Preparation and characterization of optical and electrical properties of copper selenide sulfide polycrystalline thin films

Accepted Manuscript Preparation and characterization of optical and electrical properties of copper selenide sulfide polycrystalline thin films B.A. Mansour, I.K.E.L. Zawawi, Hani E. Elsayed-Ali, Talaat A. Hameed PII:

S0925-8388(17)34250-0

DOI:

10.1016/j.jallcom.2017.12.067

Reference:

JALCOM 44153

To appear in:

Journal of Alloys and Compounds

Received Date: 23 August 2017 Revised Date:

5 December 2017

Accepted Date: 7 December 2017

Please cite this article as: B.A. Mansour, I.K.E.L. Zawawi, H.E. Elsayed-Ali, T.A. Hameed, Preparation and characterization of optical and electrical properties of copper selenide sulfide polycrystalline thin films, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.067. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

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Preparation and characterization of optical and electrical properties of copper selenide sulfide polycrystalline thin films B. A. Mansoura, I. K. EL Zawawia, Hani E. Elsayed-Alib, Talaat. A. Hameeda,∗∗ a

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Solid State Department, Physics Division, National Research Centre, Dokki, Giza 12622, Egypt b Applied Research Center, Old Dominion University, Newport News, Virginia 23606 and Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia 23529 Abstract

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The structural, optical and electrical properties of Cu1.8Se1-xSx (0.25 ≤ x ≤0.75) polycrystalline thin films deposited at room temperature by vacuum evaporation were

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studied as a function of composition. Energy dispersive X -ray analysis was used to determine the elemental composition of different films. Results indicate the incorporation of sulfur at the expense of selenium. X-ray diffraction analysis reveal that the (220) diffraction peak of the Cu1.8Se1-xSx films shifts to a higher 2θ with an increase in the sulfur content. Atomic force microscopy images of the deposited films show dense and well-defined grains. Analysis of the absorption coefficient data show the existence of two

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optical transition mechanisms - direct transitions with Eg-dir ranging from 2.2 to 2.5 eV, and indirect transitions with Eg-indir ranging from 1.23 to 1.52 eV as the sulfur content was increased from 0.25 to 0.75, respectively. The Hall coefficient, Hall mobility, and electrical conductivity were measured at room temperature for different film

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compositions. All investigated films are highly degenerated p-type semiconductors with the number of free carriers up to 1020 cm-3. Combining the results of the optical and

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electrical measurements, the effective mass for the highly degenerate compositions is found to be 1.718 to 0.587 as the sulfur content varies from 0.25 to 0.75. Key words: Copper selenide sulfide; Thermal evaporation; Atomic force microscopy; Optical properties; dc conductivity.

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Corresponding Author: [email protected]

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1. Introduction Considerable works have been devoted to the synthesis and characterization of thin metal chalcogenides films due to their distinct properties and significant applications.

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Among such compounds, copper selenide (Cu2Se) has been utilized in several devices such as thermoelectric converters [1-4] solar cells [5-7], photodetectors [8-10], optical filters, super ionic conductors [11,12], window coatings [13] and electro-optical devices [14, 15].

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Copper selenide is a p-type semiconductor with a strong self-compensation of ptype conduction resulting from the high concentration of copper vacancies supplying the

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holes where the density of free holes exceeds 8 x 1021 cm-3 [16]. Much effort has been directed towards studying copper selenide because it possess a variety of stoichiometric compositions such as CuSe, Cu2Se, Cu3Se2, Cu7Se4, Cu5Se4 and Cu2Se [17-19], as well as nonstoichiometric compositions such as Cu2−xSe (copper (I) selenide) [13,20,21]. Copper selenide exists in many crystallographic forms namely, cubic [13,22], tetragonal [12,21], orthorhombic [23,24], monoclinic [25,26] and hexagonal [20,27]. The variation in

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composition and crystallography give rise to the variety of physical, chemical and thermal properties of copper selenide and its related compounds. Several methods have been reported for the deposition of copper selenide including flash evaporation [28], melting of Cu and Se [29], vacuum evaporation [30],

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catholic deposition [31], electrodeposition [32], selenization [33], chemical bath deposition [34] and direct synthesis [35]. The unique feature of copper selenide is its

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feasibility to form compounds such as CuInSe2, CuAlSe2 and CuGaSe2 [36-38], Cu(In,Ga)Se2 and Cu(In,Al)Se2 [39,40] and Cu(In,Ga,Al)Se2 and Cu(In,Ga)(Se,S)2 [4144]. However, the alloying of copper selenide with sulfur in the form of thin films has not been studied, in spite of the closeness of the atomic radii of selenium and sulfur and their complete miscibility [45]. The present work deals with the alloying of copper selenide with different concentrations of sulfur ranging from x = 0.2 to 0.75. By gradually substituting S by Se, the optical band gap could be tuned to match the solar cell spectrum, where the 2

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photovoltaic conversion efficiency is maximum. In addition, the incorporation of sulfur at the expense of selenium results in intermediate physical properties of the new compound. The present work advances the understanding of the structure, morphology, optical and

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electrical properties of Cu1.8Se1-xSx (0.25 ≤ x ≤0.75) thin films.

2. Experimental Details

Cu1.8Se1-xSx compounds of different compositions (0.25 ≤ x ≤ 0.70) were prepared from a stoichiometric mixture of pure elements (5N Matthey chemicals Ltd.). The details

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of the preparation procedures were described previously [45]. Thin films of Cu1.8Se1-xSx of different compositions (0.25 ≤ x ≤ 0.70) were deposited by thermal evaporation, using

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Edward’s E306 coating unit, onto clean glass substrates kept at room temperature. The thin films were deposited under vacuum in the 10-6 Torr range, boat current of 85 A, and with a deposition rate of 0.2 nm/s. The source-to-substrate distance was kept at 12 cm. The thicknesses of the films were measured by a quartz thickness monitor (Edward’s FTM5). The final thickness of the fabricated thin film was set to 200 nm. The chemical composition of the films was measured by energy dispersive X-ray spectroscopy (EDS)

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unit attached to field emission scanning electron microscope (FESEM, Quanta FEG 250, FEI, USA). The structure of the deposited films was analyzed using an X-ray diffractometer (Philips PW1373). The optical properties were studied by measuring the transmittance and reflectance spectra in the wavelength range 200-2500 nm using a

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double beam spectrophotometer (JACO-570, Japan). The surface morphology and surface roughness was measured using an atomic force microscope (AFM, Dimension

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3100, Veeco, USA). The electrical conductivity (σ) was measured by the conventional four-probe method. The Hall voltage VH was measured potentiometrically by reversing both the magnetic field and current directions. Both σ and VH were measured at room temperature as a function of the sulfur content in the film. The sample holder was designed to provide pressure contact to the thin films for measuring conductivity and Hall voltage. The pressure contact of the copper electrodes with the films were ohmic.

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3. Results and Discussion 3.1. Composition Study

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The chemical constituents of the thin films were identified by EDS taken at three different positions on the surface. The values that reported in Table I are an average of the atomic percentage of each element. Figure 1(a, b) depicts the EDS profiles of Cu1.8SexS1-x thin film for sulfur contents of 0.3 and 0.5, respectively. The EDS analysis revealed that the Cu1.8SexS1-x films possess a non-stoichiometric composition and the

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ratio of Cu/(Se+S) is nearly constant. The concentration of the sulfur increases with a corresponding decrease of the selenium, while the Cu content is nearly unchanged,

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indicating that sulfur substitutes for selenium when incorporated into Cu1.8Se. The composition variation between the powder and thin films is due to the slight nonstoichiometric growth in the evaporated film.

3.2. Structure Analysis

The X-ray diffraction pattern of the powder Cu1.8Se0.8S0.2 was recorded as a

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reference to compare with the thin films prepared from them [45]. Figure 2(a) shows XRD patterns of Cu1.8SexS1-x thin films and Cu1.8Se0.8S0.2 powder with different compositions, confirming the polycrystalline nature of the studied films. Since there is no JCPDS card for Cu1.8Se1-xSx compounds, the standard card of β-Cu1.8Se, Berzelianite,

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(JCPDS card No. 71-0044) was used to identify the various diffraction peaks. All the observed d-values of thin films are matching the values for the powder with no extra

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peaks to reveal any secondary phases. However, the d-values show a slight deviation from those given in aforementioned JCPDS file, as shown in Figure 2(b) and its inset. The deviation of d- values may be attributed to the replacement of Se atoms by S atoms in the Cu1.8Se compound leading to the reduction of d-spacing due to the difference in the atomic radii of S and Se, as shown in Figure 2(c). These results are in a good agreement with previous studies [46, 47]. The dominant diffraction peak differs from the reported one in the JCPDS file, which may be due to defects and variations of lattice constant as a result of the difference in the deposition method [48]. Therefore, all prepared films are

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formed in a single FCC phase with a prominent (220) diffraction and a slight deviation in d-values, depending on the sulfur content. The mean crystallite size of polycrystalline thin films can be estimated by the


=



 

……………. (1)

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Scherrer's equation [49].

where D is the mean crystallite size, λ is the x-ray wavelength (λ = 1.54 Å for CuKα

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radiation), β is the diffraction peak full-width at half-maximum (FWHM), θ is the Bragg angle, and K is the shape factor. Figure 2(d) shows a clear reduction of the crystallite size with the incorporation of sulfur into the Cu1.8S films. This reduction results from the

3.3. Surface Morphology

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lattice distortion caused by the radius difference between the sulfur and selenium.

Figure 3 shows a 2 x 2 x 50 nm 3D atomic force microscopy image of Cu1.8SexS1x

thin films of different sulfur contents. All thin films have a uniform, compact and

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homogenous surface and the substrate surface is well covered. Moreover, all films show well-defined and uniformly distributed grains. Table 2 lists the values of the root-meansquare (RMS) roughness and particle size measured by AFM, and grain size calculated from XRD patterns using the Scherrer's equation. The measured RMS roughness

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decreases from 6.4 to 2.8 nm as the sulfur content increases from 0.2 to 0.7. It is well known that several factors affect the roughness of thin films, such as the initial surface

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roughness, growth mechanism and film thickness. Since all the films were grown under the same conditions, the reduction in the surface roughness is due to the decrease in grain size as was verified by XRD data and AFM images. The reduction of the roughness is vital for solar cell applications because it enables the complete coverage of the surface and, therefore, reduces the shunt resistance and carrier recombination at interfacial layers [41], as well as a smoother film has less light trapping at the surface. The particle size also decreases with the increasing of the sulfur content. Both AFM and XRD verified the same trend of variation of grain size with sulfur content, as shown in Figure 2(d). The slight divergence of values of grain size obtained from AFM and XRD results from the 5

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fact that the AFM measures the grain size, whereas the Scherrer's equation is used to calculate the domain size. XRD instrumental broadening and the microstrain also affect the diffraction peak width [50].

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3.4. Optical Study Figures 4(a) and (b) show the optical reflectance and transmittance spectra of the thin films as a function of wavelength in the range 400 - 2000 nm. All films have high transmittance over the visible range (400–900 nm), which gradually decreases at longer

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wavelengths due to absorption by free carriers in the degenerate films [51]. This trend is consistent with most degenerate semiconductors. The film transmittance increases with

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the increase of the sulfur content, as shown Figure 4(b), which is probably due to the decrease of both carrier concentration and surface roughness. The prominent feature of these curves is the presence of the minimum of reflectance Rmin and the maximum of transmittance Tmax approximately at the same wavelength, as shown in the inset of Figure 4(a). This trend is due to the low roughness of the deposited films, confirmed by RMS values measured by AFM, as shown in Figure (3), and reflects the good optical

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homogeneity and the quality of the deposited films [52,53]. Except for x = 0.2, the average optical transmittance values for all films is higher than 50% in the visible range for all films, a result that is in agreement with most Cu2-xSe thin films [17,51,52]. The reflectance and transmittance of all the films had a sharp absorption edge between 400 -

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800 nm, which shifts to lower wavelengths with the increase of the sulfur content. Furthermore, the transmittance curve falls sharply at the band edge verifying the

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polycrystalline nature of the films [17]. The absorption coefficients α were calculated from R and T measurements using

the following equation [43]:

 

 =  



+

   



+   ……………. (2)  

where α is the absorption coefficient and d is the film thickness. The relationship between the absorption coefficient α and the photon energy is shown in Figure 4(c).

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absorption edge shifts towards higher photon energies with increasing sulfur content. The 6

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absorption edge exceeds 5x104 cm-1 and hence is appropriate for photovoltaic devices. Two slopes are observed in Figure 4(c), which is an indication for the presence of two different optical transitions [55].The variation of the absorption coefficient with photon

(

ℎ! = "#ℎ! − %& ' ……………. (3)

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energy for band-to-band transition is given by [56]:

where hυ is the photon energy, Eg is the optical band gap, B is a parameter that depends on temperature, phonon energy and photon energy and r depends on the type of the

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transition [57]. In the present study, the values of r giving the best linear fit were 2 and 1/2, which is a characteristic behavior of direct and indirect transitions, respectively.

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Figure 4(d) and (e) show the variation of (αhν) 2 and (αhν) 1/2 with the photon energy (hν) for four different thin film compositions. These curves show a linear dependence of (αhν) 2

and (αhν) 1/2 on photon energy (hν), which confirms the presence of direct and indirect

transitions. The intercepts with the photon energy axis are taken as the values of direct and indirect band gaps of the films. It is found that the direct band gap Eg-dir and indirect band gap Eg-indir increase with the increase of sulfur concentration, as can be seen in the

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inset of Figure 4(d). The values of band gaps of the studied films lie between those of Cu2-xSe and of Cu2-xS thin films, as listed in Table 3 and shown in the inset Figure 3(d). Therefore, alloying Cu2-xSe with sulfur results in band gap that can be tuned depending on the sulfur content. The values of the direct and indirect band gaps of Cu2-xSe and Cu2are taken from previous studies [20, 58] since these studies were on films that have

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xS

almost the same thickness and grain size as the ones considered in the present study. In

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comparison with the bulk material [45], the values of direct energy gaps Eg-dir of polycrystalline thin films are the larger of the film grain size and thickness. The study of Urbach's energy Eu is used in characterizing the material disorders

and measuring the width of the band tail in the forbidden region. The Urbach's energy can be determined by [59]:  =  )*+ℎ,⁄%- ……………. (4)

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where αo is a characteristic parameter of the material, hν is the incident photon energy and Eu is the Urbach energy. The values of Urbach's energy of studied films can be obtained from the reciprocal of the slope of the linear portion in the curve of the plot ln (α) versus hν as can be seen in Figure 4(f). The values of the width of the band tail Eu

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given in Table 3 are much larger than 0.05 eV indicating the validity of Urbach's formula. The values of Urbach's energy decreases as the sulfur content in the films was increased resulting in a broadening of the band gap [60]. Accordingly, the incorporation of sulfur into the Cu2-xSe matrix leads to the decrease of concentration of localized states in the

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space-charge region.

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3.5. Electrical Study

The conductivity σ and carrier concentration p measured at room temperature for different compositions of Cu1.8Se1-xSx thin films are listed in Table 4. The Hall Coefficient RH has a positive sign, which is in agreement with previous works on Cu2-xSe [61, 62]. Therefore, the majority of the charge carriers in Cu1.8Se1-xSx thin films are free holes due to copper vacancies [63].

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The values of conductivity σ, Hall mobility µH and hole concentration p were listed in Table 4, which shows that σ and p decrease as the sulfur content (x) was increased. The reason may be the low carrier concentration of copper sulfide compared to copper selenide leading to the reduction of hole concentration when the selenium is

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replaced by sulfur. These results are in agreement with the results on polycrystalline bulk material [45]. However, the Hall mobility µH shows an opposite trend, increasing by a

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factor of ~2 as sulfur content x was increased from 0.25 to 0.70. This may be due to the reduction of the effective mass m*, as shown in Table 4, and enhanced mean-free-path resulting from the decrease in carrier density. In addition, the reduction of defects in the band gap, as shown in Urbach energy study, decreases the trapping centers. The values of Hall mobility µH are small compared with their corresponding bulks due to the effect of grain boundary limited mobility. From the data given in the Table 4, the room temperature electrical conductivity has values ranging from 2.4 x 102 to 4.1 x 102 (Ω.cm)1,

which is orders of magnitude less than previously reported for Cu2−xSe thin films [10,

20], but close to the low values of conductivity reported for polycrystalline copper 8

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selenide thin films [22]. Significant variations in the values of the thin film conductivity are due to its high sensitivity to the compositions of the film, thickness, grain size, roughness and defects.

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For a degenerate semiconductor, in which the Fermi level lies within the band gap, more than several times kT from the back edge, m*/m can be deduced when the optical measurements is related to the electrical measurements by [64, 65]: 6

− %8 ' ……………. (5)

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/=

6

01 3 ∗     #%7 2 5

where p is the hole concentration, h is the plank’s constant, Ev and Ef are the valance and

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Fermi level edge, respectively. The values of Ev - Ef were determined from the square root of the absorption coefficient versus photon energy, assuming the Fermi level lies in the middle of the indirect gap Eg-indir. The obtained values of m*/m, given in Table 4, show that m*/m is reduced with the sulfur content x. These results agreed well with the

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mobility data, namely, the lower the effective mass m*, the higher the Hall mobility µH.

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Conclusion Copper selenide sulfide polycrystalline thin films of various sulfur contents were deposited on a clean glass substrate by thermal evaporation. The influence of sulfur

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concentration on the structure, topography, optical and electrical properties was investigated. The EDS analysis showed that the films are non-stoichiometric in composition and that the sulfur replaced selenium in the Cu2-xSe matrix. XRD showed that the films are polycrystalline having a cubic structure with small changes in the d-

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spacing depending of the sulfur content. The rms film roughness measured by AFM decreased upon the increase of sulfur content. We were able to tune both the optical and electrical properties of copper (I) selenide by alloying it with different concentrations of

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sulfur. A direct transition with a band gap of 2.2 - 2.5 eV, and an indirect transition with a band gap of 1.23 - 1.52 eV were observed as the sulfur content was increased from 0.25 to 0.75. The electrical study shows that the dc conductivity and carrier concentrations decrease with the incorporation of sulfur, while Hall mobility exhibits an opposite

Acknowledgement

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behavior.

The authors thankfully acknowledge Dr. Wei Cao for providing the atomic force microscope measurements. This work was funded by the Egyptian National Research

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Centre.

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Captions Figures

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Figure 1(a, b) The EDS profile of Cu1.8SexS1-x thin films of composition x = 0.2 and (b) x = 0.5.

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Figure 2(a) XRD patterns of different compositions of Cu1.8SexS1-x thin films and Cu1.8Se0.8S0.2 powder; (b) the normalized (220) diffraction peak shows shift in peak position with increasing sulfur content. The inset shows the expanded (220) diffraction order; (c) the reduction in d-spacing of (111) and (220) diffraction orders with sulfur content, and (d) the variation of crystallite size with sulfur content. TF stands for thin film, x is sulfur content. Figure 3 3D AFM images of Cu1.8SexS1-x thin films of different sulfur content x. The RMS roughness decreases as the sulfur content increases.

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Figure 4(a, b) The spectral behavior of the reflectance R and transmittance T. The inset of Fig. 4a Shows that Tmax and Rmin occur almost at the same wavelength, (c, d, e, f) the variation of α, (αhν)2 and (αhν)1/2 ln(α) with photon energy hν, respectively. The inset of Figure 4(d) represented the variation of direct and indirect band gap with sulfur content, where the values of the direct and indirect band gap of Cu1.8Se and Cu1.8S are taken from the V.M. Garcmh and et al. [20], and Ivan Grozdanov [63]. Tables

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Table 1 EDS data of Cu1.8Se1-xSx thin films.

Table 2 The RMS roughness and particle size determined by AFM and grain size determined from XRD using the Scherrer's equation.

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Table 3 Direct and indirect optical band gap and Urbach tail of bulk and as-deposited Cu1.8Se1-xSx thin films.

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Table 4 Conductivity σ, Hall coefficient RH, carrier concentration p, Hall mobility µH and m*/mo of Cu1.8Se1-xSx thin films.

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Cu (%) 64.14 64.39 64.91 64.75

S (%) 7.25 10.53 17.60 25.2

Se (%) 28.61 25.08 17.49 10.05

Table 2 RMS roughness (nm) 6.4 5.5 5.2 2.8

0.2 0.3 0.5 0.7

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Bulk Eg-dir (eV) 1.30 1.35 1.43 1.49

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Table 4 x 0.2 0.3 0.5 0.7

Particle size (AFM, nm) 23 16 14 10

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Table 3 Content x 0.20 0.30 0.50 0.70

Cu/(S+Se) 1.79 1.81 1.84 1.83

Gain size (XRD, nm) 17 15 11 7

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Sulfur Content x

S/Se 0.25 0.42 0.99 2.50

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x 0.2 0.3 0.5 0.7

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Table 1

σ x102(Ω.cm)-1 4.1 3.8 3.0 2.4

Eg-dir (eV) 2.22 2.33 2.43 2.50

RH x10-3 (cm3/C) 1.4 2.3 2.9 4.3

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Thin films. Eg-indir (eV) 1.23 1.27 1.42 1.52

P x1021 (cm3) 4.4 2.8 2.2 1.5

Eu (eV) 0.52 0.49 0.47 0.43

µ (cm2/ V.s) 0.6 0.8 0.9 1.1

m*/mo 1.7 1.1 0.8 0.6

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Figure 1

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Figure 2 (b)

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Figure 3 x = 0.3

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Figure 4 (b)

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Highlights • Cu1.8Se1-xSx thin films are prepared by thermal evaporation. • Cu1.8SexS1-x films are formed in a single cubic phase.

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• Cu1.8Se become smoother when alloying with sulfur.

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• The direct bandgap can be tuned from 2.2 to 2.5 eV by adjusting sulfur content.