The influence of thickness and substrate temperatures of CuIn(SexS1-x)2on optical and electrical propertiesAbstract Effect of thickness and substrate temperatures on optical and electrical properties of CuIn(SexS1-x)2 (x= 0.25 and 0.5) had been studied. The films were deposited by thermal evaporation method. Structure of samples was examined by XRD. Transmission of compositions was measured in wavelength range (200 ‰¤ “‰¤ 1400 nm). Transmittance decreases with increasing both of substrate temperatures (Ts) and thickness (d). Energy band gaps (Eg) decrease with increasing Ts whilst increase with increasing d.
Urbach energy gaps of polycrystalline films increase with the increase of Ts and thickness. On the other hand, optical constants such as absorption coefficient, refractive index and extinction coefficient as a function of photon energy or wavelength of all films were calculated. Single-oscillator Wemple and DiDomenico model (WDD) was used to determine oscillator energy (E—), dispersion energy (Ed), and the ratio of free carrier concentration to electron effective mass (N/m*). Moreover, static refractive index (n—), high frequency dielectric constant at infinite wavelength (µ€ћ) and lattice dielectric constant (µL) can be obtained.
The effect of thickness and (Ts) on electrical conductivity at room temperature was investigated. The conductivity () decreases with increasing both substrate temperatures and thickness. Key words: polycrystalline CuIn(SexS1-x)2 thin films, substrate temperature, Thickness, Optical properties, Electrical conductivity.Introduction Chalcopyrite semiconductors display a possibility of obtaining absorber materials of demanded properties which can be used in production of low cost solar cells with high efficiency . Because of the optoelectronic properties and good stability which makes them suitable for photovoltaic and optoelectronic device applications, thin films of copper based chalcopyrite materials are attractive for many researchers. CuInSe2 is a direct band gap semiconductor which crystallizes in chalcopyrite structure. Copper indium diselenide, Cu (In,Ga)Se2 and CuIn(Se,S)2 have a large absorption coefficient (>105 cm€’1) and direct optical energy, resulting in a greater amount of solar radiation absorbed with a smaller thickness [2-4]. Because of an absorbing property of CuInSe2 compound, the composition may be used in the field of solar energy . Also, CuInS2, CuInSe2, and Cu(InGa)Se2 thin films offer alternative solar energy conversion material saving both efficiencies up to 19.2% and excellent longevity [6,7]. In addition, Copper indium sulde/selenide materials have high absorption coecients and excellent stability under solar radiation so they have been applied in manufacturing of solar cells [8, 9]. The target of this work is examining effect of thickness and different substrate temperatures (Ts) on optical parameters such as carrier concentration / effective mass (N/m*), high frequency dielectric constant at infinite wavelength (µ€ћ), lattice dielectric constant (µL), static refractive index (n—), and electrical conductivity at room temperature . Results and discussion Structural CuIn (Se0.25S0.75)2 deposited at Ts (100, 200 and 300°C) were analyzed by using X-ray diffraction analysis. Fig.() represents diffraction pattern of powder and films. The films are amorphous at room temperature and start to be crystalline at 100°C. Also, we observed an intensity of peaks increases as the substrate temperatures increase. It indicates that the quality of crystalline films was improved and there is no secondary phase. X-ray diffraction of CuIn(Se0.25S0.75)2 with different thickness (1330, 2800, 6280 and 9120є) deposited at room temperature is shown in Fig.(). The intensity of diffraction peaks increases and becomes sharper with increasing thickness and this confirms that an improvement in the crystallinity. 3.2. Optical properties3.2.1. Effect of substrate temperatures on the optical properties Transmittance of spectrum films was investigated in wavelength range (200 ‰¤ ” ‰¤1400nm). Transmission spectra of CuIn(Se0.25S0.75)2 with different substrate temperatures are seen in Fig.( ). It is clear that T decreases with increasing Ts. This may be assigned to the increase of scattering and reflection of light at grain boundaries due to the increase of surface roughness with increasing substrate temperatures [10, 11]. From Fig.( ), it is obvious that absorption coefficient (±) increases as substrate temperatures increase. We noticed that at high hЅ, ± has a high value (± 104cm-1) which may deduce that the direct transition of electrons occurs . The optical energy band gaps (Eg) are estimated by:±hЅ = A(hЅ”Eg)n (1)where Eg, A and hЅ are optical energy gap, constant and photon energy respectively. Fig.() represents a relation between (±hЅ)0.5 and (hЅ) for amorphous thin film at room temperature. The films become polycrystalline at different substrate temperatures. Optical absorption spectra of polycrystalline CuIn (Se0.25S0.75)2 films show three energy gaps at (Ts =100°C) and four at (Ts= 200 and 300°C) which referred to fundamental edge, band splitting by crystal field and spin orbit splitting and to transitions from copper 3d levels respectively. Moreover, figure ( ) shows the first transition according to direct fundamental band gap (direct valence to conduction band transition Eg1). The second (Eg2), third energy (Eg3) and the fourth (Eg4) band gaps can be obtained as shown in figure (). Eg values are recorded in table 1. We observed that Eg1, Eg2, Eg3 and Eg4 decrease with increasing substrate temperatures, this attributed to the formation of localized levels which are able to receive electrons and produce localized energy tails in the optical energy gap which work on absorption of low energy photons and this leads to this decrement. Representative curve lin ± vs. h… is shown in Fig. (). From inverse slope of this curve the energy band tail (width of localized states Eu) can be calculated. Urbach energy tail (Eu) of films can be determined from the equation :± = ±o exp(E/ Eu) (2)where (E), (±o) and (Eu) are photon energy, constant and Urbach energy respectively. Values of Eu are tabulated in table 1. The increase in Eu is indication to the increase of localized levels in an optical energy gap with increasing substrate temperatures. Variation of refractive index and extinction coefficient (k) with ” of films at Ts= 100°C is given in Fig.() as example. We observed that the small values of k (‰€ 10-3) are an indication of excellent surface smoothness of films . Also, the film has a maximum in short wavelength range then sharp decrease and followed by increasing. This behavior is due to interactions between photon and electrons in the crystallites. Wemple- DiDomenico (W-DD) model of single oscillator is used to determine refractive index in the normal dispersion region [15, 16]:n2 (hЅ) =1+ EdE°[E°2-(hЅ)2] (3)Values of E° and Ed (which is the measure of the average strength of inter-band optical transitions) are calculated according to (W-DD) method, where denoted as energy of effective dispersion oscillator and a dispersion energy respectively. Relation between (n2-1)-1 and (hЅ)2 is seen in figure (). From the slope and intercept of linear relation the values E°, Ed and µ€ћ can be determined. Where n€ћ2=1+EdE° and then n€ћ2=µ€ћ is dielectric constant at infinite wavelength can be determined. Table 1 shows that Ed decreases as the substrate temperatures increase this may be referred to the distribution charge carriers within each unit cell and that it related to chemical bonding . Fig.() shows the dependence of n2 on “2 as a representative example. Also, µL can be obtained from the intercept of extrapolation the straight line to n2. (N/m*) values can be estimated from the relation:n2 = ›L ” (e2N c2m*) “2 (4)where c, e and m* are velocity of light, electronic charge and electron effective mass. Values of (N/m*) depend on substrate temperatures as seen in table 1. The first and third order of moments (M-1) and (M-3) can be derived from eqs. :E°2= M-1/ M-3 , Ed2 = M-13/ M-3 (5)Also, from the relation n—= ›€ћ , the static refractive index can be calculated. n—, (M-1) and (M-3) values decrease with increasing substrate temperatures as shown in table 22.214.171.124. Effect of thickness on the optical properties Fig.( ) illustrates transmittance of CuIn(Se0.25S0.75)2 with different thickness ( d = 1330, 2800, 6280 and 9120 …) . Transmission increases with decreasing d of films. This may be due to the change in crystatllinty of films. An absorption coefficient ± is used to evaluate Eg. The relation between ± and photon energy (hЅ) is shown in figure (). All films have a high absorption coefficient (104cm-1). We noticed that ± decreased with increasing d because of inverse relation between transmission and absorption . Eg for different thickness are calculated according to equation 1. The films with low thickness (1330 and 2800 …) is mixture of amorphous and minor crystalline as clear from XRD so, Eg can be calculated according the relation (±hЅ)0.5 and (hЅ) as observed in Fig.(). For higher thickness (6280 and 9120 …) the films transform to polycrystalline structure and have three energy gaps due to fundamental edge, band splitting by crystal field and spin orbit splitting. Figs. () clarify the dependence of (±hЅ)2 on (hЅ). Eg values are stated in table 2. We observed that the direct energy gaps increase and this can be due to the film thickness  this explained by an increase in disorder with decreasing thickness . Also, the increase in Eg may be interpreted to defects which originate localized states in band-gap and therefore increase band-gaps [20, 21]. Also, the thicker film generates a more homogeneous network by saturating the dangling bonds and that way minimizing the number of defects. So, the concentrations of localized states are reduced, and Eg consequently increases. Lin ± vs. (hЅ) of a film with thickness 9120 … is displayed in figure () as example. Eu values can be calculated using eq.2 and listed in table 2. It is clear that an increase in the values of energy band tail for polycrystalline films. Representative curve () represents n and k vs. wavelength. The refractive index increases with increasing ” and then decrease until reach to be constant. The increase of refractive index points to the improvement in crystallinity of lms as conrmed by XRD results. From (W-DD) single oscillator model [15, 16] refractive index in the normal dispersion region can be determined by using equation 3. E°, Ed, and µ€ћ values can be calculated by a same method which explained before. Fig.( ) represents (n2-1)-1 vs. (hЅ)2 of films with thickness 9120 … as an example curve. By using E° and Ed values and eq.5 we can calculate the moments of optical spectra (M-1) and (M-3). Results of moments are listed in table.2. The obtained results of polycrystalline films showed that Ed and the moments of optical dispersion spectra all decreased with film thickness while E° increased. By plotting n2 vs. “2 of films with (d = 9120…) as an example, µL can be calculated and (N/m*) can be obtained by using equation 4. µL, n— and (N/m*) values are summarized in table 2. We observed that these values are decreased for polycrystalline films with increasing thickness. It can be deduced that both high frequency dielectric constant and (N/m*) ratio are correlated to the internal microstructure . As a result of free charge carrier’s contribution to the polarization process, it is observed that there is a difference between µL and µ€ћ [23-25]. Electrical conductivity Electrical properties of lms are required for improving the preparation conditions to use it in several applications. of all samples is calculated by using Van der Pauw technique . Table 1 summarized the values of conductivity of CuIn(Se0.25S0.75)2 deposited at different substrates temperatures. The results show that conductivity decreases with increasing of substrate temperatures. This denotes that a film has the time to make some atomic rearrangement . The density of localized states in the band gap decreases by removing some defects. For the same compound with various thickness evaporated at room temperature the results of are tabulated in table 2. We notice that values of decrease with increasing d of samples. At lower film thickness the variations of conductivity is less as compared to films of higher thickness because of electron scattering by surface is more at lower thickness . Liu et al.  reported that the increase in resistivity below 20 nm to surface roughness of films. As increasing d of films, there is a slight increase of , which could be referred to the fact that a large amount of material moves towards preferential areas and the diffusion of grain boundaries are more announced, as a result of the increase in thickness. Conductivity of CuIn(Se0.25S0.75)2 films is the reciprocal of resistivityConclusion Optical transmission spectra of CuIn(SexS1-x)2 films were measured in wavelength range (200-1400nm ) to estimate absorption coefficient and refractive index. A transmittance spectrum has been decreased with increasing both of substrate temperatures (Ts) and thickness of samples. The results represented that energy band gaps decrease with increasing Ts while increase with increasing thickness. Urbach energy gaps of polycrystalline films have been increased with the increasing of Ts and thickness. CuIn (Se0.25S0.75)2 films have three energy gaps at Ts = 100єC and four energy gaps at (200 and 300 єC) attributed to fundamental edge, band splitting by crystal field and spin orbit splitting and to the transitions from copper 3d levels. Moreover, dispersion energy (Ed) decreased with increasing substrate temperatures this may be due to the distribution charge carriers within each unit cell. Oscillator energy (E—), ratio of free carrier concentration to electron effective mass (N/m*), static refractive index (n—), high frequency dielectric constant at infinite wavelength (µ€ћ) and lattice dielectric constant (µL) values can be obtained according to Wemple- DiDomenico single oscillator model. The electrical conductivity decreased with increasing both of thickness and substrate temperatures.