Role of Sn Inclusions on Structural, Electrical and Optical Properties of Sb₂S₃

Raies Abdullah ¹ , Waqar Adil Syed ² , Ahsan Ali ¹ , Awais Ghani ¹ , Wasif­-ur­-Rehman ³ , Wajid Ali ⁴ , Muhammad Idrees ⁵ , Naveed Hussain ?? ⁶ , Shehzad Ahmed ?? ¹

1 State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an , 710049, China

2 Department of Physics, International Islamic University, Islamabad, 44000, Pakistan

3 MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China

4 Xi'an Key Laboratory of Sustainable Energy Materials Chemistry, School of Chemistry, Xi'an Jiaotong University, Xi'an 710049, China

5 Institute of Micro scale Optoelectronics, Shenzhen University, Shenzhen, 518060, PR China

6 Institute of Fundamental and Frontier Sciences (IFFS), University of Electronic Science and Technology of China, Chengdu, 610054, China

Abstract

Tin antimony sulfide (TAS) thin films are deposited on a glass substrate using a two-source thermal evaporation method. The doping was done to investigate its results on the structural, electrical, and optical properties of tin-antimony sulfide thin films. XRD studies disclosed that annealed films exist in the Sn₂Sb₂S₅ phase. The accurate information about the composition and concentration depth of the as-deposited and annealed Sn₂Sb₂S₅thin-films was recorded by the Rutherford backscattering spectroscopy (RBS) technique. The photoconductivity response of these films was also excellent and enhanced with the increasing concentration of tin sulfide. The obtained bandgap was in the range of 1.68-2.31 eV. The thickness of the films was observed to be 240 nm to 336 nm, and the thin films possessed P-type conductivity. Electrical properties are significantly enhanced for the ternary compound Sn₂Sb₂S₅ as compared to Sb₂S₃. Results show that antimony tin sulfide has excellent potential in photovoltaic applications.

Keywords

Two sources thermal evaporation, Band­gap, Ternary compound, p-type, Thin film

INTRODUCTION

The demand for sustainable energy using solar cells has risen from a year ago. Since the production of first-generation research and development for enhancement of cell efficiency, new paths towards cost-effective approaches have opened.1, 2, 3, 4, 5, 6 Continuous improvements in photovoltaic devices have made solar energy economically useful. Silicon solar cells have attracted significant attention in recent years and have reached their theoretical limits, but their cost remains high. Current photovoltaics research focuses on earth-abundant materials, dye-sensitized solar cells, non-toxic materials, and nanostructured materials.7, 8, 9, 10, 11 In thin-film photovoltaics, CdTe, GaAs, CuInGaSe, and InP are the leading materials for solar cells. But these materials have high toxicity and involve rare elements, which makes them expensive and harmful to the environment.12, 13Sulfides based composites are non-toxic, although associated efficiencies of these photovoltaics are up to 10.1%. As a result of their non-toxicity and abundance, sulfide materials are preferred as absorber layers in solar cells.14, 15, 16, 17, 18 Thin films of pure and doped with Sb₂S₃ are commonly used for photovoltaic devices. The reason for using Sb₂S₃ thin films for energy conversion in solar cells is their properties, such as having a direct bandgap of 1.78–2 eV and a high absorption coefficient of 10⁴–10⁵cm^-1. Solar cells must be manufactured with less expensive materials to be cost effective and easily commercializable, should have a direct bandgap, and high absorption of light.19 To obtain the required physical properties, the improved method was introduced as the substitution of an atom with fewer or more electrons in its valence shell compared to the host atom. Sb₂S₃ is a member of the V-VI layered semiconductor family. In the family of available chalcogenides, Sb₂S₃ soft films are used in many essential applications, such as highly reactive cooling systems, solar power conversion, solid-state devices, and TV type screens.20 SnS is also a well-practiced chalcogenide material that has a high optical absorption coefficient, p-type conductivity, and a minimal direct band gap of 1.3-1.4 eV. The energy conversion efficiency of tin sulfide is expected to be up to 32%, which is comparable to crystalline silicon. Whereas SnS is stable in both acid and alkali conditions, All these factors indicate that the substitution of tin sulfide to form SbSnS semiconductors will significantly affect photovoltaic applications.21, 22 In previous literature, many procedures have been used for Sb₂S₃ and SnS deposition, such as chemical bath deposition, chemical vapor deposition, electric deposition, electron beam, spray pyrolysis. In the present studies, Sn-doped using S as a precursor in Sb₂S₃ deposited onto a glass substrate using two source methods, characterized by XRD and SEM for structural and morphological properties, and Rutherford back scattering (RBS) for elemental and compositional analysis. The optical parameters like refractive index (n), extinction coefficient (k), absorption coefficient (α), and optical conductivity was calculated by ellipsometry. Tin generates acceptors level that helps to enhance optical processing. Formation Sb₂Sn₂S₅ with different compositional wait percentages of Sn significantly improved the optical structural and electrical properties. Results evidenced that Sb₂Sn₂S₅ could bring a potential enhancement in the efficiency of solar cells.

EXPERIMENTAL SETUP

The two-source thermal evaporator was used for doping by the sublimation technique. All elements, i.e., S and Sb, have 4N purity, were used to prepare the initial ingot of Sb₂S_3. Because sulfur can explode when heated rapidly, so as a precaution, we increased the temperature at a relatively slow (15 °C/h) rate. A complete uniformity in composition is accomplished by keeping the mixture at 350 °C for 24 hours. X-rays of powder analysis was performed to study the crystal structure of fabricated materials. X-rays charachterization results showed that only the homogeneous Sb₂S₃ phase was formed in the ingot. Henceforth, the grinded smooth powder form of this ingot is used as the starting material for thermal evaporation.23, 24 SnS and Sb₂S₃ powders were co-evaporated and deposited using a two-source method on a glass substrate, as shown in Figure 1, using sulfur as a precursor in SnS. The quantity of the SnS was divided as 0.1%–0.4% molecular weight compared with the Sb₂S₃ compound source. These materials were vaporized from Al₂O₃ crucibles in the vacuum chamber for sublimation on glass substrates. The pressure of the vacuum-chamber was kept at about 2 x 10^-4 mbar. After deposition of thin films, we annealed all samples at 425 °C for 24 hours in vacuum-packed glass vessels containing argon gas. 

Figure 1: Representation of experimental route using two-Source sublimation technique and formation of the thin film of Sb₂Sn₂S₅ using Sb₂S₃ and SnS as starting materials.

RESULTS AND DISCUSSION

The discussion is mainly focused on physical properties, i.e., structural, optical, and electrical. X-ray diffraction (XRD), scanning electron microscopy (SEM), Rutherford backscattering (RBS), ellipsometry, and Hall measurement techniques were used to obtain experimental data on these important physical properties. The results are discussed in detail to get a comprehensive analysis of the properties of the target material as thin-film. The structure of prepared specimens has been analyzed using x-ray diffraction (XRD) with a Cu K α λ=1.54 A° radiation source. The diffraction spectra of all samples and calculated parameters are indexed in Figure 2(a–c). The orthorhombic phase of Sn₂Sb₂S₅ was well-coordinated with (44-0829, JCPDS-ICDD), whereas Sb₂S₃ matched with pdf # 42-1393. Furthermore, it conforms to space group pnma (62) with cell parameters a = 19.59, b = 3.93, and c=11.426. Peaks at 29.2°, 30.6°, 32.1°, 40.6°, 41.7°, 43.9°, 45.0°, and 51.5° have correspondence with planes (114), (312), (042), (143), (108), (047), and (215) which are attributed to Sb₂Sn₂S₅.

Figure 2: (a) XRD pattern for Tin antimony sulfide thin films with varying concentration of Sn, (b-c) Dislocation and induced lattice strain without substitution of Sn as 0% and after incorporation of Sn from 0.1 to 0.4 %

The sharp peaks in Figure 2(a) support the polycrystalline behavior of these films. The preferential orientation of crystal planes in these thin films is (114). Qualitatively, XRD patterns show that the compositions of Sn₂Sb₂S₅ thin films are almost the same as those of the initial material. The diffraction angle 2θ slightly shifted towards higher intensity with the addition of tin sulfide in the Sb₂S₃ because of the addition of Sn atoms in lattice sites to reorient Sb₂S₃ planes. The deviation of dislocation density and associated strain with increasing tin sulfide indicates that values are exponentially dropped, while the minimum is recorded for the film with 0.4% Sn. The lower amount of dislocation density and strain obtained at the highest Sn concentration reveals the good crystalline effect of tin-antimony sulfide films. While the increase in tin ratio significantly increases the grain size from 21 nm to 41 nm. Small crystallites can fuse to make larger crystallites, resulting in micro-cracks and surface roughness. Annealing is well-known for reducing the stress on film as well as decreasing the d-spacing. XRD patterns indicate that the increase in tin sulfide leads to the improvement of the film's growth in the favored orientation. For surface morphology and compositional analyses, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) are used.25, 26, 27, 28 The SEM images of the samples are shown in Figure 3. 

Figure 3: SEM images (a) Thin-film surface of Sb₂S₃ without Sn (b-e) surface of Sb₂Sn₂S₅ with Sn (0.1 to 0.4%) (f) EDS mapping of Sn, Sb, and S.

The surface of the samples looks smooth and free from roughness, but at the micro-level, the surface samples show little blisters on the surface. Grain size and grain boundary enhancement with an increase in the concentration of SnS small grains merged, and also larger grains split into smaller grains and reoriented themselves, which had an effect on the whole structure. SEM images also indicate a decrease in the roughness of the layer by increasing the concentration of SnS. According to SEM images, we calculated that the average grain size is almost 84.34 nm with beam energy of 20 keV. The creation of a large number of tightly bound particles of different sizes can be seen in these images. The grain size is observed to increase with Sn concentration, as shown in Figure 3. The increase in grain size is due to Sn concentration with improved crystallinity, as calculated from the XRD results. The RBS measurement was carried out on the 5 MV tandem accelerator (5UDH-2, NEC) using a 2.023 MeV He⁺ collimated beam.

RBS is a well-described, non-destructive, analytical, quantitative, and unique fundamental investigational accelerator dependent tool. For material examination, it is a distinctive technique broadly used in thin films for thickness, depth profiling of essential elements, and elemental composition. The analyzed depth for incident protons and He⁺ ions is typically near 20 m and 2 m, respectively.21, 24 The structural analysis of the films was performed using a D-8 Discover diffractometer with Cu K α radiation (wavelength, λ=1.54 A° ). The RBS experiments were done on Sn₂Sb₂S₅ thin films of varying compositions deposited on glass substrates. The gathered RBS spectra were then fitted by the code RUMP to locate the relative concentrations of a mixture of elements in the films.29, 30 Figure 4 shows the RBS spectra of annealed films of Sn₂Sb₂S_5, which proves that the film thickness and concentration of SnS shift the peaks in the higher region. RBS measured a film thickness ranging from 238 nm to 290 nm, which was compared to the thickness measured by ellipsometry, as shown in Figure 4(b). The peaks of the heavy elements (Sn, Sb, and S) can noticeably be separated by the He⁺ beam and accordingly can be used to resolve the relative thickness and stoichiometry of the films. The edge arising at a channel just below 1200 is due to the thickness of films present on the substrate.31

Figure 4: (a) Rutherford backscatteringspectra(RBS) of all S1 samples, (b) Film thickness over increased concentration of Sn by RBS, and its comparison with ellipsometry.

Figure 5: (a-d) Diffuse reflectance spectroscopy (DRS) (a-e) optical band gap estimation of Sb₂S₃ and Sb₂Sn₂S₅ using tauc-plot, (d) schematic diagram of band-edge with corresponding NHE potentials.

The energy channel correspondence for tin is 1585; antimony is 1594; while for sulfur, it is 1052. A noticeable change in elemental ratio concerning stoichiometry within the layers has been observed. It was noticed that an edge occurs at 1200. The edge arises due to the excessive enrichment of Sn in the Sn₂Sb₂S₅ and corresponds to the unbound Sn content which subsists on the surface. According to the RUMP simulation software, thin-film of Sn 0.4% exhibits the most homogeneous Sn concentration as a function of depth as compared to other studied samples. The thickness of the films estimated by the RBS technique and spectroscopic ellipsometry are in good agreement. Moreover, the deposited films were homogeneous, even, and well adherent to the substrate. The bandgap of the samples was calculated by DRS, which is shown in Figure 5. The calculation was done by using a kabulka maunk and tauc plot method using reflection data.32, 33 The obtained bandgap of a sample with Sn=0%, which represents Sb₂S₃, is 1.68 eV, whereas the formation of Sb₂Sn₂S₅ significantly changes the bandgap from 1.68 to 2.31eV, as shown in Figure 5(a-e). The band edges of all samples are shown in Figure 5(f), where conduction and valance band edges widen as tin ratios increase. Substantial favorable placement of the Fermi level near the valance band, i.e., 0.91eV, indicates that it has P-type behavior. The bandgap of samples lies within a visible region, which makes it favorable as an absorber layer of solar cells. Further optical parameters like refractive index, extinction coefficient, and dielectric coefficients were calculated by the ellipsometry technique using an apparatus model named (J.A. Woolam M-200VI) given in Figure 6. 

Figure 6: Ellipsometry of Sb₂Sn₂S₅ thin films (a) Refractive index (n) (b) Extinction coefficient (k) (c-d) Dielectric constant ε1 and ε2.

The thickness of the Sn₂Sb₂S₅ films is measured by ellipsometry as given above, which is in the range of 251 nm to 330 nm. Optical properties were analyzed from 300 nm to 900 nm in wavelength at room temperature. The measurements were carried out on prepared thin films with varying tin sulfide concentrations. In Figure 6(a-b), the refractive index (n) and the extinction coefficient "K" were plotted against the wavelength of refraction tells us the penetration of light in the material and the amount that is reflected from the surface. In this context, the refractive index of Sb₂Sn₂S₃ has a dominant decrease after substitution of Sn and follows up with an increase ratio. At the same time, a marginal increase was observed in the extinction coefficient for increasing Sn concentration, which indicates the enhanced absorption of light in thin films. The higher value recorded was for Sn=0.4% concentration. Dielectric constants are shown in Figure 6(c-d). They are divided into real (?₁) and imaginary (?₂) parts. It decides the response of the material under electromagnetic radiation where real and imaginary are inverse to each other. ε₂ increased with a thickness which directly relates to enhanced absorption of light, but ε₁ describes the retardation of light in the film.10, 11, 12

Table 1: Hall Measurements of the Sb2Sn₂S₅ thin films.

The electrical analysis was done by hall measurement using a two-probe method, and by finding the Current-Voltage (IV) response for all samples and results are shown in table 1 and Figure 7. According to the IV curve, we perceive that the examples are highly conductive. By hall measurement, we confirmed that the samples are conductive, having low resistive values, high carrier concentration, and high sheet carrier mobility. The consequences of tin density on thin-film resistivity, mobility, and carrier concentration were measured. Some of the essential parameters are that the area of thin films was 1 cm² along with 0.55-tesla constant magnetic field, and one mA current was applied. Calculations show that resistivity lowers its components with increasing tin concentration.34, 35 The resistivity decreases mainly due to an increase in the crystallinity of the material. That enhances free charge carriers, as confirmed by XRD and SEM results. The relation between mobility and resistivity shows that the mobility of carriers and concentration is enhanced. The carrier concentration increases with the tin component until it attains a maximum.

Figure 7: (a) I-V measurements of all samples with different concentrations of Sn, (b) conductivity versus resistivity with a different weight percentage of Sn.

Where Rs stands for sheet resistance; Rho stands for resistivity; Con stands for conductivity; Ns stands for sheet carrier concentration; and µs stands for sheet carrier. Mobility The mobility value without tin is 11.9 cm² V^-1s^-1, while Sn=0.4% is 20.4 cm² V^-1 s^-1. This increase in mobility is attributed to the availability of free electron charge carriers coming from the tin ions. In Sb₂S₃ sites, tin ions are placed and decrease the potential grain barrier. The increase in mobility has a lower value for samples 2 and 3 as compared to other samples, due to the increasing rate of scattering for charge carriers along with the formation of secondary levels, as confirmed by the X-ray diffraction technique. In I-V measurement, the evaluated electrical parameters support that TAS thin films prepared by the two-source method demonstrate better electrical conductivity due to better crystalline quality.

CONCLUSIONS

Tin antimony sulfide was deposited on the glass surface by a two-source evaporation vacuum thermal technique and then annealed in an argon (Ar) gas environment at 350 °C. The crystallite size is calculated by x-ray diffraction and increases with Sn concentration. However, the average particle derived from SEM images is 76.15 nm. We see the homogenous structure of thin films by SEM images and grains are uniformly distributed on thin films. The physical formation of tin-antimony sulfide on thin films is crystalline in nature, with an orthorhombic phase. The material has a direct bandgap that covers the visible region and shows P-type conductivity. The refractive index, extinction coefficient, and dielectric constants prove beneficial in enhancing photon-surface interaction. The electrical properties of Sb₂Sn₂S₅ improved after the substitution of Sn with an increase in ratio. These observations suggest that thin-films of Sb₂S₃, in combination with SnS, offer results which open up various possibilities for future work on this theme of the multi-layer of the solar cell.

References

1. Ali,  N, Jabeen,  M, Khesro,  A, Ahmed,  R, Alrobei,  H, Kalam,  A & Al-Sehemi,  A G . 2021. Combinatorial synthesis of tin antimony sulfide thin films for solar cell application. International Journal of Energy Research45(15):21527–21533.

2. Nwofe,  P Akata & Sugiyama,  M . 2020. Microstructural, Optical, and Electrical Properties of Chemically Deposited Tin Antimony Sulfide Thin Films for Use in Optoelectronic Devices. Physica Status Solidi217(9):1900881.

3. Ali,  N, Hussain,  A, Ahmed,  R, Shamsuri,  W N W, Salam,  N M & Khenata,  R . 2017. Fabrication and characterization of 150 nm tin antimony sulfide thin films, a promising window layer material for homojunction solar cells. Applied Physics A 123(4):282.

4. Devasia,  S, Shaji,  S, Avellaneda,  D A, Martinez,  J A Aguilar & Krishnan,  B . 2020. Tin antimony sulfide (Sn6Sb10S21) thin films by heating chemically deposited Sb2S3/SnS layers: Studies on the structure and their optoelectronic properties. Journal of Alloys and Compounds 827:154256.

5. Ali,  N, Ahmed,  R, Shaari,  A, Rahim,  I, Shah,  M, Hussain,  A, Ahmad,  N & Abbas,  S M . 2014. Annealing Effects on the Structural and Optical Properties of Thermally Deposited Tin Antimony Sulfide Thin Films. Brazilian Journal of Physics 44(6):733–738.

6. Syed,  W A A, Ali,  A, Rafiq,  N, Shah,  W H, Shah,  N A & Yasir,  M . 2019. BaCl2 an efficient replacement of CdCl2 treatment step for thermally deposited CdTe thin film. Materials Research Express 6(8):086438.

7. Ali,  N, Hussain,  A, Hussain,  S T, Iqbal,  M A, Shah,  M, Rahim,  I, Ahmad,  N, Ali,  Z, Hutching,  K, Lane,  D & Syed,  W A A . 2013. Physical Properties of the Absorber Layer Sn2Sb2S5 thin Films for Photovoltaics. Current Nanoscience 9(1):149–152.

8. Schock,  H W . 1996. Thin film photovoltaics. Applied Surface Science 92:606–616.

9. Dhere,  N G . 2006. Present status and future prospects of CIGSS thin film solar cells. Solar Energy Materials and Solar Cells 90(15):2181–2190.

10. Fthenakis,  V . 2009. Sustainability of photovoltaics: The case for thin-film solar cells. In: Sustainability of photovoltaics: The case for thin-film solar cells. (Vol. 13, pp. 2746-2750) Elsevier

11. Ali,  N, Hussain,  S T, Khan,  Y, Ahmad,  N, Iqbal,  M A & Abbas,  S M . 2013. Effect of air annealing on the band gap and optical properties of SnSb2S4 thin films for solar cell application. Materials Letters100:148–151.

12. Fthenakis,  V . 2012. Sustainability metrics for extending thin-film photovoltaics to terawatt levels. MRS Bulletin 37(4):425–430.

13. Green,  M A . 2006. Consolidation of thin-film photovoltaic technology: the coming decade of opportunity. Progress in Photovoltaics: Research and Applications 14:383–392.

14. Aousgi,  F & Kanzari,  M . 2011.  Study of the Optical Properties of Sn-doped Sb2S3 Thin Films. Energy Procedings 10:313–332.

15. Avellaneda,  D, Nair,  M T S & Nair,  P K . 2009. Photovoltaic structures using chemically deposited tin sulfide thin films. Thin Solid Films 517(7):2500–2502.

16. Vinayakumar,  V, Hernández,  C R O, Shaji,  S, Avellaneda,  D A, Martinez,  J A A & Krishnan,  B . 2018. Effects of rapid thermal processing on chemically deposited antimony sulfide thin films. Materials Science in Semiconductor Processing 80:9–17.

17. Hussain,  N, Liang,  T X, Zhang,  Q Y, Anwar,  T, Huang,  Y, Lang,  J L, Huang,  K & Wu,  H . 2017. Ultrathin Bi Nanosheets with Superior Photoluminescence. Small 13(36):1701349.

18. Khan,  M A, Ahmed,  A, Ali,  N, Iqbal,  T, Khan,  A A, Ullah,  M & Shafique,  M . 2016. Improved Optical Properties of Tin Antimony Sulphide Thin Films for Photovoltaics. American Journal of Materials Science and Engineering 4(1):1–6.

19. Arslan,  M, Muhammad,  R, Mahmood,  A & Rasheed,  R . 2013. Effect of thermal annealing on the physical properties of Zn1−x Cu x Se thin films deposited by close spaced sublimation technique. Acta Metallurgica Sinica (English Letters) 26(6):699–706.

20. Westland,  J C . 2015. An Introduction to Structural Equation Models. In: Structural Equation Models. (Vol. 22, pp. 1-8) Springer, Cham  10.1007/978-3-319-16507-3_1

21. Scheu,  C & Kaplan,  W D . 2012. Introduction to Scanning Electron Microscopy. (pp. 1-37) Wiley-VCH Verlag GmbH & Co. KGaA

22. Toussaint,  U V, Krieger,  K, Fischer,  R & Dose,  V . 1999.  Depth Profile Reconstruction from Rutherford Backscattering Data. (pp. 107-114) Springer

23. Hussain,  N, Rafique,  M, Anwar,  T, Murtaza,  M, Liu,  J C, Nosheen,  F, Huang,  K, Huang,  Y, Lang,  J L & Wu,  H . 2019. A high-pressure mechanism for realizing sub-10 nm tellurium nanoflakes on arbitrary substrates. 2D Materials 6(4).

24. Hussain,  N, Zhang,  M H, Zhang,  Q Y, Zhou,  Z, Xu,  X Y, Murtaza,  M, Zhang,  R Y, Wei,  H H, Ou,  G, Wang,  D, Wang,  K, Li,  J F & Wu,  H . 2019. Large Piezoelectric Strain in Sub-10 Nanometer Two-Dimensional Polyvinylidene Fluoride Nanoflakes. ACS Nano 13(4):4496–4506.

25. Hussain,  N, Zhang,  Q Y, Lang,  J L, Zhang,  R Y, Muhammad,  M, Huang,  K, Villenoisy,  T C De, Ya,  H, Karim,  A & Wu,  H . 2018. Ultrahigh Room-Temperature Photoluminescence from Few to Single Quintuple Layer Bi2Te3 Nanosheets. Advance Optical Materials (13) 6.

26. Ali,  A, Syed,  W A, Arif,  W, Ahmed,  S & Ghufran,  M A . 2019. Environment friendly nanoparticles of quaternary compound Cu0.5MgxSnS4 for possible photovoltaic and photo-catalytic applications. Materials Research Express 6(9):950–955.

27. Ali,  A, Liang,  Y P, Ahmed,  S, Yang,  B, Guo,  B L & Yang,  Y D . 2020. Mutual contaminants relational realization and photocatalytic treatment using Cu2MgSnS4 decorated BaTiO3. Applied Materials 18:100534.

28. Li,  L, Lei,  J, Wu,  L, Pan,  F, Aliofkhazraei,  M, Ali,  N, Chipara,  M & Laidani,  N Bensaada . 2021. Spectroscopic ellipsometry. In: Handbook of Modern Coating Technologies. (pp. 45-83) Amsterdam

29. Goh,  E S M, Chen,  T P, Sun,  C Q & Liu,  Y C . 2010. Thickness effect on the band gap and optical properties of germanium thin films. Journal of Applied Physics 107(2):24305.

30. Neal,  W E J . 1984. Ellipsometry and its application in the monitoring of surface cleanliness. Surface Technology 23(1):1–28.

31. Hussain,  N, Yao,  Y S, Sagar,  R U R, Anwar,  T, Murtaza,  M, Huang,  K, Shehzad,  K, Wu,  H & Wang,  Z M . 2021. Quantum-confined blue photoemission in strain-engineered few-atomic-layer 2D germanium. Nano Energy 83.

32. Nyamekye,  C K A, Bobbitt,  J M, Zhu,  Q C & Smith,  E A . The evolution of total internal reflection Raman spectroscopy for the chemical characterization of thin films and interfaces. Analytical and Bioanalytical Chemistry 2020(24):6009–6022.

33. Knoll,  W . 1991. Polymer Thin-Films and Interfaces Characterized with Evanescent Light. Makromol Chem 192(12):2827–2856.

34. Auciello,  O & Krauss,  A R . 1995. Why in-Situ, Real-Time Characterization of Thin-Film Growth-Processes? MRS Bulletin 20(5):14–15.

35. Watts,  K E, Blackburn,  T J & Pemberton,  J E . 2019. Optical Spectroscopy of Surfaces, Interfaces, and Thin Films: A Status Report. Analytical Chemistry 91(7):4235–4265.