CuAlS2 thin films Dip coating deposition and characterization 2017 Journal of Science Advanced Materials and Devices

CuAlS2 thin films Dip coating deposition and characterization 2017 Journal of Science Advanced Materials and Devices tài...

Journal of Science: Advanced Materials and Devices (2017) 215e224 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article CuAlS2 thin films e Dip coating deposition and characterization Sunil H Chaki*, Kanchan S Mahato, Tasmira J Malek, M.P Deshpande P G Department of Physics, Sardar Patel University, Vallabh Vidyanagar, Gujarat 388 120, India a r t i c l e i n f o a b s t r a c t Article history: Received October 2016 Received in revised form 12 April 2017 Accepted 14 April 2017 Available online 24 April 2017 CuAlS2 thin films were deposited by a dip coating technique at room temperature The X-ray energy dispersive (EDAX) and X-ray diffraction (XRD) analysis showed that the deposited CuAlS2 thin film is nearly stoichiometric and possesses a tetragonal unit cell structure The crystallite sizes determined from the XRD data employing Scherrer's formula and modified forms of HalleWilliamson relation like the uniform deformation model (UDM), uniform stress deformation model (USDM), uniform deformation energy density model (UDEDM), and the sizeestrain plot method (SSP) were in good agreement with each other The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies of the thin film revealed that the deposited film is uniform without any cracks and the film covers the whole of the substrates The atomic force microscopy (AFM) of the as-synthesized thin film surfaces showed spherical grains having coalescences between them The optical absorbance spectrum analysis showed that the thin films possess both direct and indirect band gaps The semiconducting and p-type nature of the thin films was confirmed from dc e electrical resistivity versus temperature, room temperature Hall effect, and Seebeck coefficient versus temperature measurements The effect of the different illuminations on the CuAlS2 thin film showed that it can be used as a material for absorption of ultra-violet radiation All the obtained characterization results are deliberated in detail © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: CuAlS2 Thin film Dip coating XRD Microscopy Electrical transport properties Introduction I The ternary chalcopyrites belonging to MI-MIII-CVI (M e Cu, Ag; VI M e Al, Ga, In; C e S, Se, Te) compound semiconductor family have received wide interest because of convenient band structures suitable for optically active devices [1] They have been synthesized in single crystal form [2,3], but more recently experimental investigators have focused on thin films due to high potential for large area photovoltaic modules The CuAlS2 is one of the members of the ternary chalcopyrite family having the direct optical band gap of 3.5 eV [1,3] The optical band gap of this compound is the highest among those of all the chalcopyrite compound semiconductors making it an interesting material for applications Due to the wide optical band gap, the CuAlS2 has found potential applications in solar cells [4], in photovoltaic [5], as light emitting devices in the blue region of the spectrum [6], as window layers of solar cells [7] and in laser diodes operating in a short wavelength region [8] The CuAlS2 thin films have been used as oxygen gas sensor operating at room temperature showing an enhanced III * Corresponding author E-mail address: sunilchaki@yahoo.co.in (S.H Chaki) Peer review under responsibility of Vietnam National University, Hanoi sensitivity with the aging of the film [9] Nanocrystals of CuAlS2 have been employed in targeted “in-vitro” imaging of cancer cells after nano-engineering their surface [10] The CuAlS2 micro- and nano-particles have been used as the catalyst in cellulose pyrolysis [11] An additional major advantage of CuAlS2 is that its constituent elements are copious in nature and are non-toxic Inspired by the importance and potential applications of CuAlS2 [12] a study on this material in the thin film form has been undertaken in this investigation Till now, a number of methods have been employed to deposit CuAlS2 thin films These methods include iodine transport [13], metal organic decomposition (MOD) [14], single source thermal evaporation [9], sulfurization of precursors in H2S flow [15], sulfurization of sputtered metallic precursors by sulphur vapours in hermetically sealed ampoules [16], thermal evaporation of elemental mixture [17], spray pyrolysis [18,19], pulsed plasma deposition [20], horizontal Bridgman method [21], chemical bath deposition (CBD) [22,23], two stage thermal evaporation [24] and electron beam evaporation [25] The literature shows no report of deposition or study of CuAlS2 thin films by dip coating technique The advantage of dip coating deposition is that it is a low cost solution deposition technique mainly used for uniform coating of large areas [26] and to synthesize thin films of high quality [27,28] http://dx.doi.org/10.1016/j.jsamd.2017.04.002 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 216 S.H Chaki et al / Journal of Science: Advanced Materials and Devices (2017) 215e224 The high quality thin films by dip coating technique are achieved due to the layer-by-layer growth during each dip of the substrate in the aqueous solution In each dip of the substrate, the individual layer forms through ion-by-ion adsorption Thus the dip coating thin film formation is by ion-by-ion adsorption leading to layer-bylayer deposition in every dip, consequently to the possibility of minimization or even elimination of defects and imperfections in the synthesized thin films The other advantages of dip coating are the control over deposition rate and film thickness by means of control on dipping time during each dip and by regulating the number of dips, respectively Other deposition parameters like dip speed, withdrawal speed and dry duration; the time the substrate is out of solution between consecutive dips can be a handle to control the film deposition In this study, CuAlS2 thin films have been deposited on glass substrates by dip coating technique The asdeposited dip coating thin films were comprehensively characterized for elemental composition, crystal structure, surface morphology, optical and electrical properties Experimental Cupric chloride (CuCl2$2H2O) [S D Fine Chem Ltd., Mumbai, India], triethanolamine (TEA) (C6H15NO3) [Sisco Chem Pvt Ltd., Mumbai, India], aluminium chloride (AlCl3$6H2O) [Oxford Laboratory, Mumbai, India], ammonia liquid (NH3) [Chiti-Chem Corporation, Vadodara, India] and thiourea (NH2CSNH2) [Chiti-Chem Corporation, Vadodara, India] were used for the synthesis of CuAlS2 thin films by dip coating technique The chemicals were all of AR grade and were used without any further purification or processing In the synthesis of CuAlS2 thin films by using dip coating technique, firstly 10 ml of 0.5 M cupric (II) chloride (CuCl2$2H2O) solution was mixed with ml of 3.7 M TEA solution in a 100 ml clean dry glass beaker under continuous stirring for The CuCl2$2H2O acts as precursor for Cu and TEA acts as complexing agent to slow down the release of the metal ions resulting in slow precipitation of the compound by ioneion reaction and to prevent the agglomeration of the desired metal ions Then, in the above solution, 16 ml of 10 M NH3 solution was added and stirred for Here NH3 (liquid ammonia) is used as reagent to adjust the pH of the solution The pH of the solution was kept at 9.5, the reason for this being if pH < the solution becomes acidic due to which it can corrode the deposited thin films Under continuous stirring, 10 ml of 0.7 M aluminium chloride (AlCl3$6H2O) solution was added and stirred for Finally, 10 ml of 1.0 M thiourea solution was mixed and stirred for At last, the final solution was made to reach 100 ml by adding appropriate amount of deionized water The final solution of 100 ml volume was kept under programmed dip coating unit apparatus [Dip Coating Unit, Model No: HO-TH-02; Holmarc Opto-Mechatronics Pvt Ltd., Kochi, Kerela, India] for thin films deposition The dip coating parameters were maintained for the CuAlS2 thin films depositions as: dipping speed e mm/s; withdrawal speed e mm/s; dip duration e 10 s; dry duration e s and total number of dips e 600 In case of certain characterizations the numbers of dips were increased to increase the film thickness During the deposition of CuAlS2 thin films the following reaction was expected to have occurred: CuCl2$2H2O ỵ 2NH4OH ỵ TEA / [Cu (TEA)]2ỵ þ 2OH1À þ 2NH4Cl þ 2H2O AlCl3$6H2O þ 3NH4OH / Al3ỵ ỵ 6H2O ỵ 3OH1 ỵ 3NH4Cl 2(NH2)2CS ỵ 2OH1 / 2C2H2N2 ỵ 2H2O ỵ 2HS1 2HS ỵ 2OH1 / 2S2 ỵ 2H2O [Cu (TEA)]2ỵ ỵ Al3ỵ ỵ 2S2 / CuAlS2Y ỵ TEA The average thicknesses of the dip coating deposited thin films were determined by the gravimetric weight difference method [27,28] In the average thin film thickness calculation, the film density was taken as 3.48 g/cm3 determined from the XRD data analysis and will be discussed later in this paper Results and discussion 3.1 X-ray energy dispersive analysis The chemical compositions of the as-deposited CuAlS2 thin films were determined by the energy dispersive analysis of X-ray (EDAX) technique The EDAX analysis was done at five different spots of the thin films Fig 1(a) shows the EDAX spectrum The average weight % of the elements from five different spots of the as-deposited thin films with standard values are tabulated as inset of the Fig 1(a) The observed extra peaks of other elements like Si, Na, Mg, O, Ca etc in the EDAX spectrum are due to the glass substrate The values of Cu, Al and S are tabulated after deleting the glass substrate elements The obtained data clearly states that the deposited CuAlS2 thin film under this analysis is nearly stoichiometric but slightly rich in aluminium and deficient in sulphur 3.2 Structural analysis Fig 1(b) shows the XRD patterns of CuAlS2 thin films taken by the Philips X-pert-MPD X-ray diffractometer l ẳ 1:54056 ị Here CuKa (1.5405 ) radiation without any filter was used as the X-ray source The step size (2q) employed was 0.050 with the default slit setting and receiving slit height of 0.15 mm The scan speed employed was 0.2 /sec All peaks observed on the XRD patterns could be indexed as those of CuAlS2 with tetragonal unit cell structure The lattice parameters determined using the Powder e X software from the recorded XRD patterns are: a ¼ b ¼ 5.33 Å and c ¼ 10.40 Å They are in good agreement with the reported values of a ¼ b ¼ 5.325 Å and c ¼ 10.390 Å; according to JCPDS Card No 25-0014 Other parameters like the Miller indices, 2q angle, interplanar spacing (d) and % d errors for prominent XRD peaks are tabulated in Table The error of 1.57% for %d may be due to the presence of defects arising owing to grain size The X-ray density ‘r’ of the as-deposited CuAlS2 thin film was calculated to be 3.48 g/cm3 This calculated value is in good agreement with the reported value of 3.43 g/cm3 for bulk CuAlS2 [29] The peak broadening in the XRD pattern occurs due to the decrease of crystallite size arising as a result of the dislocation generated lattice strains [25] The crystallite size in the asdeposited CuAlS2 thin films was determined from the XRD peak broadening employing Scherrer's formula [30], given by: D ¼ K l=bhkl cos q (1) where D is the crystallite size, K is shape depending parameter and is taken here as considering the particles to be spherical in shape, l is the X-ray wavelength (1.5405 Å), b is the angular line width at half maximum intensity, and q is the Bragg angle in degree The value of the crystallite size D was evaluated from the slope q of the Scherrer's plot of cos l versus b for as-deposited CuAlS2 thin hkl films and results are shown in Fig 2(a) The graphically and analytically determined crystallite sizes are tabulated in Table S.H Chaki et al / Journal of Science: Advanced Materials and Devices (2017) 215e224 217 Fig (a) e EDAX spectrum along with inset table of chemical composition; (b) e XRD pattern of a CuAlS2 thin film Table Miller indices, 2q angle, inter-planar spacing (d), and %d error bhkl cos q=l ẳ 1=t ỵ sin q=l Thin lms (hkl) 2q d (Å) %d error CuAlS2 112 200 220 204 312 116 29.32 33.64 47.57 49.73 57.56 58.52 3.04 2.66 1.91 1.83 1.60 1.58 0.26 0.12 1.34 1.57 0.23 0.07 The source of strains in the thin films is due to the crystalline imperfections, distortions and dimensional constraints The dependence of the full width at half maxima (FWHM) on the strain and grain size is related by the HalleWilliamson relation [31], which represents the Uniform Deformation Model (UDM) The materials strain properties are independent of the crystallographic direction because the strain was assumed to be uniform in all crystallographic directions    (2)  4sinq versus bhkl cosq was plotted for the promiThe graph of l l nent XRD peaks of CuAlS2 thin films, and is shown in Fig 2(b) The slope and the ordinate intercept of the fitted line give the strain and the crystallite size, respectively The positive slope value reveals the presence of tensile strain produced due to the tensile stress This external tensile force tends to increase the inter-atomic distance as observed from the values of the lattice parameters derived from XRD data The origin of the extrinsic stress in a thin film comes mainly from the adhesion to the substrate, while the intrinsic stress comes from the defects, such as dislocations in the film The results of the UDM analysis for the CuAlS2 thin films are tabulated in Table The Hooke's law gives the linear proportionality relation between the stress ðsÞ and the strain ị as s ẳ Yhkl ; (3) Fig (a) e Scherrer's plot, (b) e Plot of the modified form of HalleWilliamson analysis representing UDM, (c) e Plot of the modified form of HalleWilliamson analysis using USDM; (d): Plot of the modified form of HalleWilliamson analysis using UDEDM, and (e) e The SSP plot of CuAlS2 thin films 218 S.H Chaki et al / Journal of Science: Advanced Materials and Devices (2017) 215e224 Table Crystallite size, strain, stress, energy and dislocation density in CuAlS2 thin films Methods Results Scherrer's formula HalleWilliamson relation Size-Strain Plot method Graphical Analytical UDM USDM UDEDM Graphical Crystallite size D (nm) Strain ε  10À3 Stress s (MPa) Energy density u (kJ/m3) Dislocation density d  10À4 (nm)À2 20.86 17.27 18.32 17.98 18.19 16.68 e e 0.23 0.21 0.28 0.22 e e e 21.99 23.03 e e e e e 4.11 e 27.56 33.52 29.79 30.94 30.23 23.27 where Yhkl is the modulus of elasticity or the Young's modulus Eq (3) is valid for a significantly small strain Assuming a small strain to be present in the deposited CuAlS2 thin films, Hooke's law can be employed Applying the Hooke's law approximation to HalleWilliamson relation, the equation is:   bhkl cosq K ¼ D l ỵs   4sinq lYhkl (4) Eq (4) is known as the Uniform Stress Deformation Model (USDM) For a tetragonal unit cell structure, Young's modulus [32] is evaluated by the following Eq (5), Yhkl ẳ ỵ p 4sinq u l sffiffiffiffiffiffiffiffiffi ! : Yhkl (7) qffiffiffiffiffiffi   4sinq of Eq (7) is shown in The plot of bhkl lcosq versus Yhkl l Fig 2(d) The square of the slope of the fitted line gives the energy density u and the reciprocal of the y-intercept indicates the crystallite size D Then stress and strain were calculated using Eqs (3) and (6), respectively All the obtained values are tabulated in Table The value of the crystallite size determined using UDEDM is in good agreement with the values determined using other models 2 h ỵ k2 ỵ L2 s11 h4 ỵ k4 ỵ 2s12 ỵ s66 ịh2 k2 ỵ 2s13 ỵ s44 ị h2 ỵ k2 L2 ỵ s33 L4 where L ¼ al/c, a and c are the lattice parameters; h, k and l are Miller indices taken from XRD analysis The elastic compliance constants Sij (m2/N) of CuAlS2 were taken from the reported values [32] and are presented in Table The determined value of the Young's modulus, Yhkl, for the CuAlS2 thin films having tetragonal unit cell turned out to be 102.52 GPa, which is nearly equal to the reported value 102.13 GPa     q [32] and 106.91 GPa [33] An USDM plot of bhkllcosq versus 4sin lY hkl for the CuAlS2 thin films is shown in Fig 2(c) The parameters like, stress calculated from the slope of the fitted line, the strain calculated using Eq (3) and crystallite size determined from the intercept are tabulated in Table They are in good agreement with the values obtained from UDM Another model known as Uniform Deformation Energy Density Model (UDEDM) was used to determine the crystallite size, strain and stress The energy density can also be determined by this model For an elastic system that follows Hooke's law, the energy density (u) can be given as [32], u¼   bhkl cosq K ¼ D l ε2 Yhkl : (6) The equation of HalleWilliamson relation [25], can be rewritten using Eq (6) as: (5) The grain size and the strain can also be evaluated using the SizeeStrain Plot (SSP) method In this estimation, it was assumed that the crystallite size profile is described by a Lorentzian function and the strain profile by a Gaussian function [32] Hence, dhkl bhkl cosqị ẳ   2 K d b cosq ỵ ; D hkl hkl (8) where K is a constant that depends on the shape of the particles; for spherical particles it is taken, e.g as In Fig 2(e), the graph of ðdhkl bhkl cosqÞ2 versus ðd2hkl bhkl cosqÞ is plotted by using Eq (8) for the prominent XRD peaks taken on the CuAlS2 thin films In this case, the crystallite size is derived from the slope of the line and the square root of the y-intercept will give the value of the strain The obtained values are tabulated in Table The grain size (D) and the dislocation density (d) of the films were calculated for the preferential orientations to have information about their crystallinity levels The dislocation density (d), defined as the length of dislocation lines per unit volume of the film, was evaluated by Eq (9) [34], dẳ (9) D2 nmị2 The crystallization levels of the as-deposited thin films are good because of their small d values derived from the Scherrer's formula, Table Elastic constants of CuAlS2 thin films S11 (m2/N) 1.421  10 S12 (m2/N) À11 À4.938  10 S13 (m2/N) À12 À5.802  10 S33 (m2/N) À12 1.467  10 S44 (m2/N) À11 1.769  10 S66 (m2/N) À11 1.860  10À11 S.H Chaki et al / Journal of Science: Advanced Materials and Devices (2017) 215e224 the HeW plot, and the SSP plot, and collected in Table 2, which represents also the amount of defects in the films The TEM and SAED images of the as-deposited CuAlS2 thin films are shown in Fig 3(a) and (b), respectively The TEM image and SAED pattern of as-deposited CuAlS2 thin films were recorded using the Philips, TECNAI 20 Transmission Electron Microscope The TEM and SAED samples were prepared by scratch removing the thin film The scratch removed films were allowed to float on the distilled water in a petri dish The floating thin films were then 219 swiftly taken on a copper grid The wet copper grid with the film samples was dried by keeping it on a piece of filtering paper The copper grid along with sample was then inserted into the electron microscope for TEM and SAED analysis The TEM image shows that the deposited thin film is uniform without any cracks The selected area electron diffraction (SAED) pattern for CuAlS2 thin film (Fig 3(b)), shows a concentric ring pattern along with spots, revealing that the deposited thin films are polycrystalline with large grain size in nature The rings were Fig (a) e TEM image, (b) e SAED pattern, (c): SEM image of large area, (d) and (e) e SEM images of small selected areas, (f) e 2D AFM image; (g) e height profile, (h) e 3D (x-y-z) AFM image, and (i) e 3D (y-x-z) AFM image of the as-deposited CuAlS2 films 220 S.H Chaki et al / Journal of Science: Advanced Materials and Devices (2017) 215e224 indexed as (112), (200), (220), (312), (116) and (400) indices, which are associated with the tetragonal structure All indexed planes except the (400) one are in agreement with the XRD data The Fig 3(c, d and e) present the SEM images of CuAlS2 thin films deposited on glass substrates at room temperature Fig 3(c) clearly shows that the film covers the whole surface of the substrates having a pocketed morphology variation Fig 3(d and e) show a magnified image of pocketed morphology of the thin films Fig 3(d) clearly shows the presence of a rod like structure whereas Fig 3(e) shows beautiful bunches of rods originating from the film surface The AFM images of the as-deposited CuAlS2 thin films were recorded by the Nano Surf Easyscan-2 in the tapping mode Fig 3(f) shows the two dimensional (2D) and Fig 3(h and i) show the three dimensional (3D) AFM images of the as-deposited CuAlS2 thin films, respectively The height profile variation is shown in Fig 3(g) The Fig 3(f) shows the 2D image of a film area of 1.98 mm  1.98 mm This 2D image shows clearly the presence of spherical grains having coalescences between them The Fig 3(h and i) present the 3D image of a film area of 256 mm  256 mm This 3D image shows obvious structures like hills and mountains having valleys between them The height profile parameters, illustrated in Fig 3(g), taken along the horizontal line of the AFM images of CuAlS2 thin films are tabulated in Table The parameters such as peak p, valley z or v (Rp-v), root mean square (rms), roughness (Rq) and the average roughness (Ra) values indicate the roughness in the vertical direction Fig 3(g) shows the rise in heights at the two ends of the viewed horizontal scale These heights increase may be due to presence of bunch of nanorod features at the sites as observed in the SEM image 3.3 Optical analysis The optical absorption spectrum, shown in Fig 4(a), of CuAlS2 thin films deposited by dip coating has been recorded in the wavelength range 200 nme3200 nm The spectrum shows high absorption in the ultra violet range with the absorption edge lying at 290 nm corresponding to an energy of 4.28 eV Table Surface and line roughness analysis of the AFM profiles Surface roughness Sa (nm) Sq (nm) Sy (nm) Sp (nm) Sv (nm) Sm (pm) Area (pm2) Line roughness 34.971 46.325 384.91 193.13 À191.79 223.57 Ra (nm) Rq (nm) Ry (nm) Rp (nm) Rv (nm) Rm (pm) 3.959 26.767 34.339 172.68 104.88 À67.801 223.4 The energy band gap Eg was determined from the optical absorption data using the near-band edge absorption relation, given by the Eq (10) [31] below, a$h$vịn ẳ A h$v Eg (10) where, n characterizes the transition For allowed and forbidden direct transitions, n ¼ and 2/3 respectively, and n ¼ 1/2 and 1/3 for allowed and forbidden indirect transitions, respectively The absorption coefficient ‘a’ was calculated employing the BeereLambert Eq (11) [35e37] a ¼ 2:303 A=t (11) where A is the absorbance of light passing through the sample, t is the path length of light which travels through the CuAlS2 thin film sample (average thickness of the thin film in the measurement was 260 nm) The analysis of Eq (10) shows that n ẳ and ẵ ts well for the as-deposited CuAlS2 thin films stating that the as-deposited CuAlS2 thin films possess direct and indirect allowed optical band gaps The plots of (a$h$n)2 versus h$n and (a$h$n)1/2 versus h$n are shown in Fig 4(b) The value of the direct allowed optical band gap was determined by extrapolating the straight line portions of (a$h$n)2 versus h$n The obtained value of the direct optical band gap is 3.82 eV for the CuAlS2 thin films in the present investigation which is greater than the reported value of 3.49 eV for bulk material [1] This shows that the blue shift occurred due to film thickness The value of indirect allowed optical bandgap of 3.11 eV was evaluated by extrapolating the straight line portions of (a$h$n)1/2 versus h$n for the as-deposited CuAlS2 thin films The transmittance (T%) and the reflectance (R%) spectra of the as-deposited CuAlS2 thin films are shown in Fig 5(a) The drop in the transmittance for wavelengths higher than 700 nm may presumably be due to the absorption by free carriers After 1200 nm wavelength, the transmittance is stable and so this material can be utilized as an infrared window The data from the spectra have been used to determine the optical constants of the film The refractive index is an important parameter for materials to be used for optical applications In the region of the inter-band transition that has strong absorption, the refractive index of the film can be determined by the Eq (12) [38] below, only when the illuminations of electromagnetic waves are perpendicular to the surface of the film, h¼ pffiffiffi R pffiffiffi; where R is reflectance R 1ỵ (12) The plots of the refractive index (h) and the extinction coefficient (k ¼ a$l/4p) versus wavelength (l) are shown in Fig 5(b) Fig (a) e Absorbance spectrum, (b) e Plot of direct and indirect band gap of CuAlS2 thin films S.H Chaki et al / Journal of Science: Advanced Materials and Devices (2017) 215e224 221 Fig (a) e Transmittance (T) and reflectance (R) spectra, (b) e Plots of the refractive index (h) and the extinction coefficient (k) versus wavelength, (c) e Variation of real and imaginary part of the dielectric constant with wavelength of the as-deposited CuAlS2 thin films The plots show that the refractive index (h) and the extinction coefficient (k) vary with the wavelength in the range 290e3200 nm for the as-deposited thin films The variation shows that the refractive index decreases in the wavelength range of 290 nm to nearly 700 nm The static refractive index h(0) determined using the optical dispersion relationship has been found to be h(0) ¼ 1.84 This obtained value is less than the reported value of 2.12 [38] This variation may be due to surface dissimilarity of the as-deposited thin films and the reported investigated thin films The dielectric constant dependence on frequency is defined by the Eq (13) below, uị ẳ r uị ỵ iεi ðuÞ: (13) where εr and εi are the real and the imaginary parts of the dielectric constants, respectively, and these values were calculated using the formulas of Eq (14) [39] below, r uị ẳ n2 uị k2 uị and i ẳ 2nuịkuị: (14) The variations of the r and εi values of the as-deposited CuAlS2 thin films with wavelength are shown in Fig 5(c) The εr values are higher than that of εi values 3.4 Electrical analysis The dc e electrical resistivity variation with temperature in the temperature range from ambient to 423 K was studied on CuAlS2 thin films using a four-probe set-up of the Model DFP-02 (Scientific Equipment & Services, Roorkee, India) Using the measured voltage while keeping the current constant, the resistivity (r) at each temperature value was evaluated by taking into consideration the correction factor The average thickness of the thin films was 327 nm The plots of logr versus 1000/T for as-deposited CuAlS2 thin films are shown in Fig 6(a) The resistivity decreases with increasing temperature, implying the thin film material to be semiconducting in nature The activation energy determined for the linear portion of the plot arrived at a value of 0.81 eV, which is in reasonable agreement with the reported value of 0.70 eV [40] Hall Effect analysis at room temperature was carried out on the as-deposited CuAlS2 thin films by using the Hall Effect setup, model DHE-22 (Scientific Equipment, Roorkee, India) Graphite conductive adhesive alcohol-based (Alfa Aesar) paste was used for making the Ohmic contacts in the van der Pauw geometry The Ohmic nature of the electrical contacts made on the sample were confirmed by measuring IeV characteristics between R12,12, R23,23, R34,34 and R41,41 contacts of the thin films (see Fig 6(b)) for both polarities in the current range from mA to ỵ5 mA The sample under investigation was kept in an applied magnetic field which modifies the path of the majority carriers which produce Hall voltage Fig 6(c) shows the graph of Hall voltage (VH) versus magnetic field (B) The Hall coefficient (RH), the mobility of charge carriers (mH) and the charge carrier concentration (h) were evaluated employing the standard formulae using the value of the slope of the plot in Fig 6(c), the thickness of the samples and the constant measuring current The average thickness of the thin films used for the Hall measurement was 318 nm The values obtained are tabulated in Table The positive value of the Hall coefficient implies that the deposited thin films are of p-type in nature which was also confirmed by the hot probe method The evaluated carrier concentration of thin films turned out to be in the order of 1016 cmÀ3 also revealing the samples to be semiconductors The value for the hole mobility determined from the Hall Effect measurement was 4.39 cm2/Vs for the as-deposited CuAlS2 thin films This value is in good agreement with the reported one (