Investigation the Effect of Thickness on the Structural and Optical Properties of Nano ZnO Films Prepared by d.c Magnetron Sputtering

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1 We Site: Emil: Investigtion the Effet of Thikness on the Struturl n Optil Properties of Nno ZnO Films Prepre y. Mgnetron Sputtering M.F.A.Alis 1, R.M.Aljrrh 2, H.KH.Al-Lmy 3 n K. A.W. Aem 4 1Physis Deprtment. College of Siene, University of Bgh P.O.Box ,Jiriy, Bgh, Irq 2Physis Deprtment. College of Siene, University of Kuf 3Physis Deprtment. College of Siene, University of Bgh, Bgh, Irq 4Physis Deprtment. College of Siene, University of Bgh, Bgh, Irq Astrt The ojetive of this reserh is stuying the effets of film thikness on the struturl n optil properties for nno ZnO films prepre y. mgnetron sputtering tehnique on glss sustrtes. Thiknesses (t) of ZnO thin films were ltere y vrying the eposition time from 45 min to 9 min (t=75,15,2 n 25) nm n re etermine using optil metho. The X-ry iffrtion spetr show tht ZnO thin films re polyrystlline with the hexgonl struture n show goo -xis orienttion perpeniulr to the sustrte. The most preferentil orienttion is long the (2) iretion for ll sputtere eposite ZnO films, in ition nother peks pper t thikness 2 n 25 nm with orienttions in (12) n (11) plnes. Generl oservtions inite tht mirostruture prmeters, suh s grin size, epen sensitively on the thikness of the film. The optil properties of nno ZnO thin films hve een investigte y UV/VIS spetrometer n the n gp vlues re foun to e with the rnge of 3.27 ev to 3.44eV.The ZnO film of ll thikness hs trnsmittne over 85% in the visile region, n the refrtive inex is out 2.1. Keywors: Nno ZnO films, struturl properties, optil properties 1. INTRODUCTION ZnO is n II-VI trnsprent onuting oxie toute s mteril of hoie for short-wvelength optoeletronis [1][2]. This semionutor hs severl fvorle properties: goo trnspreny, high eletron moility, wie n iret n gp of 3.37 ev t 3 K [3-6], n s onsequene it sors UV rition ue to the n-to-n trnsitions, strong room temperture luminesene, et. Those properties re lrey use in emerging pplitions for trnsprent eletroes in liqui rystl isplys n in energy-sving or het-proteting winows, n eletroni pplitions of ZnO s thin-film trnsistor n light-emitting ioe, n Beuse of its lrge exittion ining energy (6 MeV) ompre with other semionutor mterils [3]. ZnO hs gine sustntil interest in the fiel of semionutor reserh, ZnO high potentil for room temperture light emission, n more resistnt to rition, n is multifuntionl s it hs piezoeletri, ferroeletri, n ferromgneti properties. Unope ZnO is usully n- type onutive, whih is ssoite with the presene of ntive point efets (e.g., oxygen vnies n interstitil zin)[7][8].the rystl strutures of ZnO re wurtzit eh su lttie inlues four toms per unit ell n every tom of one kin (group-ii tom) is surroune y four toms of the other kin (group VI), or vie vers[9]. Mny tehniques hve een employe to proue zin oxie suh s sputtering, metl orgni hemil vpor eposition, sol gel, spry pyrolysis, n pulse lser eposition. Among these tehniques, sputtering hs prove to e simple n inexpensive metho [1]. Sputtering is one of the most verstile eposition tehniques use for the eposition of trnsprent onuting oxies (TCO). When sputtering is ompre to other tehniques, sputtering proues lyers with higher purity n etterontrolle omposition. It lso proues films with greter hesive strength, homogeneity n permits etter ontrol of film thikness n this tehnique is esy to eposit mterils whih oul not e esily eposite using other tehniques.reserhers hve foun tht the optil properties strongly epen on the thikness of ZnO films so in this pper, we introue the effet of thikness on the struturl n optil properties of ZnO films prepre y. mgnetron sputtering. 2. EXPERIMENTAL PROCEDURE Typil onitions for. sputtering of mterils re: thoe voltge of 1kV, thoe to sustrte istne of 3. m n working pressure of.8 Torr.Sustrte temperture ws vrie from room temperture (RT) to out 373 K. Volume 2, Issue 7, July 213 Pge 198

2 We Site: Emil: Mgnetron thoe n suffiiently uniform mgneti fiel re require to onfine the eletrons ner the trget surfe [9]. This moe of sputtering, known s. mgnetron sputtering, ws lso employe in the present stuy. In generl, the struturl properties s well s optil properties re strongly epenent on the proessing onitions. The thikness of the films (t) ws etermine using the optil metho: t = λδx/2x (1) where x is the fringes with, Δx is the fringes shift, n λ is lser wvelength. The strutures of the films were hrterize y X-ry iffrtion (XRD) with Cu K α rition λ =.1546 nm. From the peks position n the integrl with t hlf mximum of the respetive pek, the grin sizes of prepre ZnO films were lulte; the verge grin size in the -xis orienttion estimte using the Deye-Sherrer reltion [1]: D XRD =.9λ / B os θ (2) where D XRD is the men prtile size, θ is the Brgg iffrtion ngle n B is the full with t hlf mximum (FWHM) of the iffrtion pek. Distne () lulte for ll set of films using the Brgg eqution [11]: = nλ/2sinθ.. (3) Optil sorption t on the eposite films ws mesure using Shimzu UV165PC spetrophotometer to etermine the optil sorne of the films s funtion of wvelength t room temperture. A orretion for sustrte sorption ws me y pling n ientil unote glss sustrte in the referene em. The optil energy gp (E g ) etermine y using Tu eqution [12]: (αhυ) =B (hυ-e g ) r. (4) where B is Tu onstnt n hυ is the photon energy, α is the sorption oeffiient. For r=1/2 yiels liner epenene, whih esries the llowe iret trnsition. The optil energy gp of the film n e otine y plotting (αhν) 2 hν n extrpolting the stright line portion of this plot to the energy xis. The optil ehvior of mteril is generlly utilize to etermine its optil onstnt [refrtive inex (n), extintion oeffiient (k), rel n imginry prts of ieletri onstnt (ε 1 n ε 2 )]. The extintion oeffiient hrterizes sorption of the eletromgneti wve energy in the proess of propgtion of wve through mteril. The wve intensity ( I) fter it psses istne x in n isotropi meium is equl to[12]: I= I exp ( αx) (5) where I is the intensity t x =. For mny pplitions, the extintion oeffiient k is equl to[12]:.. (6) where λ, is the wvelength. The norml-iniene refletivity R n lso e given y[13]: (7) Then the refrtion inex vlue n e lulte from the formul [13]: (8) The omplex inex of refrtion (n ) is efine s [12]: n = n + ik = ε 1/2 = [ε 1 + iε 2 ] 1/2.. (9) where n is the rel refrtive inex. The optil onstnts, n n k, re rel, n n e etermine from optil mesurements. From Eqution (1), it follows tht [12]: ε 2 = n 2 k 2... (1) ε 1 = 2nk.. (11) 3. RESULTS AND DISCUSSION Fig. (1) shows the XRD for ZnO thin films eposite on glss sustrtes t ifferent thiknesses (75, 15, 2 n 25 nm). The ptterns show tht ll the films hve pek lote t 2θ 34.4 o with hkl (2). In ition nother peks pper t thikness (2 n 25) nm lote t 2θ=47.4 o n o with hkl{(12) n (11)} respetively. All set of Volume 2, Issue 7, July 213 Pge 199

3 We Site: Emil: films revel only n intense iffrtion pek roun 34.4º, whih orrespons to (2) of the wurtzite phse of ZnO. This inites tht the growth of ll films is highly oriente long -xis. Tle (1) shows ll the peks oserve in ll films n the stnr peks from JSPDS n its intensities [14]. Our result elre goo oiniene with the referene t n elre tht the films hve goo rystlline with hexgonl struture n the egree of rystlline inreses with inresing the thikness. The grin size ws lulte y Sherer s formul. The vlues of grin size show in Tle (1). This tle shows tht the grin size of the prepre films t (2) plne films inrese from nm to nm when the film thikness inrese from 5 to 2 nm. Therefore, it is oserve tht the inrese thikness provies etter rystlliztion for the films. Fig (1). XRD for ZnO thin films eposite on glss sustrtes t ifferent thiknesses. Tle (1). The experimentl n stnr vlues of peks, intensity n grin size for prepre ZnO thin film t ifferent thikness. Thik. (nm) θ (Degree) hkl (Exp.) (Å) hkl (St.) (Å) hkl Int (Exp. (.u.) Int (St.)% FWHM (Deg.) G.S (nm) (1) (2) (11) (1) (2) (11) (1) (2) (11) (12) (11) (1) (2) (11) (12) (11) Optil stuy of ZnO films ws rrie out in the wvelength rnge 3 11 nm t room temperture for the film eposite on glss sustrte. Figure (2) shows the trnsmittne spetr of ZnO thin films with ifferent thikness. It is notie tht ll otine films exhiit high trnsmittne over (8%) in the visile region with shrp sorption ege nerly t 36 nm. The trnsmittne spetr of the films erese s film thikness inreses, whih susequently inrese sorption. The onset of sorption ege for films eme less shrp whih is ue to the ft tht igger rystlline sizes re Volume 2, Issue 7, July 213 Pge 2

4 We Site: Emil: eposite; n euse in the se of more toms re present in the film so more sttes will e ville for the photons to e sore. The liner epenene of (αhν) 2 with hν inites tht ZnO films re iret trnsition type semionutors t ll thikness. The photon energy t the point where (αhν) 2 is zero is E g. The vlue of optil energy gp ereses with inresing of thikness for ll smples s shown in Fig. (3) n Tle (2). This is ue to the inrese in lolize ensity of sttes ner the n eges n in turn ereses the vlue of Eg with thikness. Also the erese of iret n gp with the inrese of thikness n e ttriute to n inrese of prtile size, erese of strin n inrese of lttie onstnt [15]. This mens tht the thikness lso effets on the n gp of the film. These results show goo greement with the reserh [16]. Fig. (4) shows the vrition of extintion oeffiient (k) with wvelength t ifferent thiknesses. We know tht the extintion oeffiient epens minly on sorption oeffiient oring to the reltion (6) for this reson; we notie the inresing of extintion oeffiient with inresing photon energy euse the sorption oeffiient is inrese. It is oserve from these figures tht the k ereses with the inrese film thikness. This is ue to erese the grin ounries of the film struture. This mens tht iret eletroni trnsition hppens. Vrition of the refrtive inex versus wvelength, for ifferent ZnO film thiknesses is shown in Fig. (5). We n notie from this figure tht the refrtive inex, in generl inreses with inresing of thikness n it is inrese from 1.82 to 2.19 with inresing thikness from 75nm to 25 nm t wvelength equl to 365 nm s shown in figure (5). This ehvior my e ue to the hnge in the on length ue to the erese in the pking ensity euse of inreses in egree of rystllite of films.. Our results pper goo greement with other reserhes [17][18]. The vrition of the ε r n ε i versus wvelength for ZnO films eposite t RT with ifferent thikness shown in Figs. (6) n (7). The ehvior of ε r is similr to tht of the n euse of the smller vlue of k 2 ompre with n 2. While ε i is minly epens on the k vlues. Our results re liste in Tle (2). Tle (2) The epenene of the n gp, extintion oeffiient, refrtive Inex, the rel n imginry prts of ieletri onstnt on the thikness for ZnO films. Thikness (nm) E g (ev) n k r i T% : t=75 nm : t=15 nm : t=2nm : t=25 nm λ nm Fig.(2) Trnsmittne versus wvelength for ZnO films t ifferent thiknesses. 6.E+1 (αhυ)^2 (ev.m^-1)^2 5.E+1 4.E+1 3.E+1 2.E+1 1.E+1.E hυ (ev) :t=75 nm :t=2 nm :t=25 nm Fig (3) Vrition of (αhυ) 2 versus hυ for ZnO films t ifferent thikness. Volume 2, Issue 7, July 213 Pge 21

5 We Site: Emil: k :t=75 nm :t=2 nm :t=25 nm λ (nm) Fig(4) Vrition of k versus wvelength for ZnO films t ifferent thikness. n λ :t=75 nm :t=2 nm :t=25 nm Fig(5).Vrition of n versus wvelength for ZnO films t ifferent thikness εr 2 :t=75 nm :t=2nm :t=25 nm λ (nm) Fig.(6) Vrition of r s funtion of wvelength for ZnO films t ifferent thikness εi :t=75 nm :t=2nm :t=25 nm λ (nm) Fig. (7) Vrition of i s funtion of wvelength for ZnO films t ifferent thikness. 4. CONCLUSION Conutive n trnsprent ZnO thin films were eposite y mgnetron sputtering from Zin trget, n the orresponing struturl n optil hrteristis of theprepre films s funtion of thikness re reporte.. 1- The XRD pttern is lerly shown emonstrte three importnt plnes of hexgonl struture (1,2 n 11) for vrious thiknesses. 2- The grin size inrese with inresing the film thikness. 3-The liner epenene of (αhν) 2 to hν inites tht ZnO films t ll ifferent thikness re iret trnsition type semionutors. Volume 2, Issue 7, July 213 Pge 22

6 We Site: Emil: 4- The optil n gp of the films ereses with inresing thikness. 5- The refrtive inex is out 2.1. Referenes [1.] Yu-lin Wng, Frition n Chrteriztion of Zin Oxie Light Emitting Dioes, Ph.D. Thesis, University of Flori, 29. [2.] H. Kumr Y.Dv, K. Sreenivs, n V. Gupt., Influene of Post Deposition,Anneling on the Struturl n Optil Properties of Mn ope ZnO Thin Films, Journl of Applie Physis, 99, pp.8357,26. [3.] B. Kumr, T. Ro Effet of Sustrte Temperture on Struturl Properties of Nnostrutur Zin Oxie Thin Films Prepre y Retive DC Mgnetron Sputtering, Digest Journl of Nnomterils n Biostrutures, 6(3), pp ,211. [4.] M. Kng,S. Kim, Y.Kim n J. Ryu. Depenene of the Optil Anisotropy of ZnO Thin Films on the Struturl Properties, Journl of the Koren Physil Soiety, 57, (2), pp. 389_394, 21. [5.] Y.Choi, J.Kng, D. Hwng, n S.Prk Reent Avnes in ZnO-Bse Light-Emitting Dioes], IEEE Trnstions on Eletron Devies, 57(1),, 21. [6.] R. Romero, M.C. López, D. Leinen, F. Mrt ın, J.R. Rmos Brro Eletril Properties of the n-zno/-si Heterojuntion Prepre y Chemil Spry Pyrolysis, Mterils Siene n Engineering B,11,pp 87 93,24. [7.] T.Yng, B.Yo, T.T. Zho, G.Z.Xing,, H. Wng, H.L.Pn,R.Deng,Y.R. Sui, L.L. Go.H.Z. Wng, T. Wu, D.Z. Shen, S Doping Behvior n its Effet on Crystl Struture, Conutivity n Photoluminesene of ZnO Film in Depositing n Anneling Proesses, Journl of Alloys n Compouns 59,pp: ,211. [8.] P. Wng, N. Chen, Z. Yin, F.Yng, Ch. Peng, R. Di, n Y. Bi. As-ope p-type ZnO Films y Sputtering n Therml Diffusion Proess, Journl of Applie Physis 1, pp: 4374, 26. [9.] L.P Shuler, Properties n Chrteriztion of Sputtere ZnO, Ph.D. Thesis,University of Cnterury Christhurh, New Zeln, 28. [1.] B. Ry, II-VI Compouns, Pergmum Press, 1st eition printe in Gret Britin y Neil n Co. Lt. of Einurgh, p.p:162, (1969). [11.] C. Kittle, Introution to Soli Stte Physis Thomsen Press New Delhi, 8th Eition 25. [12.] J.Tu, Amorphous n Liqui Semionutor, Plenums Press, New York n Lonon, [13.] J. Pnkove, Optil Proess in Semionutor,Prentie Hll In1975. [14.] N 1997 JCPDS-Interntionl Centre for Diffrtion Dt. All rights resve PCPDFWIN,1.3, Cr no [15.] S. Aksoy, Y. Cglr, S. Ilin, M. Cglr. Effet of Sn Dopnts on the Optil n Eletril Properties of ZnO Films, Opti Applit, XL (1),p.p:7-14,21. [16.] A. A. Alnjjr. ZnO:Al Grown y Sputtering from Two Different Trget Soures: A Comprison Stuy,Avnes in Conense Mtter Physis. (212), Artile ID , 8 pges oi:1.1155/212/ [17.] M. Cglr, Y. Cglr, S. Ilini. The Determintion of the Thikness n Optil Constnts of the ZnO Crystlline Thin Film y using Envelope Metho, Journl of Optoeletronis n Avne Mterils, 8 ( 4), pp ,26. [18.] W.X.Cheng, A. L.Ding, X. S. Zheng, P. S. Qiu n X. Y. He. Optil n Eletril Properties of ZnO Nnorystlline Texture Films Prepre y DC Retive Mgnetron Sputtering],Journl of Physis: Conferene Series 152 (29) 1236 oi: 1.188/ /152/1/1236. Volume 2, Issue 7, July 213 Pge 23

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