Journal of Crystal Growth 380 (2013) 118–122

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Journal of Crystal Growth

0022-02http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/jcrysgro

Pseudo-hexagonal in-plane alignment of rutile (100)Nb:TiO2

on hexagonal (0001)Al2O3 plane

Chaojun Wang a, Joonghoe Dho a,n, Sang Geul Lee b

a Department of Physics, Kyungpook National University, Daegu 702-701, South Koreab Daegu center, Korea Basic Science Institute, Daegu 702-701 South Korea

a r t i c l e i n f o

Article history:Received 28 March 2013Received in revised form15 May 2013Accepted 10 June 2013

Communicated by M.S. Goorsky

suggests that the rutile Nb:TiO2 film on the hexagonal (0001)Al2O3 can be re-interpreted by a certain

Available online 17 June 2013

Keywords:A3. CharacterizationA3. Laser epitaxyB1 OxidesB1. Semiconducting materials

48/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jcrysgro.2013.06.009

esponding author. Tel.: +82 539507354.ail address: jhdho@knu.ac.kr (J. Dho).

a b s t r a c t

Nb-doped TiO2 (Nb:TiO2) films were grown on a hexagonal (0001)Al2O3 substrate at 650 1C and∼10−5 Torr. The Nb:TiO2 film had a small resistivity of ∼8�10−4 Ω cm at room temperature and abehavior of a slightly increasing resistance upon cooling. In addition, the Nb:TiO2 film had an opticaltransmittance of about 60% in the visible range. A careful analysis of the in-plane atomic structure

pseudo-hexagonal structure, which is discriminated from the in-plane rectangular one of the tetragonal(100)Nb:TiO2. The pseudo-hexagonal properties of the Nb:TiO2 film were characterized by negligiblemosaic structure at the interface, the same electron diffraction pattern as the hexagonal Al2O3 substrate,and perfect six-fold symmetries in the pole figure and ϕ-scan XRD patterns.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Nb-doped TiO2 (Nb:TiO2) is one of the transparent conductingoxides (TCOs) having useful properties for optoelectronic applica-tions such as low resistivity and high transmittance in the visiblerange [1,2]. It is known that TiO2 can exist in three different typesof structural phases, rutile, anatase and brookite [3–5]. Since therutile phase TiO2 is the most stable one among the threepolymorphs, the rutile phase is easily formed by treatment withhigh temperature and high oxygen pressure; additionally,a transformation from the other metastable phases to the rutilephase is irreversible. Rutile phase TiO2 has a tetragonal structurewith the space group P42/mnm and lattice constants of a¼4.594 Åand c¼2.959 Å [3]. The niobium (Nb) doping method in rutile TiO2

has been frequently used to improve its transparent and conduct-ing properties [4].

Up to now, epitaxial rutile TiO2 films have been synthesized ona hexagonal α-Al2O3 substrate by various methods [6–16], and theyalso have been made on a hexagonal (0001) GaN substrate [17,18],on cubic MgO, SrTiO3, and GaAs substrates [19–21], and on the Si(100) substrate with oxidizing epitaxial TiN films [22]. In the early1990s, researchers already found that the preferred planar align-ment relationships between the rutile TiO2 film and the hexagonalAl2O3 substrate are summarized as (100)TiO2∥(0001)Al2O3, (101)

ll rights reserved.

TiO2∥ð1102ÞAl2O3 and (101), (100)TiO2∥ð1120ÞAl2O3. In particular,Chen et al. reported that the best structural match for the (100)TiO2 film on (0001)Al2O3 is to position the in-plane rectangularTiO2 lattice such that [001]TiO2∥½1010�Al2O3 is within the plane[6,7]. After that, researchers have thought that a hetero-epitaxialTiO2 film on (0001)Al2O3 will make a mosaic structure consistingof three equivalent domains since there can be in-plane rectan-gular TiO2 unit cells that are rotated 1201 from each other. Such aconjecture was indirectly supported by the six fold symmetry inthe pole figure or ϕ-scan X-ray diffraction (XRD) patterns[6,7,15,23,24] and was similarly adopted in the (100)TiO2 filmson hexagonal GaN substrates.[17,18] In this paper, we report thatthe rutile Nb:TiO2 film synthesized on (0001)Al2O3 in vacuum hasa good transparent and conducting properties and its structure canbe re-interpreted by a certain pseudo-hexagonal one instead oftetragonal one. This means that the (100) rutile oxides such asTiO2, VO2, and CrO2 can be synthesized as high quality films ofnegligible mosaic structure on a hexagonal substrate because of itspseudo-hexagonal nature.

2. Experiments

0.5% Nb-doped TiO2 (Nb:TiO2) films were deposited on (0001)Al2O3 by pulsed laser deposition at 650 1C at a pressure of∼10−5 Torr. The Al2O3 substrate was cleaned by acetone beforedeposition, and the film growth was done without any gas injection.The laser power density was about ∼1.3 J/cm2. A polycrystalline Nb:

C. Wang et al. / Journal of Crystal Growth 380 (2013) 118–122 119

TiO2 target sintered at 1200 1C was used for the synthesis of the70 nm and 200 nm thick Nb:TiO2 films. A Panalytical X-ray diffract-ometer (Daegu center, KBSI) with Cu Kα1 radiation with a

Fig. 1. (a) θ−2θ scan XRD pattern, (b) the temperature dependent resistance, and(c) the optical transmittance for the Nb:TiO2 film on (0001)Al2O3 substrate.

Fig. 2. In-plane atomic arrangements of (0001)Al2O3 and (100)TiO2 (a) for hetero-epitaxof the pseudo-hexagonal lattice on the hexagonal lattice. The bright (colored) atoms aresurface. (For interpretation of the references to color in this figure legend, the reader is

wavelength λ¼1.54056 Å was used for the θ/2θ scan, ϕ-scan andpole figure measurements to investigate the out-of-plane and in-plane structural properties of the Nb:TiO2 film. A high resolutiontransmission electron microscopy (JEOL, Japan) was used for thecross-section measurements of the Nb:TiO2 films to study themicrostructural and epitaxial growth details.

3. Results and discussions

Fig. 1(a) shows the X-ray diffraction (XRD) pattern for the 70 nmthick Nb:TiO2 film on (0001)Al2O3. In addition to the (0006) peak ofthe Al2O3 substrate, sharp peaks coming from the Nb:TiO2 film wereclearly observed. The peaks observed at 39.11 and 84.11 were matchedwith the position of the (200) and (400) reflection planes of the rutilephase Nb:TiO2, and they had a full-width-half-maximum value of∼0.081 in the rocking curve. The XRD data suggest that the Nb:TiO2

films on the Al2O3 substrate were grown with a specific crystalorientation. Fig. 1(b) shows the temperature dependent resistance ofthe 70 nm thick Nb:TiO2 film on (0001)Al2O3. The resistance exhibitedan increasing tendency during cooling, which is a typical characteristicof temperature dependence of doped semiconductors. In addition, Halleffect measurements at room temperature suggest that the Nb:TiO2

film has a small resistivity of ∼8�10−4 Ω cm, a carrier concentration of∼1�1021 cm−3 and a mobility of ∼7 cm2/V s. It is interesting that suchgood transparent and conducting properties were observed even inthe rutile Nb:TiO2 as well as the anatase one [4]. Our 0.5% Nb dopedTiO2 film made at a low oxygen pressure showed much betterconducting properties than the previously reported 5.7% Nb dopedTiO2 film with the resistivity of ∼5�10−2 Ωm and the mobility of∼0.8 cm2/V s.[25] The poorer conducting property in the previousreport may be explained by fewer oxygen vacancies in the high Nbdoped sample and the resultant decrease of density of state at Fermilevel, which has been revealed by an ab initio calculation [26]. Fig. 1(c) shows the UV–Vis optical transmittance spectrum of the 70 nmthick Nb:TiO2 film on (0001)Al2O3. The transmittance in the visiblerangewas about 60% and the optical band gap was about 3.4 eV. Theseresults suggest that the Nb:TiO2 film on (0001)Al2O3 possesses thetypical characteristics of a transparent conducting oxide. These results

ial growth of the rectangular lattice on the hexagonal lattice and (b) for epi-growthat the top surface, while the dark (colored) atoms are located slightly below the topreferred to the web version of this article.)

Fig. 3. (a) The cross-sectional TEM image, (b) and (c) the selected area electron diffraction patterns.

C. Wang et al. / Journal of Crystal Growth 380 (2013) 118–122120

suggest that the Nb:TiO2 film possesses typical properties as atransparent conducting oxide.

It has been revealed that the hetero-epitaxial films of rutileoxides such as TiO2, VO2, and CrO2 on hexagonal Al2O3 can besuccessfully synthesized [6–16,23,24]. Previously, it has beenthought that the in-plane rectangular lattice of rutile oxides wouldbe hetero-epitaxially stabilized on the hexagonal (0001) Al2O3

plane with three possible in-plane orientations, as shown in Fig. 2(a). For the projection of the tetragonal (100) plane, possible unitcell arrangements are marked by the dashed rectangles. Hetero-epitaxial growth is supported by the six-fold rotational symmetryin the ϕ-scan XRD data of the tetragonal (110)TiO2. Therefore,researchers have believed that the (100)TiO2 film on a hexagonalsubstrate consists of three domains rotated by 1201 with respect toeach other since the (110) surface of TiO2 has a twofold symmetry[6,7,15–18,23,24]. Instead of a rectangular lattice with threepossible orientations, interestingly, we found that an in-planelattice structure of (100) rutile oxides can be re-interpreted by apseudo-hexagonal structure within the plane, as shown in Fig. 2(b). For the projection of the pseudo-hexagonal (001) plane,possible unit cell arrangements are marked by the solid hexagons.For a rectangular lattice, the in-plane lattice mismatches betweenTiO2 and Al2O3 were +3.6% and −7.1% for the two orthogonaldirections. On the other hand, the in-plane lattice mismatches forthe hexagonal lattice were +0.6% and −7.1% for the two directionsof the nearly parallelogram. Since the pseudo-hexagonal lattice

structure within the plane exists accidently in the rutile (100)TiO2

plane, it is slightly distorted in the hexagonal lattice, which resultsin an angle mismatch of ∼2.3% with the Al2O3 hexagon. Both therectangular and pseudo-hexagonal lattices for the (100)TiO2 areapproximately matched with the in-plane lattice structure of the(0001)Al2O3. However, a simple algebraic summation of all mis-matches is slightly smaller for the hexagonal lattice than for therectangular lattice. Accordingly, the growth characteristics of therutile (100)TiO2 on (0001)Al2O3 can be understood by an in-planepseudo-hexagonal structure, which is conceptually discriminatedfrom the growth of the rectangular structure film on the hexago-nal substrate.

The microstructural properties of the (100)TiO2 film on (0001)Al2O3 were analyzed by high-resolution TEM and systematic XRDmeasurements. The cross-sectional TEM image of the Nb:TiO2 filmgrown on (0001)Al2O3 is shown in Fig. 3(a). It can be seen that the(100)Nb:TiO2 film is epitaxially grown on the (0001)Al2O3 withquite a clean interface between the two materials. Of note, therewere no observable mosaic structures and domain boundaries inobtained TEM images, which are expected in the case wheretetragonal structure particles are stacked on a hexagonal substrate.Contrary to our (100)Nb:TiO2 film on the (0001)Al2O3, the TEMmeasurement for the rutile TiO2 film on ð1102Þ and ð1120ÞAl2O3

clearly shows an imperfect microstructure, i.e., a high density ofgrowth twins and stacking faults [7,8,15]. Fig. 3(b) and (c) showsthe selected area electron diffraction (SAED) patterns for both the

Fig. 4. The pole-figure and ϕ-scan XRD patterns for the pseudo-hexagonal Nb:TiO2 film on (0001)Al2O3. Based on a pseudo-hexagonal structure, (a) and (b) are related to the(101)Nb:TiO2 reflection plane, and (c) and (d) correspond to the (102) and (103) reflection planes, respectively.

C. Wang et al. / Journal of Crystal Growth 380 (2013) 118–122 121

Nb:TiO2 film and the Al2O3 substrate at the interfacial area. TheSAED patterns were taken along the ½1120� zone axis of the Al2O3

substrate. The diffraction spots from only one zone of Nb:TiO2 inthe SAED pattern are indicative of the epi-growth of Nb:TiO2 onthe Al2O3 substrate. Moreover, the rutile Nb:TiO2 has the sameSAED pattern with the hexagonal Al2O3 substrate, implying thatthe (100)Nb:TiO2 grown on (0001)Al2O3 can be well understoodby a pseudo-hexagonal structure.

The in-plane texture of the 200 nm thick Nb:TiO2 film on(0001)Al2O3 was investigated by pole-figure and ϕ-scan XRDmeasurements. The bulk Nb:TiO2 has a tetragonal structure ofa¼b≈4.593 Å and c≈2.958 Å [3]. From XRD data, the latticeconstants of our Nb:TiO2 film were estimated to a¼b≈4.60 Å andc≈2.95 Å in a tetragonal representation. If the Nb:TiO2 film on(0001)Al2O3 is considered as an in-plane hexagonal lattice shownin Fig. 2(b), the in-plane parameter a of the pseudo-hexagonalstructure will be roughly the average value of the two in-planeatomic distances, 2.958 Å and 2.732 Å, and the out-of-planeparameter c will be about 4.593 Å. Actually, the pole-figure andϕ-scan XRD measurements could be done under the basis of apseudo-hexagonal structure with a∼2.90 Å and c∼4.60 Å. The (110)surface of the tetragonal TiO2, which has been frequently used forthe ϕ-scan XRD measurement, can be translated into the (101)surface in the pseudo-hexagonal structure. Therefore, we mea-sured the pole figure and ϕ-scan XRD patterns for the pseudo-hexagonal (101) Nb:TiO2 and for the higher order miller indicessuch as (102) and (103). Fig. 4(a) and (b) shows the pole figure andϕ-scan XRD patterns for the pseudo-hexagonal (101)Nb:TiO2,indicating that the in-plane texture of the Nb:TiO2 film possessesa perfect six-fold symmetry as a usual hexagonal material. Simi-larly, Fig. 4(c) and (d) shows the pole figure patterns for thepseudo-hexagonal (102) and (103)Nb:TiO2 that also show perfectsix-fold symmetries.

4. Conclusions

The 0.5% Nb doped rutile TiO2 film grown on the Al2O3 in10−5 Torr at 650 1C has quite good transparent and conductingproperties; the transmittance was about 60% in the visible rangeand the resistivity was ∼8�10−4 Ω cm at room temperature. Thestructural properties of the rutile Nb:TiO2 film on the hexagonalAl2O3 substrate were re-interpreted by a certain pseudo-hexagonalstructure; the (100)Nb:TiO2 film exhibited negligible mosaicstructure at the interface, the same electron diffraction patternwith the hexagonal Al2O3 substrate, and the pole figure and ϕ-scanXRD patterns of perfect six-fold symmetries just as the usualhexagonal material. Our results show that the epi-growth of amaterial is possible even on a substrate with a totally differentcrystal structure as long as the in-plane lattice structures aresimilar.

Acknowledgment

Authors thank Mr. Jian Li for his experimental assistance. Thiswork was supported by Kyungpook National University ResearchFund, 2012, and Basic Science Research Program through theNational Research Foundation of Korea (NRF) funded by theMinistry of Education, Science and Technology (2010-0007902).

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