007) 8567–8572www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Electrical, optical and morphological properties of nanoparticleindium–tin–oxide layers

Michael Gross ⁎, Albrecht Winnacker, Peter J. Wellmann

Department of Materials Science 6, University Erlangen-Nuremberg, Martensstr. 7, D-91058 Erlangen, Germany

Available online 1 April 2007

Abstract

Porous layers were prepared from DEGUSSA's ITO (In2O3:Sn) nanoparticle dispersion by doctor blading followed by annealing in air. Weinvestigated the influence of various annealing parameters on electrical, optical and morphological thin film properties.

Conductance rises with increasing annealing temperature and time by more than three orders of magnitude up to 44 Ω−1cm−1. Besides this wefound an abrupt decrease in free charge carrier concentration above a critical annealing temperature of 250 °C, which leads to a step inconductance curve. In spite of particle growing during annealing no decrease in porosity was observed and in opposite to compact material,nanoparticle layers do not exhibit an appreciable shrinkage below recrystallisation temperature. These both indicate a densification hinderingparticle pinning effect, which is believed to be currently the main obstruction to achieve higher electrical conductivities.© 2007 Elsevier B.V. All rights reserved.

Keywords: Indium–tin–oxide; ITO; Nanoparticle; Conductance; Transmittance; Porosity; Transport; Cluster

1. Introduction

Layers from transparent conducting oxides (TCOs) arevery important in modern electronic industry due to the neces-sity of transparent electrodes in many new applications likeTFTs, OLED-displays, organic solar cells, electrolumines-cence lamps and some more. Tin doped indium oxide In2O3:Sn(ITO) layers exhibit best conductance among all the TCOs atcoexistant good transmittance properties. They are classicallydeposited by PVD techniques as magnetron sputtering [1] orevaporation [2]. These methods are cost intensive due to in-evitable vacuum process, but lead to very good layer conduc-tance of ∼104 Ω−1cm−1 [1,2]. Novel deposition techniqueswith a high cost reduction potential are the wet depositionmethods like sol–gel-technique [3–5] and coating of nano-particle dispersions [5–7]. They both combine the advantagesof omittable vacuum step and printing possibility, which offers

⁎ Corresponding author. Tel.: +49 9131 8527719; fax: +49 9131 8528495.E-mail address: michael.grosz@ww.uni-erlangen.de (M. Gross).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.03.136

large area layer manufacturing in a continuous process anddirect patterning ability.

Wet deposited films normally need some annealing stepsafter deposition for getting better conductivity and opticalproperties. In this paper the effects of various annealing para-meters on electrical, optical and morphological properties ofnanocrystalline ITO layers are characterised and discussed.

2. Experimental

Investigated layers were made by doctor blading a com-mercial (DEGUSSA Creavis) ethanolic nanoparticle ITO dis-persion on glass, silica as well as alumina substrates. Glasssubstrates with a size of 75 mm×25 mm were used for an-nealing up to 550 °C. At higher annealing temperatures20 mm×10 mm sized silica substrates and 58 mm×24 mmalumina substrates were used due to temperature stability.Samples on alumina substrates were made for verification theresults of silica substrate samples. Substrate thicknesses all were1 mm. Using various substrates was unproblematic, becauseno influence of substrate material on the ITO layer propertieswas observed.

Fig. 1. ITO-layer conductance versus annealing temperature (annealing time:60 min).

Fig. 3. Transmittance spectra of ITO layers annealed at various temperatures for60 min.

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Depending on dilution of ITO dispersion and doctor bladingprocess parameters, such as casting velocity and blade gapheight, layer thickness could be varied from 800 nm to about6 μm. Thicknesses were determined with a DekTak mechan-ical profilometer and kept approximately constant for everysample batch. Conductance measurements in air were done ina linear four-point setup using a Keithley SMU 236. Lowtemperature electrical measurements, however, were carriedout in vacuum with a van-der-Pauw four-point setup, a Keith-ley 220 current source, a Keithley 196 System DMM and aKeithley 705 scanner device. Optical transmittance measure-ments were performed using a Perkin Elmer Lambda 19 UV-VIS-IR spectrometer in a wavelength range of 200 nm to3000 nm.

3. Results and discussion

3.1. Electrical properties

The electrical conductance of nanoparticle ITO layers isstrongly impacted by the annealing parameters after deposition,especially duration time, temperature and kind of atmosphere.The as-deposited layer conductance is about 0.02Ω−1cm−1 andrises up to 44Ω−1cm−1, if annealed at 1000 °C for 60 min in air

Fig. 2. ITO layer conductance versus annealing time (annealing temperature:550 °C).

(see Fig. 1). This conductivity corresponds to a sheet resistanceof nearly 50 Ω/□. The conductance increase can easily beexplained by formation of sinternecks between nanoparticleaggregates or the nanoparticles itselves, which leads to a largercontact cross-section area and hence lower resistance. Sinterneckgeneration, marked as process 1 in Fig. 1 and Fig. 2, is a relativefast process, as Fig. 2 indicates. Already after 5 min of annealingat 550 °C conductance raises more than 100 times. 550 °C isparticular interesting, because it is the maximum working tem-perature for common low coast sodium silicate glass.

At medium annealing temperatures above 250 °C for 60 minor at 550 °C for more than 30 min the conductance decreasesagain, which leads to a step in the characteristics (marked asprocess 2 in Figs. 1 and 2). This is presumably a consequence ofa decrease in number of free charge carriers. In ITO this num-ber strongly depends on quantity of indium substituting tinatoms as well as oxygen vacancy concentration. Both latticedefects create free electrons, which contribute to chargetransport. Whereas tin doping level is constant and adjusted toan optimum in conductivity, oxygen vacancy concentration isimpacted by the environment like oxygen partial pressure andtemperature. Depending on direction of oxygen gradient, hightemperatures kinetically enables diffusion of oxygen into or outof the material.

Fig. 4. Transmittance spectra of ITO layers annealed for various times at 550 °C.

Fig. 5. SEM pictures of ITO layers annealed at (a) 20 °C, (b) 550 °C, (c) 750 °C, (d) 1000 °C and (e and f) 1200 °C.

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However, above a critical temperature of 250 °C in air,oxygen diffuses into the ITO lattice, reduces the number ofoxygen vacancies and leads to a decrease in charge carrierconcentration and hence conductivity. The optical transmittancemeasurements shown in Figs. 3 and 4 support this statement.The free electron absorption onset shifts from about 1100to 2000 nm, which denotes an abrupt decrease in charge carrierconcentration. This shift comes along with a colour switchingfrom blue to yellowish. Admittedly the free electron absorp-tion onset shift is faster than the measurable decrease inconductivity (see Fig. 4). It occurs already after 5 min, whereasconductance decrease exhibit soonest after 30 min (see Fig. 2).The reasons for that phenomenon is still unclear.

After this step in conductance characteristics, conductivityrises again with increasing annealing temperatures up to1000 °C. The latter is related to further contact area increaseon the one hand and grain enlargement due to Oswald ripening

Fig. 6. Envelopes Tmax and Tmin in a sample transmittance spectrum.

on the other hand. As the grains become larger, the number ofgrain boundaries decreases and therewith grain boundaryelectron scattering. This enhances charge carrier mobility andhence conductivity.

3.2. Optical properties

As-deposited nanoparticle ITO layers are blue coloured.Layers with a thickness of approximately 3 μm exhibit a trans-mittance of around 80% at 600 nm wavelength and a highinfrared absorption (see Fig. 3). The free electron absorptiononset is located at about 1100 nm. After annealing for 60 minwith more than 300 °C, maximum transmittance increases toabout 90%, whereas annealing with 750 °C leads to highestvalues of 91%. During annealing the free electron absorptiononset shifts to about 2000 nm due to reduction of charge carrierconcentration as explained above: The colour of the layersswitches to yellowish.

Transmittance of thinner layers with a thickness of about1 μm are naturally a little bit higher. Even as-deposited layershave more than 90% transmittance in the maximum (see Fig. 4,these samples have layer thickness of about 1 μm). Thick layeras well as thin layer transmittance are high enough for usual ITOapplications in touchscreens, TFT- and LCD-displays andOLEDs.

Table 1Calculated porosity of ITO layers for various annealing temperatures (annealingtime: 60 min)

Annealing temperature [°C] 100 150 300 450 550 750 1000Porosity [%] 45 47 45 44 46 46 49

Fig. 7. Shrinkage of ITO compacts and layers.Fig. 9. Relative specific resistance: measurements (continuous lines) and fits(dots) with cluster model.

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3.3. Morphological properties — porosity

Fig. 5a–d shows SEM images of ITO layers annealed atvarious temperatures. Particle growth at temperatures above550 °C is obvious, but between 1000 °C and 1200 °C layermorphology changes completely. ITO crystals arise with a sizeup to 1 μm and a completely different habitus than the originalnanoparticles (see Fig. 5e and f), which indicates a kind ofrecrystallisation. At 1200 °C the annealed layer is no moretransparent, because of light scattering at the μm-sized crystals.The conductance decreases partially to 27 Ω−1cm−1, whereasthe exact reason is not clear so far.

Another result derived from the SEM pictures is, that nopronounced densification seems to takes place at high annealingtemperatures. To confirm this assumption, porosity values ρ ofsome ITO-samples were calculated from their refractive indicesnl by using the Eq. (1):

q ¼ 1� n2l � 1

n2b � 1; ð1Þ

where nb is the refractive index of bulk ITO (nb=1.89 [8]).This is the highest found value in literature [4,8,10] and

Fig. 8. Temperature dependant specific resistance of ITO-layers annealed atvarious temperatures.

therefore the calculated porosities are upper limits of the realvalues. Refractive indices nl themselves were obtained fromtransmission spectra by the method of Svanepoel [8,9]:

nl ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiN þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiN 2 � n2edn

2s

q;

rð2Þ

where ne is the refractive index of the environment, i.e. air inour case, ns is the refractive index of the substrate and N itselfis given by:

N ¼ n2e þ n2s2

þ 2nensdTmax � Tmin

TmaxdTmin; ð3Þ

where Tmax and Tmin are the maximum and minimum valuesrespectively of the transmittance spectrum's envelopes at afixed wavelength (see Fig. 6). For each ITO sample therefractive index nl was calculated at three different wave-length and averaged.

The calculated porosity values are summarised in Table 1.Unfortunately it is not possible to present a value for the sampleannealed at 1200 °C, because the opaque nature due to lightscattering on μm-sized crystals did not allow optical transmis-sion measurement. As presumed after the analysis of the SEMimages, no porosity decrease can be found with increasingannealing temperatures. This result is quite surprising, since it iswell known from other ceramic particle systems, that sinteringof the so called green body leads to a significant shrinkage andhence decrease in porosity. However, besides the missingporosity decrease, nanoparticle ITO layers also do not exhibitan appreciable shrinkage (thickness decrease) below the

Table 2Fitting parameters for fluctuation induced tunnelling model

Annealing temperature[°C]

20 350 550 750 1000

C 8.57 1.747 0.0485 0.0229 0.01972T1 408 382 258 35 51T0 209 209 274 63 214

Fig. 10. AFM pictures of ITO layers annealed at a) 550 °C and b) 750 °C.

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recrystallisation temperature between 1000 and 1200 °C, aspointed out in Fig. 7.

These both suggest a kind of mechanical pinning effect,which hinders restructuring of particles and aggregates. Acompact of ITO nanoparticles on the other hand shows shrink-age already at temperatures of 800 °C (see Fig. 7), whichsuggests, that particle pinning in the layer presumably is causedby substrate–particle-adhesion. The obvious lack of aggregatemoving possibility along x- and y-direction also leads to animmobility in z-direction. As a consequence, no densificationcan take place, which is believed to be currently the mainobstruction to achieve higher electrical conductivities in nano-particle ITO layers.

3.4. Temperature dependent resistance measurements —carrier transport

Temperature dependant resistance measurements of ITOlayers were performed in the temperature range of 53 K to260 K (see Fig. 8). All samples exhibit a negative temperaturecoefficient of resistance (TCR), indicating a semiconductingmaterial. The slope of TCR becomes smaller with increasingannealing temperatures. This is in good agreement tomeasurements on nanoparticle ITO layers of Ederth et al.[6]. He also discussed different transport models leading to anegative TCR, especially ionized impurity scattering, vari-able-range hopping, nearest neighbour hopping, a granular–metal system and a model in which metallic conductingnanoparticles form internally connected clusters which areseparated by thin isolating barriers. In this model chargetransport is dominated by fluctuation induced tunnellingbetween the metallic regions (clusters). The resistance is givenby [11]:

q Tð Þ ¼ CdexpT1

T þ T0

� �; ð4Þ

where C is a constant and T0 and T1 are impacted by area,height and width of the insulating barrier. As Ederth et al. [6],we also confirm a good agreement between theoretical fits andmeasurements. Fig. 9 shows the relative specific resistanceversus temperature measurements (continuous lines) and bestfits (dots) using Eq. (4), following the fluctuation induced

tunnelling model. The corresponding fitting parameters aregiven in Table 2. It is evident that this model is able todescribe carrier transport in nanoparticle ITO layers very well.

In the AFM images (see Fig. 10) we found a structure ofhigher order than primary particle structure. Particles have anaverage size of only 10 nm to 20 nm if annealed at 550 °C and40 nm respectively if annealed at 750 °C (compare Fig. 5b andc), whereas AFM feature size is found to exhibit a structuredimension of 150 nm or even 180 nm. This confirms the pre-sence of the presumed clusters from the fluctuation tunnellingmodel, although cluster sizes discussed by Ederth et al. [6] shouldhave a size of several micrometers for applicability to the model.However, size difference between presumed and found clusterstructure is only one order of magnitude.

4. Summary

Transparent and conducting ITO layers were prepared from ananoparticle dispersion by using doctor blading technique.Impact of annealing parameters, especially temperature andtime on electrical, optical and morphological properties wasinvestigated. Conductance rises from 0.02 Ω−1cm−1 up to44 Ω−1cm−1, if annealed at 1000 °C for 60 min. This corre-sponds to a sheet resistance of nearly 50 Ω/□. On sodiumsilicate glass substrates, with an annealing temperature limit of550 °C, layers with 17Ω−1cm−1, corresponding to ∼150Ω/□,were realised. Conductance increase is consequence of sinter-neck generation and crystal growth. A small contrary step inconductance-temperature-characteristics takes place due to de-crease of charge carrier concentration by oxygen diffusion intothe ITO. Optical transmittance of annealed layers evidentlydepends on layer thickness, but is usually more than 90% in thevisible range.

In opposite to compact ITO material, nanoparticle layers donot exhibit an appreciable shrinkage below recrystallisationtemperature between 1000 °C and 1200 °C. This indicates akind of particle pinning effect by substrate adhesion. As aconsequence, no porosity decrease takes place during annealingand hence no material densification. This is believed to be themain obstruction to achieve higher electrical conductivities.

In agreement with Ederth et al. [6] a transport model wasassumed, where fluctuation induced carrier tunnelling throughthin insulating barriers between clusters of metallic conductingnanoparticles limits charge carrier transport.

Today, nanoparticle ITO layers do not yet show the verygood properties of ITO layers fabricated with physical vapordeposition techniques, but for some modest applications theyare an alternative with a high cost reduction potential. If onesucceeds in the future to densify nanoparticle derived layersduring annealing, high-technical applications as TFT- or LCD-display electrodes may also be realised.

Acknowledgement

This work was financially supported by DFG (contractnumber GRK 1161) and DEGUSSA AG. Fruitful discussionswith Dr. Thomas Lüthge, Dr. Dieter Adam and Dr. Anna Prodi-

8572 M. Gross et al. / Thin Solid Films 515 (2007) 8567–8572

Schwab from DEGUSSA Creavis and assistance in experimen-tal work by Ilja Maksimenko are gratefully acknowledged.

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