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Effect of Reaction Temperature and Time on the Structural Proper-Ties of Cu (In, Ga) Se2 Thin Films Deposited by Sequential Elemental Layer Technique [J]

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Effect of Reaction Temperature and Time on the Structural Proper-Ties of Cu (In, Ga) Se2 Thin Films Deposited by Sequential Elemental Layer Technique [J]
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  J. Mater. Sci. Technol., Vol.23 No.4, 2007  499 Effect of Reaction Temperature and Time on the Structural Proper-ties of Cu(In,Ga)Se 2  Thin Films Deposited by SequentialElemental Layer Technique Saira RIAZ  † and Shahzad NASEEM  Centre for Solid State Physics, University of the Punjab, QAC, Lahore-54590, Pakistan [Manuscript received June 27, 2006, in revised form December 12, 2006] Thin films of copper indium gallium selenide Cu(In,Ga)Se 2  (CIGS) were prepared by sequential elemental layerdeposition in vacuum at room temperature. The as-deposited films were heated in vacuum for compoundformation, and were studied at temperature as high as 1250 ◦ C for the first time. These films were concurrentlystudied for their structural properties by X-ray diffraction (XRD) technique. The XRD analyses include phasetransition studies, grain size variation and microstrain measurements with the reaction temperature and time.It has been observed that there are three distinct regions of variation in all these parameters. These regionsbelong to three temperature regimes:  < 450 ◦ C, 450–950 ◦ C, and  > 950 ◦ C. It is also seen that the compoundformation starts at 250 ◦ C, with ternary phases appearing at 350 ◦ C or above. Whereas, there is another phaseshift at 950 ◦ C without any preference to the quaternary compound. KEY WORDS:  Cu(In,Ga)Se 2  (CIGS); X-ray Diffraction; Thin films; Structural analysis 1. Introduction CuInSe 2  is a I-III-VI 2  ternary semiconductor com-pound and has an extremely high absorptivity that al-lows 99% of the available light to be absorbed in thefirst micron of the material. Adding small amountof Ga to the absorbing CuInSe 2  layer leads to aninherent grading of the band gap, which improvesthe voltage and therefore the efficiency of solar cells.For this purpose, Cu-chalcopyrite solar cells based onCu(In,Ga)Se 2  and related compounds are rated as aleading material for the development of economicalterrestrial photovoltaic energy conversion, and haveachieved efficiencies of around 19% [1] .The stacked elemental layer (SEL) method of de-positing different materials on various substrates hasbeen applied to the preparation of thin films. SELmethod is the one in which layers of different com-ponent elements are deposited onto the substrate(s)and crystallinity is achieved by annealing. Resultshave been reported with efficiency up to 10% for so-lar cells prepared by this technique having CuInSe 2 as absorber material [2 , 3] .CuInSe 2  films have previously been prepared bySEL technique and were air and vacuum annealed attemperatures ranging from 200 to 500 ◦ C [4] . It wasfound that well-structured chalcopyrite CuInSe 2  withdesired electro-optical properties could be formed.The great advantage of SEL method is that sincecomposition is dependent on as-deposited thicknessof only single elements, it can be very accurately con-trolled. Perhaps one of the obvious drawbacks of thistechnique, not yet rectified, is the small crystallitesize produced. However, crystallinity increases as afunction of annealing temperature.In this paper we report the results of structuralcharacterization of Cu(In,Ga)Se 2  thin films. The films †  Ph.D., Present Address: State Key Laboratory for Mag-netism, Institute of Physics, Chinese Academy of Sciences,China, E-mail: saira − cssp@yahoo.com. were formed by sequentially depositing the elementalconstituents of the compound. The films were heat-treated for the compound formation for the first timein the range of 150–1250 ◦ C, and the structural resultsare correlated to the heat treatments. 2. Experimental The thin films of Cu(In,Ga)Se 2  were preparedby SEL technique using resistive heating method inthe sequence Cu-Se-Ga-In. The films were depositedonto single crystal ( < 100 > -oriented) Si substrates atroom temperature. The Si substrates were cut to asize of 1.5 cm × 2.5 cm that fits in the high tempera-ture attachment of the diffractometer (Rigaku XRD-D/MAX IIA). The weights of these elements weretaken according to the formula Cu(In,Ga)Se 2  in or-der to give a total thickness of about 1.5  µ m. Thecleaning of the substrates was performed chemically,as described in literature [5,6].Edwards 306 coating unit was used to deposit thematerials. This unit is equipped with diffusion androtary pumps, and the ultimate attainable vacuum isbetter than 10 − 6 Torr (133 × 10 − 6 Pa). Once the basepressure is reduced, the source is heated to a temper-ature near the melting point of the evaporant. Thedeposition is started after the source is thermally sta-bilized.The CIGS thin film samples were reacted in vac-uum at 150–1250 ◦ C for 15 min followed by 40 minof annealing. Some samples were heated for longertime in order to check the effect of heating time onthe properties. All the samples were heated using thehigh temperature XRD attachment after depositionof the constituent elements.The films were characterized for their structuralanalysis by XRD and the diffractometer was operatedunder the following conditions.X-ray source: Cu Kα  (Ni filtered)Wavelength: 0.15405 nm  500  J. Mater. Sci. Technol., Vol.23 No.4, 2007 Fig.1  XRD pattern of the as-deposited sample showing, ◦ —In Fig.2  Variation cycle of reaction and annealing temper-atures. ‘a’ represents 15 min of reaction time and‘b’ represents 40 min of recording time Fig.3  3D XRD traces of CIGS films at: (a) roomtemperature, (b) 150 ◦ C, (c) 250 ◦ C, (d) 350 ◦ C,(e) 450 ◦ C, (f) 550 ◦ C, (g) 650 ◦ C, (h) 750 ◦ C,(i) 850 ◦ C, (j) 950 ◦ C, (k) 1050 ◦ C, (l) 1150 ◦ C and(m) 1250 ◦ C,         —Cu(In,Ga)Se 2 Tube voltage: 35 kVTube current: 25 mAAngular range (2 θ ): 5 ◦ –80 ◦ Step width (2 θ ): 0.02 ◦ Preset time: 0.4 sDetector: Scintillation: Counter 3. Results and Discussion The CIGS thin films of thickness 1.5  µ m were de-posited by SEL technique using vacuum evaporation, Fig.4  Intensity of major peaks in Fig.3 and the XRD result of as-deposited sample is shownin Fig.1. This figure shows peaks of In [7] as expectedfrom the structural layout of these samples.The phase variation, in the temperature range of 150–1250 ◦ C, was then studied in vacuum by usinghigh temperature attachment of X-ray diffractome-ter. The heating at every temperature was performedfor 15 min and the diffractogram was then taken afterbringing the temperature down by 50 ◦ C as shown inFig.2.The diffraction patterns for all 12 reaction tem-peratures, along with the as-deposited pattern, areshown in Fig.3. The identification of various peakspresent in this figure shows that when the sample isheated at 150 ◦ C the binary compound of InSe appearsalong with the elemental peaks of Se and In. For fur-ther heating, it was found that the binary compoundsof the type In 2 Se 3  and InSe still dominate at 250 ◦ C.Whereas, the ternary compounds,  e.g . CuInSe 2  andInGaSe 2  start appearing at 350 ◦ C. There appears tobe more peaks of the ternary structure at 450 ◦ C.These effects can be clearly seen in Fig.4, where themajor peaks from Fig.3 are plotted against the re-action temperatures. This figure also shows that at950 ◦ C there is another chemical shift that takes place.This shows that there is a structural re-arrangementin the film when the reaction temperature is increased,and the maximum dominating plane shifts from onephase to another. Further, there is a strengtheningof the peaks belonging to the quaternary compound(CIGS), at or above 950 ◦ C.In Fig.4, there appears to be three distinct regionsof phase transition. In order to find out the effect of time variation at these three temperatures,  i.e.  250,450 and 950 ◦ C, more samples of CIGS were heated atvarying time intervals. The resulting diffractogramsare shown in 3D plots of Fig.5(a)–(c). Once again, thephase variations can be observed when major peaksare plotted against reaction time (Fig.6). It is clearfrom the plots of Fig.6 that these values are almostconstant at all temperatures, which means that thereis no practical effect of changing the reaction timefrom 15 min onwards.It can be seen from Fig.4 that there is an increasein the intensity of various binary/ternary peaks, sep-arately in the three regions with increasing tempera-ture. This indicates grain growth and coalescence of small grains into larger ones for a particular temper-  J. Mater. Sci. Technol., Vol.23 No.4, 2007  501 Fig.5  3D XRD profiles of CIGS films at 250 ◦ C (a), 450 ◦ C (b) and 950 ◦ C (c) for: (1) as-deposited, (2) 15 min,(3) 30 min, (4) 45 min and (5) 60 min,  ∗ —In 2 Se 3 ,  ∗∗ —InSe,  ◦ —In,  • —InGaSe 2 ,         —CuInSe 2 ,         —CIGS Fig.6  (a) Intensities of major peaks of Fig.5(a), (b) intensities of major peaks of Fig.5(b), (c) intensities of majorpeaks of Fig.5(c)  502  J. Mater. Sci. Technol., Vol.23 No.4, 2007 Fig.7  Variation in grain size with reaction temperature  Fig.9  Microstrain plotted against reaction temperature Fig.8  Variation in grain size with reaction time ature regime. Grain growth is driven by neighbor-ing grains that posses different energies due to thecurvature of energetic grain boundaries and/or dif-ferent amounts of accumulated strain energy. It isevident from the relevant figures that grain growthoccurs along with the phase transitions. The grainsizes at various temperatures were calculated from theWilliamson-Hall relation [8] as follows:2 ω f  cos θKλ  = 1 D  + 4 eKλ sin θ  (1)where, 2 ω f   is in radians,  K   the shape factor of thecrystalline particles,  λ  the wavelength,  e  the micros-train equal to ∆ d/d ,  D  the particle size and  θ  theBragg angle.The extrapolation method [9] has been adopted foreliminating the instrumental broadening of diffrac-tion lines since it does not require a standard sample.This method is valid for the conventional powder X-ray diffractometer with the Bragg-Brentano focusinggeometry [9 , 10] , so it was used in the present case.The grain size, as calculated from the above rela-tion, has been plotted with reaction temperature andshown in Fig.7. It is evident from this figure againthat there are three distinct varying regions. Thesevariations are consistent with changes in the variousphases with temperature. There is almost a constantgrain size up to reaction temperature of 450 ◦ C afterwhich there is a rather slow increase in the grain sizefirstly (up to 950 ◦ C) and then a rapid rise is seenfrom 950 ◦ C. There is a decrease in grain size, ev-ery time the phase shift takes place and then the sizestarts increasing. The variation in grain size with thechange in reaction time at various temperatures hasalso been calculated and shown in Fig.8. This fig-ure shows that there is little effect of reaction timeon the grain growth until a temperature of 950 ◦ C isachieved. This once again confirms somewhat rapidgrain growth beyond 950 ◦ C, as discussed above.The variations in grain growth can be attributedto the relaxation and stresses produced in the filmduring phase transitions. The stress/strain are strongfunction of grain growth since the film accommodatesthe high temperature stresses in the grains. The mi-crostrain can also be calculated from the Williamson-Hall relation given above. The calculated values areplotted against the reaction temperature as shown inFig.9. It is seen that three distinct regions are appar-ent in this plot as well. These regions are consistentwith the phase and grain size variations with temper-ature. Initially, there is a slight variation in the strainvalue due to low temperature, and then a suddenchange in the values appears. This confirms the resultof grain growth because of the high temperature as-sociated with the accommodating grain growth. Theresults show that there is a uniform strain produced inthe film [11] . This fact is evident from the shapes andshifts of the peaks of various reflection planes whencompared with the standard shapes [12] . 4. Conclusions (1) The quality of CIGS based thin films and de-vices is becoming decoupled from the method of filmdeposition. This will lead to novel, fast and low costmethods for absorber preparation regarding the ap-plication of solar cells. XRD technique has not onlybeen used for identifying the compound formation butalso for the detailed variations in the structure.(2) Detailed analysis has been performed for struc-tural properties of SEL deposited CIGS thin films atvarious temperatures and reaction times. The ini-tially binary compounds are formed at temperatureup to 250 ◦ C. Mixed phases of ternary and quaternarycompounds subsequently form from 350 ◦ C. However,there is another phase variation at 950 ◦ C without anypreference to quaternary CIGS.  J. Mater. Sci. Technol., Vol.23 No.4, 2007  503(3) The half width of the X-ray diffraction linedecreased and the line  s chalcopyrite character ap-peared after annealing. Microstrain of the films atvarious temperatures has also been calculated and thechanges could be attributed to grain formation andgrowth. REFERENCES[1 ] K.Ramanathan, J.Keane and R.Noufi:  Proc. 31stIEEE Photovoltaic Specialists Conference,  Orlando,FL, USA, 2005, 7.[2 ] B.J.Stanbery:  Crit. Rev. Solid State,  2002,  27,  73.[3 ] A.Knowles, H.Oumous, M.J.Carter and R.Hill:  Semi-cond. Sci. Technol.,  1988,  3,  1143.[4 ] M.S.Sadigov, M.Ozkan, E.Bacaksiz, M.Altunba,A.I.Kopya:  J. Mater. Sci.,  1999,  34,  4579.[5 ] S.Naseem, D.Nazir, R.Mumtaz and K.Hussain:  J.Mater. Sci. Technol.,  1996,  12,  89.[6 ] S.Riaz and S.Naseem:  Sci. Int.,  2003,  15,  99.[7 ]  Powder Diffraction File: Joint Committee on Pow-der Diffraction Standards,  ASTM, Philadelphia, 1967,JCPDS Card No.5-642.[8 ] H.P.Klug and L.E.Alexander:  X-ray Diffraction Pro-cedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1974.[9 ] M.ˇCerˇnanskˆy:  Mater. Struct.,  2000,  7,  3.[10] S.Riaz and S.Naseem:  Sci. Int.,  2007. (in Press)[11] B.D.Cullity:  Elements of X-ray Diffraction,  AddisonWesley Publishing Co., USA, 1978.
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