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JOMThe Journal of The Minerals, Metals &Materials Society (TMS) ISSN 1047-4838Volume 68Number 6 JOM (2016) 68:1647-1652DOI 10.1007/s11837-016-1910-5

Effect of Argon/Oxygen Flow Rate Ratioson DC Magnetron Sputtered NanoCrystalline Zirconium Titanate Thin Films

D. Jhansi Rani, A. GuruSampath Kumar,T. Sofi Sarmash, K. Chandra BabuNaidu, M. Maddaiah & T. Subba Rao

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Effect of Argon/Oxygen Flow Rate Ratios on DC MagnetronSputtered Nano Crystalline Zirconium Titanate Thin Films

D. JHANSI RANI,1,3 A. GURUSAMPATH KUMAR,1 T. SOFI SARMASH,1

K. CHANDRA BABU NAIDU,1 M. MADDAIAH,2 and T. SUBBA RAO1

1.—Materials Research Laboratory, Department of Physics, Sri Krishnadevaraya University,Anantapuramu 515 003, AP, India. 2.—Department of Physics, Govt. Degree College,Kovelakuntla, Kurnool, AP, India. 3.—e-mail: jhansiranidvr@gmail.com

High transmitting, non absorbent, nano crystalline zirconium titanate (ZT) thinfilms suitable for anti reflection coatings (ARC) were deposited on to glass substratesby direct current (DC) magnetron reactive sputtering technique, under distinct Ar-gon to Oxygen (Ar/O2) gas flow rate ratios of 31/1, 30/2, 29/3 and 28/4, with a net gasflow (Ar + O2) of 32sccm, at an optimum substrate temperature of 250�C. Theinfluence of the gas mixture ratio on the film properties has been investigated byemploying x-ray diffraction (XRD), ultra violet visible (UV–vis) spectroscopy, atomicforce microscopy (AFM), energy dispersive x-ray analysis (EDX) and four point probemethods. The films showed a predominant peak at 30.85�with (111) orientation. Thecrystallite size reduced from 22.94 nm to 13.5 nm and the surface roughness in-creasedfrom11.53 nmto50.58 nmwithincrease inoxygencontentrespectively.Thefilms deposited at 31/1 and 30/2 showed almost similar chemical composition. In-creased oxygen content results an increase in electrical resistivity from 3.59 9 103 to2.1 9 106 Xm. The film deposited at Ar/O2 of 28/4 exhibited higher average opticaltransmittance of 91%, but its refractive index is higher than that of what is requiredfor ARC. The films deposited at 31/1 and 30/2 of Ar/O2 possess higher transmittance(low absorbance) apart from suitable refractive index. Thus, these films are prefer-able candidates for ARC.

INTRODUCTION

Anti reflection coating surfaces demand less reflectiv-ity and the refractive index in the range of 1.6 to 1.9. Therefractive indexofSiliconoxidebasedmaterialsmatcheswith this range, but, is not suitable due to the consid-erable lossesandhighabsorption.Zirconiumoxide isoneof the few non absorbent materials with less reflectivity,small dispersion of refractive index and relatively lowloss.1–3 The materials suitable for high performance antireflection coatings are zirconia (ZrO2), Al2O3, titania(TiO2) and MgF2. TiO2 is also a non dispersive material.The compound of zirconia and titania, zirconiumtitanate has good chemical and mechanical durability,the material has been extensively studied to supersedeexisting SiO2 based gate dielectric materials.4 Zirconiathin films have high permittivity and mechanicaldurability and hence find applications in dynamicrandom access memory and field effect transistor.5

Titania films have high dielectric constant and exhibitsexcellent photo catalytic activity.6

Single phase, high-k dielectric zirconium titanateinherits mechanical, thermal and chemical durabil-ity from parent oxides, which are the key require-ments for optical applications. Besides an optimalcandidate for hard metallic and anti reflectioncoatings (ARC), it also finds application as dielectricresonators in microwave communications. ZT hashigh resistivity, high dielectric constant and lowleakage currents in the microwave frequencyregime. This provides higher charge storage capac-ity.7 As ZT contends the above mentioned prerequi-sites for coatings, a deeper knowledge regarding theeffect of deposition parameters on its propertiescould obviously be helpful for applicative purposeand hence, the present study has been carried out tounderstand the influence of partial gas pressureratios on the film properties.

ZT is an a-PbO type with Ortho rhombic symme-try and pbcn space group with Ti4+ and Zr4+ ionsdistributed randomly within the lattice. It hasincommensurate (IC) phase. ZT thin films have

JOM, Vol. 68, No. 6, 2016

DOI: 10.1007/s11837-016-1910-5� 2016 The Minerals, Metals & Materials Society

(Published online April 18, 2016) 1647

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been deposited by employing distinct methods likeRF magnetron sputtering,8–10 polymeric precursormethod,11 sol gel processing,12–15 chemical vapordeposition,16 direct current (DC) magnetron reac-tive sputtering,17 pulsed laser ablation technique18

and surface sol gel processes.7 Among these depo-sition techniques, DC magnetron sputtering has itsown advantages of simplicity, less expensive methodto obtain strongly adhesive, well controlled stoi-chiometry of the films, uniformity of the films andprovides a wide range of controlled variable pro-cessing parameters.

In this context, in the present study, we demon-strate the characteristics of ZT thin films depositedunder various Ar/O2 partial pressure ratios in orderto find the optimum conditions for optical coatingpurpose and to study the influence of the gas partialpressure ratio on thin film properties.

EXPERIMENTAL DETAILS

The vacuum unit of DC magnetron sputteringsystem comprises the combination of rotary and oildiffusion pumps. By employing this combination,the pressure in the working chamber has beenpumped down to a base pressure of 1 9 10�5mbar,as it is sufficient for sputtering to take place. Thepressure inside the chamber has been measured bypirani and penning gauges. After achieving the basepressure, an ultra pure sputtering gas, Argon(99.99% pure) and a reactive gas, Oxygen (99.99%)were admitted into the deposition chamber. Massflow controllers were used to have a control over theflow of the gases, to be admitted into the vacuumchamber. While depositing oxide films by DC mag-netron sputtering, a compound layer forms on thetarget surface and the target gets poisoned (oxi-dized), this eventually drops the rate of sputtering.The reactive magnetron sputtering helps to reducethe rate of target oxidation as the ejected atomsreact with oxygen to form their correspondingoxides. On the other hand, incorporation of largeramounts of reactive gases may lead to rapid targetpoisoning and also leads to porous nature in thefilms. Also, the net pressure inside the chamber isone of the sensitive parameters, which describe thefilm properties. So, a net flow of gases must bemaintained as constant to maintain the workingpressure in the chamber. Hence, in order to sustainthe plasma in the chamber and to deposit goodquality films, the range of Ar/O2 must be selectedsuch that reactive gas (oxygen) content must besufficiently smaller than the sputtering gas (argon)content. Thus, the Ar/O2-range has been selected as31/1, 30/2, 29/3 and 28/4 with a net flow of 32 sccm.Accordingly, the films have been deposited at Ar/O2

flow rate ratios of 31/1, 30/2, 29/3 and 28/4(Ar + O2 = 32 sccm). Two individual, high pure(99.99%) Zr and Ti targets, each of 2 inch diameterand 4 mm thickness were used for sputtering. Theglass substrates were fixed on a rotatable substrate

holder, located at 60 mm underneath the targets.The substrate temperature was maintained at250�C for all the films and was controlled byproportional-integral-derivative (PID) controller.The sputtering powers for all films were maintainedat 125 Watt and 100 Watt for Zr and Ti respectively.Ar atoms get ionized by an applied negative DCpotential and bombard the target surface to ejectatoms and secondary electrons. Magnetrons con-strain the motion of the electrons to a circular pathin the vicinity of the target, which enables furtherionization, which produces dense plasma of theejected atoms to get deposited on to cleaved glasssubstrates. The heat evolved during deposition wascompensated by circulating a coolant around thevacuum chamber and diffusion pump. The completedetails of this DC magnetron sputtering systemhave been reported in our earlier papers.19–22

The x-ray diffraction (XRD) patterns of the filmswere recorded with Cu-ka radiation in glancingangle mode by Rigaku smart lab (9KW) with ascanning speed of 0.02� (2h)/s. The surface rough-ness and surface morphology of the films have beendetermined by NTEGRA-PRIMA (NT-MDT) atomicforce microscopy (AFM). The composition of theelements in the films has been determined byenergy dispersive analysis carried out by FESEM-SUPRA 55. The optical transmittance spectra wererecorded by ultra violet visible (UV–VIS)–NIR spec-trophotometer (Hitachi U-3400). The electricalresistivity values were determined from four pointprobe method.

RESULTS AND DISCUSSION

The recorded XRD diffractograms of the depositedZT thin films are represented in Fig. 1. From the XRDpatterns, it is evident that, the intensity of (111) peakincreased with decrease in oxygen content as crys-tallinity increases with decrease in oxygen content.The film deposited at gas mixture ratio of 31/1 showeda high intense, (111) oriented peak at a scatteringangle of 30.85�and a low intensity (020) peak began toappear at 50.19�. With increase in oxygen content ofAr/O2 ratio, the crystallinity reduced and (020) peakdisappeared. The films exhibited orthorhombic struc-ture. The crystallite size was calculated by Debye–Scherrer’s formula,

D ¼ kkb cos h

nm ð1Þ

where k is a constant = 0.94, k is an x-ray wavelength = 0.154 nm. b is the full-width at half max-imum and h is the Bragg’s angle. The crystallite sizevaried as 22.94, 19.61, 17.23 and 13.5 nm for filmswith Ar/O2 ratio of 31/1, 30/2, 29/3 and 28/4,respectively. These values are in good agreementwith those reported ones through variation ofoxygen percentage in DC magnetron sputtered ZTthin films by Pamu, et al.17 The micro strain in thefilms has been estimated by Ref. 23.

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e ¼ b cos h4

lines�2m�4 ð2Þ

It is observed that the strain significantly reducedfrom 5.8 lines�2 m�4 to 0.81 lines�2 m�4 withincrease in oxygen content, indicating structuralrelaxation in the films. Plot of Ar/O2 versus crys-tallite size is shown in Fig. 2.

The dislocation density has been determined by23

d ¼ 1

D2lines m�2 ð3Þ

It increased from 0.02 lines m�2 to 1.21 lines m�2

as increased oxygen content induces pores anddefects in the film. Dislocation density along withstrain is represented in Fig. 3. This increment isalso attributed to the reduced deposition rates.Further increase in the reactive gas induces targetoxidation as both substrate and target were atsimilar environment. This significantly reducesdeposition rates. Pamu et al.17 reported that the

films need activation in order to get crystallized andthe films had been crystallized by utilizing the heatevolved during deposition. In the present work, thesubstrates were heated to 250�C for the activation ofatoms to get crystallized. Kim et al. reported thatthe ZT films start to crystallize above 200�C oftemperature.24

The general intensity spectral region in glassesextends from the near UV across the visible region tothe near IR, due to different absorption mecha-nisms.25 The optical transmittance spectra of thefilms have been recorded in the wave length range of200–900 nm and are represented in Fig. 4. All thefilms exhibited high transmittance, which implieslow absorbance in the visible region. The filmdeposited at 28/4 showed higher optical transmit-tance of 91% in the visible region. The transmittancedecreased with decrease in the oxygen content. Thefilm deposited at 31/1 exhibited a transmittance of86%. Hence, the reflectance of the films may varyfrom �9% to 14%, the low reflectivity ensures thesuitability of the films for ARC. High transmittancein the wide range of wave lengths in visible regionalso makes it possible to use the films in solar cellapplications. The optical band gap energies havebeen estimated from optical absorption edge asshown in Fig. 5. The band gap energies and refractiveindices were estimated to be 2.85, 2.90, 3.0 and 3.2 eVand 1.85, 1.87, 1.99 and 2.02 for 31/1, 30/2, 29/3 and28/4 of gas mixtures, respectively. The extinctioncoefficient followed the same trend as refractiveindex with oxygen and varied from 7.04 9 10�3 to1.53 9 10�3. The extinction coefficient values arerelatively closer to the reported ones but it showed adecreasing trend with increase in oxygen alike thereported one.17 From the refractive index values, it isrevealed that ZT thin films deposited at 31/1 and 30/2of Ar/O2 are suitable for ARC, since, the materialswith refractive index values in the range of 1.6 to 1.9are the promising candidates for ARC. The opticalconstants were calculated by using swanepoel’smethod.25–27 From the optical constants, the thick-ness of the films can be estimated by

Fig. 1. XRD patterns of ZT thin films deposited at Ar/O2 ratios of 31/1, 30/2, 29/3 and 28/4.

Fig. 2. Plot of Ar/O partial pressure ratio versus crystallite size of ZTfilms.

Fig. 3. Ar/O2 partial pressure dependent strain and dislocationdensity of ZT thin films.

Effect of Argon/Oxygen Flow Rate Ratios on DC Magnetron Sputtered Nano CrystallineZirconium Titanate Thin Films

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d ¼ k1k2

2ðk1n2 � k2n1Þnm ð4Þ

where n1 and n2 are refractive indices at twoadjacent maxima or minima at wave lengths, k1

and k2. As the sputtering powers and depositiontimes have been maintained as constant for all thefilms, the thickness of the films was found to be�350 nm. The dependence of refractive index,extinction coefficient and optical packing densityon Ar/O2 partial pressure is shown in Fig. 6a–c. Theoptical packing densities were determined by17:

p ¼ðn2

f � 1Þðn2

f þ 2Þ� ðn2

b þ 2Þðn2

b � 1Þ% ð5Þ

where nb is the bulk refractive index (2.31) and nf isthe refractive index of the thin films. The filmdeposited at 31/1 of Ar/O2 ratio possesses 93.94%packing density and the porosity of the filmsincreased with increase in oxygen content. This isevident from the decrement in density from 91% to89.75% for the film deposited at 28/4.

The surface morphology and rms roughness of thefilms have been studied by 2-D AFM micrographs,scanned in the area of 8 lm 9 8 lm and were shownin Fig. 7a–d. The AFM image of the film with 31/1 ofAr/O2 revealed well crystallized grains with asmoother surface. The grains observed by AFMwere almost spherical in shape alike the reportedSEM observed triangular grains.17 This may beattributed to the fact that, the grain boundaries andsizes are very much sensitive to the applied sub-strate temperatures and deposition conditions.From the micro graphs, the rms roughness values

Fig. 4. Transmittance spectra of ZT thin films deposited on glasssubstrates.

Fig. 5. Plot of (ahm) 2 versus (hm) for band gap determination of ZTfilms.

Fig. 6. Plot of Ar/O2 gas mixture versus (a) refractive index, (b)extinction coefficient and (c) optical packing density of ZT thin films.

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have been estimated as 11.53, 13.54, 28.6 and50.58 nm for the films with Ar/O2 of 31/1, 30/2, 29/3 and 28/4 respectively. The roughness increasedwith increase in oxygen flow rate, as the moreoxygen leads to porous nature of the films. But allthe films have exhibited uniform thickness.

The energy dispersive x-ray analysis (EDX) spec-troscopy analysis has been carried out to estimatethe elemental composition of the films suitable forARCs (31/1 and 30/2). The EDX spectra shown inFig. 8a–b revealed that the composition of the filmsremained unchanged as because the sputteringpowers and deposition times have been maintainedconstant during the deposition of the films. Thefilms comprise only the Zr, Ti and O elements withweight percentages of 54.28, 42.46 and 3.26 atcorresponding binding energies of 0.7 keV (O),2 keV (Zr) and 4.5 and 5 keV (Ti) respectively. Theextra peak of Si at 1.8 keV appeared due to the glasssubstrate.

The four point probe method has been used todetermine the electrical resistivity of the films.According to this method, the resistivity is given by23

q ¼ q0

G7ws

� �Xm ð6Þ

where G7 is a correction factor for samples with non-conducting bottom surface and q0 ¼ v

i X2ps. Thesheet resistance would be given by Ref. 23

Rs ¼qtX=sq ð7Þ

where t is the thickness of the films. The resistivityof the films varied from 3.59 9 103 to 2.1 9 106 Xmfor the film deposited at 31/1 to 28/4 resp. the sheetresistance also showed the similar trend with valuesfrom 3.64 9 109 to 4.46 9 1012 X/sq. The Ar/O2

versus resistivity and sheet resistance are repre-sented in Fig. 9a–b.

CONCLUSION

The Zirconium Titanate thin films have beendeposited on to glass substrates at distinct Ar/O2

gas partial pressure ratios of 31/1, 30/2, 29/3 and 28/4. The influence of the gas mixture ratios on theproperties of the films has been investigated bycharacterizing the films by XRD, UV–VIS–NIRspectroscopy, AFM and four point probe methods

Fig. 7. AFM micrographs of ZT thin films deposited at Ar/O2 of (a)31/1, (b) 30/2, (c) 29/3 and (d) 28/4.

Fig. 8. EDX spectra of ZT thin films deposited at Ar/O2 ratio of 31/1and 30/2.

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to recover the optimum gas flow rate and sputteringparameters desired for optical coatings. The crys-tallite size reduced from 22.94 nm to 13.05 nm withincrease in oxygen partial pressure. The straindecreased and dislocation density increased signif-icantly with increase in oxygen. The optical bandgap was found to vary between 2.85 eV and 3.2 eV.The rms roughness has been obtained from AFMmicro graphs. EDX spectra illustrated the elementalcompositional consistency of the films (31/1 and 30/2). The electrical resistivity was measured from fourprobe method. All the films exhibited a high averageoptical transmittance (86–91%) in the visible region.Although the film deposited at 28/4 possesses 91% oftransmittance, it is not suitable for ARC due to itshigher refractive index. The films deposited at 31/1and 30/2 were found to be optimal candidates forARC due to both higher transmittance and suit-able refractive index.

ACKNOWLEDGEMENTS

The author, D. Jhansi Rani, would like toacknowledge the Department of Science and Tech-nology (DST), New Delhi, India for financial supportunder INSPIRE Fellowship program.

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Fig. 9. Ar/O2 flow rate ratio dependent (a) electrical resistivity and(b) sheet resistance of ZT thin films.

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