Effect of composition on the optical and electrical conductivity of CuIn(SexS1-x)2Abstract Optical and electrical conductivity of chalcopyrite CuIn(SexS1-x)2 (0 ‰¤ x ‰¤ 1) amorphous thin films are investigated. The transmission has been measured in a wavelength range (200 ‰¤” ‰¤ 2500 nm). The calculated optical energy band gap Eg and Urbach tail (band tail width) Eu decrease with increasing Se content. Refractive index, n, is calculated according to Wemple- DiDomenico single oscillator model. The oscillator energy (E—), dispersion energy (Ed), and the ratio of free carrier concentration to electron effective mass (N/m*) are calculated from optical data for all thin films.
Also, high frequency dielectric constant at infinite wavelength (µ€ћ), lattice high frequency dielectric constant (µL) and static refractive index (n—) are calculated. The electrical conductivity is investigated at room temperature.Key words: CuIn (SexS1-x)2, Amorphous thin films, optical properties, electrical properties.Introduction CuIn(SexS1-x)2 chalcopyrite semiconductors are wide band gap compounds and it is a promising material for photovoltaic and solar cell applications.
There are several methods for the preparation of this compound; such as solid state reaction at high temperature, Bridgman method, electrodeposition, spray pyrolysis, sputtering, coevaporation and organometallic precursors [1-8]. Also, CuInS2 is one of the promising chalcopyrite type semiconductors. CuInS2 has a direct band gap 1.53 eV which it can be suitable for photovoltaic’s application . Moreover, CuInSe2and CuInxGa1-xSe2 chalcopyrite thin films have high potential for terrestrial and space applications . On the other hand, CuIn(SexS1-x)2 thin films are considered as solar energy conversion material and have efficiencies up to 19.2% [ 11,12]. The aim of this work is investigating the effect of Se content on optical parameters of amorphous thin films, calculations of free carrier concentration to electron effective mass (N/m*), high frequency dielectric constant at infinite wavelength (µ€ћ), lattice dielectric constant (µL), and static refractive index (n—) from the optical data. Also, investigating the electrical conductivity of amorphous thin films at room temperature.3. Results and discussion3.1. Composition analysis CuIn (SexS1-x)2 thin films of compositions ( 0‰¤ x ‰¤ 1) were examined by energy dispersive X- ray analysis (EDAX). Calculated contents of Cu, In, Se and S wt% were comparable with wt% of the starting materials. Fig.1 displays representative EDAX curve of CuInSe2. The results of (EDAX) for CuInSe2 thin film are shown in table 1. The thin films have approximately stoichiometric compositions as obvious from table1. The obtained data for all compositions revealed an excess of In.3.2. Structural The structure of both powder and thin films were investigated by X-ray diffraction. Fig.2 represents XRD pattern of powder and thin films evaporated at room temperature for CuInSe2 as example. It is clear that from this pattern the thin films are amorphous in nature. The powder diffraction peaks was investigated by an ICCD database . It was found that the compound matched with the card no.00-040-1487 of CuInSe2 tetragonal phase and no secondary phase. 3.3. Optical properties The optical transmittance spectra were measured in the wavelength range 200-2500 nm by using UV-Vis spectrophotometer. Fig.3 shows the transmission spectra of CuIn (SexS1-x)2 thin films with different compositions (0 ‰¤ x ‰¤ 1). The interference phenomenon is responsible for the variation of transmission . The absorption coefficient ± was estimated from optical transmission data (T). The curve of absorption coefficients ± vs. photon energy h… of thin films with different compositions is shown in Fig.4. All thin films have a high absorption coefficient in the range (104 to 105 cm-1) and increasing with increasing Se content. This value of absorption coefficient is close to the reported values [15-18]. The absorption coefficient can determine the nature of electron transition if the values of absorption coefficient are low (± < 104 cm-1), it is predicted that transition of electron is indirect and the electronic momentum is maintained with the assistance of the phonon . In addition, the values of absorption coefficient are small and constant at low photon energy while at high photon energy the absorption coefficient values are bigger and a great possibility for electron transition. The values of ± exceed 104 cm-1 for all thin films, so they are suitable for fabrication of photovoltaic devices . The investigation of the relation between absorption coefficient and photon energy in the high absorption region is important to determine information about energy band gap. The optical band gap Eg of thin films is estimated by using Tauc model and Davis and Mott model in the high absorbance region [21, 22] :±hЅ =B (hЅ-Eg)m (1)Where, h’, Eg’ are Planck’s constant and optical band gap of thin film respectively, B’ is constant, m’ is exponent. Exponent m’ may have values such as 1/2, 3/2, 2 and 3 depending on the nature of electronic transition responsible for absorption of light. The value of m=1/2 for allowed and direct transition, m=3/2 for direct forbidden transition m=2 for indirect allowed transition, m=3 for indirect forbidden transition. Fig.5 gives plot of (±h…)0.5 vs. h… for determination of indirect band gap of CuIn (SexS1-x)2. We notice that optical energy band gap decreases with the increase of Se ratio. In all thin films, the absorption edge shifted to greater wavelength which is indication of the band gap decreased. This energy band gap decrement may be due to the electronegativity dierence of Se and S and due to a high concentration of localized states in the band structure. Optical energy band gap values are recorded in table 2. As example, figure 6 (a,b) represents the relation between ln± and hЅ for CuIn(Se0.75S0.25)2 and CuIn(Se0.5S0.5)2. The sub-band gap photon energy (Urbach tail) describes the degree of disorder in an amorphous semiconductors was calculated by using the equation [23, 24] :± = ±— exp (hЅ /Eu) (2)Where ±—, Eu are constant and Urbach energy respectively. This equation depicts the transition between occupied states in valence band tail and unoccupied state of conduction band edge. Eu values were estimated from the inverse slope of ln ± vs. hЅ and given in table 2. It is shown that the values of Eu decrease with increasing Se content. The refractive index was calculated using the method of Swanepoel  which depends on envelope curves through the upper (Tmax) and the lower (Tmin) in the transmission spectrum. The refractive index calculated according to the equations:n1 = [N + (N2- n22)1/2]1/2N = 2n2Tmax-Tmin Tmax Tmin+ n22+12 (3)Where n1 is the refractive index of the thin films, n2 is the refractive of the substrate (n2= 1.5 for glass substrate). Fig. (7) illustrates a representative curve as example for refractive index n(“) and absorption index k according to wavelength of CuInS2. On the other hand the low value of k is an indication of excellent surface smoothness of thin films . From Fig.(7) it is obvious that the refractive index increase towards lower wavelength values which is proportionate with normal dispersion of material. The refractive index at higher wavelengths tends to decrease and then become a constant or static. In normal dispersion region, refractive index can be calculated by using the model of Wemple- DiDomenico single oscillator [27, 28]:n2 (hЅ) =1+EdE°[E°2-hЅ)2 (4)Where E—, Ed are oscillator energy and dispersion energy respectively. Figure 8 as example represents (n2-1)-1 vs. (hЅ)2 of CuIn(Se0.5S0.5)2. E— and Ed are estimated from both the slope (Ed E—)-1 and the intercept on the vertical axis (E—/ Ed) as shown in figure 8. From this figure the values of n€ћ2= 1+ (Ed /E—) can be found by extrapolating the linear part to intercept the axis. Also, dielectric constant at infinite wavelength (›€ћ) can be known where n°2€ћ=›€ћ. Moreover, lattice high frequency dielectric constant (›L) and (N/m*) can be obtained from a relation between the refractive index, n2 and wavelength, “2 :n2 = ›L ” (e2N c2m*) “2 (5) The relation between n2 and “2of CuIn (Se0.75S0.25)2 as example is shown in figure 9. Also, (›L) can be determined from the intercept of extrapolation of straight line to n2axis. Ed, E—, ›€ћ, ›L and (N/m*) values are summarized in table 3. It is clear that (N/m*) depends on Se content. The complex dielectric constant is responsible for transparency and absorption [30, 31]. The imaginary part of the complex dielectric constant ›i can be calculated from the relation: ›i= 2nk (6)The values of first order of moments M-1 and third order of moments M-3 are the measure of inter-band transition strengths can be deduced from equations :E°2=M-1/ M-3 , Ed2 = M-13/ M-3 (7)Static refractive index (n—) can be calculated from the equation n—=›€ћ and from another equation : (8) The values of n— from the two equations are the same. The values of M-1, M-3 and n— are recorded in table 4.3.4. Electrical Conductivity Several experiments revealed that chalcopyrite semiconducting properties are basically governed by intrinsic native defects . The electrical conductivity is investigated for the quaternary CuIn (SexS1-x)2 thin films evaporated at room temperature . The electrical resistance was measured by Van der Pauw method. The conductivity was calculated by using the relation: = 1/ (9)Where is the resistivity. Fig. (10) represents the dependence of conductivity on composition x. The electrical conductivity decreases with increasing the Se content as shown in figure 10, this may be due to the presence of intrinsic lattice defects in the solid solution which caused by increasing the substitution of sulfur by selenium. The relation between the conductivity and the composition x can be fitted exponentially by: = – 0.01239 +0.2677e(-x/0.21704)+ 0.41978e(-x/0.22085) (10)The values of the conductivity are shown in table 4. Moreover for CuInSe2, Nishitani et al  have reported that the electrical conductivity of the thin films are sensitive not only to In/Cu ratio but also to Se/(Cu+In) ratio and have observed the conductivity to lie in the range of 10 – 100 ©-1 cm-1 and 10-4 ©-1 cm-1 for copper rich and indium rich films respectively. It is found that the value of the electrical conductivity of CuInSe2 has the same order 10-4 ©-1 cm-1as Nishitani et al reported  and the results of EDAX ensure that the excess of In.Conclusion Thermal evaporation method has been employed for deposition of CuIn (SexS1-x)2 thin films at room temperature. Optical and electrical properties of amorphous thin films were investigated as a function of composition at room temperature. X-ray analysis showed the amorphous nature for all the thin films. The optical data revealed that the absorption coefficient of the thin films ranged from104 to 105 cm -1. The band gap and Urbach energy were determined from the transmittance measurements. Optical energy band gap and the Urbach energy decrease with increasing Se content. It has been found that the refractive index increase towards lower wavelength values i.e. higher frequencies. It has been observed that the ratio of free carrier concentration to electron effective mass (N/m*) decreases with increasing Se content. (E—), (Ed), (›L), (N/m*) and (›€ћ) values have been determined by single oscillator Wemple-DiDomenico model. In addition, the electrical conductivity decreases with the increase of Se ratio.