Article pubs.acs.org/cm
Thermoelectric Properties of Strontium Titanate Superlattices Incorporating Niobium Oxide Nanolayers S. R. Sarath Kumar,† M. N. Hedhili,† Dongkyu Cha,† Terry M. Tritt,‡ and H. N. Alshareef*,† †
Materials Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡ Department of Physics and Astronomy, Clemson University, South Carolina 29634, United States S Supporting Information *
ABSTRACT: A novel superlattice structure based on epitaxial nanoscale layers of NbOx and Nb-doped SrTiO3 is fabricated using a layer-by-layer approach on lattice matched LAO substrates. The absolute Seebeck coefficient and electrical conductivity of the [(NbOx)a/(Nb-doped SrTiO3)b]20 superlattices (SLs) were found to increase with decreasing layer thickness ratio (a/b ratio), reaching, at high temperatures, a power factor that is comparable to epitaxial Nb-doped SrTiO3 (STNO) films (∼0.7 W m−1 K−1). High temperature studies reveal that the SLs behave as n-type semiconductors and undergo an irreversible change at a varying crossover temperature that depends on the a/b ratio. By use of high resolution X-ray photoelectron spectroscopy and X-ray diffraction, the irreversible changes are identified to be due to a phase transformation from cubic NbO to orthorhombic Nb2O5, which limits the highest temperature of stable operation of the superlattice to 950 K.
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INTRODUCTION
The effectiveness of the SLs in energy conversion is often expressed by its dimensionless figure of merit, ZT = α2σTκ−1, where α, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and total thermal conductivity.19 The total thermal conductivity consists of two components, an electronic component κe and a lattice component κL, i.e., κ = κe + κL. At the very heart of the research in the field of thermoelectrics lies the inherent interdependence of the electronic properties and lattice dynamics of a material, which is detrimental, and hence the efforts to decouple them.20 Efforts to maximize ZT involve enhancing the electronic properties by doping and band gap engineering and also reducing the lattice thermal conductivity by structural modifications, paving different pathways.21−23 SrTiO3 possesses a wide band gap,24 high mobility,25 and a huge Seebeck coefficient.15 The carrier concentration and hence the n-type conductivity can be enhanced by δ doping with atoms such as La in the Sr (A) site26 or Nb in the Ti (B) site18 or by introducing oxygen vacancies24 but at the expense of Seebeck coefficient, limiting the power factor (α2σT). Even though the power factor of SrTiO3 thermoelectrics is still reasonably high,15 the major hurdle in the development of ntype thermoelectric oxides is their relatively large lattice thermal conductivity.27 For bulk materials and thin films, nanostructuring28 and formation of heterostructures,29 respectively, are considered to be promising approaches for reducing the
The diverse physical properties exhibited by oxides offer novel opportunities to realize tailor-made materials for diverse applications. Unlike conventional semiconductors, various emergent phenomena arise in oxides and oxide interfaces1 which offer a potential route for versatile and improved technologies.2 In the emerging field of oxide electronics, correlated electron effects3,4 at tailored oxide interfaces have attracted considerable attention.5 For instance, the formation of two-dimensional electron gas (2DEG), i.e., a thin sheet of highly conducting region, at the interface between two insulating oxides6 has been hugely debated for the past few years.7,8 SrTiO3, a cubic perovskite oxide with the highest known mobility for oxides, is one of the most studied oxides.9 The electrical properties of SrTiO3 can be tailored at the bare surface10 or at δ-doped layers11 or at the interface12 with a polar oxide such as LaAlO3 (LAO) and may be controlled by epitaxial strain13 or by varying the number of unit cells in artificial superlattices (SLs).14 The tunability of electrical properties and the possible large Seebeck coefficient15 make SrTiO3 based films and SLs one of the promising candidates for high efficiency thermoelectrics, wherein solid state materials are used for waste heat harnessing in diverse environments as well as for solid state refrigeration, by exploiting respectively the Seebeck and the Peltier effects. While thin films of doped SrTiO3 are reported15−18 to be good thermoelectric materials, Ohta et al.’s report14 of large Seebeck coefficient of 2DEGs confined within SrTiO3 and Nb doped SrTiO3 SLs has generated a renewed interest in SLs for thermoelectric applications. © 2014 American Chemical Society
Received: February 23, 2014 Revised: March 27, 2014 Published: April 9, 2014 2726
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Electrical conductivity and Seebeck coefficient were measured inplane, in the temperature range 300−1000 K using a commercial setup (Ozawa Science). Pt−Pt/Rh thermocouples were used as voltage and current probes as well as temperature sensors. The setup helps the measurement of electrical conductivity and Seebeck coefficient simultaneously at any given temperature, the former by four-probe and the latter by differential methods, wherein the temperature difference between the hot and cold probes is varied in the range 4−10 K. The SLs that underwent high temperature measurements are treated as annealed ones, since the measurements in Ar/H2 (96/4) ambient took ∼20 h to complete.
lattice thermal conductivity. The challenge, however, will be to maintain the electrical properties (or the power factor) of the resultant structures. Here, we report fabrication of SLs, containing niobium oxide (NbOx) and 20% Nb doped SrTiO3−δ (STNO) layers, represented as [(NbOx)a/(STNO)b]c, where a and b represent the number of unit cells of NbOx and STNO, respectively, and c represents the total number of NbOx/STNO pairs. Both STNO and niobium monoxide (NbO) crystallize in the cubic structure, and hence an epitaxial layer-by-layer growth of the SLs is possible, which can help preserve the electrical transport properties. The introduction of alternating layers of semiconducting NbOx in STNO films is also expected to help reduce the thermal conductivity of the system due to the enhanced scattering of phonons at the interfaces. The influence of the NbOx layers in determining the high temperature thermoelectric properties and chemical and structural stability of the SLs are discussed.
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RESULTS AND DISCUSSION The schematic of the SLs grown by pulsed laser deposition is shown in Figure 1a. The high quality layer-by-layer growth of STNO and NbOx layers is evident from both (i) the high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) cross-section image (Figure 1b) and (ii) the reflection high energy electron diffraction (RHEED) pattern observed after the end of growth of two pairs (Figure 1c). The elemental maps (Figure 1d−f), obtained for a selected area of cross section, confirm the presence of alternating layers of STNO and NbOx. The energy dispersive spectroscopy (EDS) line scan across the SL cross section is shown in Figure 1g. Peaks of the Ti and Sr signals occur at the same point where troughs of the Nb signal occur, further confirming the growth of alternating layers of STNO and NbOx. A line scan within the STNO layer (inset to Figure 1g) reveals that the peaks of Sr signal coincide with the troughs of Ti signal and vice versa, indicating that SrTiO3 is a layered perovskite structure with alternating SrO and TiO2 planes, which is well established in literature. Interestingly, the Nb signal is in phase with the Ti signal, indicating that Nb substitutes for Ti (B site of the perovskite structure) in STNO. Though recent studies30 on STNO have identified the B site substitution by Nb dopants, it is nevertheless worthwhile to establish it with a conclusive and direct experimental evidence. The temperature dependence of electrical conductivity of the SLs and the STNO film is shown in Figure 2a. For STNO film and the SL with a/b = 0.08, the electrical conductivity decreases with temperature, suggesting degenerate conduction, consistent with the reported behavior for STNO films.31 In STNO layers, the carrier concentration is much higher, since both Nb dopants and oxygen vacancies act as electron donors.32 The NbOx layers, however, possess a lower conductivity compared to the STNO layers, which accounts for the observed decrease in conductivity with increasing ratio of a/b (Figure 2b, where the electrical conductivity of the SLs at 300 K is plotted as a function of a/b). For higher a/b values, the SLs exhibit a semiconducting behavior up to a certain temperature, beyond which the conductivity drops. This crossover temperature exhibits a strong dependence on the ratio of a/b, with the reversal of conductivity setting in at much lower temperatures for SLs with higher values of a/b. Here, the observed drop in conductivity beyond the crossover temperature is not due to the semiconductor−metal transition often reported for STNO films but rather is driven by a diffusion of oxygen from the STNO layers to the NbOx layers at higher temperatures, which results in a deterioration of the electrical transport properties of the NbOx layers, as will be discussed later. Higher oxides of niobium (NbO2 and Nb2O5) are more electrically insulating than NbO.33,34 While NbO is metallic,33 NbO2 behaves as an intrinsic semiconductor33 and Nb2O5 acts as a dielectric.35 Interestingly, the electrical conductivity of the SLs vary with a/
EXPERIMENTAL SECTION
[(NbOx)a/(STNO)b]20 SLs were fabricated by pulsed laser deposition, using a KrF excimer laser (λ = 248 nm, pulse width of 20 ns, repetition rate of 10 Hz, and fluence of 3 J cm−2 pulse−1). The SLs were deposited on polished LAO (100) substrates held at 973 K in ambient argon (20 mTorr), after evacuating the chamber to a high vacuum (better than 2 × 10−8 Torr). The targets used were Nb2O5 for NbOx layers and 20% Nb doped SrTiO3 ceramic for the STNO layers. Reflection high energy electron diffraction (RHEED) intensity oscillations were used to monitor the layer by layer growth of SLs and to control the number of unit cells of the deposited oxide layers. The number of unit cells of NbOx and STNO was varied to form SLs with different a/b ratios. The SLs were characterized by conventional out of plane (θ−2θ) X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXRD) with a grazing angle of 0.8°, high resolution electron microscopy combined with energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). For crosssection transmission electron microscopy (TEM) analysis, the samples were prepared by focused ion beam (FIB, Helios 400s, FEI) with liftout method. After FIB preparation, the samples were gently milled by Ar ion with low energy (50 eV) (Nanomill 1040, Fischione Co.) to remove any amorphized area by Ga ion. We used TEM (Titan, FEI) with Cs probe corrector which can provide subangstrom resolution (0.8 Å) in scanning transmission electron microscopy (STEM) mode with 300 kV probe for TEM analysis. In STEM mode, coherent focused probe is raster-scanned across the specimen and X-ray emission spectrum from each probe point is recorded. These spectra were used to construct the elemental map and line profile depending on the scanned area. For EDS chemical analysis, a high sensitive, high speed EDS system (ChemiSTEM, FEI) was used and line profile and elemental mapping was done with 0.23 nA beam current and 25 μs dwell time in STEM mode. XPS studies were carried out in a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) operating at 150 W, a multichannel plate, and delay line detector under 1.0 × 10−9 Torr vacuum. Measurements were performed in hybrid mode using electrostatic and magnetic lenses, and the takeoff angle (angle between the sample surface normal and the electron optical axis of the spectrometer) was 0°. All spectra were recorded using an aperture slot of 300 μm × 700 μm. The survey and high-resolution spectra were collected at fixed analyzer pass energies of 160 and 20 eV, respectively. Samples were mounted in floating mode in order to avoid differential charging. Charge neutralization was required for all samples. Binding energies were referenced to the C 1s binding energy of adventitious carbon contamination which was taken to be 284.8 eV. The data were analyzed with the commercially available software, CasaXPS. The individual peaks were fitted by a Gaussian (70%)−Lorentzian (30%) (GL30) function after linear or Shirley type background subtraction. 2727
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b ratio and not on the numbers of layers (refer AC). Since the diffusion of oxygen is irreversible, a reduced electrical conductivity was obtained during cooling (refer AC) once the crossover temperature is exceeded while heating. It is to be noted here that semiconductor−metal transition is a very specific temperature dependent electronic transport phenomenon, which is reversible and hence cannot account for the observed data. The temperature dependence of Seebeck coefficient of the SLs is shown in Figure 3a. All SLs show a negative Seebeck coefficient indicating n-type conduction. The absolute value of Seebeck coefficient progressively falls with increasing a/b ratio, consistent with the variation of electrical conductivity, indicating that higher relative numbers of STNO layers is required for obtaining both high Seebeck coefficient and electrical conductivity. For all SLs, the Seebeck coefficient decreases linearly with temperature in a diffusive-like behavior up to the crossover temperature beyond which the slope changes significantly, similar to that observed in the electrical conductivity. Interestingly, the temperature of the onset of the slope change (marked by the arrows) progressively decreases with increasing a/b ratio. The behavior is consistent with the corresponding changes observed in electrical conductivity, albeit more drastic, and hence can be explained using the oxidation of NbOx to higher oxides. The crossover in the Seebeck coefficient from its lower temperature diffusive-like behavior is more pronounced than in the electrical conductivity. This is not surprising, since in many cases the Seebeck coefficient can be thought of as a derivative of the electrical conductivity. The slope change occurs in the temperature range of 750−950 K, consistent with the temperature range reported for the oxidation of NbOx to Nb2O5.34 The steep rise in absolute Seebeck coefficient is attributed to the irreversible fall in electrical conductivity of the SLs. Our results suggest that oxidation of NbOx depends on the concentration of the starting material (NbOx) and the oxygen donor (STNO) and hence is not surprising. From the in-plane electrical conductivity and Seebeck coefficient studies, it is evident that reducing the number of NbOx layers and increasing the number of STNO layers is the right approach to enhancing the electronic properties. In the present work, the absence of in-plane thermal conductivity values of the SLs prevents calculation of in-plane ZT values. Nevertheless, power factor values act as an important metric of the performance of a thermoelectric material. The in-plane power factor (α2σT, in units of W m−1 K−1) of the SLs shown in Figure 3b serves to explain the dependence more clearly. With decreasing a/b ratio, the power factor is observed to increase. Also, the power factor increases with temperature for the SLs and reaches a maximum, which is comparable to the STNO film, at ∼950 K, beyond which it starts to decrease rapidly with increasing temperature. Evidently, the decrease is due to the irreversible changes affecting the SLs, and hence, the SLs can only be used for applications at temperatures below 950 K. Nevertheless, this is a very high temperature for potential high temperature applications using SL-based devices. It may be noted here that it is extremely difficult to maintain a high temperature gradient across the thickness of the SL or film for practical applications and recent approaches make use of lateral architectures, where heat flows parallel to the film surface, to circumvent the problem.36 In order to investigate the degradation of the SLs after annealing beyond 950 K, we performed detailed structural
Figure 1. (a) Schematic of the [(NbOx)a/(STNO)b]20 SLs deposited on LAO substrates. (b) HAADF-STEM image of the cross section of the SLs revealing the layer by layer growth of the individual layers. (c) RHEED pattern (azimuth ⟨100⟩) obtained for [(NbO x ) a / (STNO)b]20, suggesting a highly oriented growth of the SLs. Elemental maps of (d) Sr, (e) Ti, and (f) Nb, obtained from a selected area of cross section. Bright areas correspond to regions with high concentration of the elements, clearly indicating that Nb is rich at regions where Sr and Ti are poor, suggesting a layer by layer growth. The dimension of scale bars in (d)−(f) is 5 nm. (g) EDS line scan of the cross section of the SLs. Signals corresponding to Ti Kα, Sr Lα, Nb Lα, and O Kα are plotted as a function of point number of the line. Inset to (g) shows the fine line scan of a single STNO layer of 12 unit cells. 2728
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Figure 2. (a) Electrical conductivity of the STNO film and the SLs, with varying numbers (a, b) of unit cells, respectively, of STNO and NbOx as a function of temperature. As shown, there is the appearance of a crossover temperature where the electrical conductivity begins to decrease with increasing temperature. (b) Dependence of electrical conductivity at 300 K on the a/b ratio. The solid line is a guide to the eye.
Figure 3. (a) High temperature Seebeck coefficient and (b) power factor of the STNO film and the SLs, with varying a/b ratios. The temperature at which the slope of the Seebeck coefficient vs temperature changes significantly (i.e., crossover temperature) is indicated by the arrows.
Figure 4. (a) θ−2θ X-ray diffraction of the SLs. (b) GIXRD of the asdeposited and annealed SLs.
analysis. Figure 4 shows the XRD pattern in the θ−2θ geometry for the as-deposited SLs. The XRD spectra suggest a preferential epitaxial growth of STNO and layers on LAO substrate. The calculated lattice constants are 3.79 Å for LAO, 3.95 Å for STNO, and 4.19 Å for NbOx, comparable to the reported values of 3.791, 3.905, and 4.210 Å, respectively, for cubic perovskite LAO, cubic perovskite SrTiO3, and cubic NbO. The increased lattice constant for STNO layers compared to stoichiometric SrTiO3 is due to the higher ionic radius of Nb5+ ions (0.78 Å) that substitute the Ti4+ ions (0.745 Å). Also, oxygen vacancies create enhanced cation−cation repulsion in the TiO2 planes of STNO, stretching the lattice. Of
particular interest in the ensuing discussion is the much lower lattice constant obtained for the NbOx layer compared to the reported value for cubic NbO. Evidently, the NbOx layers are under huge compressive strain. There is also the possibility of the presence of higher oxides of Nb, since the target used is Nb2O5. The GIXRD patterns of the as-deposited and annealed SLs shown in Figure 4b suggest that the SLs are not grown perfectly epitaxial, but additional phases, though in small concentration, are indeed present. For as-deposited SLs, analysis of GIXRD reveals that the NbOx layer is composed of a mixture of NbO (ICDD no. 01-071-2146; cubic; space group Pm-3m (221); a = 4.210 Å), β-NbO2 (ICDD no. 00-0442729
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Figure 5. (a) Lattice resolved HRTEM image of the SLs (colored to distinguish between the layers). (c) HRTEM image of the SLs annealed to 1000 K. The mechanism of destruction of the SL structure is illustrated in (c)−(e). (c) shows the NbOx layer in the initial structure of NbOx/STNO. (d) shows the expansion of NbOx layer upon oxidation beyond ∼950 K, and (e) shows that beyond 1000 K, the superlattice structure is completely destroyed because of the huge tensile stress, resulting in a polycrystalline mixed phase of STNO and higher oxides of Nb.
1053; body centered tetragonal; space group I41 (80); a = 9.693 Å and c = 5.985 Å) and T-Nb2O5 (ICDD no. 01-0710336; orthorhombic; space group Pbam (55); a = 6.175 Å, b = 29.175 Å, and c = 3.930 Å). However, for the annealed SLs, a strong peak corresponding to T-Nb2O5 and reflections from βNbO2, but none attributable to cubic NbO, are observed, suggesting oxidation of NbO to form higher oxides. It is well established that NbOx oxidizes gradually to form Nb2O5 nonuniformly below ∼900 K, while above this temperature, the NbO2 formed transforms to Nb2O5, leading to breakaway oxidation,37 depending upon the availability of oxygen. Since the annealing is done in the thermoelectric measurement setup in a reducing Ar/H2 ambient, the only source of oxygen for formation of higher niobium oxides is the diffusion from STNO layers. The reaction mechanisms can be summarized as
dependence of electrical conductivity and XRD analysis, XPS spectra have been obtained from the SLs before and after annealing. The high resolution XPS spectra of Sr 3d, Ti 2p3/2, and Nb 3d core levels from the as-deposited SLs are shown along with the deconvoluted components in parts a, b, and c of Figure 6, respectively. The insets of the figures show the corresponding spectra obtained from annealed SLs. The Sr 3d core level before and after annealing showed identical features and could be decomposed into two doublets (Sr 3d5/2 and Sr 3d3/2) with a fixed area ratio equal to 3:2 and doublet separation of 1.75 eV. The separation between the doublets remained within 0.2 eV, i.e. ∼1.0 ± 0.1 eV, which is higher than the main Sr peak. The Sr 3d5/2 core level dominated by the peak centered at 133.1 eV corresponds to the bulk Sr from STNO with the valence state +2.38 The high binding energy peak can be attributed to SrO for SrO terminated STNO as observed in the case of SrTiO3 39 and/or to the formation of surface Sr-COH species.40 The results suggest that the Sr site is unaffected by the annealing. The Ti 2p3/2 core level was also fitted using two components. The dominant peak centered at 458.3 eV is associated with Ti ions with a formal valency of +4,30,41 while the peak at lower binding energy (∼456.4 eV) is associated with Ti ions with a reduced charge state (Ti3+).30,41 In STNO, the Nb dopant enters the lattice as Nb5+ ions30 and substitutes for the B-site Ti ions, thus creating a fraction of Ti ions with 3+ valence state.30 Also, since the growth of the SLs is done in 20 mTorr of Ar at 700 K, the STNO layers are grown oxygen deficient. The oxygen deficiency induced nonstoichiometry of SrTiO3 also leads to the formation of Ti3+ ions.42 After annealing, a small positive shift (∼0.3 eV) of the Ti 2p3/2 components was observed. For the as-deposited SLs, the Nb 3d core level could be decomposed into three doublets (Nb 3d5/2 − Nb 3d3/2) with a fixed area ratio equal to 3:2 and doublet separation of 2.75 eV. The binding energies of Nb 3d5/2 for the three components are 203.3, 205.0, and 206.8 eV which is attributed to NbO1−α, NbO2−β, and Nb2O5−γ, respectively (where α, β, and γ represent the varying levels of oxygen vacancies), since the binding energies are lower compared to the stoichiometric NbO (204.7 eV),43,44 NbO2 (205.7),44,45 and Nb2O544,46 phases. Though the Nb signal detected is from both STNO and NbOx layers, the NbO1−α and NbO2−β
2NbO + O2 → 2NbO2 4NbO2 + O2 → 2Nb2 O5
The formation of the noncubic higher niobium oxides is confirmed using HRTEM analysis. The HRTEM image of the as-deposited SLs is shown in Figure 5a. The preferential epitaxial growth of the individual layers is clearly visible for the as-deposited SLs. After the thermoelectric measurement up to 1000 K, the SL structure is destroyed to form a polycrystalline film containing a mixture of STNO and niobium oxide (predominantly Nb2O5) phases (Figure 5b). The gradual destruction of the SL structure (as illustrated by Figure 5c−e) is attributed to the formation of higher oxides of Nb which leads to a huge tensile stress in NbOx layers of the SLs, since the higher oxides possess a much larger unit cell volume compared to NbO. Both in-plane and c-axis elongation of the NbOx layers lead to the destruction of the layered structure. Hence, the highest temperature of stable operation for the SLs is set as 900 K. For stability at higher operating temperatures, SLs may be formed with differently doped STO layers instead of NbOx layers. Diffusion of oxygen and oxidation to form noncubic structures may not happen with differently doped STO layers. In order to confirm the oxidation of the NbOx layers of the SLs upon annealing, as suggested by the temperature 2730
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ASSOCIATED CONTENT
S Supporting Information *
Information on high temperature electrical conductivity of SLs having different thickness of individual layers but same a/b ratio and the irreversible changes in the films once heated above the crossover temperature. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +966-(0)12-808-4477. E-mail: husam.alshareef@kaust. edu.sa. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.R.S.K., H.N.A., and T.M.T. acknowledge the financial support of the KAUST Competitive Faculty-Initiated Collaboration Grant.
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REFERENCES
(1) Ramirez, A. P. Science 2007, 315 (5817), 1377−1378. (2) Mannhart, J.; Schlom, D. G. Science 2010, 327 (5973), 1607− 1611. (3) Dagotto, E.; Tokura, Y. MRS Bull. 2008, 33 (11), 1037−1045. (4) Jang, H. W.; Felker, D. A.; Bark, C. W.; Wang, Y.; Niranjan, M. K.; Nelson, C. T.; Zhang, Y.; Su, D.; Folkman, C. M.; Baek, S. H.; Lee, S.; Janicka, K.; Zhu, Y.; Pan, X. Q.; Fong, D. D.; Tsymbal, E. Y.; Rzchowski, M. S.; Eom, C. B. Science 2011, 331 (6019), 886−889. (5) Mannhart, J.; Blank, D. H. A.; Hwang, H. Y.; Millis, A. J.; Triscone, J.-M. MRS Bull. 2008, 33 (11), 1027−1034. (6) Reyren, N.; Thiel, S.; Caviglia, A. D.; Kourkoutis, L. F.; Hammerl, G.; Richter, C.; Schneider, C. W.; Kopp, T.; Rüetschi, A.-S.; Jaccard, D.; Gabay, M.; Muller, D. A.; Triscone, J.-M.; Mannhart, J. Science 2007, 317 (5842), 1196−1199. (7) Goniakowski, J.; Finocchi, F.; Noguera, C. Rep. Prog. Phys. 2008, 71 (1), 016501. (8) Zubko, P.; Gariglio, S.; Gabay, M.; Ghosez, P.; Triscone, J.-M. Annu. Rev. Condens. Matter Phys. 2011, 2 (1), 141−165. (9) Schlom, D. G.; Chen, L.-Q.; Pan, X.; Schmehl, A.; Zurbuchen, M. A. J. Am. Ceram. Soc. 2008, 91 (8), 2429−2454. (10) Meevasana, W.; King, P. D. C.; He, R. H.; Mo, S. K.; Hashimoto, M.; Tamai, A.; Songsiriritthigul, P.; Baumberger, F.; Shen, Z. X. Nat. Mater. 2011, 10 (2), 114−118. (11) Jalan, B.; Stemmer, S.; Mack, S.; Allen, S. J. Phys. Rev. B 2010, 82 (8), 081103. (12) Park, J. W.; Bogorin, D. F.; Cen, C.; Felker, D. A.; Zhang, Y.; Nelson, C. T.; Bark, C. W.; Folkman, C. M.; Pan, X. Q.; Rzchowski, M. S.; Levy, J.; Eom, C. B. Nat. Commun. 2010, 1, 94. (13) Bark, C. W.; Felker, D. A.; Wang, Y.; Zhang, Y.; Jang, H. W.; Folkman, C. M.; Park, J. W.; Baek, S. H.; Zhou, H.; Fong, D. D.; Pan, X. Q.; Tsymbal, E. Y.; Rzchowski, M. S.; Eom, C. B. Proc. Natl. Acad. Sci. U.S.A. 2011, 108 (12), 4720−4724. (14) Ohta, H.; Kim, S.; Mune, Y.; Mizoguchi, T.; Nomura, K.; Ohta, S.; Nomura, T.; Nakanishi, Y.; Ikuhara, Y.; Hirano, M.; Hosono, H.; Koumoto, K. Nat. Mater. 2007, 6 (2), 129−134. (15) Ohta, H. Mater. Today 2007, 10 (10), 44−49. (16) Kumar, S. R. S.; Barasheed, A. Z.; Alshareef, H. N. ACS Appl. Mater. Interfaces 2013, 5, 7268−7273. (17) Kumar, S. R. S.; Abutaha, A. I.; Hedhili, M. N.; Alshareef, H. N. J. Appl. Phys. 2012, 112 (11), 114104−114107. (18) Ohta, S.; Nomura, T.; Ohta, H.; Hirano, M.; Hosono, H.; Koumoto, K. Appl. Phys. Lett. 2005, 87 (9), 092108. (19) Rowe, D. M. Thermoelectrics Handbook: Macro to Nano; CRC Press/Taylor and Francis: Boca Raton, FL, 2006.
Figure 6. High resolution XPS spectra of (a) Sr 3d, (b) Ti 2p3/2, and (c) Nb 3d for the as-deposited SLs. The insets show the corresponding spectra for the annealed SLs.
contribution to Nb core level are from the NbOx layers while the Nb5+ signal is mainly from the STNO layer. After annealing, the Nb 3d core level could be decomposed into only two doublets. The Nb 3d5/2 core level is dominated by the peak centered at 207.1 eV from Nb2O5 44,46 and with a small contribution (3 atom %) at 205.6 eV from NbO2.44,45 The disappearance of NbO1−α peaks shows the effect of annealing which converts Nb from lower oxidation states to Nb5+.
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CONCLUSION The successful layer-by-layer growth of [(NbOx)a/(Nb-doped SrTiO3)b]20 superlattices on LaAlO3 substrates by pulsed laser deposition was demonstrated for the first time. A preferred epitaxial growth of the superlattices was observed, and the substitution of Nb on Ti sites in STNO layers was established. The in-plane electrical conductivity and absolute Seebeck coefficient were found to increase with the decrease in a/b ratio. The maximum temperature below which the superlattices can be used was found to be 950 K. Above this temperature, diffusion of oxygen from STNO layers was found to oxidize NbOx interlayers, affecting the stability of the superlattices and causing a deterioration of electrical conductivity and a corresponding steep rise in the absolute Seebeck coefficient. 2731
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