Optical and electrical properties of chalcopyrite CuIn(SexS1-x)2 (0 ? x ? 1) amorphous thin films are investigated. The transmission has been measured as a function of the wavelength range from 200 to 2500 nm. The calculated optical energy band gap Eg and the Urbach tail (band tail width) Eu are found to decrease with increasing Se content. The refractive index, n, is calculated according to the Wemple- DiDomenico single oscillator model. The oscillator energy (E?), the dispersion energy (Ed), and the ratio of the free carrier concentration to the electron effective mass (N/m*) are estimated from the optical data for all thin films.
Also, high frequency dielectric constant at infinite wavelength (??), the lattice high frequency dielectric constant (?L) 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.
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 different methods for the preparation of this compound; such as solid state reaction at high temperature, the Bridgman method, electrodeposition, spray pyrolysis, sputtering, coevaporation and organometallic precursors [1-7]. Moreover, it is reported that CulnSe2 thin film has the excellent absorption coefficients and high light generated current in the visible light range (400-1200nm) .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 .CuInSe2and CuInxGa1-xSe2chalcopyrite thin films have a 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 studying the effect of composition on the optical properties of amorphous thin films and the calculation of the ratio of the free carrier concentration to the electron effective mass (N/m*), high frequency dielectric constant at infinite wavelength (??), lattice dielectric constant (?L), and static refractive index (n?) from the optical data. Also, investing the electrical conductivity ? of the amorphous thin films at room temperature.
CuIn (SexS1-x)2 ( x = 0, 0.25,0 .5,0.75, and 1) bulk was synthesized by direct fusion method. The mixture of high pure Cu, In, Se and S (99.999%, Matthey chemicals Ltd.) in stoichiometric ratio was placed in silica tube under a pressure of about 10-3 Pa. Amorphous CuIn (SexS1-x)2 thin films of different compositions ( 0 ? x ?1 ) were deposited by thermal evaporation under a vacuum of (6 ? 10-6m.bar) onto glass substrate at room temperature. The thickness of the thin films was measured by quartz crystal thickness monitor and equals ? 2000 ?. The rate of the deposition is 10-20?/sec. The CuIn(SexS1-x)2 thin films were analyzed by XRD Philips PW 1373 diffractometer. Energy dispersive X-ray analysis (EDAX) was made by using SEM model Philips XL 30 attached with EDX unit accelerating voltage 30K.V, magnification 10 xs up to 400.000 xs. The transmittance spectra were determined at room temperature in the range 200- 2500 nm using a double beam spectrophotometer, JASCO V-570 model. The electrical resistance was measured at room temperature by Van der Pauw method.
The compounds of CuIn (SexS1-x)2 thin films of compositions ( x = 0, 0.25,0.5,0.75 and 1 ) were determined by energy dispersive X- ray analysis (EDAX). The calculated contents of Cu, In, Se and S wt% were comparable with wt% of the starting materials. Fig.1 shows the representative EDAX curve of CuInSe2. The results of (EDAX) for CuInSe2 compounds are shown in table 1. It is noticed that the thin films have approximately stoichiometric compositions. The obtained data for all compositions revealed an excess of In.
The structure of the powder and thin films were examined by X-ray diffraction. Fig.2 represents the 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.
The optical transmittance spectra were measured in the wavelength range 200-2500 nm by using UV-Vis spectrophotometer. Fig.3shows 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 the transmission . The absorption coefficient ? was calculated using the measured transmission values T. Fig.4 shows the relation between the absorption coefficient ? and the photon energy h? of the thin films with different compositions. It can be shown that all thin films have a high absorption coefficient between 104 cm-1 and 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 the 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 the 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 the fabrication of photovoltaic devices . The analysis of the relation between the absorption coefficient and the photon energy in the high absorption region was done to determine information about the band gap energy. The optical band gap Eg of the thin films is determined by using Tauc model and Davis and Mott model in the high absorbance region [21, 22] :
?h? =B (h?-Eg)m (1)
Where, ‘h’ is Planck’s constant, ‘Eg’ is optical band gap of the thin film, ‘B’ is constant depends on the transition probability, ‘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 the 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 the plot of (?h?)0.5vs. h? for the determination of the indirect band gap of CuIn (SexS1-x)2. It is noticeable that the optical energy band gap decreases with increasing the 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 di?erence of Se and S and due to the presence of a high concentration of localized states in the band structure. The values of the energy band gap are recorded in table 2.
As example, figure 6 (a, b) shows the relation between ln? and h? 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 (h? /Eu) (2)
Where ?? is constant and Eu is the Urbach energy i.e. the width of the tails localized states in the band gap. This equation depicts the transition between occupied states in the valence band tail and unoccupied state of the conduction band edge. The value of Eu was estimated from the inverse slope of ln ? vs. h? and given in table 2. It is shown that the values of Eu decrease with increasing the Se content. The refractive index was calculated using the method proposed by 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:
N = 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 the absorption index k according to the wavelength of CuInS2. On the other hand the low value of k is an indication of excellent surface smoothness of the thin films . From Fig.(7) it is obvious that the refractive index increase towards lower wavelength values i.e. higher frequencies which is proportionate with normal dispersion of material. The refractive index at higher wavelengths tends to decrease and then become a constant or static i.e. the thin films become non dispersive at high wavelengths. In normal dispersion region, the refractive index can be calculated by using the model of Wemple- DiDomenico single oscillator [27, 28]:
Where E? is the oscillator energy and Ed is the dispersion energy. Figure 8 as example represents (n2-1)-1 vs.(h?)2of CuIn(Se0.5S0.5)2. The values of E? and Ed can be estimated from both the slope (Ed E?)-1and the intercept on the vertical axis (E?/ Ed) as shown in figure 8. From this figure the value of n?2= 1+ (Ed /E?) can be found by extrapolating the linear part to intercept the axis. Also, the dielectric constant at infinite wavelength (??) can be known where n°2?=??. Moreover, the lattice high frequency dielectric constant (?L) and (N/m*) the ratio of carrier concentration to the electron effective mass can be calculated from the relation between the refractive index, n2 and wavelength, ?2 :
The relation between n2 and ?2of CuIn (Se0.75S0.25)2 is shown in figure 9. The lattice high frequency dielectric constant (?L) can be obtained from the intercept of the extrapolation of straight line to n2axis. The values of Ed, E?, ??, ?L and (N/m*) are summarized in table 3. It is clear that (N/m*) depends on the 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 the first order of moments M-1 and the third order of moments M-3 are the measure of the inter-band transition strengths can be derived from the relations :
The static refractive index (n?) can be calculated from the equation n?=?? and from another equation :
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.
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) shows the relation between the conductivity and the composition x. It is observed that the conductivity decreases with increasing the Se content, 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.
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. The values of the optical energy band gap and the Urbach energy decrease with increasing the 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 the free carrier concentration to the electron effective mass (N/m*) decreases with increasing the Se content. The values of the single oscillator energy (E?), dispersion energy (Ed), lattice high frequency dielectric constant (?L), (N/m*), and high-frequency dielectric constant (??) have been determined by the single oscillator Wemple-DiDomenico model. In addition, the electrical conductivity decreases with the increase of Se ratio.