Thickness Effect of CuAlTe2 Thin Films on Morphological, Structural and Visual Properties

CuAlTe2 thin films were evaporation on glass substrates using the technique of thermal evaporation at different range of thickness (200,300,400and500) ±2nm. The structures of these films were investigated by X-ray diffraction method; showing that films possess a good crystalline in tetragonal structure. AFM showed that the grain size increased from (70.5599.40) nm and the roughness increased from (2.08-3.65) nm by increasing the thickness from (200-500) nm. The optical properties measurements, such as absorbance, transmtance, reflectance, and optical constant as a function of wavelength showed that the direct energy gap decreased from (2.4-2.34) eV by the gain of the thickness. Key word: Thin Film, Optical Properties, Thermal Evaporation.


Introduction
I-III-VI2 compound are ternary isoelectronic equivalents of the II-VI binary compounds. They crystallize in these chalcopyrite structure that closely for zinc blend. Completely these materials have direct band gaps [1]. Ternary ceramic is attractive materials in promising photovoltaic and optoelectronic requests [2,3]. Like smart windows, coating films for IR reflection, transparent electrodes in plane panel show, functional glasses and solar cells [4,5]. Single crystal has a direct forbidden band gap Eg =2.06 eV. It can be used for example in window with other ternary compounds, CuAlTe2 film is found to be p-type, and some of electrical properties annealed CuAlTe2 thin film at 300C, it also found that there are two Xray diffraction peaks at 2Ɵ=24.60and 45.20 corresponding respectively to (112) and (220,204) orientation of the CuAlTe2 chalcopyrite phase. Thin film was deposited using several methods: sputtering [6]. Elemental co-evaporation [7]. Flash evaporation [8]. And electrodeposition [9]. Thin films employed in this study were prepared from a single source technique using thermal evaporation.

Theoretical
Absorbance (A) is defined as the proportion between the intensity of the absorbed radiation (IA) and of the incident radiation (I0) [10].

A=IA/I0
(1) Transmission (T): is the ratio between transmission intensity radiation of the film (IT) to the incident radiant intensity (I0), otherwise reflectance (R) is the ratio between reflected radiation intensities (IR) and (I0) as in formula [10].

R=IR/I0
(2) The reflectivity was created for all prepared films as in the equation [10]. A+T+R=1 (3) The absorption coefficients of CuAlTe2 thin film depend on the values of the incident photon (hυ) and the type of optical electronic transfer. When the photon beam incident on the thin film component part of it is reflected, other transmitted and third absorbed by the film material. Absorption coefficient (α) represented the attenuation that happened in the energy of incident photon for unit thickness, which attributed to the acculturation processes. We can deduce from the absorption or transmission spectra using the Lambert Law [11,12]. I=I0 e -αt (4) Where, Ӏ is the total radiation, t is the film thickness. αt=ln(I0/I) (5) αt=2.303Log(I0/I) (6) The absorption of the film (A) could be defined as [10]. α=2.303 (7) To calculate the optical energy gap for direct transition used the following equation [13]. αhυ = D (hυ -Eg ) r (8) r represents constant and can take values 1/2,3/2,2 and 3 depending on the type of the optical transition and the material. Extinction coefficient is the exponential decay of the wave that passes through the medium [14]. k=αλ/4 (9) The complex refractive index with real and imaginary parts as in the equation [15]. ncomplex n0ik (10) Where no could be evinced by the formula [16]. n0= (4R/(1-R) 2 -k 2 ) 1/2 + (1+R/1-R) (11) The R is the The reflectivity, as show below [17]. R= (no-1) 2 +k 2 /(nO+1) 2 +k 2 (12) The real (εr) and imaginary part (εi) of dielectric constant can be estimated utilizing the following equation [18][19]. εcomplex=εr-εi (13) εr=n0 2 -k 2 (14) εi=2nok (15) AFM is an imaging technique employed to evaluate the physical properties of dielectrics, conductors and semiconductor surfaces; it supplies us with very exact data about surface roughness average (r.m.s), as well as grain size [20]. It is an important tool for nanoscale, that it bears a very high-resolution amount of (0.1nm), Zoom power is estimated at (5 × 102 -108) with the power to operate under normal atmospheric pressure without the need for the high vacuum [20]. The structure of the CuAlTe2 deposited films were studied by XRD method by Siemens x-ray diffractometer method, that recorded the intensity as a functions of Bragg angles [21]. From two of consecutive lattice plane, the term 2dsinθ of the diffraction of Xrays for crystalline materials are [22]. nλX -Ray = 2dsinθ (16) The lattice constant values in Tetragonal system can be computed from the following equations using the Miller incidents (hkl) and the inter-planar spacing (d) [23]. 1/d 2 = (h 2 +k 2 /a 2 ) +l 2 /c 2 (17) 3-Result and discussion Figure 1. Show the XRD pattern for thin CuAlTe2 film that deposited on glass substrate with many thickness (200, 300,400, and 500) nm. The blueprints showed that all the films have polycrystalline mode. The first peak located at 2θ ≈ 25.96 with the (112) preferred orientation, when the second peak appeared at 2θ ≈ 27.16 with the (103) one. Table 1. showed the structural parameters, the results showed that the film had a good crystalline in tetragonal structure and the intensity increased with the increasing of the thickness [24,25,26] with a simple shift at the distinctive peaks. This was due to the different conditions of preparation of the  In parliamentary law, to examine the surface topography, roughness of thin films and the impression of film thickness on it, we used the atomic force microscope (AFM), which possessed the ability to get very precise statistical values about the texture size and surface roughness values depending on the root mean square (r.m.s.). Figure 2. Represented AFM images in two and three-dimensional forms for CuAlTe2 thin film with different thickness (t = 200,300,400,500) nm. The average grains size was improved as the increase of film thickness increased as presented, this was too supported by the X-ray diffraction pattern and indicated the improved structural properties of the films of high thickness by decreasing synthetics defects such as the vacancy and grainboundary, leading to an increase in the particle size of the film. The roughness of the surface was increased as; these results are listed in Table 2.  The absorbance spectra for thin film with different thickness were presented in Figure 3. It was demonstrated that absorbance increased with the decrease of wavelength when it was closer to the visible spectrum (400-700) nm which indicated the possibility of the use of CuAlTe2 films in the solar cell industry, while decreased to the lowest values at the wavelengths (near IR). Also it was observing from Figure 3. That absorbance increased with increasing of thickness reaching to high absorbance over (80%) in the visible region when t= 500 nm, that agreement with Murugan and Murali [27][28][29]. This increasing can be attributed to the improved crystalline arrangement and the decreasing of crystalline defects, the bigger crystalline sizes were formed and in this case extra atoms existing in the CuAlTe2 films so extra states would be available of the photons to be absorbed [30,31]. Within the visible region (400-700) nm of the electromagnetic spectrum [32,33].The increase of the absorption in general by increasing the thickness of the film, which was due to the increase of the degree of crystallization of the film preparation -by increasing the thickness -and then the increase of the particle size of the resulting. Consequently the photon falling on the surface of the film preparation would suffer from successive absorption by the crystals within the grain and thus the possibility of reflection within the grain and thus the possibility of reflection or transmission without being absorbed by electrons. The compound atoms will be few, especially by increasing the size of the grain (ie, increasing the number of crystals in it by increasing thickness. As the number of these crystals increases, the falling photon will capability more absorption attempts than before the atoms of those crystals, which leads to the absorption of the whole and then increases the absorption factor, especially when low photonic energies, these high absorptions indicate the possibility of use these prepared films in photovoltaic solar cells or as an antireflection cover in the visible region of the electromagnetic spectrum.     The absorption coefficient (α) behavior is very similar to the Absorption spectrum (A) because of the proportionally between them according to the equation (7). From the high values of absorption coefficient (α>10 4 cm -1 ), it can be recorded that the band gap is direct and this is consistent with [35,36]. We likewise note that the values of absorption coefficient (α) of the films is increasing when it is high to the visible spectrum region. The increases of absorption coefficient with the increasing of thickness as indicated in Figure 6. This is consistent with the effects of obtaining structural test. The cause for this is ascribable to the nature of the relationship between Absorption and absorbance factor adds to the improvement in the level of crystallization Material of the film by increasing thickness.   Table 3. Presents the decreasing of the allowed direct band gap and optical constant at wavelength (500nm) with the gain of thickness, which can be attributed to the increase of particle size [37]. One can notice the Eg opt value of film with thickness 200 nm has 2.4 eV; this value is in good agreement with [38].The reason for this decrease is due to the so-called effect size Quantum, meaning that if the particle size is much larger than the diameter of Bohr and the equal to half of the Angstrom, then the quantitative effect will appear and the value of the energy gap will change in reverse proportion with the grain radius square according to the Schrödinger equation for the energy level. The consequences of the structural tests of the atomic force microscope showed that there was a clear increase in particle size by increasing thickness -as in Table 2. Which led to the decrease observed in the value of the energy gap. We can note from Figure 8. That the variation of extinction coefficient with film thickness is non-regular, this is credited to the similar reason mentioned earlier in the absorption coefficient because the behavior of extinction coefficient is similar to the absorption coefficient, and the reason for increase the absorption coefficient is the case of direct electronic transitions. The refractive index that is shown in Figure 9. Indicates the increase in the values with the gain of the thickness, which can explain the prepared samples denser (the increasing of the packing density), which in turn decreases propagation velocity of light through them, which resulting in the increasing of no values since not represent the ratio of light velocity through the vacuum to velocity through any medium.  . Shows the behavior of the real part of the dielectric constant as a subroutine of the wavelength. We note that the mode of (εr) is similar to that of the refractive index, which is due to the decreased values of k 2 compared with n 2 in (equation14).  Figure 11. Depicts the imaginary dielectric constant value as functions for wavelengths. The imaginary part of the dielectric constant is a measure of the absorption of radiation energy by free carriers of material atoms. It has a behavior similar to that of extinction coefficient, which is related to the variation of absorption coefficient also the figure determines, that increased with the increasing of thickness.

Conclusion
Thin films [39]. Have polycrystalline with Tetragonal unit structures with preferential orientation in the (112) direction. Grain size and roughness increased with the increasing of thickness. It was noted that all prepared thin films have high absorption, in the visible range of the electromagnetic spectrum, which made it desirable for the fabrication of solar cells, the optical transitions in CuAlTe2 were direct optical energy gap, and it decreased from (2.34-2.4) eV with the increasing of thickness from (200-500) nm.