Raman Investigations of Metal Chalcogenide Thin Films (A Short Review)

Introduction

The binary,^1-8 ternary,^9-15 quaternary^16-18 and penternary compounds^19-22 have received great attention owning to their unique properties. These films are used in a wide variety of applications such as solar cell,^23-25 sensors,²⁶ photodiode arrays,²⁷ photoconductors²⁸ and optoelectronic devices.²⁹ The chalcogen is the elements (oxygen, sulfur, selenium and tellurium) in Group 16 of the periodic table. The band gap energy was estimated to be in the range of 1-3 eV.^30,31 Currently, the solar cell market is dominated by silicon solar cell³² due to abundant, non-toxic³³ and shown remarkably higher power conversion efficiency. However, these solar cells are more expensive that other types of cells.³⁴ Thin film solar cell has always been cheaper but less efficient. This solar cell is favorable due to its minimum material usage. The two major thin film technologies such as cadmium telluride and copper indium gallium diselenide successfully contributed about 10 % of the global production market share. The major drawback is high toxic material such as tellurium, cadmium,³⁵ and selenium.

In this work, thin films were investigated using Raman spectroscopy. The purity of sample and the phase identification were also examined.

Literature Survey

Raman spectroscopy provides useful information on molecular interactions,³⁶ crystallinity, crystal phase³⁷ and chemical structure.³⁸ Raman is based upon the interaction of light with the chemical bonds within a material. Raman spectra exhibit the intensity of the scattered photons versus the frequency difference to the incident photons. Typically, the peaks fall within a range of 500 to 2000 cm^-1 and only appear if vibrational modes are sensitive to the laser wavelength used.³⁹

Raman spectroscopy is chemical analysis technique⁴⁰ and is employed to measure the scattering radiation⁴¹ of a matter (Figure 1). Generally, when light is scattered by matter, almost all of the scattering is an elastic process⁴² and does not give useful information (commonly known as Rayleigh Scatter). However, a very small percentage of scattering⁴³ is an inelastic process (known as Raman effect). Raman spectroscopy is both qualitative (measuring the frequency⁴⁴ of the scattered radiations) and quantitative (measurement of the analyte concentration in the sample by quantifying the intensity of the scattered radiations). The obtained Raman spectra give a unique data^45,46 to identify a material and distinguish it from others. Table 1 shows the advantages and limitations of Raman spectroscopy technique.

Figure 1: Raman is a light scattering technique. Click here to view figure

 

Table 1: Advantage and disadvantage of Raman spectroscopic. Click here to view table

Several efforts have been made by researchers to synthesize semiconductor thin films such as sulfur, selenium and tellurium-based nanostructured films. Typical deposition technique is shown in Table 2. In this study, the utilization of Raman spectroscopy for the characterization of thin films was discussed.

Table 2: Raman spectroscopic investigation of thin films prepared under various deposition conditions.

Thin films	Raman spectroscopy explained	Deposition method	Reference
Binary thin films
Thalium selenide	TlSe2 bending mode at 92 cm-1Tl-Se symmetric stretching modes at 158 and 140 cm-1Tl-Se asymmetric stretching modes at 208 and 185 cm-1Se-Se stretching modes at 255 and 240 cm-1 In terms of crystal structure, Tl-Se bond was observed in 2.66-2.73 Å	Thermal evaporation method	51
Arsenic triselenide (As2Se3)	The peak at 224 cm-1 corresponded to asymmetric (for amorphous phase)Weak peak at 480 cm-1 represented Se-Se vibrationThe peak at 221 cm-1 could be seen in In-doped As2Se3 filmsThere are two peaks at 216-219 cm-1 and 240-242 cm-1 for the films prepared with indium content of 2 %.	Thermal evaporation	52
Antimony triselenide (Sb2Se3)	Peak at 188 cm-1: Sb-Se stretching modePeak at 150 cm-1: Sb-Sb bondPeak at 120 and 210 cm-1: vibration mode Se-Se bondRaman spectra reflected selenium rich in sample (peaks at 70, 102, 129, 252 cm-1).	RF magnetron sputtering	53
Tungsten disulfide  (WS2)	Peak at 175 cm-1 represented vibration modePeak at 419 cm-1 attributed to WS2 phase	Pulsed laser deposition	54
SnS2 films	Citric acid was used as complexing agent during the deposition process.The films prepared using 0.375, 0.5 and 0.625 ml/L citric acid indicated the peak at 315 cm-1 (mode of hexagonal SnS2 phase).Researchers confirm the phase purity of sample.	Chemical bath deposition	55
SnS	The peak was observed at 307 cm-1 for the films prepared using 40 and 50 mTorr, indicating that the formation of Sn2S3.There are two peaks (93 and 224 cm-1) could be seen for the films prepared using 6 and 10 mTorr, respectively.An additional small peak in the spectra (135 cm-1) was detected for the films synthesized under 40, and 50 mTorr.	RF sputtering	56
FeS2	There are two main phases (pyrite [343, 379 & 430 cm-1] and marcasite [323 & 386 cm-1]) of development for the synthesis of iron sulphide.Pyrite was preferentially produced (pyrite:marcasite about 99:1) at high temperature (420 °C).At low deposition temperature (250 °C), marcasite was grown preferentially.	Evaporation method	57
InS	There are very broad peaks at 200-500 cm-1 may indicate the presence of amorphous or nano crystalline materials.Researchers were able to identify some peaks at 200-400 cm-1 contributed to the In2S3 Peak at 460 cm-1 attributed to the S-S mode of sulphur	Chemical bath deposition	58
In2S3	There are 3 peaks (115, 135 and 180 cm-1) could be observed for the films prepared at 60, 70 and 80°C.The presence of In2O3 could be detected for the films deposited at 60°CThe intensity of peak is improved in annealed films.	Chemical bath deposition	59
As2S3	There are 3 peaks (336, 230, 485 cm-1) could be seen for the films prepared using laser lightPeaks at 335-340 cm-1 associated with As-S bond stretching vibration (pyramidal phase).Peaks at 180 and 230 cm-1 attributed to As-As homopolar bond vibrationPeak at 485 cm-1 corresponded to S-S vibration of AsS4	Thermal evaporation	60
CdTe	Both the transverse (142 cm-1) and longitudinal (170.5 cm-1) mode could be found.Peak at 120 cm-1 attributed to phonon of tellurium	Thermal evaporation	61
ZnS films	Two peaks at 773 and 1078 cm-1 were detected in annealed and as-deposited films.The lattice constant value for both as-deposited (5.3 Å) and annealed films (5.66 Å) was determined.	Physical vapor deposition	62
Bi2Te3	Peak at 77 cm-1 attributed to vibration mode of BiTePeaks at 65 and 131 cm-1 due to vibration mode of trigonal directionPeak at 102 cm-1 corresponded to vibration mode in the basal plane	Electro deposition	63
CdS	Three peaks (299, 600 and 900 cm-1) were found and could be observed as longitudinal optical phonons.	Chemical bath deposition	64
CdS	There are 2 peaks could be identified (296 and 593 cm-1), for the films prepared under various pH values.The intensity of the Raman peak decreases with increasing the pH value from pH 9 to 11.	Chemical bath deposition	65
Ternary thin films
Ag-Ge-Se	The films showed GeSe4/2 corner sharing tetrahedral at 192-201 cm-1, vibration of Se atom at 210-218 cm-1, Se-Se bond at 255-270 cm-1 and Ge-Ge mode at 178 cm-1	Pulsed laser deposition	66
Ge-Sb-Se	Four main modes such as symmetric stretching mode of GeSe4/2 tetrahedral (200, 215 cm-1), stretching mode of Ge-Ge bond (170 cm-1), Se-Se stretching mode (235-245 cm-1) and   Se-Se bond vibration (265 cm-1) are observed in these films	RF magnetron sputtering	67
Ge2Sb2Te5	They observe some peaks such as 80 (GeTe4 tetrahedral), 125 (GeTe4-nGen(n=1,2)), 153 (presence of Sb2Te3) and 300 cm-1 (presence of Ge-Ge) in spectra	Thermal evaporation	68
GexAs35-xSe65	The films exhibited symmetric vibrational stretching GeSe4/2 at 198 cm-1, vibration mode of AsSe3/2 at 230 cm-1, vibration mode of selenium at 260 cm-1 and vibration mode of GeSe4/2 tetrahedral at 215 cm-1	Thermal evaporation	69
As50Se40Te10	Three major modes such as asymmetric stretching As-Te-Se mode (127 cm-1), As-Se vibration mode (228 cm-1) and Te-Te vibrational mode (472 cm-1) are observed in these films	Thermal evaporation	[70]
Cu2SnS3	The Raman peaks exhibit strong (vibration mode in monoclinic) and weak peak (Cu2SnS3 phase).	Evaporation method	71
CuSbSe2	They observe several peaks such as 188, 250, 372, 450 cm-1 (Sb2Se3), 185 cm-1 (Cu3SbSe3), and 82, 117, 144 cm-1 (presence of CuSbSe2) in spectra	E-beam evaporation deposition	72
Cu12Sb4S13	There are many peaks such as 351 cm-1 (Cu12Sb4S13),354 cm-1 (Sb-S bond stretching), 315 cm-1 (Sb-S bond bending mode), 330 cm-1 (Cu3SbS4), 468 cm-1 (CuS) could be confirmed.	e-beam evaporation	73
Cu2SnS3	There are several peaks could be identified such as 290 and 350 cm-1 (monoclinic), 325 cm-1 (tetragonal phase), 223 and 371 cm-1 (Cu2Sn3S7)	Radio frequency magnetron sputtering	74
Cu2SnS3	The films indicated orthorhombic Cu3SnS4 phase at 295 cm-1, monoclinic Cu2SnS3 at 289 cm-1 and vibration mode of tetragonal Cu2SnS3 in anneal films at 326 cm-1	Spray pyrolysis	75
Cu2SnSe3	Peak at 2926 cm-1 was observed (attributed to C-H stretching vibration of CH2) for the films prepared using 0.5 M of sodium citrate.However, this peak could not be detected when using higher concentration (0.1 M).	Electro deposition	76
Quaternary thin films
Cu2ZnSnS4	The films showed a prominent kesterite phase (331-336 cm-1), indicating microwave treatment enhanced nucleation and grow process.	Sol gel method	77
Cu2ZnSnS4	There is no Raman peak could be detected for the films prepared at room temperature and within the first 15 minutes.The films prepared under 30 minutes of sulfurization show peak at 330 cm-1The films synthesized under 60 and 180 minutes of sulfurization show higher intensities if compared to other conditions.There are no other impurities (ZnS, SnS2 and CuS) indicating high purity of sample.	Sol gel technique	78
Cu2FeSnS4	Two main modes such as sulfur pure anion around the copper metal (285 cm-1) and asymmetry vibration mode of sulphur around the tin metal (319 cm-1) are observed in these films.	Spray pyrolysis	79
CuGaxIn1-xSe2	The presence of Cu2Se (290 cm-1) and Cu(In,Ga)Se2 phase (175 cm-1) could be detected when x=1, and x=0, 0.3, respectively.	Close spaced vapor transport technique	80
Cu2ZnSnS4	Kesterite phase (332, 285, 356 and 368 cm-1), copper tin sulphide (303 cm-1) and copper sulphide (468 cm-1) were observed in Raman spectra.	Spray pyrolysis	81
CuIn(S,Se)2	There are many peaks at 473 cm-1 (Cu2-xS), 390-475 cm-1 (sulfur rich Cu2-x(S,Se)), 228 cm-1 (CuIn3Se5) and 150-400 cm-1 (amorphous In2S3)could be confirmed.	Chemical bath deposition	82
Penternary thin films
Cu2ZnSn(S,Se)4	Raman study confirmed existence of the Cu2ZnSnSe4 (196 cm-1), Cu2ZnSnS4 (338 cm-1) and CZTSSe films (202 and 329 cm-1).	Spin coating technique	83
Cu2ZnSn(S,Se)4	Raman investigation showed existence of the polycrystalline filmsStrong peaks could be seen at 196 (vibration with asymmetry of CZTSe) and 338 cm-1 (vibration with asymmetry of CZTS).Cu2ZnSnS4 (CZTS) shows narrow peak, however Cu2ZnSnSe4 (CZTSe) exhibits broad peak as the concentration of H2S was increased from 1 to 5 %.	Co-evaporation technique	84
Cu2ZnSn(S,Se)4	CZTS has three peaks (288, 337 and 367 cm-1), while CZTSSe has two peaks (208 and 329 cm-1).Pure CZTSSe phase was confirmed.	Successive ionic layer adsorption and reaction	85
Cu2ZnSn(SxSe1-x)4	Two strong peaks at 336 cm-1 (CZTS) and 195 cm-1 (CZTSe) could be identified.They report that most of the peaks shift to lower frequency due to selenium replaced sulfur in samples.	Solvothermal method	86

 

Conclusion

Raman spectroscopy is a powerful technique for characterization of thin films. Here, author reviews the Raman spectra of nanostructured films. Also, author indicates how to use it to determine structure and composition of samples. The purity of the compound and phase formation can be verified by this technique.

Acknowledgements

INTI International University is gratefully acknowledged for the financial support of this work.

Conflict of Interest

There is no conflict of interest.

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