33 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 The Influence of Annealing and Doping by Copper on Electrical Conductivity of CdTe Thin Films Bushra. K.H.Al-Maiyaly Dept. of physics/ College of Education For Pure Science (Ibn Al-Haitham)/ University of Baghdad Received in:7September 2014 , Acceptedin: 20 October 2014 Abstract In this research CdTe and CdTe: Cu thin films with different doping ratios (1, 2, 3, 4 and 5) %, were deposited by thermal evaporation technique under vacuum on glass substrates at room temperature in thickness 450 nm. The measurements of electrical conductivity (σ), and activation energies (Ea1, Ea2), have been investigated on (CdTe) thin films as a function of doping ratios, as well as the effect of the heat treatment at (373, 423, and 473) K° for one hour on these measurements were calculated and all results are discussed. The electrical conductivity measurements show all films prepared contain two types of transport mechanisms, and the electrical conductivity (σ) increases whereas the activation energy (Ea) would decrease as the increasing (Cu) percentage in the sample except 5%. It is also noticed that the electrical conductivity (σ) showed a decreasing trend with increasing annealing temperature, while the activation energies (Ea1, Ea2) showed opposite trend, where the activation energies increased with annealing temperature. Also the electrical conductivity values was found increased about 3- 4 orders when pure CdTe films are doped with (3, 4) % Cu and annealing at 473 K°. Key words: - Cadmium telluride, Electrical Conductivity, heat treatment, thermal evaporation technique. 34 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 Introduction In the recent years, there was an increasing interest in the preparation and study of the physical properties of the semiconducting compounds belonging to the cadmium chalcogenides family as good materials for making semiconducting devices. [1, 2] Cadmium telluride (CdTe) is unique among II–VI compounds which makes it important and quite suitable for several applications in optoelectronic devices such as photo detectors, photovoltaic solar cells, IR and γ detectors, photo-electrochemical cells, field effect transistors, detectors, and photodiodes [3-7]. It is having specific properties, such as a high average atomic number, good charge-transport properties, high resistivity, an ideal direct band gap of 1.5 eV, just in the middle of the solar spectrum which is optimum for single junction solar cell efficiency, processes high absorption coefficient (> 104) [2,4,8,9]. One of the advantages of this material is the possibility to vary its band gap with various dopant concentrations and changes the structural and physical properties of CdTe thin films when doping with different metal atoms that make it useful in the technology of thin film devices. [1, 10] Electrical and optical properties of CdTe are strongly dependent on the structure, and method of preparation [3], the experimental investigations on the electrical and optical properties of CdTe films obtained by different deposition techniques such as chemical bath deposition [11], pulsed laser deposition [12], hot wall epitaxy [13], thermal evaporation [14], physical vapor deposition [3], electrodepositing technique [15], RF-sputtering [16], successive ionic layer adsorption and reaction method [17], and close spaced sublimation [18]. In this research the thermal evaporation technique was used to prepore undoped and Cu- doped CdTe thin films as it ensures stoichiometry. The effect of Cu- doping and annealing temperature on the electrical conductivity and activation energies of CdTe films has been investigated. Experiment CdTe thin films were prepared on glass substrates of thickness (450) nm by thermal evaporation in a high vacuum system of (3*10-6) torr using Edward coating unit model E 306 A from CdTe alloy which is prepared by fusing the mixture of the appropriate quantities of the elements Cd and Te of high purity (99.999 %), (99.95 %) respectively in evacuated fused quartz ampoules at (1373 K°) by a rate of about 4 K°/ min. The ampoules kept at these temperatures for (5-6) hours from the optimum temperature, then the ampoules quenched rapidly in cold water. The compound formation was tested by X-ray diffraction. The distance from molybdenum boat which is used as the evaporation source to substrate was about 15 cm. The deposition rate was about 2nm/s for all the films. Cu doped CdTe films with different doping ratios (1, 2, 3, 4 and 5) % have been prepared by thermal evaporation of CdTe and Cu from two sources (co-evaporation). Annealing processes were carried out on these films at different temperatures (373, 423, and 473) K° for one hour by using (Kilns Furnaces). For electrical resistance measurements Al electrodes were used as contact material for making the electrical connections. Keithly model (614) have been used to measure the variation of electrical resistance (R) with temperature range (308-473) K°, then the resistivity (ρ) of the films is calculated by using the relation [19]:    L tWR  R.A/L ……………………………………… (1) Where A: is the area of the film (W.t) R: is the resistance t: is film thickness, 35 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 w: is electrodes width L :is distance between two Al electrodes. The conductivity (σ) of the films was determined by using the following equation:-   1 c.d ………………………………………… (2) The activation energy (Ea) could be calculated using the formula [19]: - σ = σο exp (-Ea / KBT)……………………………. (3) Where σο: is constant, but change slowly with temperature. KB: is Boltzman's constant. T: is absolute temperature in kelvin. Thickness of the films was measured by using the weighing method according to the following relation: [20] t= (m / A .ρ) ……………………. (4) Where t= thickness of film, m= mass of film, ρ= density of films, A= area of film. Using a sensitive balance whose sensitivity is of the order (10-4) gm. Results and Discussion Fig (1) shows the plots of lnσ versus 103/T at different doping ratio by Cu for (CdTe) films, It is observed that the electrical conductivity (σ) increases from (3.27*10-4) ohm-1.cm-1 for undoped films to (1.44*10-1) ohm-1.cm-1 as doping ratio increases from (0-4)% .This indicated that the doped film have high electrical conductivity compared to that of un-doped film except ratio 5% Cu which showed opposite trend, as shown in Fig (2) and table (1) hence the Cu content in the film, strongly influences the values of σ. The increasing trend in σ after doping with (1,2,3,4)%Cu is attributed to that Cu introduces interband energy levels in the band gap of CdTe thin films which were responsible for the shift of Eg to lower band gap energy due to increase number of carriers available for transport is in agreement with our result on optical properties. A similar manner was found by other researchers for different dopant atoms [1, 4, and 21]. Figure (3) shows the plots of lnσ versus 103/T as a function of annealing temperature for (CdTe) films, the noticeable remark is that the electrical conductivity decreases with increasing annealing temperature. Figures (1, 3) also shows two temperature ranges characterized by different conductivity slopes. These films contain two types of transport mechanisms at lower temperature range (289-383) K°, due to carriers excited into the localized states at the edge of the band by hopping and at higher temperature range (393-473) K° due to carriers excited into extended states beyond the mobility edge by thermal excitation, with two values of activation energy (Ea1, Ea2). It is clear from Fig (4) and table (1) that decrease in the activation energy with increasing doping ratio except ratio 5% Cu, this behavior is a result of the increased-doping concentration , a stronger interaction among impurities and this behavior can be attributed to the improvement in the films structure with increasing doping ratio due to decreasing scattering at grain boundaries, decrease the trapping centers of charge carriers ,is in agreement with our result on structural properties. The influence of different annealing temperature on the electrical conductivity and activation energies of CdTe films is shown in figures (5, 6) respectively and table (2). From these figures we can notice that σ decrease after heat treatment, while the activation energies showed opposite trend for all films prepared because of the decreased number of carriers available for transport, this may be due to the fact that correlation between the structure and the electrical properties of the film, from this reason this behavior can be attributed to the 36 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 improvement in the films structure with increasing Ta due to decreasing the density of states in the gap, reducing of dangling bonds, and defects like vacancy sites in the films structure after heat treatment. In conclusion, it is seen from figure (7) that the annealed Cu- doped (3%, 4%) samples at (473) K°, had higher conductivities than those before annealing. It is also noticed that the electrical conductivity values increase about 3- 4 orders as compared to as deposited films and reached maximum value (2.11 *10-1) ohm-1.cm-1 for film doped 4% Cu at Ta=473 K°. Conclusion Using the thermal evaporation technique, un-doped and Cu-doped CdTe thin films with different doping ratio were obtained. The influence of doping ratio (1, 2, 3, 4 and 5) % Cu and annealing temperature (373, 423, 473) K° on the electrical conductivity, and activation energies for CdTe was investigated. The electrical conductivity measurements indicate that the films contain two types of transport mechanisms and the electrical conductivity of dopant films is higher than those of un-doped films and it is increased with the increase of doping ratio, but the activation energies of dopant films is lower than those of un-dopant films and it is decreased with the increase of impurity percentages. We should mention that the doping at ratio higher of about 4% Cu showed opposite trend. The electrical conductivity are strongly dependent on the annealing temperature, it shows as decreasing behavior with the increase of annealing temperature, whereas the activation energies showed an opposite trend. Finally, the electrical conductivity for heat-treated doped films increases more than two times as compared to the electrical conductivity for pure films. References 1. Rusu, G. (2006), Structural, Electronic Transport and Optical Properties of Zn-doped CdTe Thin Films, Journal of Optoelectronics and Advanced Materials, 8 , 931. 2. Alnajjar, A.A.; Al-Shaikley, F. Y. and Alias, M.F.A. (2012), Optical Properties and photo- conductivity of Undoped and n-doped CdTe thin films", Journal of Electron Devices, 16, 1306. 3. Rusu, G. G. , (2001), On The Electrical and Optical Properties of nano crystalline CdTe thin films, Journal of Optoelectronics and Advanced Materials, 3, 4, p. 861 – 866. 4. Abbas Shah, N.; Ali, A.; Ali, Z.; Maqsood, A.,and Aqili, A.K.S. (2005), Properties of Te- rich cadmium telluride thin films fabricated by closedspace sublimation technique, Journal of Crystal Growth, 284 477–485. 5. Bonnet, D.and Meyers, P. (1998), Cadmium Telluride-mater for thin film solar cell, J. Mater. Res., 13, 2740. 6. Sahay, P.P. ;Jha ,S. and Shamsuddin, M. (2001),Electrical Properties of Vacuum- evaporated CdTe thin films, Journal Of Materials Science Letters,20,1933-1935. 7. Hanaa, I. Mohammed,( 2010), Study the Structural and Electrical Properties of CdTe:Ag Thin Films, Journal of Al-Nahrain University, 13, 2, 129-135. 8. Pinheiro, W.A.; Falcao, V.D.; Cruz, L. R. O. and Ferreira ,C. L. (2006), Comparative Study of CdTe Sources Used for Deposition of CdTe Thin Films by Close Spaced Sublimation Technique , Materials Research, 9, 1, 47-49. 9. Lane, D. (2006), A review of the optical band gap of thin film CdSxTe1−x, Solar Energy Materials and Solar Cells, 90, 9, 1169–1175. 10. Al-Douri, A. A. J. , Al-Shakily, F. Y. , Alias, M. F. A. and Alnajjar, A. A. ,(2010) ,Optical Properties of Al- and Sb-Doped CdTe Thin Films," Advances in Condensed Matter Physics, Article ID 947684, 5 pages. 37 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 11. Deivanayaki, S.; Jayamurugan, P.; Mariappan, R.; and Ponnuswamy, V. (2010), Optical and Structural Characterization of CdTe Thin Films by Chemical Bath Deposition Technique, Chalcogenide Letters 7, 159. 12. Pandey, S.; Tiwari, U.; Raman, R; Prakash, C; Krishma, V; Dutta, V; and Zimik K,2005 , Growth of Cubic and Hexagonal CdTe Thin Films by Pulsed Laser Deposition”, Thin Solid Films ,473, 54. 13. Ferreira, S.; Leal, F.; Faria, T .Oliveria; J, Motisuke P;and Abramof E.,(2006), Characterization of CdTe Thin Films Grown on Glass by Hot Wall Epitaxy”, Brazilian Journal of Physics ,36, 317. 14. Singh,S; Kumar,R;and Sood,K. (2010), Structural and Electrical Studies of Thermally Evaporated Nanostructure CdTe Thin Films, Thin Solid Films ,519, 1078. 15. Gangawane,S.; Dhaygude,H; Patit S;and Fulari,V. ,(2011), Surface Deformation of Cadmium Telluride Thin Films by DEHI Technique, Archives of Applied Science Research ,3, 186. 16. Melendez-Lira M; Becerril-Silva M; Zapata-Torres M; Mendoza-Galvan,A;and Jimenez- Sandoval,S. (2005), Semiconductor Thin Films Grown by RF-co-sputtering of CdTe and Al Targets, Superficies Y Vacio 18, 22. 17. Ubale, A.and Kulkarni, D. (2006), Studies on Size Dependent Properties of Cadmium Telluride Thin Films Deposited by Using Successive Ionic Layer Adsorption and Reaction Method, Indian Journal of Pure and Applied Physics 44, 254. 18. Pinheiro,W; Falcao,V; Cruc,L;and Ferreira C. , (2006), ”Comparative Study of CdTe Sources Used for Deposition of CdTe Thin Films by Close Spaced Sublimation Technique, Materials Research ,9, 47. 19. William, D. and Callister,(2003), Materials Science and Engineering, an Introduction”, 6th edition, John Wiley & Sons, Inc. 20. AL-Mizban, E.S. (1997), A study of optical and electrical properties of Cr2O3 and Co3O4 thin films and their mixture”M.Sc thesis University of Baghdad,p.47-49. 21. Abbas Shaha N.; Alia A.; Aqilib A.K.S.; and Maqsood A. , (2006), "Physical properties of Ag-doped cadmium telluride thin films fabricated by closed-space sublimation technique, Journal of Crystal Growth,290 , 452–458. Table No. (1): The electrical conductivity and activation energies of (CdTe) films State σ at R.T (Ω.cm)-1 Ea1 Tem. rang Ea2 Tem. rang ( eV ) ( K° ) ( eV ) ( K° ) undoped 3.27*10-4 0.02007 298-383 0.3006 393-473 Doped 1%Cu 1.31*10-3 0.0165 298-383 0.245 393-473 Doped 2%Cu 9.65*10-3 0.01319 298-383 0.1491 393-473 Doped 3%Cu 4.23*10-2 0.00267 298-383 0.1354 393-473 Doped 4%Cu 1.44*10-1 0.00242 298-383 0.1013 393-473 Doped 5%Cu 1.03*10-4 0.010945 298-383 0.2484 393-473 38 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 Table No. (2): The electrical conductivity and activation energies of (CdTe) films at different annealing temperature films )CdTe/T as a function of doping ratio for (3lnσ versus 10 Figure No.(1): Variation Figure No.( (2): Electrical conductivity as a function of doping ratio for (CdTe) films -12 -10 -8 -6 -4 -2 0 1.5 2 2.5 3 3.5 ln  (o h m .c m )- 1 1000/T (K)-1 pure doped 1% cu doped 2% cu doped 3% cu doped 4% cu 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 0 1 2 3 4 5 6  ( o h m .c m ) -1 Doping Percentag% Annealing temperature K° σ at R.T (Ω.cm)-1 Ea1 Tem. rang Ea2 Tem. rang ( eV ) ( K° ) ( eV ) ( K° ) R.T 3.27*10-4 0.02007 298-383 0.3006 393-473 373 2.6*10-4 0.062 298-383 0.34 393-473 423 2.02*10-4 0.095 298-383 0.375 393-473 473 1.37*10-4 0.12 298-383 0.42 393-473 39 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 Figure No. (3): lnσ versus 103/T as a function of annealing temperature for (CdTe) films Figure No.(4): Variation activation energies as a function of doping ratio for (CdTe) films -10 -9.5 -9 -8.5 -8 -7.5 -7 -6.5 -6 -5.5 -5 1.5 2 2.5 3 3.5 ln  (o h m .c m )- 1 1000/T (K)-1 R.T Ta=373 K Ta= 423 K Ta=473 K 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0 1 2 3 4 5 E a ( e V ) Doping Percentag % Ea1 Ea2 40 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 Figure No.(5): Electrical conductivity as a function of annealing temperature for (CdTe) films Figure No.(6): Activation energies as a function of annealing temperature for (CdTe) films 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 273 323 373 423 473  ( o h m .c m ) -1 Ta (K) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 273 323 373 423 473 E a ( e V ) Ta (K) Ea1 Ea2 41 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 Figure No. (7): lnσ versus 103/T for (CdTe) films un-doped and Cu-doped before and after annealing. -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1.5 2 2.5 3 3.5 ln  (o h m .c m )- 1 1000/T (K)-1 undoped at R.T doped 3% Cu 3%cu+Ta(473 K) doped 4% Cu 4%cu+Ta(473 K) 42 | Physics 2015) عام 1(العدد 28المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol. 28 (1) 2015 ى التوصيلية الكھربائية ف والتشويب بالنحاسالتلدين تأثير الرقيقة CdTe ألغشية بشرى كاظم حسون الميالي / جامعة بغداد) بن الھيثم ا الصرفة( للعلوم كلية التربيةقسم علوم الفيزياء / 2014تشرين االول20قبل في: 2014ايلول 7استلم في : الخالصة % (5 ,1,2,3,4)الرقيقة غير المشوبة والمشوبة بالنحاس بنسب تشويب مختلفة CdTeحضرت اغشية في ھذا البحث قياسات تحسبو. عند درجة حرارة الغرفة 450nmبتقنية التبخير الحراري بالفراغ على ارضيات من الزجاج وبسمك اثير . نسب التشويبلتغير دالة )CdTe( الغشية Ea)Ea1 ,2 (التنشيطوطاقات , (σ)التوصيلية الكھربائية وكذلك حسب ت ومدة ساعة واحدة على ھذه القياسات ونوقشت جميع ھذه النتائج. °K (and 473 ,423 ,373)المعاملة الحرارية بدرجات ادة التوصيلية آليتين لالنت المحضرة كل االغشيةل ان التوصيلية الكھربائيةقياسات وقد أظھرت قال االلكتروني ولوحظ زي ادة لوحظ نقصان التوصيلية كما .%5 باالنموذج ماعدا النسبة (Cu) نسبة الكھربائية مع نقصان طاقات التنشيط بزي أزدادت طاقات التنشيط مع إذ اُ,معاكس اُ الكھربائية مع زيادة درجات حرارة التلدين ، بينما أظھرت طاقات التنشيط سلوك CdTeمراتب عند تطعيم اغشية 4 -3 تزداد بمقدار التوصيلية الكھربائيةكذلك وجد ان قيم زيادة درجة حرارة التلدين. .°K 473وتلدينھا عند درجة حرارة Cu % (4 ,3)النقية بنسبة المعاملة الحرارية , تقنية التبخير الحراري .ية ، ، التوصيلية الكھربائ تيليرايد الكادميوم -: الكلمات المفتاحية