Tranparent conducting coatings have been prepared by sol gel methods either by a conventional sol-gel process (Antimony doped Tin Oxide - ATO, Aluminium doped Zinc Oxide - AZO) or a new wet chemical process using fully dispersed crystalline nanoparticles (ATO, Indium Tin Oxide - ITO). The dip coating technique has been used as deposition technique with single coating thickness varying from a few nanometer to ca. 400 nm. The layers have been fired in a furnace. Structural properties have been determined by x-ray diffraction and TEM analysis and the electrical properties by the van der Pauw/Hall measurement. Three different coating procedures have been used to investigate the effect on the structure, morphology and the electrical properties of the coatings. It is shown that the individual layer thickness in multilayer coatings influences dramatically the mentioned properties. Very thin individual layers favour a heterogeneous nucleation with dense columnar growth of the crystallites leading to low electrical resistivity (&Omega; &asymp; 10 -³ &Omega; cm), while thick individual layers result in a porous morphology made of small crystallites leading to resistivities in the 10&asymp;2 &Omega; cm range

Aegerter, Michel A.

Gasparro, Guido

Goebbert, Christian

Krajewski, Thomas

Schuler, Thomas

Scientific documents from the Saarland University,

Influence of the Layer Morphology on the Electrical Properties of Sol Gel
Transparent Conducting Oxide Coatings
CHRISTIAN GOEBBERT, GUIDO GASPARRO, THOMAS SCHULER, THOMAS KRAJEWSKI AND
MICHEL A. AEGERTER⁄
Department of Coating Technology, Institut fu¨r Neue Materialien-INM, Im Stadtwald, Gebaeude 43,
D-66123 Saarbruecken, Germany
aegerter@inm-gmbh.de
Abstract. Tranparent conducting coatings have been prepared by sol gel methods either by a conventional sol-gel
process (Antimony doped Tin Oxide—ATO, Aluminium doped Zinc Oxide—AZO) or a new wet chemical process
using fully dispersed crystalline nanoparticles (ATO, Indium Tin Oxide—ITO). The dip coating technique has been
used as deposition technique with single coating thickness varying from a few nanometer to ca. 400 nm. The layers
have been fired in a furnace. Structural properties have been determined by x-ray diffraction and TEM analysis and
the electrical properties by the van der Pauw/Hall measurement. Three different coating procedures have been used
to investigate the effect on the structure, morphology and the electrical properties of the coatings. It is shown that
the individual layer thickness in multilayer coatings influences dramatically the mentioned properties. Very thin
individual layers favour a heterogeneous nucleation with dense columnar growth of the crystallites leading to low
electrical resistivity (‰ … 10¡3˜ cm), while thick individual layers result in a porous morphology made of small
crystallites leading to resistivities in the 10¡2 ˜ cm range.
Keywords: morphology, electrical properties, transparent conductive coatings
Introduction
Transparent conducting coatings like antimony doped
tin oxide (ATO), aluminium doped zinc oxide (AZO)
and tin doped indium oxide have (ITO) found a wide
range of application in opto-electronic devices. They
also find applications as gas sensors, catalysts and heat
shields [1].
The sol gel technique allows produces thin homo-
geneous layers with high transmission in the visible,
but the resistivity is always found at least one order of
magnitude higher than that obtained with coatings pre-
pared by deposition methods such as CVD, sputtering
or spray pyrolysis. One of the factors which influences
the electrical properties is the morphology of the lay-
⁄To whom all correspondence should be addressed.
ers. The influence of the sintering temperature and the
heating rate on the morphology of the layers have been
already investigated [2, 3].
The aim of this work is to investigate the influence of
the single layer thickness on the morphology and elec-
trical properties of ATO, ITO and AZO sol gel multi-
layer coatings.
Experimental
Single and multilayer coatings have been deposited by
dip coating technique on borosilicate (ATO, ITO) and
on fused silica (AZO) respectively using an ethanolic
solution of the metal salt and dopant (5 mole % Sb
for ATO, 1 mole % Al for AZO, 8 mole % Sn for
ITO). In order to investigate the influence of already
crystalline particles a dispersion of redispersed ATO
436 Goebbert et al.
Figure 1. Flow diagram of the various procedures.
nanoparticles was used as well [4]. Three different
types of procedures have been used (Fig. 1).
After their deposition the AZO coatings have been
tempered in a furnace under forming gas to reduce their
resistivity. The thickness of the single layers was deter-
mined by a stylus profiler (Tencor P10). The resistivity
was determined by the 4-point, van der Pauw and Hall
measurements. X-ray diffraction and HRTEM cross
sections have been used to investigate the morphology
of the samples.
Results and Discussion
AZO Coatings
TEM cross section of a single and a 10 layers coat-
ing made by process I with 25 nm thick single layers
are shown in Fig. 2(a) and (b) respectively. The sin-
gle layer coating is made of spherical particles with
15–25 nm in diameter scattered over the surface of
the substrate. The 10 times dipped multilayer shows
a columnar texture perpendicular to the surface. An
increase of the thickness of the single layers to 40 nm
drastically changes the morphology of the multilayer.
No columnar texture is observed (Fig. 2(c)) but the
coating practically consists of elongated crystallites in
the direction perpendicular to the surface. From these
data the critical single layer thickness for columnar tex-
ture of ZnO : Al is approximately 15–20 nm. The film
resistivity decreases from 5£ 10¡1 ˜ cm for the single
layer (d D 25 nm) to 4£ 10¡2 ˜ cm for the multilayer
made of thin single layers (d D 17 nm).
ATO and ITO Coatings
The electrical properties and the morphology of ATO
single layers with thickness >10 nm is independent
on the densification process. For all three processes
the single layers are made of small almost spherical
crystallites that are loosely packed. The layer is porous
and its top surface is denser than the rest of the layer.
With a sintering temperature of 550–C resistivities as
low as ‰ D 9£ 10¡3 ˜ cm could be reached.
On the hand the sintering conditions and the thick-
ness of the single layers influence the morphology and
the electrical properties of the ATO multilayer coatings.
Multilayers made with single layer thickness of 20 nm
by process I are homogeneous and built with small
spherical crystallites that are loosely packed (Fig. 3(a)).
The morphology of the multilayers obtained by pro-
cess II or III reveals that each individual layer consist-
ing of a 10 nm relatively dense interface layer on top
of a porous bulk layer (Fig. 3(b)). The resistivity of
the 200 nm thick multilayer made by process I and II
(or III) is 1:4£ 102 ˜ cm and 1:8£ 10¡3 respectively.
A reducing of the thickness of the single layers in the
multilayer system leads to a change in the morphology
of the coatings. Multilayers made with single layers
having a thickness •5 nm and a columnar growth is
observed (Fig. 3(c)). The resistivity has a value of
2:9£ 10¡3 ˜ cm.
The reason for the change in the morphology in the
SnO2 : Sb system might be the same as in the ZnO : Al
system. The thickness of the single layer is smaller
than the crystallite size in the single layer. The criti-
cal thickness can be observed by a 2–4 nm single layer
thickness.
The morphology of coatings made from redis-
persed ATO and ITO nanoparticles using processes I,
II and III is identical to that shown for thick ATO
sol gel coatings (Fig. 3(a) and (b)). The results
on thin coatings could not be confirmed because it
was not possible to produce thin layers with the re-
dispersed particles. The electricaal properties are si-
miliar as well, the resistivity for a 150 nm single
layer is ‰D 1:8£ 10¡2 ˜ cm, which decrease to
‰D 6:8£ 10¡3 ˜ cm for a 10£ 150 nm multilayer.
Conclusion
The morphology and texture of ATO, ITO and AZO
coatings have been related to the coating procedure.
Influence of the Layer Morphology on Sol Gel Oxide Coatings 437
(a) (b)
(c)
Figure 2. AZO HRTEM cross-section of (a) a 25 nm thick single layer, (b) a multilayer made of 10£ 17 nm thin single layers, (c) a multilayer
made of 4£ 40 nm thick single layers.
Using conventional sols the morphology of ATO
and AZO coatings can be tailored from an agglom-
eration of spherical particles to a columnar type
of arrangement. The morphology of ATO and ITO
coatings made of redispersed nanoparticles always
consists of loosely packed and not orientated crys-
tallites. These structures have an important influence
on the electrical properties of the coatings. Thick
438 Goebbert et al.
(a) (b)
(c)
Figure 3. HRTEM cross section of ATO multilayers made of (a) 10 £ 20 nm thick single layers densified by process I, (b) 6 £ 20 nm thick
single layer densified by process II, (c) 40£ 2 nm thin single layer densified by process II.
single layers are always made of an agglomera-
tion of spherical particles with a high void fraction
and exhibit a low density and high resistivity. In
very thin single layers (<20 nm) heterogeneously
nucleated crystals predominate and their size is
limited by the thickness of the layer. As the single layer
thickness increases the formation of homogeneously
nucleated crystals becomes more likely.
Influence of the Layer Morphology on Sol Gel Oxide Coatings 439
References
1. H.L. Hartnagel, A.L. Dawar, A.K. Jain, and G. Jagadish,
Semiconducting Transparent Thin Films (Institute of Physics,
Bristol and Philadelphia, 1995).
2. M.A. Aegerter, A. Reich, G. Ganz, G. Gasparro, and J. Pu¨tz, J.
Non Cryst. Solids 218, 123 (1997).
3. J. Pu¨tz, D. Ganz, G. Gasparro, and M.A. Aegerter, in
Proc. Int. Conference Sol-Gel 97, Sheffield, UK, 31.08–05.09
1997.
4. C. Goebbert, M.A. Aergerter, D. Burgard, R. Nass, and
H. Schmidt, J. Mater. Chem. 9, 253 (1999).


Influence of the layer morphology on the electrical properties of sol-gel transparent conducting oxide coatings

Universaar

Abstract. Tranparent conducting coatings have been prepared by sol gel methods either by a conventional sol-gel process (Antimony doped Tin Oxide—ATO, Aluminium doped Zinc Oxide—AZO) or a new wet chemical process using fully dispersed crystalline nanoparticles (ATO, Indium Tin Oxide—ITO). The dip coating technique has been used as deposition technique with single coating thickness varying from a few nanometer to ca. 400 nm. The layers have been fired in a furnace. Structural properties have been determined by x-ray diffraction and TEM analysis and the electrical properties by the van der Pauw/Hall measurement. Three different coating procedures have been used to investigate the effect on the structure, morphology and the electrical properties of the coatings. It is shown that the individual layer thickness in multilayer coatings influences dramatically the mentioned properties. Very thin individual layers favour a heterogeneous nucleation with dense columnar growth of the crystallites leading to low electrical resistivity ( ‰  … 10¡3 ˜ cm), while thick individual layers result in a porous morphology made of small crystallites leading to resistivities in the 10¡2 ˜ cm range

Christian Goebbert

Guido Gasparro

Thomas Schuler

Thomas Krajewski

Michel A. Aegerter

CiteSeerX

Influence of the Layer Morphology on the Electrical Properties of Sol Gel Transparent Conducting Oxide Coatings

Acronym

Influence of the Layer Morphology on the Electrical Properties of Sol GelTransparent Conducting Oxide CoatingsCHRISTIAN GOEBBERT, GUIDO GASPARRO, THOMAS SCHULER, THOMAS KRAJEWSKI ANDMICHEL A. AEGERTER⁄Department of Coating Technology, Institut fu¨r Neue Materialien-INM, Im Stadtwald, Gebaeude 43,D-66123 Saarbruecken, Germanyaegerter@inm-gmbh.deAbstract. Tranparent conducting coatings have been prepared by sol gel methods either by a conventional sol-gelprocess (Antimony doped Tin Oxide—ATO, Aluminium doped Zinc Oxide—AZO) or a new wet chemical processusing fully dispersed crystalline nanoparticles (ATO, Indium Tin Oxide—ITO). The dip coating technique has beenused as deposition technique with single coating thickness varying from a few nanometer to ca. 400 nm. The layershave been fired in a furnace. Structural properties have been determined by x-ray diffraction and TEM analysis andthe electrical properties by the van der Pauw/Hall measurement. Three different coating procedures have been usedto investigate the effect on the structure, morphology and the electrical properties of the coatings. It is shown thatthe individual layer thickness in multilayer coatings influences dramatically the mentioned properties. Very thinindividual layers favour a heterogeneous nucleation with dense columnar growth of the crystallites leading to lowelectrical resistivity (‰ … 10¡3˜ cm), while thick individual layers result in a porous morphology made of smallcrystallites leading to resistivities in the 10¡2 ˜ cm range.Keywords: morphology, electrical properties, transparent conductive coatingsIntroductionTransparent conducting coatings like antimony dopedtin oxide (ATO), aluminium doped zinc oxide (AZO)and tin doped indium oxide have (ITO) found a widerange of application in opto-electronic devices. Theyalso find applications as gas sensors, catalysts and heatshields [1].The sol gel technique allows produces thin homo-geneous layers with high transmission in the visible,but the resistivity is always found at least one order ofmagnitude higher than that obtained with coatings pre-pared by deposition methods such as CVD, sputteringor spray pyrolysis. One of the factors which influencesthe electrical properties is the morphology of the lay-⁄To whom all correspondence should be addressed.ers. The influence of the sintering temperature and theheating rate on the morphology of the layers have beenalready investigated [2, 3].The aim of this work is to investigate the influence ofthe single layer thickness on the morphology and elec-trical properties of ATO, ITO and AZO sol gel multi-layer coatings.ExperimentalSingle and multilayer coatings have been deposited bydip coating technique on borosilicate (ATO, ITO) andon fused silica (AZO) respectively using an ethanolicsolution of the metal salt and dopant (5 mole % Sbfor ATO, 1 mole % Al for AZO, 8 mole % Sn forITO). In order to investigate the influence of alreadycrystalline particles a dispersion of redispersed ATO436 Goebbert et al.Figure 1. Flow diagram of the various procedures.nanoparticles was used as well [4]. Three differenttypes of procedures have been used (Fig. 1).After their deposition the AZO coatings have beentempered in a furnace under forming gas to reduce theirresistivity. The thickness of the single layers was deter-mined by a stylus profiler (Tencor P10). The resistivitywas determined by the 4-point, van der Pauw and Hallmeasurements. X-ray diffraction and HRTEM crosssections have been used to investigate the morphologyof the samples.Results and DiscussionAZO CoatingsTEM cross section of a single and a 10 layers coat-ing made by process I with 25 nm thick single layersare shown in Fig. 2(a) and (b) respectively. The sin-gle layer coating is made of spherical particles with15–25 nm in diameter scattered over the surface ofthe substrate. The 10 times dipped multilayer showsa columnar texture perpendicular to the surface. Anincrease of the thickness of the single layers to 40 nmdrastically changes the morphology of the multilayer.No columnar texture is observed (Fig. 2(c)) but thecoating practically consists of elongated crystallites inthe direction perpendicular to the surface. From thesedata the critical single layer thickness for columnar tex-ture of ZnO : Al is approximately 15–20 nm. The filmresistivity decreases from 5£ 10¡1 ˜ cm for the singlelayer (d D 25 nm) to 4£ 10¡2 ˜ cm for the multilayermade of thin single layers (d D 17 nm).ATO and ITO CoatingsThe electrical properties and the morphology of ATOsingle layers with thickness >10 nm is independenton the densification process. For all three processesthe single layers are made of small almost sphericalcrystallites that are loosely packed. The layer is porousand its top surface is denser than the rest of the layer.With a sintering temperature of 550–C resistivities aslow as ‰ D 9£ 10¡3 ˜ cm could be reached.On the hand the sintering conditions and the thick-ness of the single layers influence the morphology andthe electrical properties of the ATO multilayer coatings.Multilayers made with single layer thickness of 20 nmby process I are homogeneous and built with smallspherical crystallites that are loosely packed (Fig. 3(a)).The morphology of the multilayers obtained by pro-cess II or III reveals that each individual layer consist-ing of a 10 nm relatively dense interface layer on topof a porous bulk layer (Fig. 3(b)). The resistivity ofthe 200 nm thick multilayer made by process I and II(or III) is 1:4£ 102 ˜ cm and 1:8£ 10¡3 respectively.A reducing of the thickness of the single layers in themultilayer system leads to a change in the morphologyof the coatings. Multilayers made with single layershaving a thickness •5 nm and a columnar growth isobserved (Fig. 3(c)). The resistivity has a value of2:9£ 10¡3 ˜ cm.The reason for the change in the morphology in theSnO2 : Sb system might be the same as in the ZnO : Alsystem. The thickness of the single layer is smallerthan the crystallite size in the single layer. The criti-cal thickness can be observed by a 2–4 nm single layerthickness.The morphology of coatings made from redis-persed ATO and ITO nanoparticles using processes I,II and III is identical to that shown for thick ATOsol gel coatings (Fig. 3(a) and (b)). The resultson thin coatings could not be confirmed because itwas not possible to produce thin layers with the re-dispersed particles. The electricaal properties are si-miliar as well, the resistivity for a 150 nm singlelayer is ‰D 1:8£ 10¡2 ˜ cm, which decrease to‰D 6:8£ 10¡3 ˜ cm for a 10£ 150 nm multilayer.ConclusionThe morphology and texture of ATO, ITO and AZOcoatings have been related to the coating procedure.Influence of the Layer Morphology on Sol Gel Oxide Coatings 437(a) (b)(c)Figure 2. AZO HRTEM cross-section of (a) a 25 nm thick single layer, (b) a multilayer made of 10£ 17 nm thin single layers, (c) a multilayermade of 4£ 40 nm thick single layers.Using conventional sols the morphology of ATOand AZO coatings can be tailored from an agglom-eration of spherical particles to a columnar typeof arrangement. The morphology of ATO and ITOcoatings made of redispersed nanoparticles alwaysconsists of loosely packed and not orientated crys-tallites. These structures have an important influenceon the electrical properties of the coatings. Thick438 Goebbert et al.(a) (b)(c)Figure 3. HRTEM cross section of ATO multilayers made of (a) 10 £ 20 nm thick single layers densified by process I, (b) 6 £ 20 nm thicksingle layer densified by process II, (c) 40£ 2 nm thin single layer densified by process II.single layers are always made of an agglomera-tion of spherical particles with a high void fractionand exhibit a low density and high resistivity. Invery thin single layers (<20 nm) heterogeneouslynucleated crystals predominate and their size islimited by the thickness of the layer. As the single layerthickness increases the formation of homogeneouslynucleated crystals becomes more likely.Influence of the Layer Morphology on Sol Gel Oxide Coatings 439References1. H.L. Hartnagel, A.L. Dawar, A.K. Jain, and G. Jagadish,Semiconducting Transparent Thin Films (Institute of Physics,Bristol and Philadelphia, 1995).2. M.A. Aegerter, A. Reich, G. Ganz, G. Gasparro, and J. Pu¨tz, J.Non Cryst. Solids 218, 123 (1997).3. J. Pu¨tz, D. Ganz, G. Gasparro, and M.A. Aegerter, inProc. Int. Conference Sol-Gel 97, Sheffield, UK, 31.08–05.091997.4. C. Goebbert, M.A. Aergerter, D. Burgard, R. Nass, andH. Schmidt, J. Mater. Chem. 9, 253 (1999).

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Influence of the layer morphology on the electrical properties of sol-gel transparent conducting oxide coatings

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