Materials Chemistry and Physics 70 (2001) 285–289

Preparation and characterisation of tin-doped indium oxide films

A. Kachouanea,∗, M. Addoua, A. Bougrinea,B. El idrissia, R. Messoussia, M. Regraguia, J.C. Bérnedeb

a Département de Physique, Faculté des sciences, Laboratoire d’Optoélectronique et de Physico-Chimiedes Matériaux, Université Ibn Tofail, BP 133, 14000 Kénitra, Morocco

b Equipe de Physique des Solides pour l’Electronique, Groupe couches minces et MatériauxNouveaux, Université de Nantes, FSTN, 2 rue la Houssinière, BP 9209 44322, Nantes Cedex 3, France

Received 25 February 2000; received in revised form 21 July 2000; accepted 25 September 2000

Abstract

Tin-doped In2O3 (ITO) thin films were prepared by reactive evaporation from new pulverulent mixture of indium oxide, tin oxide andmetallic indium at different partial pressures of oxygen. The films were annealed in a secondary vacuum just after deposition. Underoptimal conditions of evaporation, these films are stoichiometric, show a good crystallinity and feature high transmission in visible region(T > 90%) and high reflection in near IR region versus oxygen pressure. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Thin films; ITO; Transmission; XPS

1. Introduction

The high transmittance in the visible spectral region incombination with a high conductivity, and a high reflectancein the IR region of non-stoichiometric and doped films ofoxides of indium, tin, and zinc is still of wide interest dueto their broad applications in electronic and optical devices,like transparent electrodes for display devices, gas sensors,heating elements in aircraft and car windows for defoggingand deicing [1]. These coatings have been deposited by var-ious methods such as reactive RF and DC sputtering [2,3],reactive evaporation [4], pulsed laser ablation [5], spraypyrolysis [1] and sol–gel processes [6].

In this paper, we present and discuss investigation on thecomposition and optical properties of In2O3 films dopedwith tin prepared by evaporating the mixture of In2O3, SnO2and metallic indium in partial pressure of oxygen.

2. Experimental details

Reactive evaporation using metallic sources was em-ployed to deposit transparent conducting films of tin-dopedIn2O3. The most important control parameters are theevaporation rate, substrate temperature, source-to-substratedistance and oxygen partial pressure, to produce films

∗ Corresponding author. Fax:+212-739-2770.E-mail address:kachouaneamina@yahoo.fr (A. Kachouane).

having the proper stoichiometric composition. To obtainhighly transparent conducting oxide films, partial pressuresof oxygen sufficient to oxidise almost all indium and tinspecies, combined with appropriate substrate temperaturesand fast evaporation rates, are needed. Tin-doped In2O3films were prepared by reactive evaporation of mixture(In2O3, SnO2 + 10% In) from electrically heated crucibleson glass substrates heated at 350◦C and at partial pres-sures of oxygen between 10−4 and 5× 10−4 mbar. Thesource-to-substrate distance was 20 cm. These films aretin-doped (10%) and were annealed for 1 h at 350◦C in asecondary vacuum (10−6 mbar) immediately after deposi-tion. We have chosen the mixture of (In2O3, SnO2 and In)on account of the difference between the evaporation tem-peratures of the three compounds (T evp(In2O3) = 200◦C,T evp(SnO2) = 600◦C, T evp(In) = 742◦C) at partial pres-sures in the order of 10−4 Torr. This so as to insure thetraining of stoichiometric films.

The film structure and composition were, respectively, de-termined by X-ray diffraction analysis and X-ray photoelec-tron spectroscopy (XPS). The optical properties of tin-dopedIn2O3 films were characterised by optical transmittance.

3. Results and discussion

The diffraction patterns of tin-doped In2O3 films showthat the films feature a cubic structure witha = 1.003 nm,

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286 A. Kachouane et al. / Materials Chemistry and Physics 70 (2001) 285–289

Fig. 1. Diffraction patterns of tin-doped In2O3 prepared at different partial pressures of oxygen: (a) 10−4 mbar; (b) 5× 10−4 mbar.

the orientation of the (2 2 2) crystal plane was predominant.A systematic study according to oxygen pressure showsthat the films prepared in the interval of oxygen pressure(10−4–5×10−4 mbar) have a good crystallinity as comparedto these prepared to more raised pressures or weaker. Ex-amples are shown in Fig. 1(a and b).

The XP spectra before and after sanding for tin-dopedIn2O3 films are shown in Fig. 2. These spectra reveal theexistence of main elements (indium and oxygen) and dopantelement (tin) and the presence of impurities as traces suchas sodium and carbon.

Fig. 2 shows that the films are stoichiometric and present ahigh concentration of tin in bulk, while at the surface indiumconcentration is low as compared to bulk concentration. Thisaccumulation of tin at the surface can be explained by thehigh mobility of tin and its weak ionic radius.

We have also observed an important decrease of carbonconcentration in bulk. Its presence is apparently due to acontamination by hydrocarbons on the surface.

The binding energy of In 3d5/2 is at 444.7 eV, which cor-responds to results from previous studies of indium bondedas In2O3.The binding energy of Sn 3d5/2 is at 486.4 eV andcorresponds to that in SnO2 [7].

Figs. 3 and 4 (a and b) show, respectively, the results froma fitting of an XPS of O 1s and C 1s in ITO film. Beforesanding, the experimental contour of oxygen spectrum fitsfour peaks. In Fig. 3, the first peak is at 530.5 eV and cor-responds to oxygen bonded as In2O3, the second peak is at530.6 eV and is related to oxygen bonded as SnO2 [7,8], thethird and fourth peaks are consequence of contamination ofthe surface with carbon compounds. These peaks are, re-spectively, located at 532.1 eV in the form of C–OH and at

A. Kachouane et al. / Materials Chemistry and Physics 70 (2001) 285–289 287

Fig. 2. XP spectra of tin-doped In2O3: (a) before sanding, (b) after sanding.

534.2 eV in the form of O=C–O [9]. After sanding, the dis-appearance of the fourth peaks shows the excess of oxygenin surface by absorption.

As revealed in Fig. 4, deconvolution of carbon spectrumshows three peaks: the first is located at 285 eV and corre-sponds to carbon C–C/C–H, the second and the third corre-spond to the liaisons with oxygen atom in the form of C–OHat (286.8 eV) and O=C–O at (289.3 eV), respectively [10].After sanding, the third peak disappeared.

Fig. 3. O 1s XP spectra of tin-doped In2O3: (—) fitted spectrum; (- - -) resolved peaks; (a) before sanding, (b) after sanding.

4. Optical properties

Fig. 5 shows the spectra of optical transmission oftin-doped In2O3 films obtained for different oxygen pres-sures (10−4–5×10−4 mbar) in the optimal conditions. Thesespectra show the existence of interference fringes withan important transmission in visible region (the averagetransmissionT > 90%) for oxygen partial pressure of10−4 mbar. The decrease of optical transmission for oxygen

288 A. Kachouane et al. / Materials Chemistry and Physics 70 (2001) 285–289

Fig. 4. C 1s XP spectra of tin-doped In2O3: (—) fitted spectrum; (- - -) resolved peaks; (a) before sanding, (b) after sanding.

Fig. 5. Optical transmittance spectra of tin-doped In2O3 at: (a)P(O2) = 10−4 mbar; (b)P(O2) = 5 × 10−4 mbar.

Fig. 6. Dispersion of the refractive index of tin-doped In2O3 at: (a)P(O2) = 10−4 mbar; (b)P(O2) = 5 × 10−4 mbar.

Fig. 7. Dispersion of the extinction coefficient of tin-doped In2O3 at: (a)P(O2) = 10−4 mbar; (b)P(O2) = 5 × 10−4 mbar.

pressure of 5× 10−4 mbar is due to the increase of lightscattering by the excess of oxygen. We have also observeda high absorption by the free-carriers in the IR region.

The variations of the refractive indexn and extinctioncoefficientk versus wavelength are shown in Figs. 6 and7. In the visible and near IR regions, the refractive indexesare almost constant and located between 1.6 and 1.85. Theincrease in extinction coefficient, as shown in Fig. 7, in theUV and IR region is due, respectively, to a high absorptionin UV region and to the plasma effect resulting from thehigh concentration of mobile electron in the IR region [11].

5. Conclusions

We have optimised the parameters of tin-doped In2O3films elaboration by the reactive evaporation of the newmixture of In2O3, SnO2 and metallic indium in partialpressure of oxygen. The samples obtained for optimal con-ditions have a good crystallinity and exhibit cubic structurewith a preferential orientation along (2 2 2) direction. The

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optical study shows a very important transmission ex-ceeding 90% in the visible region and high absorption byfree-carrier in the IR region. These results allow the utilisa-tion of this semiconductor in devices applications: selectivefilters for solar thermal collectors (heat-mirrors), solar cells,conducting electrodes and antireflection coating, etc.

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