Transparent conducting thin sol-gel SnO2:Sb multilayer coatings having the same total thickness, but different numbers of layers have been deposited on borosilicate glass by the dip-coating method using an ethanolic solution of SnCl2 (OAc)2 doped with 5mol%  SbCl3. The densification of the multilayers was done by 3 different processes. In process I each layer was dried at 200°C and the coating process repeated to obtain a tick dry coating which was then sintered as a whole at 550°C. In process II every single layer was dried at 120°C and then directly sintered at 550°C before repeating the coating process. In process III every single layer was directly sintered at 550°C without drying step. The electrical properties and the morphologies of the coatings depend highly on the sintering and drying conditions. With process I all layers are homogenous and have a high resistivity (1.4x10 -² Omega cm) whatever their thickness. The multilayers produced by process II and III show a sandwich structure with alternated dense and porous layers. The resistivity of a 10 times dipped multilayers obtained by these processes is about 3,2X^0 -³ Omega cm. Moreover the electrical properties vary with the number of layers whereas in process I no such correlation can be found. A model was developed which allows to caclulate the electrical properties of the dense and porous layers. The chemical composition of the homogeneous coating and of the multilayers with the alternated structure is also reported

Aegerter, Michel A.

Ganz, Dietmar

Gasparro, Guido

Pütz, Jörg

Universaar

Multilayer SnO2:Sb transparent electronic conducting coatings made by the sol-gel process

Acronym

MULTILAYER Sn02:Sb TRANSPARENT ELECTRONIC CONDUCTING COATINGS MADE BY THESOL-GEL PROCESSG. Gasparro, D. Ganz, J. Putz, M. A. Aegerter *lnstitut fur Neue Materialien - INMDepartment of Coating Technology[m Stadtwald, Gebaude 43D-661 23 SaarbruckenABSTRACTTransparent conducting thin sol-gel SnOp:Sb multilayer coatings having the same total thickness, butdifferent numbers of layers have been deposited on borosilicate glass by the dip-coating method using anethanolic solution of SnClz (OAC)Z doped with 5 mol% SbCl~. The densification of the multilayers was doneby 3 different processes. In process I each layer was dried at 200°C and the coating process repeated toobtain a thick dry coating which was then sintered as a whole at 550”C. In process II every single layerwas dried at 120”C and then directly sintered at 550”C before repeating the coating process. In process IIIevery single layer was directly sintered at 550°C without a drying step. The electrical properties and themorphologies of the coatings depend highly on the sintering and drying conditions. With process I alllayers are homogeneous and have a high resistivity (1.4x1 O-2S2cm)whatever their thickness. Themultilayers produced by process II and Ill show a sandwich structure with alternated dense and porouslayers. The resistivity of a 10 times dipped multilayers obtained by these processes is about 3,2x103Qcm.Moreover the electrical properties vary with the number of layers whereas in process I no such correlationcan be found. A model was developed which allows to calculate the electrical properties of the dense andporous layers. The chemical composition of the homogeneous coating and of the multilayers with thealternated structure is also reported.INTRODUCTIONThin transparent conducting oxide coatings such as tin doped iridium oxide (ITO), fluor or antimonydoped tin dioxide (FTO, ATO) are of broad interest because of their high optical transmission, highconductivity and numerous of application fields [1]. They are used as electrodes in solar cells [2], LCD’s,optical electrical devices [3], catalysts or as heat shields [4]. Commercial coatings are essentially made byphysical or chemical deposition techniques like sputtering [5], spray pyrolysis [2] or chemical vapourdeposition (CVD) [6]. The vacuum techniques give coatings with good electrical and optical quality butbear the disadvantage of high production costs. The spray techniques are cheaper and give also highlyconductive coatings but with lower homogeneity.The sol-gel technique [7-1 5] allows to produce layers with high transmission in the visible part buttheir resistivity was always found at least one order of magnitude higher than those obtained for coatingsprepared by the other methods. Therefore numerous investigations are underway to understand thereasons of this high resistivity and to improve the electrical properties of sol-gel coatings. The morphologyof the sol-gel layer is one of the main factor influencing the conductivity of the layer. They are porous andconsist of small crystalline particles whereas spray coated films show a densely packed columnarstructure [14]. Recent studies indicated that it is possible to improve the structure of sol-gel coatings byincreasing the sintering temperature and the heating rate of the sintering process [15] using for exampleC02 laser densification [16, 17].The resistivity of the coatings depends also on the layer thickness. Itsharply decreases with increasing total thickness up to a value of 200-300 nm. Therefore one way toimprove the sheet resistance is to increase the total layer thickness by repeating the coating cycles [18].In this paper we investigate the influence of the sintering conditions on the electrical and structuralproperties and on the chemical composition of Sn02:Sb multilayers made by the sol-gel process.EXPERIMENTALMultilayer sol-gel coatings have been deposited on borosilicate glass by dip coating using a 0.2 molarethanolic solution of SnClp(OAc)z containing 5 mol% SbCl~ in a controlled atmosphere (T=20”C,RH= 40 %). The densification of the coatings was performed by three different processes (Fig. 1) whereeach coating consisted of a different number of layers having the same total thickness about 250 nm indry state. The thickness of the individual layers was adjusted by changing the withdrawal speed. [t isworthwhile to note that for process I a drying stage at a temperature lower than 200”C (e.g. 120”C) leadsto cracked layers. The final heat treatment was always realised by introducing the layers directly into apreheated furnace at 550”C.?sol:0.2molar ethanolic solution of SnC12(CH3COO)2doped with 5 mol % SbC13I IproJess 1 proc~ss II proc~ss Ill1f \ --J- f--J---Fig. 1: Flow diagram of the various proceduresThe layer thickness was determined by a surface profiler (Tencor PI O). The electrical properties ofthe coatings were measured by the 4 probe method and the Van der Pauw/Hall technique (MMRTechnology with a 1.5 T magnetic field). The texture has been observed by high resolution transmissionmicroscopy and the crystallite size was determined from XRD spectra (Siemens D500). The chemicalcomposition was investigated by Secondary Neutral Mass Spectrometry (SNMS) using a Leybold INA 3-equipment connected to a quadropole-mass spectrometer. The SNMS measurement were performed witha different batch of samples than the electrical measurements.RESULTS AND DISCUSSIONThe drying and sintering temperature influence the thickness of the multilayer. For example amultilayer coating prepared with 10 coating cycles with the same withdrawal speed and sintered accordingto process II or Ill has a total thickness of 180 nm while the same coating prepared according to process Ihas a total thickness of 232 nm. The total thickness for all multilayer coatings made according to process Iwas always found about 25 ?40 higher than the thickness of those obtained by the other processes, whichhardly differ from each other. This can be explained by differences in the layer morphology as shown bythe HRTEM cross sections. (Fig. 2 and Fig. 3). The multilayer coating made by process I is porous buthomogeneous and built with small (6-8 nm) almost spherical crystallite that are loosely packed (Fig. 2).On the contrary, the morphology of the multilayers obtained according to process Ill reveals that eachindividual layer consists of a two layer structure: a thin 10 nm relatively dense interface lying on top of amore porous bulk layer (Fig. 3). The crystallite’s size is about 6 nm. Because the multilayers made byprocesses II and Ill are equally thick it is assumed that the structure of the coating prepared by process IIis also built of consecutive porous and dense layers. The morphologies of the multilayers is similar to thatfound for single layers sintered and heated in the same conditions [10].Fig. 2: HRTEM cross section of a 10 layers Fig, 3: HRTEM cross section of a 6 layerscoating prepared according to process I coating prepared according to process IIIThese different morphologies have a dramatic effect on the electrical parameters of the coatings. Formultilayer coatings obtained by process I the resistivity is practically independent of the number of layersi.e. the resistivity is the same 1.4x10-2 Qcm for a coating made of 4 x 58 nm or a 10 x 23.2 nm thick layers(Fig. 5). The electron mobility p = 1,7 cm2A/s and the carrier density (electron) N = 2,5x1020 cm3 are alsopractically constant for each coating. A different behaviour has been found for coatings made by processII and Ill. In both cases the electrical properties are strongly dependent on the number of layers (Fig. 5).An increasing number of layers leads to a decrease of the resistivity from p = 3,6x 103 f2cm for a4 x 45 nm thick coating to p = 1.8x 10’3 Qcm for a 10x 18 nm thick coating. The electron mobilityincreases from p = 2,93 cm2/Vs for the 4 layers coating to p = 4,5 cm2/Vs for a 10 layers coating. Howeverthe carrier density remains constant N = (4x 1020ems). A higher conductivity is therefore observed whenthe number of the dense interfaces increases. The thin layers should have consequently a lower resistivitythan the internal part of each layers.The resistivity p. of the dense interfaces and the resistivity pi (pi >p.) of the porous bulk can bedetermined by assuming that the sequence of the single layers is connected electrically in parallel (Fig. 4).In this case the macroscopic sheet resistance is given by the sum of the single layer sheet resistance. Fora given multilayer built with n layers with alternating porous and dense regions, the macroscopic resistivityis given by:1()Idcll———pn-fl+T ~-; “n(1)A plot of the measured resistivity of the multilayers made by the three processes versus the numberof layers is shown in Fig. 5. For each process a straight line is obtained with almost equal intercept -50(Qcm)”’. For process II and Ill the slope has nearly the same value of 22 (f2cm)-i, while for process I thereciprocal resistivity is practically constant.Fig. 4:dScheme of a 3 layer system Fig. 5:q W==lA prccEsslll prcce5sIllwith a total thickness dobtained by process II and Ill.The denser surface layer ofthickness d. is shaded.0! I 1 I I I0 2 4 6 8 10 1numberof IayemReciprocal of the measured resistivity pn.l vs. thenumber of layers deposited. The straight lines are afit accordingly to equation 1, The total thickness ofeach sintered coatings is 232 nm (Process 1) and180 nm (Process II and Ill).The resistivity p, of the dense interface of each layer was calculated assuming that the thickness ofthe dense interfaces measured from the HRTEM pictures was 10 nm (Table 1).Table 1: Values of pi and p, calculated from (1) and Fig, .5for coatings made according to process 1,11,Ill.Process I II Illpi (f2cm) 0,014 0,014 0,014p, (Qcm) - 0,0018 0,0022The chemical composition of the coatings (C, O, Si, Sn, Sb) and the ratio Sn:Sb was examined by SNMSfor two 4 times dipped multilayers made by process I and process Ill respectively (Fig. 6 and Fig. 7). Inboth samples no carbon was detected and the surface shows an excess of O probably due to atmosphericwater and OH-group absorbed after the obtention of the coatings.For the multilayer coating produced by process I the depth profile of the ratio Sn:Sb is practicallyconstant and corresponds to the composition of the sol. The Sn concentration slightly increasescontinuously toward the substrate while the O concentration has an opposite behaviour. Near the2substrate the ratio Sn:O corresponds practically to the SnOz composition. In this process the 4 layerswhere first dried at 200°C and at this stage considerable amount of organic materials are still included inthe coating. During the sintering process at 550”C the organic material lying at the air/coating interface willreact more strongly with the oxygen from air than the organic material lying deeper in the layer, which willmostly react with oxygen inside the coating.When each layer is sintered at 550° C without a predrying step (process Ill) the shape of theconcentration profile of Sn, O, Sb is totally different (Fig. ‘7). The oxygen concentration does not decreasein the direction to the substrate and the 4 sequential structures of the coating are clearly observed by thealternating maxima and minima for O, Sn and the ratio Sb:Sn. As the first minimum of this ratio lies at thelayer’s surface it is assumed that their minima correspond to the dense part of the multilayer and theirmaxima to the porous part of the layer. At the minima, the Sn:Sb ratio agrees with the value of the solcomposition. The Sb and Sn concentration proceeds according to the Sn:Sb curve while the Oconcentration has an opposite behaviour. The composition of the dense and porous part is thereforedifferent. As the amount of ’Sb is small compared to the amount of Sn the difference in the composition ofthe dense vs. bulk part of the layers is essentially related to change in Sn . In the dense interface the ratioSn:O corresponds almost perfectly to the SnOz composition whereas in the porous structures the ratioSn:O corresponds to a composition SnOz.X,with x> 0.5, In the porous part of the layer the Sb concentrationis higher than in the dense part. More Sb seems necessary to keep the electrical neutrality of the layer,due probably to the higher surface of the particles (lower density).o lm m m m mSpLtk!n%irm [s].$0,7-—.— 0—* SI—- %*SMO~o m m ?03 al m msputteringtime [s]Fig. 6: SNMS of a 4 layers coating 187 nm thick Fig. 7: SNMS of a 4 layers coating 230 nm thickprepared by process I prepared by process IllCONCLUSIONThe influence of drying and sintering conditions on the electrical and morphological properties andthe chemical composition profiles of sol gel Sn02:Sb multilayers has been reported. Multilayers coatingmade of layers of same thickness, individually sintered in air at 550”C are composed of alternated layersconsisting of a dense external layer on top of a porous bulk structure. A model was proposed to calculatethe resistivity of the dense layer p, = 2,2x 10-3Qcm and the porous layer pi = 1,4x 10-2Qcm. Thechemical composition of both layers is different. In the dense part the composition is nearly SnOz with aratio Sb:Sn = 5 O/O. In the porous layer the composition corresponds to Sn02_X, with x >0,5 and a ratioSb:Sn = 7 Yo. For coating made with single layers all dried at 200”C and where the whole coating was thensintered at 550”C, the resistivity is independent of the number of layers and the final total thickness. Thevalue is pz = 1.4 x 10’2 Qcm. The morphology of the coating is homogeneous as well as their chemicalcomposition. it corresponds to Sn02 with a ratio Sb:Sn = 5 Yo.ACKNOWLEDGEMENTSWe are grateful to the Institut fur Oberfl~chen and Schichtanalytik GmbH in Kaiserslautern, Germany,where the SNMS measurements were performed and to Dr. T. Krajewski for the TEM analysis, Thefinancial support of the Bundesministerium fur Bildung und Forschung (BMBF, Project 2A 67/03 N0940)and the State of Saarland is gratefully acknowledged.REFERENCES[1] K.L. Chopra, S. Major and D,K, Pandya, “Transparent conductors - a status review”, Thin So/idFi/ms, 102 1-46(1983).[2] J. Dutta, P. Roubeau, T. Emeraud, J.-M. Laurent, A. Smith, F. Leblanc and J. Perrin, “Application ofpyrosol deposition process for large-area deposition of fluorine-doped tin dioxide thin films”, ThinSo/id Fi/ms, 239 150-5(1994).[3] C.G. Grandquist, “Transparent conductive electrodes for electrocromic devices: A review”, ,4pp/ied.Physics /A, 57 19-24(1993).[4] C.G. Grandquist, “Window coatings for the future”, Thin So/id Fi/ms, 193/1 94730-41 (1990).[5] L. Meng and M.P. dos Santos, “The effect of substrate temperature on the properties of sputteredtin oxide films”, Thin So/id Fi/ms, 237 112-7(1994).[6] F.J. Yusta, M.L. Hitchman and S.H. Shamlian, “CVD preparation and characterization of tin dioxidefilms for electrochemical applications”, Jouma/ of Materia/s Chemistry, 71421-7 (1997).[7] C.J.R. Gonzalez-Oliver and 1.Kate, “Tin(antimony)-oxide sol-gel coatings on glass”, Journa/ of Non-Crysta//ine So/ids, 82 400-1O(1986).[8] Y.J. Lin and C.J. Wu, “The Properties of antimony-doped tin oxide thin-films from the sol-gelprocess”, Surface & Coatings Technology, 88 239-47(1997).[9] S.S. Park and J.D. Mackenzie, “Sol-gel-derived tin oxide thin-films”, Thin So/id Fi/ms, 258268-73(1995).[1O] T.M. Racheva and G.W. Critchlow, “SnO, thin films prepared by the sol- gel process”, Thin So/idFi/ms, 292299-302 (1997).[11 ] T.D. Senguttuvan and L.K. Malhotra, “Sol-gel deposition of pure and antimony doped tin dioxidethin-films by non alkoxide precursors”, Thin So/id Films, 289 22-8(1996).[12] Y. Takahashi and Y. Wada, “Dip-coating of Sb-doped SnOz films by ethanolamine-alkoxidemethod”, Journa/ of the Hectrochemica/ Society, 137267-72 (1990).[13] C. Terrier, J.P. Chatelon and J.A. Roger, “Electrical and optical-properties of Sb-SnOz thin-filmsobtained by the sol-gel method’, Thin So/id Films, 29595-100 (1997).[14] M.A, Aegerter, A. Reich, G. Ganz, G. Gasparro and J. Putz, “Comparative study of SnO,:Sbtransparent conducting films produced by various coating and heat treatment techniques”, Jouma/of Non-Crystalline Solids, 218123-9 (1997).[15] J. Putz, D. Ganz, G. Gasparro and M.A. Aegerter, “Influence of the heating rate on themicrostructure and on macroscopic properties of sol-gel SnOz:Sb coatings”, Journa/ of So/-Ge/Science and Technology, accepted.[16] G. Ganz, A, Reich and M.A. Aegerter, “Laser firing of transparent conducting SnOz sol-gelcoatings”, Journal of Non-Crystalline Solids, 218242-7 (1997).[17] D. Ganz and M.A. Aegerter, “Laser sintering of SnOz:Sb sol-gel coatings”, Journa/ of So/-Ge/Science and Technology, accepted.[18] S.S. Park and J.D. Mackenzie, “Microstructure effects in multidipped tin oxide-films”, Journa/ of theAmerican Ceramic Society, 782669-72 (1995).

Scientific documents from the Saarland University,

MULTILAYER Sn02:Sb TRANSPARENT ELECTRONIC CONDUCTING COATINGS MADE BY THE
SOL-GEL PROCESS
G. Gasparro, D. Ganz, J. Putz, M. A. Aegerter *
lnstitut fur Neue Materialien - INM
Department of Coating Technology
[m Stadtwald, Gebaude 43
D-661 23 Saarbrucken
ABSTRACT
Transparent conducting thin sol-gel SnOp:Sb multilayer coatings having the same total thickness, but
different numbers of layers have been deposited on borosilicate glass by the dip-coating method using an
ethanolic solution of SnClz (OAC)Z doped with 5 mol% SbCl~. The densification of the multilayers was done
by 3 different processes. In process I each layer was dried at 200°C and the coating process repeated to
obtain a thick dry coating which was then sintered as a whole at 550”C. In process II every single layer
was dried at 120”C and then directly sintered at 550”C before repeating the coating process. In process III
every single layer was directly sintered at 550°C without a drying step. The electrical properties and the
morphologies of the coatings depend highly on the sintering and drying conditions. With process I all
layers are homogeneous and have a high resistivity (1.4x1 O-2S2cm)whatever their thickness. The
multilayers produced by process II and Ill show a sandwich structure with alternated dense and porous
layers. The resistivity of a 10 times dipped multilayers obtained by these processes is about 3,2x103Qcm.
Moreover the electrical properties vary with the number of layers whereas in process I no such correlation
can be found. A model was developed which allows to calculate the electrical properties of the dense and
porous layers. The chemical composition of the homogeneous coating and of the multilayers with the
alternated structure is also reported.
INTRODUCTION
Thin transparent conducting oxide coatings such as tin doped iridium oxide (ITO), fluor or antimony
doped tin dioxide (FTO, ATO) are of broad interest because of their high optical transmission, high
conductivity and numerous of application fields [1]. They are used as electrodes in solar cells [2], LCD’s,
optical electrical devices [3], catalysts or as heat shields [4]. Commercial coatings are essentially made by
physical or chemical deposition techniques like sputtering [5], spray pyrolysis [2] or chemical vapour
deposition (CVD) [6]. The vacuum techniques give coatings with good electrical and optical quality but
bear the disadvantage of high production costs. The spray techniques are cheaper and give also highly
conductive coatings but with lower homogeneity.
The sol-gel technique [7-1 5] allows to produce layers with high transmission in the visible part but
their resistivity was always found at least one order of magnitude higher than those obtained for coatings
prepared by the other methods. Therefore numerous investigations are underway to understand the
reasons of this high resistivity and to improve the electrical properties of sol-gel coatings. The morphology
of the sol-gel layer is one of the main factor influencing the conductivity of the layer. They are porous and
consist of small crystalline particles whereas spray coated films show a densely packed columnar
structure [14]. Recent studies indicated that it is possible to improve the structure of sol-gel coatings by
increasing the sintering temperature and the heating rate of the sintering process [15] using for example
C02 laser densification [16, 17].The resistivity of the coatings depends also on the layer thickness. It
sharply decreases with increasing total thickness up to a value of 200-300 nm. Therefore one way to
improve the sheet resistance is to increase the total layer thickness by repeating the coating cycles [18].
In this paper we investigate the influence of the sintering conditions on the electrical and structural
properties and on the chemical composition of Sn02:Sb multilayers made by the sol-gel process.
EXPERIMENTAL
Multilayer sol-gel coatings have been deposited on borosilicate glass by dip coating using a 0.2 molar
ethanolic solution of SnClp(OAc)z containing 5 mol% SbCl~ in a controlled atmosphere (T=20”C,
RH= 40 %). The densification of the coatings was performed by three different processes (Fig. 1) where
each coating consisted of a different number of layers having the same total thickness about 250 nm in
dry state. The thickness of the individual layers was adjusted by changing the withdrawal speed. [t is
worthwhile to note that for process I a drying stage at a temperature lower than 200”C (e.g. 120”C) leads
to cracked layers. The final heat treatment was always realised by introducing the layers directly into a
preheated furnace at 550”C.
?
sol:
0.2molar ethanolic solution of SnC12(CH3COO)2
doped with 5 mol % SbC13
I I
proJess 1 proc~ss II proc~ss Ill
1f \ --J- f--J---
Fig. 1: Flow diagram of the various procedures
The layer thickness was determined by a surface profiler (Tencor PI O). The electrical properties of
the coatings were measured by the 4 probe method and the Van der Pauw/Hall technique (MMR
Technology with a 1.5 T magnetic field). The texture has been observed by high resolution transmission
microscopy and the crystallite size was determined from XRD spectra (Siemens D500). The chemical
composition was investigated by Secondary Neutral Mass Spectrometry (SNMS) using a Leybold INA 3-
equipment connected to a quadropole-mass spectrometer. The SNMS measurement were performed with
a different batch of samples than the electrical measurements.
RESULTS AND DISCUSSION
The drying and sintering temperature influence the thickness of the multilayer. For example a
multilayer coating prepared with 10 coating cycles with the same withdrawal speed and sintered according
to process II or Ill has a total thickness of 180 nm while the same coating prepared according to process I
has a total thickness of 232 nm. The total thickness for all multilayer coatings made according to process I
was always found about 25 ?40 higher than the thickness of those obtained by the other processes, which
hardly differ from each other. This can be explained by differences in the layer morphology as shown by
the HRTEM cross sections. (Fig. 2 and Fig. 3). The multilayer coating made by process I is porous but
homogeneous and built with small (6-8 nm) almost spherical crystallite that are loosely packed (Fig. 2).
On the contrary, the morphology of the multilayers obtained according to process Ill reveals that each
individual layer consists of a two layer structure: a thin 10 nm relatively dense interface lying on top of a
more porous bulk layer (Fig. 3). The crystallite’s size is about 6 nm. Because the multilayers made by
processes II and Ill are equally thick it is assumed that the structure of the coating prepared by process II
is also built of consecutive porous and dense layers. The morphologies of the multilayers is similar to that
found for single layers sintered and heated in the same conditions [10].
Fig. 2: HRTEM cross section of a 10 layers Fig, 3: HRTEM cross section of a 6 layers
coating prepared according to process I coating prepared according to process III
These different morphologies have a dramatic effect on the electrical parameters of the coatings. For
multilayer coatings obtained by process I the resistivity is practically independent of the number of layers
i.e. the resistivity is the same 1.4x10-2 Qcm for a coating made of 4 x 58 nm or a 10 x 23.2 nm thick layers
(Fig. 5). The electron mobility p = 1,7 cm2A/s and the carrier density (electron) N = 2,5x1020 cm3 are also
practically constant for each coating. A different behaviour has been found for coatings made by process
II and Ill. In both cases the electrical properties are strongly dependent on the number of layers (Fig. 5).
An increasing number of layers leads to a decrease of the resistivity from p = 3,6x 103 f2cm for a
4 x 45 nm thick coating to p = 1.8x 10’3 Qcm for a 10x 18 nm thick coating. The electron mobility
increases from p = 2,93 cm2/Vs for the 4 layers coating to p = 4,5 cm2/Vs for a 10 layers coating. However
the carrier density remains constant N = (4x 1020ems). A higher conductivity is therefore observed when
the number of the dense interfaces increases. The thin layers should have consequently a lower resistivity
than the internal part of each layers.
The resistivity p. of the dense interfaces and the resistivity pi (pi >p.) of the porous bulk can be
determined by assuming that the sequence of the single layers is connected electrically in parallel (Fig. 4).
In this case the macroscopic sheet resistance is given by the sum of the single layer sheet resistance. For
a given multilayer built with n layers with alternating porous and dense regions, the macroscopic resistivity
is given by:
1()
Idcll———
pn-fl+T ~-; “n
(1)
A plot of the measured resistivity of the multilayers made by the three processes versus the number
of layers is shown in Fig. 5. For each process a straight line is obtained with almost equal intercept -50
(Qcm)”’. For process II and Ill the slope has nearly the same value of 22 (f2cm)-i, while for process I the
reciprocal resistivity is practically constant.
Fig. 4:
d
Scheme of a 3 layer system Fig. 5:
q W==l
A prccEssll
l prcce5sIll
with a total thickness d
obtained by process II and Ill.
The denser surface layer of
thickness d. is shaded.
0! I 1 I I I
0 2 4 6 8 10 1
numberof Iayem
Reciprocal of the measured resistivity pn.l vs. the
number of layers deposited. The straight lines are a
fit accordingly to equation 1, The total thickness of
each sintered coatings is 232 nm (Process 1) and
180 nm (Process II and Ill).
The resistivity p, of the dense interface of each layer was calculated assuming that the thickness of
the dense interfaces measured from the HRTEM pictures was 10 nm (Table 1).
Table 1: Values of pi and p, calculated from (1) and Fig, .5for coatings made according to process 1,11,Ill.
Process I II Ill
pi (f2cm) 0,014 0,014 0,014
p, (Qcm) - 0,0018 0,0022
The chemical composition of the coatings (C, O, Si, Sn, Sb) and the ratio Sn:Sb was examined by SNMS
for two 4 times dipped multilayers made by process I and process Ill respectively (Fig. 6 and Fig. 7). In
both samples no carbon was detected and the surface shows an excess of O probably due to atmospheric
water and OH-group absorbed after the obtention of the coatings.
For the multilayer coating produced by process I the depth profile of the ratio Sn:Sb is practically
constant and corresponds to the composition of the sol. The Sn concentration slightly increases
continuously toward the substrate while the O concentration has an opposite behaviour. Near the
2
substrate the ratio Sn:O corresponds practically to the SnOz composition. In this process the 4 layers
where first dried at 200°C and at this stage considerable amount of organic materials are still included in
the coating. During the sintering process at 550”C the organic material lying at the air/coating interface will
react more strongly with the oxygen from air than the organic material lying deeper in the layer, which will
mostly react with oxygen inside the coating.
When each layer is sintered at 550° C without a predrying step (process Ill) the shape of the
concentration profile of Sn, O, Sb is totally different (Fig. ‘7). The oxygen concentration does not decrease
in the direction to the substrate and the 4 sequential structures of the coating are clearly observed by the
alternating maxima and minima for O, Sn and the ratio Sb:Sn. As the first minimum of this ratio lies at the
layer’s surface it is assumed that their minima correspond to the dense part of the multilayer and their
maxima to the porous part of the layer. At the minima, the Sn:Sb ratio agrees with the value of the sol
composition. The Sb and Sn concentration proceeds according to the Sn:Sb curve while the O
concentration has an opposite behaviour. The composition of the dense and porous part is therefore
different. As the amount of ’Sb is small compared to the amount of Sn the difference in the composition of
the dense vs. bulk part of the layers is essentially related to change in Sn . In the dense interface the ratio
Sn:O corresponds almost perfectly to the SnOz composition whereas in the porous structures the ratio
Sn:O corresponds to a composition SnOz.X,with x> 0.5, In the porous part of the layer the Sb concentration
is higher than in the dense part. More Sb seems necessary to keep the electrical neutrality of the layer,
due probably to the higher surface of the particles (lower density).
o lm m m m m
SpLtk!n%irm [s]
.$
0,7-
—.— 0
—* SI
—- %
*SMO
~
o m m ?03 al m m
sputteringtime [s]
Fig. 6: SNMS of a 4 layers coating 187 nm thick Fig. 7: SNMS of a 4 layers coating 230 nm thick
prepared by process I prepared by process Ill
CONCLUSION
The influence of drying and sintering conditions on the electrical and morphological properties and
the chemical composition profiles of sol gel Sn02:Sb multilayers has been reported. Multilayers coating
made of layers of same thickness, individually sintered in air at 550”C are composed of alternated layers
consisting of a dense external layer on top of a porous bulk structure. A model was proposed to calculate
the resistivity of the dense layer p, = 2,2x 10-3Qcm and the porous layer pi = 1,4x 10-2Qcm. The
chemical composition of both layers is different. In the dense part the composition is nearly SnOz with a
ratio Sb:Sn = 5 O/O. In the porous layer the composition corresponds to Sn02_X, with x >0,5 and a ratio
Sb:Sn = 7 Yo. For coating made with single layers all dried at 200”C and where the whole coating was then
sintered at 550”C, the resistivity is independent of the number of layers and the final total thickness. The
value is pz = 1.4 x 10’2 Qcm. The morphology of the coating is homogeneous as well as their chemical
composition. it corresponds to Sn02 with a ratio Sb:Sn = 5 Yo.
ACKNOWLEDGEMENTS
We are grateful to the Institut fur Oberfl~chen and Schichtanalytik GmbH in Kaiserslautern, Germany,
where the SNMS measurements were performed and to Dr. T. Krajewski for the TEM analysis, The
financial support of the Bundesministerium fur Bildung und Forschung (BMBF, Project 2A 67/03 N0940)
and the State of Saarland is gratefully acknowledged.
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Multilayer SnO2:Sb transparent electronic conducting coatings made by the sol-gel process

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