Microsoft Word - 41-51 Physics | 41 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Study of Some Structural and Optical Properties of AgAlSe2 Thin Films Iman H. Khudayer Bushra H. Hussien Dept. of Physics/ College of Education For Pure Science (Ibn Al-Haitham) /University of Baghdad Received in:25/November/ 2015,Accepted in:30/March/2016 Abstract The structural properties of ternary chalcopyrite AgAlSe2 compound alloys and thin films that prepared by the thermal evaporation method at room temperature on glass substrate with a deposition rate (5±0.1) nm s-1 for different values of thickness (250,500 and 750±20) nm, have been studied, using X-ray diffraction technology. As well as, the optical properties of the prepared films have been investigated. The structural investigated shows that the alloy has polycrystalline structure of tetragonal type with preferential orientation (112), while the films have amorphous structure. Optical measurement shows that AgAlSe2 films have high absorption in the range of wavelength (350-700 nm). The optical energy gap for allowed direct transition were evaluated, which decreases with film thickness increasing, i.e. it decreases from 2.5 eV to 2.2 eV when thickness varies from 250±20 nm to 750±20 nm. Keywords: AgAlSe2, thin films, energy gap, structure and optical properties. Physics | 42 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Introduction In recent years, many groups worked on chalcopyrite semiconductors with a wide band gap to increase solar cell conversion efficiency by using an absorber close to solar spectrum [1]. Wide gap chalcopyrite materials are required for tandem solar cells and high-voltage devices [2]. Photovoltaic research has moved beyond the use of single crystalline materials such as group IV like Si and group III-V compounds like GaAs to much more complex compounds of the group I-III-VI2 with chalcopyrite structure. The ternary ABC2 chalcopyrites (A = Cu,Ag; B = In, Ga or Al; C= S, Se or Te) form a large group of semiconductor materials with diverse structural and electrical properties. These materials are attractive for thin film photovoltaic application for a number of reasons [3]. Cu based chalcopyrites are most intensively studied, but few experimental and theoretical studies of ABC2 type semiconductors where group A element is Ag and the group B element is Al have been carried out [4] Ag chalcopyrites has several advantages over Cu chalcopyrites such as: The band gap energy of Ag chalcopyrites films is wider than Cu chalcopyrites [5] and the melting point of the Ag chalcopyrites film is lower than Cu chalcopyrites which make them commercially more preferable [6]. Most of the I-III-VI2 compounds are direct gap semiconductors and they crystallize with the chalcopyrite structure [7]. This research aim to analyzes the effect of thickness on the structural and optical properties of AgAlSe2 thin films prepared by thermal evaporation method. Experimental AgAlSe2 films of different thicknesses (250, 500 and 750) nm were prepared by the alloy which is obtained by fusing the mixture of the appropriate quantities of the elements Ag, Al and Se of high purity (99.999%) in evacuating fused quartz ampoules, heated at (1200 K) for five hours, AgAlSe2 films were prepared onto a glass slide substrate by thermal evaporation technique in vacuum system of 3x10-6 Torr using the Edward coating unit model (E 306) from molybdenum boat. The distance from molybdenum boat to the substrate was about 15 cm. The deposition rate was about 5 nm/sec for all the films in room temperature (R.T). X-ray Diffraction (XRD), was used to the position and intensity of diffracted intensity spectra versus Bragg’s angle θ, using (Shimadzu6000 X-ray Diffraction with copper target of the wavelength (λ=1.5406)Ǻ, gives information on the crystal structure such as phase crystalline, polycrystalline, amorphous, crystalline size, and lattice parameter. The inter planer distance d (hkl) for different planes was measured by Bragg’s law [8]: n λ= 2θ d Sin…………………………………………. (1) where n is the order of diffraction, λ is the wavelength of the X-rays, d is the spacing between consecutive parallel planes and θ is the angle of incidence.The average crystalline size can be calculated using the Scherrer’s Formula [9]: )2....(.................................................. cos 94.0 . )( BFWHM RayX B SC    where λ is the wavelength of the X-ray and β is the full width at half maximum intensity in radians. Optical experiments measurement have been done using UV-Visible 1800spectro photometer,. The optical absorption spectrum is used to determine the optical energy gap and the absorption coefficient. Knowledge of these band gaps is extremely important for understanding the electrical properties of a semiconductor, and is therefore of great practical interest [10]. The Tauc formula, equation [11]: αhν=B (hν –Egopt)1/r ………………………………………….(3) Physics | 43 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 where, B is a constant inversely proportional to amorphousity, hν is the photon energy (eV), Egopt is the optical energy gap (eV) ,and r is constant and may take values 2, 3,1/2, 3/2 depending on the material and the type of the optical transition. There are two types of the optical transitions, direct and indirect transition, according to the type of materials and optical transition. The optical behavior of a material is generally utilized to determine its optical constants [refractive index (no), extinction coefficient (k) and, real (εr) and imaginary parts (εi) of dielectric constant]. The absorption coefficient value can be calculated from the formula [12]: )4....(................................................................................303.2 t A  where A is the optical absorbance and t is the film thickness. The refractive index value can be calculated from the formula [13]:   )5.........(........................................ 1 1 1 4 2 1 2 2                 R R k R R no where k represents the extinction coefficient, which is calculated by the relation [12]: )6......(................................................................................ 4  k where R is the reflectance. which is calculated by the relation: R=1-A-T…………………………………………….(7) The real εr and imaginary εi part of dielectric constant can be calculated by using the following equations[14]: )8(................................................................................ir i  )9(................................................................................20 2 0 knr  )10...(................................................................................2 00 kni  Result and discussion X-ray diffraction pattern of AgAlSe2 alloy is shown in Figure (1). The spectrum is seen to exhibit sharp peaks at (101) (112), (211), (220), (204), (312), (400), (404), and  (415) corresponding to 2θ equal to 16.94, 26.78, 34.56, 42.78, 45.206, 51.149, 62.11, 72.366 and 80 polycrystalline structure tetragonal unit structure of AgAlSe2 were absolved as compared with the standard values in ASTM cards. . The X-ray diffraction parameters inter planer spacing (d), Miller indices and crystalline size) for AgAlSe2 alloy are listed in Table (1). The prefer orientation at 112 plane. The experimental absorption spectra for the AgAlSe2 thin films with different thicknesses were made at room temperature in the spectral range (350-1100) nm are shown in Figuree (2). All spectra show better absorption in the ultraviolet and visible regions. When the thickness increases the absorption increases and close for visible region. The behavior of the absorption spectra is opposite completely to that of the transmission and reflection spectra as shown in Figures (3 and 4). From Figure (5) it is observed that the absorption coefficient (α) values, which were calculated using equation (4), were fairly have high values reached above 104 cm-1, which indicated the optical transition in the extended band region. The coefficient absorption values increase with the increase of optical energy gap. This result agrees with that shown by other research [15].The coefficient values decrease with increase of thin thickness that it varied from 5.7×104 to 2.7 ×104cm-1 increased from 250 to750 nm respectively. This behavior agrees with that shown by other researcher [16]. This behavior attributed to the variation of the crystal structure and that the films atoms density increases with film thickness. Physics | 44 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 The absorption coefficient decreases with increase of the thickness as shown in Figure (5) this is due to the relationship between α and the absorbance if we consider the direct proportionality of the absorbance with respect to thickness as shown in Figure (2) It is obvious that A increases with the increase of the film thickness and that can be attributed to the increase of the defects and localized centers [16]. The optical energy gap of material can be determined by using the equation (3), by plotting of (αhυ)2 versus hʋ for AgAlSe2 thin films with different thicknesses is shown in Figure (6). The plot is linear at the absorption edge which confirms that AgAlSe2 is a semiconductor with a direct band gap. Extrapolation of the line to the hʋ axis gives the direct optical band gap and the transition is allowed direct. It can be noticed from the values of Egopt that it decreases with the increase of thickness as shown in table (2). This is due to an increase in practical size [17], and one can notice the Egopt value for film with thickness 250 nm has 2.5 eV, this value is in good agreement with Sharma . et al (4). As well as for thin films, it varies with thickness due to the changes in barrier height at grain boundaries with the increase of film thickness. This is due to the increase in localized density of states near the band edges and in turn decreases the value of Egopt with thickness. Also, the decrease of direct band gap with the increase of film thickness can be attributed to the increase of particle size, decrease of the strain and increase of lattice constant [18]. The variation of the refractive index (n) as a function of the photon energy for AgAlSe2 films at different thicknesses is shown in Figure (7), which indicates that n decreases with the increase of thicknesses This behavior is may be due to decrease in the reflection which the refractive index depends on it. Graph of extinction coefficient (k) as a function of photon energy for different thicknesses of AgAlSe2 films is plotted in Figure (8). This figure revealed that k in general decreased and then increased as film thickness increased (250,500 and 750) nm. The variation of k with film thickness is non- systematic, as shown Table (2). This is attributed to the same reason mentioned previously in the absorption coefficient because the behavior of k is similar to α. The dielectric constant consists of a real (εr) and imaginary part (εi) depends on the frequency of the electromagnetic wave. The variables of εr and εi versus photon electron at different thicknesses are shown in Figures. (9 and 10) respectively. The behavior of εr is similar to that of the refractive index because of the small value of k2 compared with n2,while εi behavior is similar to that of extinction coefficient because it mainly depends on the k value, which is related to the variation of absorption coefficient. The variation of εr and εi with film thickness are non-systematic. This means that this material possesses a specialized property with thickness. Conclusions AgAlSe2 alloy was prepared successfully and used for preparation of thin films by thermal evaporation method. XRD tests for alloy showed that polycrystalline and have the tetragonal structure with preferential orientation in the [112] direction respectively. The influence of thickness on the values of optical parameters of AgAlSe2 thin films is investigated. All thin films exhibited allowed direct optical energy band gap and high absorption in the ultraviolet and visible regions, thus, making the films suitable for optoelectronic devices, for instance as window layers of solar cells. Physics | 45 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 References 1. Siebentritt, Susanne, and Rau, Uwe (Eds.), (2006), Wide-Gap Chalcopyrites, Springer Series in Materials Science. 2. Yamada, K.; Hoshino, N. and Nakada, T. (2006), Crystallographic and electrical properties of wide gap Ag (In 1-x , Ga x ) Se2 thin films and solar cells, Science and Technology of Advanced Materials, 7, (42–45). 3. Aldrin Antony, (2004), Preparation and characterisation of certain II-VI, I-III-VI2 semiconductor thin films and transparent conducting oxides, Thesis presented to the department of physics of the Cochin University of Science and Technology, in partial fulfillment of the requirements for the degree of Doctor of philosophy in physics, India. 4. Sheetal Sharma; Verma, A.S.;Bhandari ,R.; Sarita Kumari and Tindal ,V.K. (2014) Ab initio studies of structural, electronic, optical, elastic and thermal properties of Ag- chalcopyrites (AgAlX2: X=S, Se, Materials Science in Semiconductor Processing, 26, 187– 198. 5. Moller, H., (1993), Semiconductor for Solar Cells, Boston: Artech House. 6. Kalel Murat, (2010, Investigation of electracial and optical properties of Ag-In-Se based devise, thesis presented to the the graduate school of neutral and applied sciences of middle east technical university, in partial fulfillment of the requirements for the degree of Doctor of philosophy in physics, Turkish. 7. Hamid, S. AL-Jumaili and Ahmed, Kh. AL-Rawi, (2011), Effect of thermal annealing and laser radiation on the optical properties of AgAlS2 thin films, Iraqi Journal of Physics, 9, (79- 83). 8. Cullity, B. D., (1978), elements of X-Ray diffraction, 2nd edition, copyright ©, by Addison – Wesley Publishing company , Inc. 9.  Dwivedi, D.K.;Vipinkumar, M.D. and Pathak, H.P., (2011), Structural, electrical and optical investigations of CdSe nanoparticles, Chalcogenide Letters, 8, 9, ( 521-527). 10. Jona, F. and Shirane, G., (1983), Ferroelectric Crystals, Plenum Press, New York 11.Ray, J.R; Panchal, C.J.; Desai, M.S. and Trivedi, U.B., (2011), Simulation of (CIGS) Thin film solar cells using AMPS-1D, J.Nano-Electron. Phys., 3, (747-754). 12.Sze, S.M.,(1981), Physics of semiconductor devices,2nded.,John Wiley and Sons, Inc. New York. 13. Buba, A.D.A., and Adelabu, J.S.A., (2010), Optical and Electrical Properties of Chemically Deposited ZnO Thin Films, The Pacific Journal of Science and Technology, 11, ( 429-434). 14. Ahmed, M. ; Sauli ,Z.; Hashim, U. and add Al-Douri, Y., (2009), Investigation of the absorption coefficient, refractive index, energy band gap, and film thickness for A l0.11Ga 0.89N, A l0.03Ga 0.97 N, and GaN by optical transmission method, Int. J. Nanoelectronics and Materials, 2, 189-195. 15. Kittle, C. (2005), Introduction To Solid State Physics, John Wiley and Sons Inc., 8th edition . 16. Al husseini Mustafa, (2013), Study the optical, electrical and structural properties of silicon- based CdTe solar cell, Thesis Submitted to the College of Science/University of Baghdad, in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Physics, Iraq. 17. Velumani, X.M.; Sebastian,P.J. ;Narayandass,S.K. and.Mangalaraj, D., (2003), Structural and optical properties of hot wall deposited CdSe thin film, Solar Energy Mat.& Sol. Cells, 76, ( 347-358). 18. Aksoy, S.; Cagalar ,Y.; Ilican, S. and Caglar, M. , (2010), Optica Applicat, XL(1), 7 Physics | 46 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Table (1) Structural parameters of AgAlSe2 alloy a,c (exp.) (Å) a,c (ASTM) (Å) hkl d(exp) (Å) d (stand) (Å) 2θ(exp.) (degree) 2θ(stand.) (degree) FWHM (deg.) Crystalline size (nm) a=5.967 c=10.755 a=5.968 c=10.77 101 5.267 5.26 16.94 16.89 0.188 49.4432 112 3.325 3.32 26.78 26.83 211 2.59 2.59 34.56 34.603 220 2.11 2.11 42.78 42.822 204 200 1.99 45.206 45.400 312 1.78 1.78 51.149 51.314 400 1.49 1.49 62.11 62.07 404 1.304 1.303 72.366 72.478 415 1.1979 1.199 80 79.947 Table (2) The optical parameters (Egopt, α, k, n, εr and εi )for AgAgSe2 thin films for different thicknesses at λ =380nm. Figure (1) X-ray diffraction pattern of AgAlSe2 alloy. λ=380nm t(nm) Egopt (eV) Absortion٪ α×104 (cm)-1 n k εr εi 250 2.5 62.25 5.7 2.18 1.73 1.78 7.5 500 2.3 70.95 2.9 1.89 0.89 2.77 3.4 750 2.2 80.3 2.7 1.64 0.82 2.02 2.7 2θ(deg.) (415) (101) (112) (220) (400) (211) (312) (204) (404) Physics | 47 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Figure (2) Absorbance spectra for AgAlSe2 thin films at different thicknesses (250, 500, 750) nm. Figure (3) Transmittance spectra for AgAlSe2 thin films at different thicknesses (250, 500, 750) nm. Figure (4) Reflectance spectra for AgAlSe2 thin films at different thicknesses (250, 500, 750) nm. 0 0.2 0.4 0.6 0.8 1 350 450 550 650 750 850 950 1050 1150 A b s o rb a n c e Wavelength (nm) t=250nm t=500nm t=750nm 0 10 20 30 40 50 60 70 80 90 100 350 450 550 650 750 850 950 1050 1150 T ra n s m it ta n c e ( % ) Wavelength (nm) t=250nm t=500nm t=750nm 0 5 10 15 20 25 350 450 550 650 750 850 950 1050 1150 R e fl e c ti o n ( % ) Wavelength (nm) t=250nm t=500nm t=750nm Physics | 48 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Figure (5) Variation of absorption coefficient with photon energy for AgAlSe2 thin films at different thicknesses (250, 500, 750) nm. Figure (6) Variation of (αhv )2 with photon energy for AgAlSe2 thin films at different thicknesses (250, 500, 750 )nm. Figure (7) Variation of refractive index with photon energy for AgAlSe2 thin films at different thicknesses (250, 500, 750) nm. 0 1 2 3 4 5 6 7 8 9 10 1 1.5 2 2.5 3 Photon energy (eV) t=250nm t=500nm t=750nm 0 50 100 150 200 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 Photon energy (eV) t=250nm t=500nm t=750nm 1 1.5 2 2.5 3 0.5 1 1.5 2 2.5 3 R e fr a c ti v e I n d e x ( n ) Photon energy (eV) t=250nm t=500nm t=750nm Physics | 49 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Figure (8) Variation of Extinction coefficient with photon energy for AgAlSe2 thin films at different thicknesses (250, 500, 750)nm. Figure (9) Variation of real part of dielectric constant with photon energy for AgAlSe2 thin films at different thicknesses (250, 500, 750)nm. 0 0.5 1 1.5 2 0.5 1 1.5 2 2.5 3 E x ti n c ti o n c o e ff ic ie n t (K ) Photon energy (eV) t=250nm t=500nm t=750nm 0 2 4 6 8 0.5 1 1.5 2 2.5 3 R e a l P a rt o f D ie le c tr ic C o n s ta n t Photon energy (eV) t=250nm t=500nm t=750nm Physics | 50 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 AgAlSe2 thin Figure (10) Variation of of imaginary part of dielectric constant with photon energy for films at different thicknesses (250, 500, 750)nm 0 1 2 3 4 5 6 7 8 0.5 1 1.5 2 2.5 3 Im a g e n ry P a rt o f D ie le c tr ic C o n s ta n t Photon energy (eV) t=250nm t=500nm t=750nm Physics | 51 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 2AgAlSeدراسة بعض الخواص التركيبية والبصرية لألغشية الرقيقة ايمان حميد خضير بشرى هاشم حسين بغداد جامعة )/ الهيثم ابن (الصرفة للعلوم التربية كلية / الفيزياء قسم 2016/اذار/30في , قبل 2015/الثاني/تشرين 25استلم في : الخـالصـــة واغشيته التي حضرت بوساطة طريقة التبخير 2AgAlSeالتركيبية لسبيكة المركب الثالثي درست الخواص وبسمك .nm sec (0.1±5)-1الحراري في الفراغ عند درجة حرارة الغرفة على قواعد من الزجاج بمعدل ترسيب لألغشية الخواص البصرية حيود األشعة السينية كما درست نانومتر بأستعمال تقنية (750,500,250 20±) مختلف المحضرة. بينت الفحوصات التركيبية أن السبيكة تملك تركيبا متعدد التبلور من النظام البلوري الرباعي (Tetragonal) أوضحت القياسات البصرية أن ، بينما تمتلك األغشية المحضرة تركيبا عشوائيا. (112)مع هيمنة االتجاه وتم حساب فجوة .نانومتر 700)- (350ذات امتصاصية عالية لألطوال الموجية في المنطقة المرئية 2AgAlSeأغشية (2.2eV)إلى (2.5eV)وان قيمة هذه الفجوة تتناقص مع تزايد السمك،اذ قلت من الطاقة ذات االنتقال المباشر المسموح، على التوالي.(20±750) نانومتر الى (20±250)بزيادة السمك من .اغشية رقيقة, فجوة طاقة, الخواص التركيبية والبصرية AgAlSe ,2 :المفتاحية الكلمات