorded in the MIR region using Agilent make Fourier transform infrared spectrometer (FTIR, Cary, 630). Raman spectroscopy measurements were carried out on RENISHAW make inVia Reflex model micro-Raman spectrometer. Micro-Raman spectra were recorded for the study of different phonon mode vibration in lattice structure. 3.0 Results and Discussion 3.1 XRD Analysis HRXRD patterns of the SnTe thin films having thickness 275nm, 145 nm, 55 nm and 33 nm are shown in Fig. 1a–d and the estimated lattice parameters from these patterns of each of the films along with the space groups are given in Table 1. The x-ray diffraction pattern revealed the well5

resolved peaks of SnTe in thin films indicating the formation of the cubic crystal structure with space group,

. The observed results have been compared with standard JCPDS file no:

(08-0487) and found in good agreement with the standard values. HRXRD pattern of SnTe thin film of thickness 275 nm (Fig. 1a), shows that the film is oriented along the (220) plane representing highest intense peak while other planes (200), (222), (420), (422), (400) indicate relatively lower intensity peaks. Similarly, HRXRD patterns of the SnTe thin films of thicknesses, 145 nm, 55 nm and 33 nm (Fig. 1b-d) indicate the highest intense peak along the plane (200) however, the planes represented as (220), (222), (420), (422), (400) shows lower intensity peaks. Moreover, the peak broadening as well as the more intense peak of the plane (200), has also been noticed in SnTe thin film of thickness 33 nm. It may be possible due to strain in thin film at this particular thickness range [10, 12, 19-32], whereas HRXRD pattern of SnTe thin film of thickness 55 nm (Fig. 1c) shows the significant reduction in overall intensity of all the peaks as compared with 275, 145 and 33 nm which leads to change in crystallite size and morphological features of the grains. It also revealed that at higher 2 angle, the planes (400) and (422) are almost disappears resulting in fewer grains responsible for these lattice structures. The average crystallite size of SnTe thin films of thickness 275 nm, 145 nm, 55 nm and 33 nm were calculated as 14 nm, 14 nm, 6 nm and 13 nm, respectively using Scherrer‟s formula (Table 1). This indicates that SnTe thin film of thickness 55 nm has the smallest crystallite size. It may be due to the fact that variation in crystallite size becomes random below 55 nm thickness for SnTe thin films. Further, it is necessary to mention here that in the case of 275 nm and 145 nm thick film, the stress/ strain may be uniform, but for the 55 nm thick film the stress/ strain may be higher and hence constrained in the growth of crystallite size was observed. However, on further reducing the thickness of thin film up to 33 nm, the developed stress/strain is not enough to constrain the growth of individual crystallite and is presumably the reason for the larger crystallite size in 33 nm thick film as comparison to the 55 nm thick film [20-32, 39]. However, it is further a subject of thorough investigation and studies are being carried out. Moreover, according to Scherrer‟s formula crystallite size is inversely proportional to the full width at half maxima of diffraction peak. Generally, when the thickness of thin film decreases, the crystallinity decrease and micro-strain/ stress play a dominant role. After a critical thickness is reached, the crystalline feature may be reduced and forming an amorphous feature of thin films. 6

This is important to mention here that up to some critical thickness of thin film the effect of strain may be dominant due to substrate. But above that critical thickness, the deposited material behaves like normal polycrystalline nature. In nano-regime properties are responsible for this unexpected change in crystallite size. This abrupt change as occurred in the thin films on reduction of thickness may be associated with large surface to volume ratio of thin films of SnTe from 55 nm to 33 nm. Earlier researchers have also reported that crystallite size increases with the reduction of the thickness of the thin film and simultaneously strain decreases with the thickness of the films [14, 15, 25, 29, 30, 32].

Figure 1: HRXRD patterns of SnTe thin film of thicknesses: (a) 275 nm, (b) 145 nm, (c) 55 nm and (d) 33 nm deposited on glass substrate. 3.2 Morphology and Microstructure Analysis SEM images of SnTe thin films of thickness: 275 nm, 145 nm, 55 nm and 33 nm deposited on a glass substrate at room temperature are shown in Fig. 2(a-d), respectively. From the SEM micrographs, the SnTe thin films are found to be smooth, uniform, homogenous showing the presence of large as well as small size agglomerated nanoparticles of spherical shape. The increase in the size of nanoparticles has been 7

observed with the enhancement of the thickness of thin films. However, the SEM image of SnTe thin film of 55 nm thick (Fig. 2c) revealed the distribution of variable size nanoparticles in comparison to other thin films. Hence, the morphological analysis of SnTe thin films are in good agreement with the XRD data (Table 1) in terms of variation of grain size and crystallite size as per the thickness of the films is concerned. The crosssection view of SnTe thin films of thicknesses: 275 nm, 145 nm, 55 nm and 33 nm are shown in Fig. 2(e–h). The thickness measured by the Stylus profilometer is found to be fairly matched with the cross-section view of thin films obtained by SEM.

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Figure 2: SEM images of SnTe thin films of different thickness deposited on glass substrate: (a) 275 nm, (b) 145 nm, (c) 55 nm, (d) 33 nm, and (e) – (h) cross-section view of SnTe thin films of thickness: 275 nm, 145 nm, 55 nm and 33 nm, respectively.

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SnTe thin films deposited on a NaCl substrate at room temperature were characterized by using HRTEM for microstructural features associated with them. The microstructural details are shown in Fig. 3(a-h). TEM micrographs of SnTe thin films as shown in Fig. 3(a, c, e and g) revealed the formation of a homogeneous, uniform and randomly oriented thin film depicting polycrystalline nature. HRTEM micrographs of SnTe thin film deposited at 275 nm thickness shown in Fig. 3b indicates the lattice fringes with fringe width 3.04 Å corresponding to the (220) plane of cubic structure. Detailed analysis of the selected area electron diffraction (SAED) pattern as shown in the inset of Fig. 3a also revealed the existence of (220) plane in 275 nm thick film. The SAED pattern shown as an inset in the TEM image Fig. 3c revealed the polycrystalline nature indicating the presence of (200), (220), (222), and (420) planes. HRTEM image as depicted in Fig. 3d also shows the existence of the lattice fringes. These lattice fringe widths on indexing found to be corresponding to (200) and (222) planes. The similar planes have also been depicted in the SAED pattern in Fig. 3c. The results of SAED pattern of the SnTe thin are found to be closely matched with the XRD patterns of these films. Fig. 3(e and f) shows the HRTEM image of SnTe thin film of 55 nm thick depicting the lattice fringes. The SAED pattern (shown as an inset) on analysis revealed the presence of (200), (220), (400), (420), (422) planes. Microstructural features associated with SnTe thin film deposited on a glass substrate at a thickness of 33 nm is shown in Fig. 3(g and h). SAED pattern (inset) depicts the polycrystalline nature of the film The lattice fringes as shown in the HRTEM image revealed the presence of various planes such as (200), (220), and (400). Diffraction planes as revealed in SAED patterns of all the thin films are found to be in close agreement with standard values of JCPDS files no. 08-0487 suggesting the formation of good quality SnTe thin films of different thicknesses.

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Figure 3: TEM images and SAED patterns of SnTe thin films of different thickness deposited on NaCl substrate: (a, b) 275 nm, (c, d) 145 nm, (e, f) 55 nm and (g, h) 33 nm, respectively, while the orientation of various planes is shown in inset of SAED pattern. 11

3.3 Spectroscopic Analysis Elemental distribution analysis of SnTe thin films was carried out by the TOF-SIMS technique. Wide and narrow range depth profile of SnTe thin films of thickness 145 nm and 55 nm were measured and shown in Fig. 4(a–b) and 4(c–d), respectively. Fig. 4(b and d) shows the depth profile of SnTe thin films on a glass substrate and gives detailed information on the variation of composition with respect to a depth below the initial surface and such information is useful for the analysis of layered structures. This depth profile of the sample may be obtained simply by recording sequential SIMS spectra as the surface is gradually eroded away by the incident ion beam probe. Fig. 4(b and d) demonstrate the plot of the intensity of a given mass signal as a function of time and it is a reflection of the variation of its abundance/concentration with depth below the surface. This depth profile is similar for both the 275 nm and 145 nm thin films (Fig. 4a and b) but there is considerable intensity variation of 55 nm thin film of SnTe (Fig. 4c and d). For both the films of SnTe, the Sn and Te profile moves smoothly and drops down with time. The low-intensity B signal comes out when the primary ion beam touches the glass substrate. It is also clearly observed that both Sn and Te diffuse into the substrate at room temperature very slightly which is ignorable. The inter-diffusion of Sn and Te at the interface of the substrate is dependent on the thickness of the thin film. An increase of B intensity is very prominent when the profile starts on the glass substrate. Here, also the high-intensity inter-diffusion occurs at 145 nm thickness of the thin film in comparison to 55 nm thick film. The inter-diffusion near the substrate interface could be due to the knock-on effect during the sample deposition [18]. Positive high mass resolution spectra were also acquired from a 500 μm × 500 μm area on the sample using a cycle time of 100 μs. TOF-SIMS spectra of the SnTe thin film were recorded at the top surface without sputtering. For the thin film of thickness 275 nm many peaks from range 112 – 122 amu belongs to Sn isotopes (Sn112, Sn114, Sn115, Sn117, Sn118, Sn119, Sn120 and Sn122) and from 123 – 130 amu belongs to Te isotopes (Te123, Te124, Te125, Te126, Te128 and Te130) as shown in Fig. 5a. Antimony isotope Sb121 is also present in these thin films. In the TOF-SIMS spectra, it has been observed that the

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intensity of Te is much lower as compared to Sn. The relative sensitivity factor for Te is one order lower in comparison to Sn, so Sn is highly sensitive in comparison to Te. Fig. 5b shows the presence of various isotopes of Sn and Te in the thin film of thickness 145 nm. Here it is observed that the peaks from range 112 – 124 amu belongs to Sn isotopes (Sn112, Sn114, Sn116, Sn117, Sn118, Sn119, Sn120, Sn122 and Sn124) and from 122 – 130 amu belongs to Te isotopes (Te122, Te123, Te124, Te125, Te128 and Te130). TOF-SIMS spectra of SnTe thin films of thicknesses of 55 nm is shown in Fig. 5c depicting the presence of various isotopes of Sn and Te in the thin film. Peaks from range 112 – 124 amu represents to Sn isotopes (Sn112, Sn114, Sn115, Sn116, Sn117, Sn118, Sn119, Sn120, Sn122 and Sn124) and from 120 – 130 amu belongs to Te isotopes (Te120, Te122, Te123, Te124, Te125, Te126, Te128 and Te130). Similarly for the SnTe thin film of thickness 33 nm peaks in the range 112 – 124 amu belongs to Sn isotopes (Sn112, Sn114, Sn115, Sn116, Sn117, Sn118, Sn119, Sn120, Sn122 and Sn124) and from 120 – 130 amu represents to Te isotopes (Te120, Te122, Te123, Te124, Te125, Te126, Te128 and Te130) as depicted in Fig. 5d. These isotopes present in the thin films play a major role during symmetrical stretching molecular vibrations of the Sn-Te molecule. The presence of various isotopes of Sn and Te elements in these thin films indicates that the as-grown bulk SnTe compound (source) already contained the isotopes of Sn and Te elements [17]. Ten stable isotopes with different abundance ratios for Sn element exist and six stable isotopes with different abundance ratios for Te element exist in nature. In this case, one representative isotope has been shown in each case for this depth profile analysis. When the thin film samples were prepared, all the isotopes were also incorporated into this sample and these isotopes were later detected by TOF-SIMS. The other isotopes can also be detected here during depth profile measurement but have not been presented here for brevity. Since this is a time of flight based SIMS technique involving a bit of random sampling relative intensity ratio in the sample to sample thus this is not being discussed too much here.

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Figure 4: Depth profiles of secondary Sn+ and Te+ ions of SnTe thin films on a glass substrate for profiling regime with the sputter and probe beams raster widths 500 × 500 μm2: (а) 145 nm and (c) 55 nm films, and depth resolution of lower intensity signal, (b) 145 nm and (d) 55 nm thick thin films of SnTe.

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Fig. 5: Positive TOF-SIMS spectra of SnTe thin films of thicknesses: (a) 275 nm, (b) 145 nm, (c) 55 nm and (d) 33 nm showing the presence of various molecules and other elements up to ppm level. 3.4 Optical Analysis FTIR spectroscopy was used for the identification of the fingerprint region of the vibration of molecules. The FTIR spectrum of the thin film of thickness 275 nm deposited on a glass substrate as shown in Fig. 6a indicates that various vibrational peaks belong to different molecules exist in the thin film. In the range of 2500-3200 cm-1 wavenumbers, the band of various peaks belongs to Sn-Te symmetrical stretching molecule. But some peaks of low energy in the band may be attributed to asymmetrical stretching of SnTe molecule. The transmittance peak at wavenumber 414.8 cm-1 could be attributed to (O-Te-O) molecule. Other transmittance peaks at 560.8, 1241.7, 1677.9, 2315.9, 2359.1 and 3659.8 cm-1 correspond to vibrations of (Sn-O), (C-O), (C=C), (H-O15

H), (Si-Hx) and (SiO2- OH) molecules, respectively. Fig. 6b revealed the FTIR spectra of the SnTe thin film of thickness 145 nm deposited on a glass substrate. The transmittance peaks at wavenumber 2668 cm-1 correspond to the Sn-Te stretching mode of vibration. Other transmittance peaks at 518, 761, 875, 1095, 1262, 1947, 2121, 2334 and 3801 cm-1 correspond to (Sn-O) stretching, (Si-N- O), (O-Sn-O) stretching, (Si-O-Si), primary or secondary O-H in-plane bending, alkenes (C=C) asymmetric stretching, (Si-H) stretching, (H-O-H) and (Si-O-H) molecules respectively. Similarly, for the SnTe thin film having thickness 55 nm, the FTIR spectrum as shown in Fig. 6c revealed the transmittance peaks at wavenumbers 742, 864, 1086, 1929 and 2312 cm-1 which corresponds to (Si-N-O), (OSn-O) stretching, (Si-O-Si) asymmetric stretching, (M-H) stretching where M is any metal and (H-O-H) symmetric stretching respectively. Transmittance peak at 3484 and 3668 cm-1 could be attributed to (Si-O-H) molecule. A single transmittance peak at 2649 cm-1 corresponds to the stretching vibration of (Sn-Te) molecule. The transmittance peaks of SnTe thin film of thickness 33 nm is shown in Fig. 6d. The spectra exhibits various transmittance peaks at wavenumbers 497, 764, 861, 1102, 1355, 1936, 2097 and 2312 cm-1 which corresponds to molecular vibrations of (Sn-O), (Si-N-O), (O-Sn-O) stretching, (Si-O-Si), (-C-H) alkane bending, (C=C) alkenes asymmetric stretching, (SiH) stretching and (H-O-H) molecules respectively. The transmittance peak at 2619 cm-1 may attribute to the symmetrical stretching vibration of (Sn-Te) molecules. While another peak at 2978 cm-1 represents the (CH3) asymmetric stretching of hydrocarbon molecule. But the other two peaks at 3476 and 3645 cm-1 represent the presence of symmetric stretching of (Si-O-H) molecule in the thin film. This indicates the IR sensitiveness of SnTe thin films and the enhancement of optical properties at the level of nano-regimes. Since at the nanoscale region, the surface to volume ratio is higher, hence more surface area leads to more open sites, which indicates that dangling bonds/ unfilled bonds are present in the thin film structure. Therefore, the possibility of bond formation is quite prominent during the synthesis of a thin film on nanoscale [34, 35, 43]. From Fig. 6, an increase of oscillation number is noticed with an increase in the thicknesses of the thin films and it makes the films more homogenous [34, 35, 36]. The film of higher thicknesses has absorption edges shifted towards the low energy side, which indicates a 16

small bandgap. As the film thickness increases from 33 nm to 275 nm, the variation in optical absorption edge occurred indicating the creation of the defect in 275 nm thick film during the growth of film. Therefore, due to an insufficient number of atoms some unsaturated bonds could be produced [34]. These bonds are liable for the creation of some defects in the films and these defects invoke localized states in the films. Hence, the film of thickness 33 nm due to deficiency of defects is most suitable for MIR applications.

Figure 6: FTIR spectra of (a) 275 nm, (b) 145 nm, (c) 55 nm and (d) 33 nm thick SnTe films deposited on glass substrates. Raman spectroscopy is an informative technique for studying alloys and nanostructures. Micro-Raman spectroscopy was used for the identification of phonon modes in the material. Raman spectra were obtained at room temperature (RT) by using Ar+ laser of wavelength, 514 nm in backscattering geometry. To reduce excessive heating effects at the sample surface normally less than 15 mW incident power was used. Peak position shifting was noticed in SnTe thin films as compared to reported results and it has also been observed that the longitudinal – optical (LO) phonon scattering occurs at RT for bulk SnTe [17]. It is observed that E (LO) mode and A1 (LO) mode Raman frequencies do not much differ from each other and these modes may have the same effective charge e*. Both [222] and [200] orientation spectra were found to be diagonally polarized [17]. The observed scattering peak cannot be attributed to the transverse optical (TO) – phonon 17

mode because its frequency was temperature independent. It is also possible that this peak may arise from defects scattering. It was observed that at lower frequencies, no structure was obtained due to the background of Rayleigh scattering [17]. Therefore, 2 LO – phonon structure was not observed anywhere in the spectra for SnTe. To interpret the oxidation state and surface characteristics, the measurement range 100– 400 cm-1 was chosen. Fig. 7a shows the spectra of SnTe thin film of thickness 275 nm, indicating the peak at wavenumber 122.49 cm–1 is attributed to scattering at optical phonons of symmetry A1 which revealed the strong existence of the LO phonon frequency in SnTe. The peak at 140.06 cm–1 is associated with scattering at TO phonons of symmetry E. Generally, Raman modes at 140-172 cm-1 are assigned to substoichiometric SnOx (1 < x < 2) intermediate between SnO and SnO2. The peak position at wavenumber 140 cm-1 belongs to crystalline SnO2 molecule that depicts the Eu mode observed in the IR spectra, and the reason for the activation of small particle size. For SnO2 molecule in the Raman active modes, the Sn atoms keep at rest while oxygen atoms vibrate. But it was also observed that TeO2 nano-crystals may be caused by the quantumsize effect and/or by changes in the crystalline structure of TeO 2. The γ-TeO2 single crystals may be responsible for the Raman peak at 140.06 cm–1 and expected broadening (or an asymmetry) as well as the shifts of the Raman bands. It was observed that the infrared spectrum is modified when the size of the SnTe grain is reduced and this may be happened due to the interaction between electromagnetic radiation and the particles. All this process depends on the crystallite size, shape, and state of aggregation that occurred during the synthesis process of the thin film. Raman spectrum of SnTe thin film of thickness 145 nm is shown in Fig. 7b. The highest intense peak at 124.77 cm-1 is attributed to scattering at optical phonons of symmetry A1 which exist in the region that is used to demonstrate the surface characteristics and oxidation state. The quantization of the phonon spectra manifests itself in lowdimensional structures (thin films) under a decrease in system size (quantum size effect). The spectral redistribution inside the region of 100–180 cm–1 is probably due to the appearance of a supplementary band at 142 cm–1 typical for γ-TeO2, which enlarges the total intensity observed in the region of 140–180 cm–1. Therefore, the Raman peak at 18

142.66 cm-1 may attribute to γ-TeO2 single crystal structure [17,37, 41]. The total „enhancement‟ of the Raman spectrum seen for the nano-composite can be explained by multiple reflections of the exciting photons from disordered elements present in the synthetic opal structure, which result in increasing temporal intervals of radiationsubstance interactions. As per literature [41], the 142.66 cm-1 peak position is not attributed to SnO2 nanoparticles but the presence of SnO2 cannot be discarded at any level because the IR spectrum of this thin film clearly indicates the presence of vibrations of (O-Sn-O) molecule. On analyzing the Raman spectra as observed for SnTe thin film of thickness 55 nm as shown in Fig. 7c, the active peak at 121 cm-1 indicates the presence LO - phonon mode vibration of Sn-Te related to scattering at optical phonons of symmetry A1. However, peak at 140.52 cm-1 is found to be associated with scattering at TO - phonons of symmetry E. In the Raman active modes for SnO2 molecule, the oxygen atoms vibrate while Sn atom is at rest position. Fig. 7d revealed the Raman active phonon modes in SnTe thin film of thickness 33 nm deposited on a glass substrate. The peak at 121.04 cm-1 indicates the LO phonon mode of Sn-Te molecule associated with scattering at the optical phonon of symmetry A1. However, another Raman active peak at 139.99 cm-1 is mainly attributed to phonon mode of SnO2 molecule which is associated with scattering at TO - phonons of symmetry E. The peak position at wavenumber 139.99 cm-1 is similar to the Eu mode that belongs to SnO2 molecule and gets activated due to the small particle size. For large size particles, a higher volume for Raman scattering is available; thus, there is no effect due to the size distribution. But at wavenumber 163.09 cm-1 the Raman peak could be attributed to phonon modes of -TeO2 molecules as per literature [37, 13, 34, 41].

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Figure 7: Raman spectra of (a) 275 nm, (b) 145 nm, (c) 55 nm and (d) 33 nm thick SnTe films deposited on glass substrates. 4.0 Conclusions A series of SnTe thin films of different thicknesses have been deposited on glass substrates using thermal evaporation technique at room temperature. HRXRD analysis confirms the crystalline features and formation of NaCl like cubic crystal structure with space group,

in all the synthesized films of SnTe. The grain size is found to vary

between 10-50 nm as revealed in the microstructural investigations of SnTe films using HRTEM. The microstructural studies and the SAED pattern of thin films performed by HRTEM suggest that the inter-planar spacing values are found to be in good agreement with HRXRD results. SIMS analysis reveals the uniform distributions of isotopes of various elements throughout the thickness of these films along the lateral and longitudin a l directions. As the thickness of thin film increased from 55 nm to 275 nm the crystallite 20

size was noticed increasing sequentially. An interesting feature has been observed in nano-regime for the 33 nm thick film, the crystallite size and agglomerated nanoparticles found to increase dramatically. The large surface-to-volume ratio at the nano-scale regime could be the reason behind these facts. Peak shifts in all the thin films have been noticed as investigated from micro-Raman, the variation in absorption edge may be attributed to the film thickness and also this behavior demonstrates an improvement in the disorder with reduction of thickness. FTIR and micro-Raman spectra revealed that the lowest thickness around 33 nm could be the suitable thickness of SnTe material for MIR applications. Acknowledgment We would like to thank Director, CSIR-National Physical Laboratory, India for providing the necessary experimental facilities to carry out the research work. Authors are also thankful to Dr. R. P. Pant, Dr. Sandeep Pundir, Mrs. Sweta Sharma, Mr. Dinesh Singh, Dr. Jai S. Tawale, Mrs. Geetanjali Shegal and Dr. Parveen Jain for their support during synthesis and characterization of work. References [1] E. Baudet, A. G. –Arroya, M. Baillieul, J. Charrier, P. Němec, L. Bodiou, J. Lemaitre, E. Rinnert, K. Michel, B. Bureau, J. L. Adam, V. Nazabal, Development of an evanescent optical integrated sensor in the mid-infrared for detection of pollution in ground water or sea water, Adv. Dev. Mater. 3 (2017) 23 – 29. [2] E. K. H. Salje, D. J. Safarik, K. A. Modic, J. E. Gubernatis, J. C. Cooley, R. D. Taylor, B. Mihaila, A. Saxena, T. Lookman, J. L. Smith, Tin Telluride: A weakly coelastic metal, Phys. Rev. B 82 (2010) 184112. [3] P. Chandra, A. K. Verma, R. K. Shukla, A. Srivastava, Characterization of Copper added Tellurium rich chalcogenide thin films, Int. Jour. Adv. Sci. Res. Manag. 2 (2017) 36-40. [4] Y. Tanaka, Z. Ren, T. Sato, K. Nakayama, S. Souma, T. Takahashi, K. Segawa, Y. Ando, Experimental realization of a topological crystalline insulator in SnTe, Nat. Phys. 8 (2012) 800 – 803. [5] M. Safdar, Q. S. Wang, M. Mirza, Z. X. Wang, K. Xu, J. He, Topological surface transport properties of single crystalline SnTe nanowire, Nano Lett. 13 (2013) 5344 – 5349. 21

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[39] M. Geilhufe, S. K. Nayak, S. Thomas, M. Däne, G. S. Tripathi, P. Entel, W. Hergert, A. Ernst, The effect of hydrostatic pressure and uniaxial strain on the electronic structure of Pb1xSnxTe,

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[40] D. B. Williams, C. B. Carter, Transmission Electron Microscopy – A text book for materials science, Springer Publication, 2nd edition 2009. [41] B. A. Sal, V. Moiseyenko, M. Dergachov, A. Yevchik, G. Dovbeshko, Manifestation of metastable γ-TeO2 phase in the Raman spectrum of crystals grown in synthetic opal pores, Ukr. J. Phys. Opt. 14 (2013) 119 – 124. [42] Y. Xu, W. Li, C. Wang, Z. Chen, Y. Wu, X. Zhang, J. Li, S. Lin, Y. Chen, Y. Pei, MnTe 2 as a novel promising thermoelectric material, J. Materiomics 4 (2018) 215 – 220. [43] S. Ghosh, L. Manna, The many “Facets” of halide ions in the chemistry of colloidal inorganic nanocrystals, Chem. Rev. 118 (2018) 7804 – 7864. List of Tables Table 1: Lattice parameters and crystallite size of variable thickness SnTe thin films. List of Figures Figure 1: HRXRD patterns of SnTe thin film of thicknesses: (a) 275 nm, (b) 145 nm, (c) 55 nm and (d) 33 nm deposited on glass substrate.

Figure 2: SEM images of SnTe thin films of different thickness deposited on glass substrate: (a) 275 nm, (b) 145 nm, (c) 55 nm, (d) 33 nm, and (e) – (h) cross-section view of SnTe thin films of thickness: 275 nm, 145 nm, 55 nm and 33 nm, respectively. Figure 3: TEM images and SAED patterns of SnTe thin films of different thickness deposited on NaCl substrate: (a, b) 275 nm, (c, d) 145 nm, (e, f) 55 nm and (g, h) 33 nm, respectively, while the orientation of various planes is shown in inset of SAED pattern. Figure 4: Depth profiles of secondary Sn+ and Te+ ions of SnTe thin films on a glass substrate for profiling regime with the sputter and probe beams raster widths 500 × 500 μm2: (а) 145 nm and (c) 55 nm films, and depth resolution of lower intensity signal, (b) 145 nm and (d) 55 nm thick thin films of SnTe.

25

Figure 5: Positive TOF-SIMS spectra of SnTe thin films of thicknesses: (a) 275 nm, (b) 145 nm, (c) 55 nm and (d) 33 nm showing the presence of various molecules and other elements up to ppm level. Figure 6: FTIR spectra of (a) 275 nm, (b) 145 nm, (c) 55 nm and (d) 33 nm thick SnTe films deposited on glass substrates. Figure 7: Raman spectra of (a) 275 nm, (b) 145 nm, (c) 55 nm and (d) 33 nm thick SnTe films deposited on glass substrates. Table 1: Lattice parameters and crystallite size of variable thickness SnTe thin films. S.No.

Film

Lattice

Standard

Crystallite

Space

Structure

Thickness

parameters

JCPDS file

size calculated

group

type

with

and angles

data

from Scherrer

error

Formula

(nm) 1.

275  8.7 nm

2.

145  5.5 nm

3.

55  3.4 nm

4.

33  3.9 nm

a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c= 6.303 Å  = 90

a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90 26

14 nm

JCPDS file no. 08-0487 having cubic crystal structure

14 nm

JCPDS file no. 08-0487 having cubic crystal structure

6 nm

JCPDS file no. 08-0487 having cubic crystal structure

13 nm

JCPDS file no. 08-0487 having cubic crystal

 = 90  = 90

 = 90  = 90

structure

Authors' contributions to the manuscript: TSF-D-18-01962-R3 as per the format of CRediT statement. Praveen Tanwar: Conceptualization, investigation, Writing an initial draft, Amrish K. Panwar: Writing & Review and editing, Analysis, Supervision, Sukhvir Singh: Resource, Visualization, Investigation, Supervision, A. K. Srivastava: Supervision, Analysis

Table 1: Lattice parameters and crystallite size of variable thickness SnTe thin films. S.No.

Film

Lattice

Standard

Crystallite

Space

Structure

Thickness

parameters

JCPDS file

size calculated

group

type

with

and angles

data

from Scherrer

error

Formula

(nm) 1.

275  8.7 nm

2.

145  5.5 nm

3.

55  3.4 nm

a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å

a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å 27

14 nm

JCPDS file no. 08-0487 having cubic crystal structure

14 nm

JCPDS file no. 08-0487 having cubic crystal structure

6 nm

JCPDS file no. 08-0487

4.

33  3.9 nm

c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c= 6.303 Å  = 90  = 90  = 90

c = 6.303 Å  = 90  = 90  = 90 a = 6.303 Å b = 6.303 Å c = 6.303 Å  = 90  = 90  = 90

28

having cubic crystal structure 13 nm

JCPDS file no. 08-0487 having cubic crystal structure