Optical dispersion characterization of NiO thin films prepared by nebulized spray technique

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1 Iteratial Jural f ChemTech Research CODEN (USA): IJCRGG ISSN : Vl.6, N.3, pp , Nvember 04 MESC 04 [4th -5 th September 04] Natial Cferece Material fr Eergy Strage ad Cversi- 04 Optical dispersi characterizati f NiO thi films prepared by ebulized spray techique V. Gwthami, M.Meeakshi, P. Perumal, R. Sivakumar, C. Sajeeviraja 3 * Departmet f Physics, Alagappa Gvt. Arts Cllege, Karaikudi , Idia. AlagappaUiversity, Karaikudi , Idia. 3 Departmet f Physics, ACCET, Karaikudi , Idia. *Crres.authr: sajeeviraja@rediffmail.cm Abstract : Nickel xide (NiO) thi films were prepared by ebulized spray pyrlysis (NSP) techique. The films are havig mre uifrm thickess ad there is pihle. Nickel xide thi films have bee depsited usig 0.3M aqueus sluti f ickel chlride NiCl.6H O t micrscpic glass substrates, which were chemically ad ultrasically cleaed befre catig. The film depsitis are carried ut at varius substrate temperatures frm 350 t 450 C i 50 C step.the ptical prperties f the NiO films were studied usig UV- Vis-NIR spectrmeter. It is fud that icrease i the substrate temperature the trasmittace decreases, csequetly the bad-gap eergy waes frm 3.54eV t 3.37eV. The refractive idex ad the exticti cefficiet have bee calculated frm the ptical measuremets. The dispersi f the refractive idex is discussed i terms f the Wemple-DiDmeic sigle-scillatr mdel. The scillatr parameters such as ptical dispersi eergies, dielectric cstat, the average value f scillatr stregth, wavelegth f sigle scillatr ad plasma frequecy have bee evaluated. Keywrds: NiO thi film, refractive idices, dispersi f the refractive idex.. Itrducti Nickel xide may exist i varius frms like NiO, NiO, NiO 4 ad Ni O 3. Amg them NiO is a atiferrmagetic material with a desity f 6.67 g/cm 3 ad havig a cubic structure. Bulk ickel xide (NiO) has a very high resistivity ad a high meltig pit (~000 C) suggestig that it ca be used i high temperature applicatis. NiO shws p-type semicductivity whe it is heated i air due t the geerati f Ni + vacacies it the NiO structure[].nio thi films have bee fabricated by varius physical ad chemical depsiti techiques, such as reactive sputterig[], spray pyrlysis[3], sl-gel[4] ad plasma-ehaced chemical vapr depsitis[5], electr beam evaprati[6]. NSP techique is a very easy, lw cst, safe, cheap, -vacuum system f depsiti methd[7], fr preparig trasparet cductig xides whe cmpared with the ther techique. The advatage f NSP ver cvetial peumatic sprayig is its lw material csumpti with better ctrl f the spray ad lw carrier gas flw, which allws thedepsiti f very thi layers f uifrm thickess[7]. Recetly, the ptical prperties are ecessary t study ptelectric devices that use semicductr thi films ad iterferece devices (atireflecti catig, laser mirrrs ad mchrmatic filters). The ptical cstats f thi films prvide us the ifrmati ccerig with micrscpic characteristics f the material. I this wrk the ptical cstats f NiO thi films were iflueced by the substrate temperature (T s ) depsited

2 V.Gwthami et al /It.J. ChemTech Res.04,6(3),pp by simple NSP techique, whereas the ptical absrpti parameters, such as ptical dispersi eergies, dielectric cstat, the average values f scillatr stregth, wavelegth f sigle scillatr, plasma frequecy, etc., had bee calculated uder the effect f substrate temperature.. Experimetal Nickel xide thi films had bee depsited frm 0.3M aqueus sluti f ickel chlride NiCl.6H O by ebulizer techique t micrscpic glass substrates, which were chemically ad ultrasically cleaed befre catig. This techique has sme advatages such as a atmizati based hydraulic pressure withut usig ay carrier gas, itermittet sprayig ad fie atmizati. Durig the film depsiti the substrate temperature was kept at 350 C, 400 C ad 450 C. The vlume f the sluti fr each depsiti was 0 ml. Oly the small amut was csumed ad 0 mis was ecessary fr sprayig 5 ml i.e 0.5 ml per mi. The zzle t substrate distace was ~7 cm ad the ptimized air flw rate was. kg/cm. After depsiti, the films were allwed t cl slwly t rm temperature. NiO thi film prperties were studied, the substrate temperature was kept at 350 C, 400 C ad 450 C. The ptical trasmittace ad bad gap were demstrated by Ocea Optics HR-000 (USA) UV-Vis-NIR spectrphtmeter. 3. Results ad discussi 3. Optical trasmittace ad Eergy Bad gap Optical trasmittace f the films depsited at varius substrate temperatures are shw i Fig.. The ptical trasmittace spectrum is recrded frm 300 t 00 m wavelegth regis. Trasmittace spectrum idicates that the trasmittace decreases as film thickess icreases; the trasmittace is varied frm 30% t 90% as the film thickess is decreased. The ptical bad gap f the film was evaluated frm the relati h h g () where is equal t ½ ad fr direct ad idirect trasitis respectively, A is a prprtiality cstat, E g is ptical eergy bad gap ad is absrpti cefficiet. The ptical absrpti cefficiet is calculated usig the fllwig expressi, = t lt () Frm the plts f ( h ) versus h direct bad gap was fud (Fig..) by extraplatig the liear prti f the curve t eergy axis fr ( h ) = 0. The curves idicate the values f the direct bad gap. The eergy bad gap values are fud as 3.54, 3.43 ad 3.37 ev fr the NiO thi films with icreasig the substrate temperature frm 350, 400 ad 450 C, which is i gd agreemet with the reprted bad gap values f ev fr NiO films[8]. Fig.. Trasmittace spectra f NiO thi films Fig.. ( h ) versus h plt f NiO thi films

3 V.Gwthami et al /It.J. ChemTech Res.04,6(3),pp Optical cstats Refractive idex is a imprtat parameter fr ptical materials desig ad it icludes valuable ifrmati fr higher efficiecy ptical materials, because it is clsely related t the electric plarizati f is ad the lcal field iside materials. The cmplex refractive idex (*) ca be expressed as[9]. * ik (3) Here, the real part f the refractive idex () ad imagiary part (k) ca be calculated usig the fllwig relatis. R R 4R ( R) k (4) k 4 (5) Where, R is the reflectace f the film ad is the wavelegth f the icidet beam. Fig.3. shws the spectral variati f the refractive idex as a fucti f wavelegth at varius substrate temperatures. It ca be see that the maximum value f refractive idex (=~.8 at T s = 450 C) at very lw wavelegth, =300m (strg absrpti regi), was bserved i all samples. The behavir f refractive idex has a higher value at very lw wavelegth (strg absrpti). This is due t the quality betwee the frequecy f icidet electrmagetic radiati ad the plasma frequecy f electrs. This leads t the cuplig f electrs i NiO films t the scillatig electric field. Fig.4 shws the variati f exticti cefficiet as a fucti f wavelegth at varius substrate temperatures. I the visible regi, it idicates the better surface hmgeeity f depsited NiO thi film [0]. The icrease i k value with icrease i substrate temperature may be due t surface rughess f the film, which ehaced the scatterig lsses thereby reducig the trasmittig ability. This iferece is csistet with ur bserved ptical trasmittace spectra decrease with icrease f substrate temperature. I geeral, the idex f refracti is higher fr shrter wavelegths f light ad decreases mtically with icreasig wavelegth.hwever, sme highly clred materials exhibit abrmal dispersi fr thse wavelegths f light that are strgly absrbed. Further, it may be metied that the refractive idex ad exticti cefficiet deped the atmic desity ad atmic masses f each cmpet elemet. The refractive idex is high fr crystallie material tha the amrphus e. This is due t the lwer atmic desity f each elemet i the amrphus state because f the higher average iteratmic distace. Fig. 3.Refractive idex () f NiO thi films Fig. 4.Exticti cefficiet (k) f NiO thi films as a fucti f wavelegth ( ) as a fucti f wavelegth ( ) 3.3 Dielectric cstats The calculated refractive idex is used fr the estimati f high frequecy dielectric cstat by adptig tw prcedures[]: The first, describes the ctributi f the free carriers ad lattice vibrati mdes f the dispersi, while the secd prcedure is based up the dispersi arisig frm the bud carriers i a empty lattice. Hwever, bth prcedures were emplyed fr btaiig lattice high frequecy

4 V.Gwthami et al /It.J. ChemTech Res.04,6(3),pp dielectric cstat. The fllwig equati shws the relati betwee the lattice dielectric cstat ( L=), wavelegth ( ) ad refractive idex () []. () e N 4 c m * (6) where L is the lattice dielectric cstat, e is the electric charge, c is the velcity f light, is the permittivity f free space (8.854 x 0 - F/m), N is the free carrier ccetrati ad m* is the effective mass f the charge carriers. The ature f the dispersi f as a fucti f wavelegth (λ ) fr differet substrate temperatures are shw i Fig.5 which verifies the liearity f the abve said expressi (6). The itercept f the straight lie t X-axis (λ =0) gives the value f lattice dielectric cstat (ε L ). Whereas the slpe f the graph results the N/m* ad these values are preseted i Table.Mahmud et al. reprted that the lattice high frequecy dielectric cstat varies frm 3.34 t 3.54 ad N/m* lies betwee 6.5 x 0 40 ad 5.49 x 0 4 cm -3 g -, fr spray pyrlysisednio thi films. A characteristic frequecy at which the material chages frm a metallic t a dielectric respse is the plasma frequecy (ω p ). I this frequecy, the real part f dielectric fucti vaishes. The plasma resace frequecy (ω p ) f all valece electrs ivlved i the ptical trasitis was calculated usig the fllwig relati ad reprted i Table. p e N / m * (7) Accrdig t this relati, plasma frequecy depeds the ccetrati f free carriers. The plasma resace frequecy icreases with icreasig the substrate temperature.determiati f the dielectric cstat culd be defied usig the dispersi relati f the icidet pht. The refractive idex was als fitted usig a fucti fr extraplati twards shrter wavelegth. The Mss mdel[] stated that: the free carriers ctributi t dispersi are relatively small. This meas that data crrespdig t the wavelegth rage lyig belw the absrpti edge f the material has t be used[].the prperties f the ivestigated NiOculd be treated as a sigle scillatr at wavelegth λ at high frequecy. The fllwig equati is used t calculate the high frequecy dielectric cstat ε, S (8) ( / ) where S is the average scillatr stregth ad λ is a average scillatr wavelegth. The equati (8) ca be expressed i a sigle term Sellmeier scillatr with the refractive idex as[3] (9) where is the static refractive idex, which prvides a gd idicati the structure ad desity f the material. The abve equati (9) ca be used t calculate high frequecy dielectric cstat (ε ). Hwever, accrdig t Mss mdel, the free carriers ctributis t dispersi are relatively small, which meas that the data crrespdig t the wavelegth rage lyig belw the absrpti edge have t be used t evaluate ε. Hece the graph betwee ( -) - ad λ - as a fucti f substrate temperature is shw i Fig.6.The itersecti with ( -) - axis is ad hece, at λ is equal t high frequecy dielectric cstat (ε ). The btaied λ ad S values are fud t be cmparable with the bserved NiO thi films depsited by spray pyrlysis at varius substrate temperatures[].

5 V.Gwthami et al /It.J. ChemTech Res.04,6(3),pp Fig. 5.Plt f versus Table : fr NiO thi films Fig. 6.Plt f ( -) - versus - fr NiO thi films Dispersi parameters f NiO thi films prepared at varius substrate temperatures E Ts C ε L ε N/m*x p (H λ 0 S 0 x0-3 E d E g E 0 /E g M - M (cm - g - ) z)x0 8 (m) - 0 (ev) (ev) (ev) (ev) Dispersi eergy parameter The dispersi f refractive idex fr NiO films was aalyzed usig the ccept f the sigle scillatr ad ca be expressed by Wemple ad Di-Dmeic (WD) mdel[4]. I this mdel, the refractive idex data have bee examied belw the iter-bad absrpti edge, where the rmal dispersi f the ptical dielectric cstat f the material ad the eergy depedece f refractive idex satisfy the relati, E EEd ( h ) (0) where E=h is the pht eergy. is the average scillatr eergy (a average f the ptical bad gap) ad E d is the dispersi eergy parameter f the material that measures the average stregth f iter-bad ptical trasitis. A plt f ( -) - versus (h ) f NiO depsited films fr varius substrate temperatures is shw i Fig. 7. It is clear that, the effect f substrate temperature the refractive idex ad semicductr dispersi prfiles were exhibited a liear displacemet i the shape f the dispersi prfile with decreasig refractive idex. The refractive idex value declies twards lger wavelegths. This is due t the ifluece f lattice absrpti. The values f E d ad E are btaied frm the itercepts, ad slps f the curves ad preseted i the Table. The variati f E d ad E with varius substrate temperatures is shw i Fig. 7. The values f E was fud t be decreased with icreasig the substrate temperature, while the values f E d icreased with icreasig the substrate temperature. The scillatr eergy, E is related t the ptical bad gap (E g ) by E.5E g, as suggested by WD mdel[4]. It is evidet frm Table that the average rati f E /E g.5, which shws a agreemet with this relati. A simple cecti betwee the sigle-scillatr parameters f E ad E d ad the statistical weightig f the imagiary part f the dielectric cstat ε, ca be expressed i terms f mmets f the ε as fllws[5] M M E () 3

6 V.Gwthami et al /It.J. ChemTech Res.04,6(3),pp M E d () M 3 The scillatr eergy E is idepedet f the scale f ε the evaluated values f M vary frm 3.04 t 3.93 (dimesiless) ad M 3 lies betwee 0. ad 0.5 (ev) (Table ). Fig. 7.Plt f ( -) - versus (h ) fr NiO thi films 4. Cclusi Nickel xide thi film was prepared glass substrate by simple ebulized spray pyrlysis (NSP) techique usig ickel chlride (NiCl.6H O) sluti at varius substrate temperatures. The ptical trasmittace decreases, csequetly the ptical bad gap eergy f NiO film decreases frm 3.54 ev t 3.37 ev with icrease i substrate temperature. I additi, ptical cstats, dielectric cstats, eergy dispersi parameter usig WD mdel, scillatr eergy, lattice eergy are crrelated betwee btaied data ad reprted values. Ackwledgemet Oe f the authrs (Prf. C. Sajeeviraja) deeply thaks the Cucil f Scietific ad Idustrial Research, New Delhi, Idia fr sactiig Emeritus Scietist Scheme. Refereces. Das NS, Saha B, hapa R, Das GC, Chattpadhyay KK. Bad gap wideig f acrystallie ickel xide thi films via phsphrus dpig, Bad gap wideig f acrystallie ickel xide thi films via phsphrus dpig.physics E, 00, 4; Zhu Y, Gu DH, Geg YY, Ga FX. Thermal, structural ad ptical prperties f NiOx thi films depsited by reactive dc-magetr sputterig. Mater Sci. Eg. B, 006, 35; Reguig BA, Khelil A, Catti L, Mrsli M, Berede JC. Prperties f NiO thi films depsited by itermittet spray pyrlysis prcess.appl. Surf. Sci., 007, 53; Park YR, Kim KJ. Sl gel preparati ad ptical characterizati f NiO ad Ni x Z x O thi films. J. Cryst. Grwth, 003, 58; Yeh WC, Matsumura M. Chemical vapr depsiti f ickel xide films frm bis-π-cyclpetadieylickel. Jp. J. Appl. Phys., 997, 36; Subramaia B, Ibrahim MM, Murali KR, Vidhya VS, Sajeeviraja C, Jayachadra M. Structural, ptelectric ad electrchemical prperties f ickel xide films.j. Mater Sci. Mater Electr., 009, 0; Gwthami V, Perumal P, Sivakumar R, Sajeeviraja C. Structural ad ptical studies ickel xide thi film prepared by ebulizer techique. Physica B, 04, 45; Puspharajah P, Radhakrisha S, Arf AK. Trasparet cductig lithium-dped ickel xide thi films by spray pyrlysis techique.j. Mater. Sci., 997, 3; El-Ghamaz NA, El-Sabati AZ, Diab MA, El-Bidary AA, Seyam HA. Optical prperties f thermally evaprated 4-(4-itrbezalideeami) atipyrie Schiff base thi films. Slid State Sci., 03, 9;9-6.

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