Chemistry and Materials Research ISSN (Print) ISSN (Online) Vol.3 No.12, 2013

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1 Effect of zinc on structurl nd some opticl properties of CdS thin films Fod Shkir Hshim, Mjeed Ali Hee, Ihsn Rzooqi Ghnim University of Bylon, college of eduction for pure sciences, deprtment of physics E-mil: Astrct This reserch dels with the study of the structurl nd opticl properties of pure nd Zinc doped CdS films with different doping rtios (3,6 nd 9) % volume percentge. Thin films of CdS were deposited on glss sustrtes t temperture 400 o C y using of chemicl spry pyrolysis with spry rte 3ml/sec nd with different thickness. The structurl properties for the prepred films were studied through test of X-Ry diffrction nd it ppered tht the films re polycrystlline hving hexgonl structure. it is lso showed the film prepred hs preferred orienttion long (002).The grin size ws found to e decresed with incresing (Zn) concentrtion, on the other hnd the tomic force microcopy (AFM) showed n decrese in (RMS) vlues with incresing (Zn) concentrtion, lso the opticl properties of these films were studied y mesuring trnsmission nd sorption spectr s function of wvelength, nd it ws found tht the trnsmittnce increses with incresing (Zn) concentrtion, s well s the opticl energy gp increses with incresing (Zn) concentrtion. Keywords: thin films, opticl energy gp, structurl properties. 1. Introduction: The term of thin films is used to descrie lyer or severl lyers of toms for certin sustnce whose thickness rnges etween 10nm nd less thn 1 μm [1].The interest in thin films rised from the wide rnge of use nd ppliction in electronic equipments nd devices. These films hve een used in vrious fields such s of mnufcturing (p-n) junctions, rectifiers, mirrors with two types ordinry nd thermlly, reflected nd ntireflected coting, photogrph, integrted circuits, nd opticl communictions s light emitting diodes, detectors solr cells,.., etc. This leds to study of electronic nd opticl properties of thin films. [2,3] Semiconductors differ from insultors only in the width of the foridden nd (E g ). An insultor hs foridden gp is so wide tht very few electrons cross it t room temperture, wheres semiconductor possesses nrrow gp which llows considerle mount of electrons to the conduction nd. At room temperture the semiconductors hve conductivity in the rnge of ( )Ω -1.cm -1[4]. Cdmium sulfide is direct nd gp semiconductor of out 2.4 ev. It exists ner the photon energy of mximum solr rdition spectrum, it cuses sorption in the short wvelength side, nd hs high sorption coefficient ( 10 5 cm -1 ) within the solr rdition to generte crriers cross the nd gp with wvelength less thn (0.520 µm) [5]. The refrctive index of CdS is 2.3,2.26 nd dielectric constnt 8.64 [6,7]. Cdmium sulfide is n idel mteril for mking photovoltic p-n junction with p-type CIS, CGS. CdS is nturlly n-type with wide opticl nd-gp nd close lttice mtch to CIS [8]. It is usully used s very suitle window lyers which re prepred s thin s possile to void opticl trnsition losses [9,10]. A numer of film deposition methods such s Therml Evportion in Vcuum Deposition (TEVD),Sputtering technique, Chemicl Vpor Deposition (CVD), Pulse Lser Deposition ( PLD ), Therml Pyrolysis Deposition (TPD), hve een used for prepring II-VI compound. Among these, spry pyrolysis is one of the most used methods [11]. 2. Bsic reltion The eqution for the energy gp [12] is given y : αhυ = B ( h υ E g opt. ) r [1] opt. Where: E g : opticl energy gp. B: constnt depended on type of mteril. r: exponentil constnt, its vlue depended on type of trnsition, r =1/2 for the llowed direct trnsition. r =3/2 for the foridden direct trnsition. The trnsmittnce prt of incident light T is depending on α through the following eq. [13] T =exp( A) [2] Where A is the sorption. Thus the sorption coefficient α corresponding to ny wvelength λ cn e clculted [13] y pplying the reltion: 67

2 Where t is the thickness. 3. Experimentl work α = A/t [4] CdS nd CdS:Zn thin films were prepred onto glss sustrte t 400 o C y chemicl spry pyrolysis with different doping concentrtion (3,6,9 %) volume percentge of Zn. The films were prepred y mixing queous cdmium nitrte-4-hydrl solution Cd(NO 3 ) 2.4H 2 O with queous thioure solution CS(NH 2 ) 2. Zinc doped CdS films were deposited y dding Aqueous zinc chloride solution s dopnt source to the solution. The spry rte ws optimized to 3 ml/min through the nozzle to ensures uniform film thickness. Glss sustrtes of 2.5 cm x 3.8 cm, re plced on the surfce of heter when spryed. The heter used ws hot plte with controller. Cd(NO 3 ) 2.4H 2 O + CS(NH 2 ) 2 CdS + CO 2 + 6H 2 O + 2N 2 O The film thickness ws mesured using BLK-CXR-SR-25 (BLACK-Comet Models Super Rnge nm) nd The Weight Method. The crystl structure of CdS films ws determined y X-ry diffrction. Atomic Force Microscopy AFM (AA3000 SPM) ws used to study the surfce morphology of the film. Opticl properties of the films were determined y (shimdzu, UV-1800 o A) spectrophotometer. 4. Result nd discussion 4.1 XRD diffrction nd AFM nlysis The X-ry diffrction (XRD) nlysis hs een performed on the CdS:Zn thin films to scertin the crystl structure nd the crystl orienttion. The XRD ptterns of s-deposited films with different doping concentrtion (0,3,6 nd 9 %) of Zn onto clened glss sustrte t 400 o C nd thickness 260nm re shown in Figs. (1,,,c nd d ). The peks of the XRD were oserved etween 20 o nd 60 o t room temperture. A comprison of the pek position (2θ) vlues of Americn Stndrd for Testing Mterils (ASTM) crd ( ) with XRD spectr dt suggests tht ll films hve (wurtzite) hexgonl structure nd exhiit polycrystlline structure. It cn e oserved from diffrctogrms tht the preferred crystllogrphic orienttion of the crystllites in CdS:Zn films grdully chnges from (002) to (100) with the chnge in the vlue of Zn from 0 to 9%. This result is in greement with those of Irhim [14], nd Perez [15]. The spectrum of CdS nd CdS:3%Zn thin films exhiited peks t (100), (002), (101), (102), (110) nd(112) with preferentil orienttion of (002) plne, nd for CdS:6% nd 9% Zn thin films exhiited peks t (100), (002), (101), (102), (110) nd(112) with preferentil orienttion of (100) plne. Chnges tht hve occurred in the intensity of the min peks re scried to the rottion of some crystlline grins during the preprtion processes. The verge crystllite size of the prepred smples mesured t thickness 260 nm comes to e 17nm,11nm,10.5nm, nd 9nm for doping concentrtion (0, 3,6 nd 9 %) of Zn respectively. the crystllite size is found to decrese with the increse in Zn composition. Therefore, it hs een found tht when the strin increses the crystllite size decrese, ecuse of the delyed crystl growth which is due to the pressures tht cn increse the lttice energy nd reduce the driving force to growth. 68

3 69

Out of the figures ove it cn e noticed tht fmilir grnulr structure with crystllite size round some of nnometers, root men squre (RMS) roughness rnged of (1.84 0.

4 Out of the figures ove it cn e noticed tht fmilir grnulr structure with crystllite size round some of nnometers, root men squre (RMS) roughness rnged of ( ) nm, nd mximum pek to pek height, Sz (ten point height) rnged of ( ) nm which shows tht the surfce of films is highly smooth, s shown in tle (1). Tle (1): AFM dt for films prepred from CdS:Zn. Concentrtion Roughness Root men Ten point Of Zn(%) verge S(nm) squre height Sz(nm) Sq(nm) Opticl properties The opticl sorsion spectr CdS nd CdS:Zn thin films, we cn e seen from Figs.(3,&) displys the vrition of Asornce spectr with wvelength of CdS:Zn thin films mesured t thickness of 160 nd 260 nm respectively. From the plots it cn e justified tht the films show higher sorption on the shorter wvelength side (ultrviolet region). The sorption edges of the films were oserved to shift towrds shorter wvelengths, which is indicted the increse of the opticl nd gp [16], with respect with the incresing of Zn concentrtion. The films show low sorption on the higher wvelength side (visile rnge). This ehvior cn e explined s follows: t high wvelength the incident photon do not hve enough energy to interct with toms, thus the photon will trnsmitted, while when the wvelength decreses (t the neighorhood of the 70

5 fundmentl sorption edge), the interction etween incident photon nd mteril will occur, nd the photon will sornce [17]. Fig.(3, &): Asornce spectr of CdS:Zn thin films mesured t thickness of 160 nd 260 nm respectively Figs.(4, &) displys the opticl trnsmittnce spectr for the deposited CdS:Zn thin films mesured t thickness of 160 nd 260 nm respectively. The films t thickness of 160 nm s in Fig.(4, ), exhiit very high opticl (87% - 90%) trnsmission in the visile rnge, depends upon rdition energy nd the doping concentrtion of Zn within the films, which is importnt for its pplictions s window lyers in solr cells. The shrp sorption edge corresponding to the nd gp confirms the good qulity of grown films. The lue shift occurs in sorption edge (corresponding to the nd gp) in CdS:Zn thin films with the increse in doping concentrtion of Zn within the films. The films t thickness of 260 nm s in Fig. (4, ), exhiit high opticl (82% - 87%) trnsmission in the visile rnge. The opticl study indictes tht the trnsmittnce vlues in the visile rnge decreses with the incresing of thickness for ll investigted smples. This ehvior is ttriuted to the increse the numer of toms with thickness, tht leds to the increse the numer of collisions etween incident photon nd toms which led to the incresing of sornce nd decresing trnsmittnce [18]. Also my e ttriuted to the cretion of level t the energy nd y incresing thickness. Figs.(5, &) show the opticl sorption coefficient s function of incident wve length on CdS nd CdS:Zn thin films t different doping concentrtion (3,6 nd 9 %) of Zn onto clened glss sustrte t 400 o C nd the thickness ws out 160 nd 260 nm respectively. We cn evidently see tht ll films hs vlue of sorption coefficient (α >10 4 cm -1 ) which cuses the increse of the proility of the occurrence direct trnsitions. The sorption coefficient ws decresed with incresed thickness. The sorption coefficient vlue depends on sorptivity nd theoreticlly its reltion is reversed with the film thickness. Fig.(4, &):Trnsmittnce spectr of CdS:Zn films mesured t thickness of of 160 nd 260 nm respectively. 71

6 Fig. (5, &): The sorption coefficient s function of wve length for CdS:Zn thin films mesured t thickness 160 nd 260 nm respectively Figs. (6,,,c nd d) nd (7,,,c,d). Extrpolting the stright line of the plot (αhν) 2 versus (hν) for zero sorption coefficient vlue ( α= 0) give the energy nd gp vlue. The liner nture of the plots t the sorption edge confirmed tht CdS:Zn is semiconductor with direct nd gp. The vlue of opticl energy gp, which hve een determined for CdS & CdS:3,6 nd 9 % Zn films t (αhν) 2 =0, were equl 2.39, 2.393, 2.98 & ev nd 2.38, 2.39, & 2.4 ev for 160 nd 260 nm thicknesses respectively. The increse in opticl energy gp upon doping with Zn hs een explined y splitting of the til levels from the nd. Increse in nd gp cn lso occur y filling the lower-lying conduction (vlence) nd levels with electrons (holes), therey requiring lrger photon energy to promote n electron from the vlence to the conduction nd (Burstein Moss shift) [ 19,20]. c d 72

7 Fig. (6,,,c nd d): Vrition of (αhν) 2 versus photon energy (hν) of CdS thin films t thickness 160 nm with different Zn concentrtion c d Fig. (7,,,c nd d): Vrition of (αhν) 2 versus photon energy (hν) of CdS thin films t thickness 260 nm with different Zn concentrtion 5. Conclusion 1- From X-ry diffrction investigtion oserved crystlline structure of CdS nd CdS:Zn films hve (polycrystlline) structure nd synthesis (hexgonl). 2- The doping with Zn did not effect on the nture of crystl structure of CdS. 3- The verge crystllite size estimted from XRD dt indicte tht the crystllite size within the structures of the nnoprticles, s well s crystllite size decrese with incresing the doping concentrtion. 4- Trnsmittnce of CdS & CdS:Zn films is rnged of 82-87% & 87-90% in the visile nd ner IR regions for thickness of 160 nd 260 nm respectively, nd high sornce in UV region which mkes it suitle s window in solr cell. 5- The trnsition hppen in these films re llowed direct trnsition. 6- Opticl energy gp increse with incresing doping concentrtion. 73

8 References [1] K.L.Chopr, "Thin Film Devices Appliction", Plenum Press, New York, (1983). [2] L. Eckortov, "Physics of Thin Films ", plenum press, (1977). [3] A.H. Anderson, Solr cell (5), (1982), ( ). [4] J.O. Strutt, Semiconductors Devices [5] J.H.Leck, Theory of Semiconductor Junction Device, John Willey & Sons, New York (1976). [6] B.O.Serphin Solr Energy Conversion, Springer Velg, Berlin Heidelerg, New York, (1979). [7] B.Ry II-VI Compounds, Pergmon Press, New York (1969). [8] R.P.Rffelle, H.Forsell, T.Potdevin, R.Friedfeld, Electrodeposited CdS on CIS p-n junction, Solr energy mterils &Solr Cells 57(1999) [9] M.Rmi,E.Benmr, M.Fhoume,F.Chri nd A.Ennoui M.J.Condensed Mtter, 3, (1) (2000) 66 [10] X.Wu, R.G.Dhere, D.S.Alin, T.A.Gessert, C.Dehrt, J.C.Kene, A.Dud,T.J.Coutts,S.Asher,D.H.Levi, H.R.Moutinho, Y.Yn, T.Mority, S.Johnston, K.E.Mery nd P.Sheldon, High Efficiency CTO/ZTO/CdS/CdTe Polycrystlline Thin Films Solr Cells, presented t NCPV progrm metting, Colrdo, (2001). [11] K.L.Chopr, "Thin films phenomen", Mc.Grw-Hill, NewYork, (1969). [12] S.R. Elliott, Physics of Amorphous mterils, Long mn Group limited (1983). [13] J.I.Pnkove, "Opticl Process in Semiconductors", Dover Pulishing, Inc., New York. (1971). [14] A. E.Irhim1, R. A.Ismil nd A. Y. Ali " Effect of Sustrte Temperture on the Structurl Surfce Morphologicl nd Opticl Properties Of nnostrctured CdS Thin Films" Tikrit Journl of Pure Science 17 (2) 2012 ISSN: , (2011). [15] A. M Perez nd I. Vleriono A. " Structurl, opticl nd electricl properties of CdS thin film otined y spry pyrolysis" Revist Mexicn de fisic 54(2), (2008), [16] Je-Hyeong Lee, Jun-Sin Yi, Ke-Joon Yng, Joon-Hoon Prkc, Ryum-Duk Ohd " Electricl nd opticl properties of oron doped CdS thin films prepred y chemicl th deposition" Thin Solid Films (2003) [17] B. G. Streetmn, nd S. Bonerjee, "Solid stte electronic devices", 5 th edition, Engle wood cliffs, Nj prentice itll, (2000). [18] Peter Y.Yu Mnul Crdon "Fundmentls of semiconductors" physics nd mterils properties springerverlg Berlin Heidelerg New York (1996). [19] S. M. H. Al- Jwd " Studies on solution growth thin films of CdS:Zn for photovoltic ppliction" Um- Slm Science Journl,6,(1).(2009) [20] N. R. Yogmlr nd A. C. Bose," Burstein Moss shift nd room temperture ner-nd- edge luminescence in lithium-doped zinc oxide" Appl. Phys. A, (2011). 74

This cdemic rticle ws pulished y The Interntionl Institute for Science, Technology nd Eduction (IISTE). The IISTE is pioneer in the Open Access Pulishing service sed in the U.S. nd Europe.

9 This cdemic rticle ws pulished y The Interntionl Institute for Science, Technology nd Eduction (IISTE). The IISTE is pioneer in the Open Access Pulishing service sed in the U.S. nd Europe. The im of the institute is Accelerting Glol Knowledge Shring. More informtion out the pulisher cn e found in the IISTE s homepge: CALL FOR JOURNAL PAPERS The IISTE is currently hosting more thn 30 peer-reviewed cdemic journls nd collorting with cdemic institutions round the world. There s no dedline for sumission. Prospective uthors of IISTE journls cn find the sumission instruction on the following pge: The IISTE editoril tem promises to the review nd pulish ll the qulified sumissions in fst mnner. All the journls rticles re ville online to the reders ll over the world without finncil, legl, or technicl rriers other thn those inseprle from gining ccess to the internet itself. Printed version of the journls is lso ville upon request of reders nd uthors. MORE RESOURCES Book puliction informtion: Recent conferences: IISTE Knowledge Shring Prtners EBSCO, Index Copernicus, Ulrich's Periodicls Directory, JournlTOCS, PKP Open Archives Hrvester, Bielefeld Acdemic Serch Engine, Elektronische Zeitschrifteniliothek EZB, Open J-Gte, OCLC WorldCt, Universe Digtil Lirry, NewJour, Google Scholr

2.57/2.570 Midterm Exam No. 1 March 31, :00 am -12:30 pm

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