Indium tin oxide (ITO) thin films have been deposited onto glass substrates at room temperature by ion beam assisted deposition technique at different deposition rates (0.1 -- 0.3 nm/s). The effects of the deposition rate on the structural, optical and electrical properties of the deposited films have been studied. The optical constants of the deposited films were calculated by fitting the transmittance spectra using the semi-quantum model. Considering the application for the electromagnetic wave shielding which needs a high IR reflectance, the optimising deposition rate is 0.2 nm/s. The films prepared at this deposition rate shows a relative high IR reflectance (60%), a good electrical conductivity (5 x 10-3 -cm), and a reasonable transmittance in the visible region (over 80%)

Meng Lijian

Santos, M. P. dos

Teixeira, Vasco M. P.

physica status solidi (a)

Universidade do Minho: RepositoriUM

Effect of the deposition rate on ITO thin films properties prepared by ion beam assisted deposition (IBAD) technique

Phys. Status Solidi A 207, No. 7, 1538–1542 (2010) / DOI 10.1002/pssa.200983704 p s sastatussolidiwww.pss-a.comphysicaapplications and materials scienceEffect of the deposition rate on ITOthin film properties prepared by ion beam assisteddeposition (IBAD) techniqueLi-Jian Meng*,1,2, V. Teixeira2, and M. P. dos Santos3,41Departamento de Fı´sica, Instituto Superior de Engenharia do Porto, Rua Dr. Anto´nio Bernardino de Almeida, 431, 4200-072 Porto,Portugal2Centro de Fı´sica, Universidade do Minho, 4800-058 Guimara˜es, Portugal3CeFITec, Universidade Nova de Lisboa, Lisboa, Portugal4Departamento de Fı´sica, Universidade de E´vora, E´vora, PortugalReceived 20 May 2009, revised 15 January 2010, accepted 18 January 2010Published online 2 June 2010Keywords electrical properties, ion-assisted deposition structure, ITO, optical properties*Corresponding author: e-mail ljm@isep.ipp.pt, Phone: þ00 351 962 325 429, Fax: þ00 351 228 321 159Indium tin oxide (ITO) thin films have been deposited ontoglass substrates at room temperature by ion beam assisteddeposition technique at different deposition rates (0.1–0.3 nm/s). The effects of the deposition rate on the structural, opticaland electrical properties of the deposited films have beenstudied. The optical constants of the deposited films werecalculated by fitting the transmittance spectra using the semi-quantum model. Considering the application for the electro-magnetic wave shielding which needs a high IR reflectance, theoptimising deposition rate is 0.2 nm/s. The films prepared at thisdeposition rate shows a relative high IR reflectance (60%), agood electrical conductivity (5 103V cm), and a reasonabletransmittance in the visible region (over 80%). 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1 Introduction Indium tin oxide (ITO) film is a wideband gap semiconductor and has been widely used in manyelectronics and optoelectronics applications, such as flatpanel displays, organic light emitting diodes (OLED) andsolar cells, because of its high transmittance in the visibleregion and high electrical conductivity [1–3]. In addition, itshigh infrared reflection has also found the applications inthermal insulation of windows and in some electromagneticwave shielding [4, 5]. ITO film also exhibits its excellentsubstrate adherence, hardness and chemical stability. Due tothese excellent properties of ITO films, the study on it hasnever been interrupted.Many physical and chemical deposition techniques havebeen used for fabricating the ITO films, such as variousevaporation and sputtering techniques, and several chemicalvapour deposition techniques. Generally, in order to get theITO films with high transmittance and good electricalconductivity using these deposition techniques, the substrateneeds to be heated during the deposition process, or post-deposition annealing is needed [6, 7]. Recently, there is agrowing interest in applying organic substrates for liquidcrystal display devices and light emitting diodes as they arelighter and thinner than glass substrates. As the organicsubstrates cannot withstand the high temperature, preparingthe ITO films at low temperature, particularly at roomtemperature, is full of challenge.Although sputtering and reactive evaporation techniquecan produce ITO films with low electrical resistivity, theoxygen gas flow during the deposition processes needs to becontrolled very precisely. Some small variations on oxygenflow will result in a big different on ITO films properties. Ithas been found that the ion beam assisted deposition (IBAD)technique is a suitable technique for low temperature deposition.By using this deposition technique, one gets more flexibility incontrolling film properties and the ITO films prepared at roomtemperature without any post-deposition annealing has a hightransmittance and good electrical conductivity. It is also a lowcost production technique [8–11]. In this work, the physicalproperties of ITO films prepared at different deposition ratesby IBAD technique have been studied. 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimPhys. Status Solidi A 207, No. 7 (2010) 1539OriginalPaperTable 1 Deposition conditions and some properties of ITO films prepared at different deposition rates.sample D1 D2 O2 D3deposition rate (nm/s) 0.3 0.2 0.15 0.1deposition time (s) 718 1 033 1 353 2 004thickness (nm) 230 215 228 224sheet resistance (V/square) 323 51 94 1.4 106resistivity (103V cm) 34 5 10 1.4 105carrier concentration (1020 cm3) 1.3 2.7 0.8 1.9 103Hall mobility (cm2/V s) 7 21 37 1surface roughness by AFM (nm) 2.5 1.4 0.3 1.2d-spacing between (222) planes (nm) (standard value d0¼ 0.2923 nm) 0.3039 0.2973 0.2967grain size along (222) direction (nm) 16 6 8measured IR reflectance at 12 000 nm 37% 63% 45% 20%calculated IR reflectance 14% 62% 45% 0%706050403020Intensity (a.u.)Diffraction Angle (2θ)D1(0,30 nm/s)D2(0,20 nm/s)O2(0,15 nm/s)D3(0,10 nm/s)(222)Figure 1 XRD patterns of the ITO films deposited at differentdeposition rates.2 Experimental details ITO films were depositedonto commercial k9 glass substrates at room temperature byIBAD technique using a vacuum coater equipped with twoelectron beam guns (only one of them was used in this work)and a Kaufman ion source. ITO powder pellet witha composition of 90wt.% In2O3 and 10wt.% SnO2 was usedas the evaporation source material. A 120mm diameterKaufman ion source was used to generate oxygen ion beam.The oxygen gas flowwas controlled by amass flow controller.The deposition rate and the film thickness were monitored andcontrolled by a quartz crystal sensor which has been linked toe-beam power supply for automatic controlling. The nominaldeposition rate was set to be 0.1, 0.15, 0.2 and 0.3 nm/s,respectively. The thickness was preset at 200 nm. The filmthickness indicated in Table 1 was obtained by fitting thetransmittance spectra. The substrate holder was rotated at aspeed of 0.3 rounds/s. The angle between the incident oxygenion beam and the normal of the substrate holder was fixedat 458. Before the deposition, the chamber was evacuated untila pressure of 1 103 Pa. After that, the oxygen gas wasintroduced into the chamber. The oxygen flow was set to be40 sccm, and the dynamic pressure in the chamber was about2.3 102 Pa.During all depositions, the ionbeamcurrent, theaccelerating voltage and the screenvoltagewerekept constantsat 100mA, 250 and 500V, respectively.The optical transmittance spectra of the films wererecorded by Perkin-Elmer Lambda 900 UV/VIS/NIR spec-trometer and the infrared reflectance wasmeasured by Perkin-Elmer SpectrumGX at angle of incidence of 608 related to thesubstrate normal. Atomic force microscopy (AFM) measure-ments were made using the equipment from DigitalInstruments Veeco Metrology Group. The X-ray diffractionwas done by SHIMADZUXRD-6000 performed between the2u values of 20 708with a step of 0.058. The Hall effect wasmeasured using Lake Shore 665 with a 5 kG magnetic fieldintensity at room temperature. All the experimental errordepends on the respective measuring equipments.3 Results and discussion X-ray diffraction patternsof the ITO films show that the films prepared at thedeposition rate lower than 0.2 nm/s have a cubic In2O3www.pss-a.comstructure [12] with a preferred orientation along the (222)direction. As exhibited in Fig. 1, the film deposited at highdeposition rate (higher than 0.3 nm/s) shows an amorphousstructure. Although the ITO powder pellet with a compo-sition of 90wt.% In2O3 and 10wt.% SnO2 was used as theevaporation source material, during the evaporation, theoxygen may get lost. As the deposition rate is increased, thee-beam power must be increased. That may result in morelost of the oxygen. It meansmore oxygenmust be introduced 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1540 L.-J. Meng et al.: Effect of the deposition rate on ITO thin film propertiesphysica ssp status solidi ainto the chamber to compensate the lost. As we kept theoxygen flow constant for all the deposition rates, it may notenough to form the polycrystalline ITO films at highdeposition rate and result in an amorphous structure. Thedistance between the (222) crystal planes has been calculatedand the crystalline dimension along the (222) direction hasbeen estimated using Scherrer formula [13] by fitting the X-ray diffraction peak. The results are given in Table 1. It hasbeen found that the distance between the (222) crystal planesfor all the polycrystalline ITO films are higher than thestandard stress-free value (0.2923 nm) [12], and thedifference becomes big as the deposition rate is increased.It means that there are compressive stresses in all thepolycrystalline films and the stress value is increased as thedeposition rate is increased. Although the ITOfilm depositedat 0.2 nm/s is subject to a big compressive stress, it has a bigcrystalline size.The surface morphology of the ITO films deposited atdifferent deposition rates was studied by AFM in a non-contact mode as shown in Fig. 2. The surface roughness [rootmean square (rms) value] has been calculated for thedifferent ITO films and is listed in Table 1. The ITO filmFigure 2 (online colour at: www.pss-a.com)AFM images (3mm 3rates. 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimprepared at 0.15 nm/s has a smooth surface and the ITO filmprepared at 0.3 nm/s has a rough surface. Generally,crystalline film has the rough surface [14]. However, ouramorphous ITO film shows a rough surface comparing to thecrystalline films. At the high deposition rate, there isno enough oxygen to form stoichiometric ITO film andmetal rich ITO film may be formed and results in a roughsurface.Figure 3 shows the specular transmittance spectra of theITO films prepared at different deposition rates. It can beseen that the transmittance decreases as the deposition rate isincreased. As we kept oxygen flow constant for all thedeposition rates, the ITO films deposited at high depositionrate can not be fully oxidized and result in a decrease of thetransmittance. It can be seen that the transmittance of the ITOfilms show a big decrease when the deposition rate is higherthan 0.2 nm/s. It means the deposition rate must be keptbelow the 0.2 nm/s in order to get ITOfilmswith good opticalproperties.In order to calculate the optical constant of the ITOfilms,the transmittance (between 400 and 1 800 nm) have beenfitted using semi-quantum model [15, 16] combined withmm) of the surface of the ITO films deposited at different depositionwww.pss-a.comPhys. Status Solidi A 207, No. 7 (2010) 1541OriginalPaper2500200015001000500020406080100Transmittance (%)Wavelength (nm)D1(0,30 nm/s)D2(0,20 nm/s)O2(0,15 nm/s)D3(0,10 nm/s)Substrate withou coatingFigure 3 Specular transmittance as a function of the wavelengthfor the ITO films deposited at different deposition rates.Table 2 Fitting parameters.sample D1 D2 O2 D3e1 0.20 3.69 3.33 4.23vp (cm1) 474 10 110 7 195 4 895gd (cm1) 1 00 000 528 731 853vLO1 (cm1) 2 58 520 6 654 7 200 5 783gLO1 (cm1) 2 00 000 4 758 5 069 3 672vTO1 (cm1) 320 6 584 7 132 5 774gTO1 (cm1) 47 787 4 653 5 052 3 760vLO2 (cm1) 4 504 35 861 36 809 25 286gLO2 (cm1) 3 378 6 650 9 974 5 174vTO2 (cm1) 14 514 33 593 33 651 25 148gTO2 (cm1) 52 428 5 750 7 731 5 088discrepancy 0.005 0.005 0.005 0.005Drude model. The dielectric function can be described asfollows:0.40.5ient (b)wwweðvÞ ¼ e1Yjv2jLO  v2  ig jLOvv2jTO  v2  ig jTOvþ v2pv2 þ igdv: (1)180016001400120010008006004001.01.52.02.50.00.10.20.3Refractive indexWavelength (nm)D1(0,30 nm/s)D2(0,20 nm/s)O2(0,15 nm/s)D3(0,10 nm/s)(a) Extinction CoefficD1(0,30 nm/s)D2(0,20 nm/s)O2(0,15 nm/s)D3(0,10 nm/s)Figure 4 Refractive index (a) and extinction coefficient (b) as afunction of the wavelength for the ITO films deposited at differentdeposition rates.The first term is the semi-quantum model and itrepresents the dielectric function as a product of individualoscillator terms. For each term there are four parameters,where vjTO, g jTO, vjLO and g jLO are the resonancefrequencies and damping constants of the transverse andlongitudinal optical modes, respectively. e1 is the ‘high-frequency’ contribution to the dielectric function. Thesecond term is the Drude model contribution which is usedto modify the free electron properties. Very good fits havebeen obtained for all the ITO films using these models. Thefitting parameters are listed in Table 2.After fitting, the refractive index and extinction coeffi-cient can be obtained as shown in Fig. 4. It can be seen thatthe extinction coefficient for ITO films increases as thedeposition rate is increased. It has been seen from Fig. 3that as the deposition rate is increased, the transmittance ofthe ITO films decreases and then results in an increase of theextinction coefficient. The refractive index of the ITO filmsdeposited at low (0.1 nm/s) and high (0.3 nm/s) depositionrate have high refractive index. At the low deposition rate,the 40 sccm oxygen flow is enough to oxidize deposited film.pss-a.comand forms the stoichiometric ITO film. The stoichiometricITO film has a high packing density and a high refractiveindex. As the deposition rate is increased, the same oxygenflow is not enough to fully oxidize deposited ITO films andresults in more oxygen vacancies in the deposited films andthen reduce the packing density and show a low refractiveindex. When the deposition rate reaches to 0.3 nm/s, thedeposited films structure changed as shown in Fig. 1 andmayresult in the high refractive index. The thickness of thedeposited ITO films is also obtained by fitting thetransmittance. The results are listed in Table 1. All the filmsshow a similar thickness about 220 nm and are little bithigher than the predefined value (200 nm).The electrical properties of the deposited ITO films arelisted in Table 1. The ITO film deposited at low deposition 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1542 L.-J. Meng et al.: Effect of the deposition rate on ITO thin film propertiesphysica ssp status solidi arate (0.1 nm/s) has very low electron concentration and then avery high electrical resistivity. At low deposition rate,deposited ITOfilms are fully oxidized and only a few oxygenvacancies exist and result in a low electron concentration. Asthe deposition rate is increased, the deposited ITO filmscannot be fully oxidized and more oxygen vacancies areformed and produce more electrons. From Table 1 it can beseen that the films deposited at 0.15–0.2 nm/s depositionrates have high Hall mobility, high electron concentrationand low electrical resistivity.Figure 5 shows the reflectance FTIR spectra for ITOfilms prepared at different deposition rates. The spectrashownwere obtained for angles of incidence of 608 related tothe substrate normal. It can be seen from the figure that thereflectance increases as the deposition rate is increased from0.1 to 0.2 nm/s.When the deposition rate is increased further,the reflectance decreases. There is a very simple relationbetween the infrared reflection R and the sheet resistanceRsr [17, 18]:1Reflectance (%)Figudiffe 20RIR ¼ ð1þ 2e0c0RsrÞ2: (2)The reflectance of ITO films deposited at differentdeposition rates has been calculated by this equation. Theresults are listed in Table 1, and to compare, the measuredreflectances at 12 000 nm are also listed in the Table. It can beseen that only for the sample prepared at 0.15–0.2 nm/sdeposition rates, the calculated value is in good accordance24000200001600012000010203040506070809000Wavelength (nm)D1(0,3 nm/s)D2(0,2 nm/s)O2(0,15 nm/s)D3(0,1 nm/s)Substrate without coatingre 5 Infrared reflectance spectra for ITO films deposited atrent deposition rates.10 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwith themeasured value. Itmeans this equation can only givegood results for samples with low sheet resistance.4 Conclusions ITO films have been deposited ontoglass substrates at different deposition rates by IBADtechnique. During all the deposition processes, the oxygenflow is kept constant at 40 sccm and the deposition rateis varied from 0.1 to 0.3 nm/s. The films deposited at lowdeposition rate (lower than 0.2 nm/s) show polycrystallinestructure and give a preferred orientation along the (222)direction. The films prepared at high deposition rates(higher than 0.3 nm/s) show an amorphous structure. Allthe polycrystalline structure ITO films are subject to acompressive residual stress and as the deposition rateincreases, the residual stress increases. The film depositedat high deposition rate leads to a high surface roughness.The films prepared at low deposition rate (0.1 nm/s) showhigh optical transmittance. As the deposition rate increases,transmittance is decreased. The films deposited at thedeposition rate of 0.15–0.2 nm/s exhibit good electricalproperties (5–10 103V cm). Considering both the opticaland electrical parameters, the deposition rate should be keptbetween 0.15 and 0.2nm/s. The ITOfilms deposited at 0.2 nm/s deposition rate gives high IR reflectance which is very usefulfor some electromagnetic wave shielding applications.References[1] I. Hamberg and C. G. Granqvist, J. Appl. Phys. 60(11), R123(1986).[2] S. H. Choi, J. T. Kim, C. S. Kim, and H. K. Baik, Appl. Phys.Lett. 90, 033513 (2007).[3] S. K. Park, J. I. Han, W. K. Kim, and M. G. Kwak, Thin SolidFilms 397, 49 (2001).[4] L. Yang, X. He, and F. He, Mater. Lett. 62(30), 4539 (2008).[5] J. L. Huang, B. S. Yau, C. Y. Chen, W. T. Lo, and D. F. Lii,Ceram. Int. 27, 363 (2001).[6] L. J.Meng,A.Mac¸arico,andR.Martins,Vacuum46, 673(1995).[7] P. Canhola, N. Martins, L. Raneiro, S. Pereira, E. Fortunato,I. Ferreira, and R. Martins, Thin Solid Films 487, 271(2005).[8] D. Kim and S. Kim, Thin Solid Films 408, 218 (2002).[9] C. Liu, T. Mihara, T. Matsutani, T. Asanuma, and M. Kiuchi,Solid State Commun. 126, 509 (2003).[10] P. J. Martin, R. P. Neterfield, and D. R. McKenzie, Thin SolidFilms 137, 207 (1986).[11] M. 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Effect of the deposition rate on ITO thin films properties prepared by ion beam assisted deposition (IBAD) technique

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