Eu-doped indium tin oxide thin films fabricated by sol –gel technique
Jung Kyun Kim,Yong Gyu Choi ⁎
Department of Materials Science and Engineering,Korea Aerospace University,Goyang,Gyeonggi 412-791,Republic of Korea
a b s t r a c t
a r t i c l e i n f o Available online 21March 2009Keywords:
Indium tin oxide Rare earth elements Europium
Sol –gel technique
Electrical conductivity Optical transmission
We have fabricated Eu-doped indium tin oxide thin films via the conventional sol –gel technique,and con firmed that the doped Eu atoms were chemically incorporated into the indium tin oxide lattice by substituting the In sites.Optical spectra indicated that the Eu-doped films were free of any impurities l
eading to additional vibrational effects.Valence states of Eu ions in our Eu-doped indium tin oxide films were discussed in connection with Eu concentration.
©2009Elsevier B.V.All rights reserved.
1.Introduction
The indium tin oxide (ITO;In 2O 3doped with tetravalent tin),one of the well known transparent conducting oxides,is widely used for numerous applications including display devices,solar cells and so on [1–3].This implies that the processing conditions for mass production of ITO powders and films are already well established for the commercial uses.Here,we notice that ITO may serve as a good host matrix for doping rare earth (RE)elements,when considering its excellent electrical conductivity and superior transmittance in the visible and near-infrared wavelengths.In addition,the crystalline structure of In 2O 3is basically the same to that of typical rare earth sesquioxides [4],and this would be helpful for the introduction of RE elements into the ITO lattice.A successful incorporation of RE elements is thus necessary to develop new material combinations emitting photons via electroluminescence as well as photoluminescence.Controlling of absorption or emission wavelengths would be viable for a display device by deploying ITO layer containing optimized amoun
t of a suitable RE element.In addition,tailoring the work function or band gap energy of ITO may be attempted through the incorporation of RE element into the ITO structure.Works needed to verify these potential opportunities should be preceded by investigation on more fundamental aspects of the RE-doped ITO.Recently,we fabricated Er-doped ITO thin films via the conventional sol –gel process,and investigated chemical and structural aspects of the dopant atoms in the ITO lattice [5].We have con firmed that the Er ions in their trivalent state are chemically incorporated into the ITO lattice,occupying the In 3+sites.The Er 3+ions doped in ITO thin films were optically active,emitting photoluminescence at around 1.5µm [6].
On the other hand,it is not surprising that divalent europium ions,the 4f con figuration of which is built up with 7electrons,are frequently found since half-filled and completely filled 4f electronic shells are
particularly stable [7].The presence of stable Eu 2+ions in some dielectric matrices,in addition to the usual valence of 3+,may indicate that both valence states are possibly formed also in the ITO lattice structure.When doped in solid-state dielectrics,the 4ƒ65d ↔4ƒ7inter-con figurational transitions of Eu 2+ions and the 4ƒ6↔4ƒ6intra-con figurational transi-tions of Eu 3+ions bring about radiative emissions in the blue and red wavelengths,respectively [8].Thus,it would be quite intriguing if both valence states were formed in the ITO lattice.Extending the previous works on the Er-doped ITO film
s,in this present study,we have aimed at fabrication of Eu-doped ITO thin films through the conventional sol –gel technique and characterized changes of optical and electrical properties accompanied with the introduction of Eu element.2.Experimental
Our Eu-doped ITO films were prepared according to the conven-tional sol –gel procedure [9,10].In(NO 3)3·2H 2O (KANTO,N 99.9%),SnCl 2·2H 2O (KISHIDA,N 99.9%)and EuCl 3·6H 2O (STREM,N 99%)were used as starting materials,which were dissolved in the mixture of ethanol and acetylacetone.To make more homogenous solutions with increased Eu solubility,volume ratio of acetylacetone to ethanol was varied.According to our preliminary experiments,the best electrical conductivity was obtained when the Sn/In ratio was 8at.%in undoped ITO films [5,6].As such,this ITO composition was selected as the host matrix for Eu doping.Each sol obtained via stirring and stabilization of 10ml solutions with different Eu concentrations was used for subsequent spin coating onto quartz substrates.A single coating process resulted in film thickness of ~25±5nm,and all the prepared film samples were coated three times,thus having thickness of 75±15nm.After each coating process,the film was dried at 170°C for 20min.As an annealing procedure,the film samples were all subjected to thermal treatment at 500°C for 20min.Scanning electron microscope images of our Eu-doped ITO thin films revealed that they consisted of particles with quite uniform size of ~20nm.
Thin Solid Films 517(2009)5084–5086
⁎Corresponding author.Tel.:+8223000169;fax:+82231583770.E-mail address:ygchoi@kau.ac.kr (Y.G.
Choi).0040-6090/$–see front matter ©2009Elsevier B.V.All rights reserved.doi:
10.1016/j.tsf.2009.03.093
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To identify crystalline phases in our samples,an X-ray diffractometer (XRD;SEIFERT 3000PTS)was used.Scans for the samples were carried out in the 2θrange of 20–70°with identical scanning speed of 0.05°/s.Crystalline structures were then assigned with the aid of the JCPDS cards.Four-point-probe (CMT SR1000)was used to measure the sheet resistance of samples.Optical transmittance spectra were recorded by using a Fourier-transform infrared spectrometer (FTIR;Perkin-Elmer Spectra100)and UV/VIS/NIR spectrophotometer (Shimadzu,UV3600).3.Results and discussion
In our previous works concerning the introduction of Er atoms into ITO lattice,we found that phase separation into SnO and/or Er 2O 3was sensitive especially to stirring and drying ,duration and temperature.Speci fically,stirring and drying of precursor solution at too low temperature or for insuf ficient time resulted in such a phase separation [5].Following the processing conditions established before,in the present study,we successfully fabricated Eu-doped ITO thin films consisting of only the In 2O 3phase with no additional crystalline phase such as SnO,Eu 2O 3or (Eu 0.5In 0.5)O 3(Fig.1).We thus inferred that Eu atoms were chemically incorporated in the ITO lattice,occupying the In sites.
Er L 3-edge extended X-ray absorption fine structure spectroscopic analysis carried out upon Er-doped ITO powders indicated that Er ion tended to occupy more distorted site out of two different sixfold-coordinated sites available for In in the ITO lattice [5].Based on this previous observation,we believe that Eu ions also occupy the more distorted In sites in the ITO lattice.
Sol –gel technique is particularly vulnerable to impurities such as hydroxyls and other organic residues.These impurities usually lead to additional vibrational effects,and may deteriorate the performance of Eu-doped ITO thin films.Fig.2shows FTIR spectra of the Eu-doped ITO samples.After being subtracted from the effects of quartz substrate,the spectra con firm that the Eu-containing ITO films have negligible vibrational impurities inside.Here,it is noteworthy that the presence of electrical charge ,free electrons in this case,is responsible for the decrease of transmittance with decreasing wavenumber observed especially from samples containing low Eu concentrations [11].
As mentioned above,a least sheet resistance was observed from our thin film samples with 8at.%Sn inside.This Sn concentration
is
Fig.1.X-ray diffraction patterns obtained from the Eu-doped ITO samples in which the ratio of Sn to In was fixed at 8at.%.The circular dots indicate major peak positions originating from the crystalline structure of cubic In 2O 3(JCPDS No.
06-0416).
Fig.2.FTIR spectra of Eu-doped ITO thin films obtained after the effects of the quartz substrate are ruled out.The inset shows the as-measured FTIR spectra of the samples and the substrate.Note that the absorption peak located at about 3680cm −1is due to the OH −
vibration.
Fig.3.Measured sheet resistance of the Eu-containing ITO thin films plotted as a function of Eu conce
ntration.Note that uncertainty involved in each data point is less than the size of the symbols.The inset displays the measured sheet resistance values for low Eu concentrations for a clearer
presentation.
Fig.4.Transmission spectra of the Eu-containing ITO thin films in the range from ultraviolet to near-infrared wavelengths.Note that each spectrum is shifted by 10in the y axis scale for a clear presentation.
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J.K.Kim,Y.G.Choi /Thin Solid Films 517(2009)5084–5086
within the optimal concentration range determined in the previous studies [12,13].It is known that the introduction of tetravalent Sn gives rise to generation of free electrons,thus enhancing the electrical conductivity of ITO [14].Decrease of electrical conductivity at too large amount of Sn is attributed to segregation of Sn atoms at the grain boundaries and/or reduction of Sn 4+to Sn 2+[15].On the other hand,it is also known that the electrical resistance of sol –gel processed ITO films is greater in general than those prepared by ion plating or magnetron sputtering methods [16].Fig.3shows sheet resistance of our Eu-doped ITO thin films.The sheet resistance of ITO film doped with 0.5at.%Eu was measured to be 780±150Ω/sq.,which is quite comparable to the values for sol –gel processed ITO films reported previously [16].The sheet resistance increased gradually with increasing Eu concentrat
ion up to 5at.%,but featured a rapid increase for Eu concentrations between 5and 10at.%.
Transmission spectra measured in the range from the ultraviolet to near-infrared wavelengths (Fig.4)exhibit that our Eu-doped ITO films have above 80%transparency at visible wavelengths.Also exhibited in Fig.4is the increase of transmittance in the near-infrared wavelengths with increasing Eu concentration,which is consistent with that observed in the FTIR spectra.
From a closer look at the absorption spectra,we found that there was a small but clear absorption peak in the wavelength range from 270nm to 290nm (Fig.5).Intensity of this peak tended to decrease as Eu concentration increased.We tentatively assign this peak to absorption due to the 4ƒ65d ↔4ƒ7inter-con figurational transition of Eu 2+ions [17].Since this transition is dipole-allowed in its nature,the corresponding oscillator strength is usually larger than those for the intra-4f -con figurational transitions [18].On the other hand,we could observe another peak located at ~570nm as displayed in the inset of Fig.5.The peak appeared only in ITO films doped with larger amount of ,5at.%and 10at.%.We interpret that this peak is presumably attributed to (7F 0,7F 1)→5D 0transition of trivalent Eu ions [19],thus indicating that Eu 3+ions are also present in the heavily doped samples.Many absorption transitions corresponding to the intra-4f -con figurational transitions of Eu 3+ions would be resolved in the measured wavelengths [19].However,when we take into
consideration the small thickness of our fabricated films,to a few tens nm at most,it is not surprising that the corresponding peaks are hardly observed.As such,we cannot rule out the existence of Eu ions with valence state 3+even in low doped ,0.5at.%and 1.0at.%in this case.Here,we recall that only trivalent state was observed when Er ions were doped into the ITO lattice [6].Unlike the Er case,the low reduction potential corresponding to Eu 3+→Eu 2+would be responsible in part for the existence of both valences of Eu ions in the ITO lattices [20].Presence of tetravalent Sn ions would also play a role:Some of the free electrons are supposed to be captured by the Eu 3+ions,thus letting these ions being reduced.However,it should be mentioned that this explanation needs to be con firmed based on more elaborated experiments.4.Conclusions
We have demonstrated that ITO can be doped with Eu ions through the conventional sol –gel technique.The added Eu ions are believed to substitute the In sites in the ITO lattice.The absorption spectra of the Eu-doped ITO thin films indicated that there are two valence states for Eu ions inside the ITO ,divalent and trivalent states.The low reduction potential for Eu 3+→Eu 2+would be responsible in part for this observation,and presence of tetravalent Sn ions would also play a role.Further works are needed to elucidate the related phenomena in more detail.Acknowledgement
This work was supported by the Korea Research Foundation Grant (KRF-2006-331-D00238)funded
by the Korean Government (MOEHRD).References
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Fig.  5.Absorption spectra of the Eu-containing ITO thin films in the range from ultraviolet to visible wavelengths.The absorption spectra are shown again in the inset for a clearer presentation.
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