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Thin Solid Films 516 (2

Pulsed laser deposited Nb doped TiO2 as a transparent conducting oxide

Matthew S. Dabney a,⁎, Maikel F.A.M. van Hest a, Charles W. Teplin a,S. Phil Arenkiel b, John D. Perkins a, David S. Ginley a

a National Renewable Energy Laboratory, Golden CO, USAb South Dakota School of Mines and Technology, Rapid City SD, USA

Received 17 May 2007; received in revised form 12 September 2007; accepted 15 October 2007Available online 26 October 2007

Abstract

Nb doped TiO2 (Nb:TiO2) is a promising indium-free transparent conducting oxide. We have examined the growth of Nb:TiO2 thin films bypulsed laser deposition (PLD) on SrTiO3, LaAlO3, and fused silica. For b004N oriented anatase Nb:TiO2 films grown on SrTiO3 by PLD at550 °C, the conductivity can be as high as 2500 S/cm. A nearly thickness independent conductivity for Nb:TiO2 demonstrates that theconductivity is a bulk property and not a substrate interface effect. In addition, Nb:TiO2 films deposited at room temperature were annealed attemperatures up to 750 °C in either vacuum or 1.3×10−3 Pa O2. For these films, conductivities as high as 3300 S/cm on SrTiO3 and 85 S/cm onLaAlO3 substrates were obtained for the highest temperature vacuum anneals, albeit with some loss in transparency.Published by Elsevier B.V.

Keywords: Transparent conducting oxides; Anatase; Titanium dioxide; Pulsed laser deposition; Epitaxial

1. Introduction

Transparent conducting oxides (TCOs) are key componentsin many optoelectric devices including flat panel displays, smartdisplays, photovoltaics, and a variety of hand held devices [1].Aluminum doped ZnO, flourine doped SnO2, and tin dopedIn2O3 (ITO) have been the most common TCO materials forover 20 years, but the increasing diversity in technologicalapplications has resulted in a need for improved or specialtyTCOs [1]. On top of this, the rising price of indium, now nearly$1000/kg, is driving interest in indium-free TCO materials.Collectively, these factors have combined to stimulate arenewed interest in the materials science of TCOs with adrive towards both higher-performance and lower-cost materi-als with improved ancillary properties as well. Recently,substitutionally doped anatase TiO2 has been demonstrated asa potential TCO material [2]. Anatase TiO2 has more favorableelectronic properties for use as an n-type TCO than the rutileTiO2 phase due to a substantially lower conduction bandeffective mass, me

*∼1me for anatase [3,4] compared withme*∼8–20me for rutile [5–7]. Furthermore, in anatase TiO2, Nb

⁎ Corresponding author. Tel.: +1 303 384 6866.E-mail address: matthew_dabney@nrel.gov (M.S. Dabney).

0040-6090/$ - see front matter. Published by Elsevier B.V.doi:10.1016/j.tsf.2007.10.093

in particular is expected to be a shallow donor [3,8] or evenresonant level within conduction band [9] and hence promisingas an effective dopant to create a high density of free carriers.TiO2 based TCOs are also of fundamental interest because inTiO2 the conduction band minimum is formed largely fromempty Ti d-states [4,9] whereas in conventional TCO materialsthe metal atom d-states are full and the conduction band isformed from metal atom s-states [10].

Here, we present and discuss the electrical, structural andoptical properties as a function of thickness and depositiontemperature of a series of Nb doped TiO2 (Nb:TiO2) films thatwere grown by pulsed laser deposition (PLD) on fused silica,SrTiO3 (STO), and LaAlO3 (LAO) substrates. Cubic STO(a=3.905 Å) is reasonably well latticed matched to tetragonalanatase TiO2 (a=3.785 Å, c=9.514 Å). Rhombohedral LAO,which is nearly cubic with an effective cubic lattice constanta=3.791 Å, is also well matched to anatase TiO2 and, inaddition, avoids the possible diffusion of Nb from the film intothe substrate, which has been suggested as an explanation forthe measured conductivity of Nb:TiO2 films on SrTiO3 [11,12].In this work, highly-textured (004)-oriented anatase Nb:TiO2

films grown by PLD on STO substrates had conductivities of upto 2500 S/cm. For comparison, conductivities of 13,000 S/cmhave been reported for epitaxial ITO on single crystal yttria-

Fig. 1. Conductivity of the Nb:TiO2 versus film thickness for films grown onSTO, LAO, and fused silica.

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stabilized zirconia [13], and 6200 S/cm for commerciallyavailable ITO on glass (Colorado Concept Coatings). In ad-dition, these conducting Nb:TiO2 films on STO were quitetransparent in the visible, ∼90% relative to the substrate.Hence, these basic results confirm the initial report ofconcurrent transparency and conductivity in anatase Nb:TiO2reported by Furubayashi et al. [14]. Furthermore, for Nb:TiO2

films on STO, the conductivity was found to be independent ofthe thickness for films 11 to 220 nm thick which demonstratesthat the observed conductivity, σ≈2500 S/cm, of the Nb:TiO2

films is an intrinsic property of Nb:TiO2, not an interfacial orsurface effect. Finally, to investigate the viability of lowertemperature deposition which would be less expensive inpractice, the Nb:TiO2 films deposited at ambient temperaturewere subsequently annealed at temperatures up to 750 °C. Ofthese, the films annealed in vacuum exhibited conductivitiessimilar to those for high temperature PLD films (3300 S/cm)albeit with some degradation of their optical transparency.

2. Experimental details

Ti0.95Nb0.05Ox thin films were grown by PLD with a KrFLambda Physics Compex 201 laser (248 nm, 2 Hz pulses).Depositions were performed on 10 mm×10 mm×0.5 mmdouble-sided polished single crystal STO (100), double-sidedpolished LAO (100), and fused silica substrates. The laseroutput was maintained at 320 mJ per pulse, but due to lossesfrom optics, focused to a fluence of 1 J/cm2 in a 3 mm×2 mmspot at the target surface for all depositions. The incident angleof the beam to the rotating target surface was fixed at 45°. Acommercial hot-pressed 1-inch Ti0.95Nb0.05Ox target fromPlasmaterials, Inc. was used for these experiments. The sub-strate to target distance was set to 6 cm along the normal fromthe target. Ultra pure oxygen (grade 5.0) was introduced into thevacuum chamber with a mass flow controller to maintain thedesired partial pressure (1.3×10−3 Pa). The substrates weresilver pasted onto a Neocera resistive heater parallel to thetarget. They were heated to the desired growth temperature(Ts=550 °C) and held at growth temperature for 10 min bothbefore and after the deposition. After the total heat treatmentprocess, the substrates were cooled down over the course of 1 hin the deposition atmosphere. The films with the highestconductivity (σ=2500 S/cm) were grown at a 6 cm substrate totarget distance, Ts=550 °C, and in 1.3×10−3 Pa O2. Unlessotherwise noted, these are the parameters at which the filmsdiscussed were grown. For further details of the growthchamber see Warmsingh et al. [15].

Film conductivities were measured with a standard 4-pointprobe and the carrier concentration was determined using a Bio-Rad Hall probe system. The crystal structure was characterizedwith a Bruker AXS D8 Discovery X-Ray Diffractometer (XRD)with a large area 2D detector. Cross-sectional TransmissionElectron Microscope (TEM) images and diffraction patternswere acquired on a Philips CM200, operated at 200 kV. Sampleswere prepared for the TEM analysis by mechanical polishingand ion milling. Optical transmission and reflection spectrawere measured with a CCD-based spectrometer and variable

angle ellipsometry data was acquired using a J. A. WoollamWVASE system. Surface profiles were measured with a WYKO1100 optical profilometer.

Post deposition anneals were done in the growth chamber onall three substrate materials. One series of anneals were run invacuum (1.3×10−4 Pa) and the other series in 1.3×10−3 Pa ofO2. The samples for these annealing experiments were grown atroom temperature (Ts =23 °C) in 1.3×10− 3 Pa O2 with5000 pulses to give a film thickness of 55 nm. The sampleswere removed from the chamber and analyzed by XRD forcrystallinity and with a 4-pt probe for conductivity. The first heattreatment was done at 300 °C for 15 h in vacuum. The samplewas then removed and characterized by XRD and 4-point probe.For subsequent anneals, the same sample was silver pasted backonto the substrate heater and cured at 220 °C in ambientatmosphere before placing it back into the chamber, pumpingdown to the base pressure of 1.3×10−4 Pa (and raised to1.3×10−3 Pa of O2 with the ultra pure O2 for the O2 annealingseries), and heating to the next higher annealing temperaturefor another 15 h soak. In summary, for each annealing series,only one sample was heated at successively higher temperaturesand soaked for the 15 h at each temperature for the entire range(300–750 °C).

3. Results and discussion

The major properties of interest for most TCO applicationsare the electrical conductivity and visible transmission proper-ties. Fig. 1 shows the conductivity versus thickness of the Nb:TiO2 films on STO, LAO and fused silica substrates. The datashows that for the Nb:TiO2 films grown on STO, conductivitiesof ∼2500 S/cm were achieved for film thicknesses from 11 nmto 220 nm. A literature value of conductivity (3000 S/cm) ofa 40 nm Nb:TiO2 film is shown as well for comparison [14].Note that the conductivity of the films grown on STO(σ∼2500 S/cm) does not change significantly with filmthickness. This indicates that the electrical conductivity isintrinsic to the Nb doped TiO2, and supports the assertion theconduction is not due to interfacial diffusion doping at thesubstrate-STO interface [11]. The fused silica data shows the

Fig. 3. 2θ plot of the X-ray diffraction of a Nb:TiO2 film grown on STO, LAO,and fused silica. The patterns for films on STO and LAO are offset vertically forclarity. “S” denotes the inherent substrate peaks for the STO and the LAO. Theanatase and rutile crystal peaks for TiO2 are shown in the bottom two panelsusing a linear intensity scale.

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same trend with thickness but at a much lower conductivity(σ∼1 S/cm). The measured conductivities of the films on LAOdemonstrate a large spread, 0.6 to 2000 S/cm measured over amillimeter length scale on the 55 nm thick film, for example. Webelieve that this variance in the measured conductivity valuesfor the films grown on LAO, arises from the local interfacialdefects, which occur due to substantial variations in the polishedLAO surface, which we will later discuss in detail.

In Fig. 2a, we show the optical reflection (thin solid line) andtransmission (thick solid line) of a 220 nm thick Nb:TiO2 filmgrown on STO. The transmission of the STO substrate alone(thin dashed line) is also shown for comparison. The sharp dropin transmission at 0.4 μm is due to the STO absorption cut-off.From the transmission data, it is clear that the bandgap of theNb:TiO2 film is at least as large as that of the STO substrate, asthe transmission cut-off for the sample and substrate are indis-tinguishable. The reason for the absence of thin layer absorptionfringes seen in the usual TCOs is that the refractive index (n) isalmost the same for STO and Nb:TiO2 [16]. The transmission ofthe film and substrate combined, referenced to that of thesubstrate alone, is greater than 90% for the entire visible rangeand is shown in Fig. 2a with a thick dashed line.

Continuing to look at the optical properties of the Nb:TiO2,in Fig. 2b, we show the optical constants, n and k, of the 220 nmthick Nb:TiO2 film grown on fused silica, as determined bymodeling the ellipsometry data of films deposited on fusedsilica. The best fit to the ellipsometry data was achieved using aparametric oscillator model [17] for the Nb:TiO2 film. Forcomparison, we also show tabulated n and k values for ITO inFig. 2b. At visible wavelengths, the index of refraction of Nb:TiO2 is considerably higher (n∼2.5) than that of typical TCOs,i.e. ITO (shown in the figure), which has an n∼2. The band gapof the Nb:TiO2 film (on SiOx) is near 3.5 eV which is consistentwith spectra shown in Fig. 2a. Similar attempts to fit the

Fig. 2. a) Transmission and reflection spectra of a 110 nm thick Nb:TiO2 filmgrown on STO at best growth conditions as a function of wavelength (μm).The transmission of the film relative to the STO is shown with a bold dotted line.b) n and k for the Nb:TiO2 film (bold) and ITO (thin).

ellipsometry data for the films on STO were unsuccessfulbecause of the difficulty in ellipsometrically distinguishing thefilm from the substrate due to the similar optical constants ofSTO and Nb:TiO2 as just discussed above.

XRD patterns for 110 nm thick Nb:TiO2 films grown on singlecrystal STO, single crystal LAO, and fused silica substrates areshown in Fig. 3 using a logarithmic intensity scale. For reference,the expected diffraction peaks for the anatase and rutile phases ofTiO2 are shown in the lower panels using a linear scale. The filmsgrown on both the STO and the LAO show the TiO2 anatase (004)peak at 2θ=37.8°. There are also peaks attributed to the substrate,which are marked with an “S”, with the strong substrate peaksoccurring as doublets due to detector saturation effects. The filmgrown on LAO also shows a small (200) TiO2 rutile peak, but theTiO2 anatase (004) peak dominates the XRD spectrum. Theabsence of the other anatase or rutile peaks for the films on theSTO or LAO, indicates that these films are highly-textured. Incontrast, the films grown on fused silica show no anatase peaksand only small TiO2 rutile (200) and (110) peaks, as emphasizedby the rutile lines extended from the bottom panel. These peaksdo not match the expected peak profile intensities, implyingpossible texturing.

The conductivities of the films in Fig. 3 are 2500 S/cm onSTO, 1500 S/cm on LAO, and 1 S/cm on fused silica. Note, inparticular that there is three orders of magnitude difference inconductivity between the highly crystalline biaxially textured

Fig. 4. TEM diffraction pattern for Nb:TiO2 grown on STO (a) and LAO (b).

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anatase films on STO and the films on fused silica, which showonly weak rutile XRD lines. To determine the relativecontributions of mobility (μ) and carrier concentration (n) tothis difference in conductivity for the films on STO and thefilms on fused silica, Hall effect measurements were made. Thefilms on STO had σ∼2500 S/cm with μ∼10–20 cm2/Vs, andn∼5–10×1020 cm−3. The films on fused silica had σ∼1 S/cmwith μ∼1–2 cm2/Vs, and n∼2–7×1018 cm−3. The two ordersof magnitude decrease in the carrier concentration indicates thatthe niobium is a substantially less effective dopant for the lesscrystalline rutile films on fused silica than for the highlycrystalline anatase films on STO. The factor of ten drop in μcould be due to the decrease in crystallinity, the change incrystal structure, or the variations due to the decrease in carrierconcentration.

Further insight into the structure of the films was gained byTEM analysis. Fig. 4a shows a TEM diffraction image for a Nb:

Fig. 5. The TEM image of a Nb:TiO2 film grown on STO.

TiO2 film on STO. The diffraction pattern reveals biaxial texturedgrowth of the film on the substrate in the tetragonal anatasestructure, with the film [001] parallel to the cubic [001] of thesubstrate. Taken together with the XRD patterns of Fig. 3, whichalso show a preferred [004] orientation, these TEM results showdefinitively the film to be biaxially textured anatase TiO2 on STO.Fig. 4b shows the TEM diffraction image for Nb:TiO2 film onLAO. Again, from the pattern it is clear that the film shows a niceplanar structure that is perpendicular to the film-substrateinterface. The cross-sectional TEM image shown in Fig. 5 revealsan oriented pattern in the film that can be traced to the substrate,indicating epitaxial growth of Nb:TiO2 on (100) SrTiO3.

In Fig. 1, a wide measurement spread in the measuredconductivities for the films grown on LAO was observed. Apossible cause of this measurement spread on the LAO substrateswas determined to be associated with roughness of the double-sided polished single crystal LAO. Fig. 6 shows an image of thesurface of a 55 nm thick film grown on LAO taken with an opticalprofilometer. The surface demonstrates a faceted structure withpeak to valley heights on the order of 100 nm. Similar surfacefacet texturing was also observed on the bare LAO substrates. TheLAO substrates used have varying facet densities, directions, and

Fig. 6. Optical profilometer image of Nb:TiO2 film grown on LAO substrate.

Fig. 7. Conductivity versus annealing temperature for anneals in vacuum (opensymbols) and anneals in 1.3×10−3 Pa O2 (closed symbols).

Fig. 8. 2θ plot of the X-ray diffraction of a Nb:TiO2 films after vacuum and1.3×10−3 Pa O2 annealing studies. The patterns for films on STO and LAO areoffset vertically for clarity. The “S” denotes the inherent SrTiO3 substrate peaks.The anatase and rutile crystal peaks for TiO2 are shown in the bottom two panelsusing a linear intensity scale.

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depths that could be due to twinning of the LAO [18]. Suchsurface facets could cause discontinuities in the film surface andconsequently in the conductivity. Another possible conductivitydisruption that could be associated with the twinning or facets isthe resultant high angle grain boundaries. The step heights on theLAO surface are larger than the thickness of the thinner filmsgrown but smaller than those of the thickest films grown. Thiscould lead to a variance in measured conductivity depending onthe twinning density, facet height, and direction of test. As thefilms grow thicker than the step height of the surface defects, thevariation in the measured conductivity decreases from over threeorders of magnitude on the 55 nm thick films, to one order ofmagnitude on the 110 nm thick films, to no noticeable variation onthe 220 nm films. This trend supports the idea that the surfaceroughness causes both the decreased macroscopic conductivityand the spread in measured conductivity values. Although themeasured conductivity is still slightly higher on the films grownon STO than on the best conductivity of films on LAO, the upperlimit of the measured conductivity of the films grown on the LAOsubstrate approaches the conductivity of the films grown on STO.Revisiting the possibility that the conductivity of the Nb:TiO2

might be due to an Nb:TiO2/STO interfacial layer, the fact that themost conducting films onLAOhave comparable conductivities tofilms on STO, coupled with the thickness independence of theconductivity for films on STO, indicates that the conductivity ofNb:TiO2 films is not due to an interfacial mechanism.

To make Nb:TiO2 viable for production, a cheaper depositionmethod must be developed. Towards this end, experiments weredone to determine if Nb:TiO2 films could be grown at roomtemperature and then subsequently annealed to produce a goodTCO, especially on fused silica. In Fig. 7, the dependence of theconductivity on the annealing temperature is shown for Nb:TiO2

samples grown at Ts=28 °C, where open symbols denote resultsfor vacuum annealed films and the solid symbols show the resultsfor samples annealed in 1.3×10−3 Pa O2. All of the as-depositedsamples have conductivities of 0.2–0.5 S/cm, essentiallyindependent of the substrate material. For the vacuum annealedsamples (open symbols), the conductivity increases after the first300 °C anneal to∼3 S/cm and then there is very little change untilthe conductivity increases sharply between the 650 °C and the

700 °C anneal, from 6 to 80S/cm for films onLAO and from17 to890 S/cm for films on STO respectively. The results on fusedsilica are substantially different. First, for Nb:TiO2 films on fusedsilica, after the first vacuum anneal at 350 °C, the conductivity is72 S/cm, about 20 times higher than for theNb:TiO2 films onSTOor LAO after the 350 °C vacuum anneal. Furthermore, after the700 °C vacuum anneal, the conductivity of the film on fused silicais 24 S/cm. This is lower than after the 350 °C vacuum annealwhich is opposite to the trend with temperature for the vacuumannealed films on STO and LAO. At 700 °C, the oxygen may beable to escape from the film, resulting in a conductivityenhancement due to an increased carrier concentration arisingfrom annealing induced oxygen vacancies for the films on STOand LAO. This is supported by a decrease in transparency of thesefilms as they started to appear metallic to the eye. The decrease inthe conductivity at the higher temperatures for the films on fusedsilica is not understood at present.

For the samples annealed in O2 (solid symbols), the effect ofthe post-deposition anneals is similar for films on all threesubstrates. After the 400 °C anneal, the conductivity is ∼7 to8 S/cm, slightly higher than that after the vacuum heattreatments on STO and LAO. However, for the O2 anneals,the conductivity steadily decreases for all higher temperatureanneals and ranges from 0.02 to 0.05 S/cm after the 600 °C O2

anneal. This drop in conductivity upon annealing the Nb:TiO2

films in O2 suggests that oxygen likely diffuses into the films attemperatures greater than 400 °C.

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In addition to the oxygen stoichiometry changes as justdiscussed, annealing induced structural changes could alsocontribute to conductivity changes. To examine this possibility,XRD patterns of the films were measured after each annealingstep. Fig. 8 shows the XRD patterns for 55 nm thick Nb:TiO2

films on STO obtained after the final anneals, 750 °C and600 °C for the vacuum and O2 anneals respectively. The filmsremain largely uncrystallized, with just a small rutile (110) peakappearing in the film annealed at 750 °C. It is interesting to notethat except for this weak rutile crystallization after the 750 °Canneal, the films in both the vacuum and oxygen annealedsamples on all tested substrates remained amorphous through-out the entire annealing temperature range investigated here. Incontrast, the Nb:TiO2 films grown at Ts=550 °C on STO andLAO are crystalline anatase. We note that recently anatase Nb:TiO2 films on glass with a conductivity of 650 S/cm werereported for films deposited at TS=250 °C and then subse-quently annealed in H2 at 500 °C [19]. Unlike our ambienttemperature deposited films which are amorphous as-deposited,these films deposited at 250 °C are weakly crystalline with ananatase structure. Hence, for compositionally similar Nb:TiO2

films on glass, the initial as-deposited crystallinity and structuremay well have a large effect on the electronic properties afterannealing.

4. Summary

Transparent conductive (004) oriented anatase phase Nb:TiO2

films were grown on STO and LAO substrates by PLD at 550 °C.The films grown on the fused silica substrates show very smallXRD peaks of the (200) and (110) oriented rutile TiO2, but aremuch less crystalline and conductive (σ∼1 S/cm) than the filmsgrown on STO (σ∼2500 S/cm) and LAO (σ∼2000 S/cm) at thesame conditions. The films grown on STO show the best anatasecrystallinity and also the best conductivity. The films grown onLAO show a variance in conductivity, which is attributed to thesurface structure of the LAO substrates. The films on STO werenot only crystalline, but appeared to grow epitaxially on thelattice-matched substrate. For these Nb-TiO2 films on STO, theconductivity did not depend on the film thickness demonstratingthat the high conductivity, ∼2500 S/cm, is an intrinsic bulkproperty of the Nb:TiO2 and not a substrate interface effect.Annealing studies showed the temperature dependent effects of

oxygen evolution from or diffusion into the Nb:TiO2 films.Finally, this study of Nb doped TiO2 thin films grown by PLD hasshown, at present, conductivities of 2500 S/cm can be achievedfor films grown at high temperature, TS=550 °C, on single crystalsubstrates. Further work is needed to find the combination ofgrowth conditions, substrates and post-deposition thermalprocessing that could lead to the use of Nb doped TiO2 as apractical TCO material.

Acknowledgement

This work was supported by the Laboratory DiscretionaryResearch and Development (LLRD) program at the NationalRenewable Energy Laboratory (NREL).

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