Enhanced Cross-Plane Thermoelectric Figure of Merit Observed in an

Article Cite This: J. Phys. Chem. C 2019, 123, 14187−14194

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Enhanced Cross-Plane Thermoelectric Figure of Merit Observed in an Al2O3/ZnO Superlattice Film by Hole Carrier Blocking and Phonon Scattering Won-Yong Lee,† No-Won Park,† Soo-Young Kang,† Gil-Sung Kim,† Jung-Hyuk Koh,‡ Eiji Saitoh,§,∥,⊥ and Sang-Kwon Lee*,†

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Department of Physics and ‡School of Electrical and Electronics Engineering, Chung-Ang University, Seoul 06974, Republic of Korea § Institute for Materials Research and ∥WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ⊥ Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan ABSTRACT: We experimentally investigate the cross-plane figure of merit (ZT) for an Al2O3/ZnO (AO/ZnO) superlattice film by measuring cross-plane electrical and thermal conductivity and Seebeck coefficient using the 3-ω method and an in-house Seebeck coefficient measurement system recently developed for 300−500 K and examine how ZT factors depend on the AO layer inside the AO/ZnO superlattice film using measured thermoelectric properties. The AO/ZnO superlattice film exhibited maximum power factor of ∼276.2 μW/m K2 with low thermal conductivity (∼0.31 W/m K), producing ZT ≥ 0.45 at 500 K, which is approximately 2,650% improvement compared with an undoped ZnO film (∼0.017). The enhanced ZT performance of the AO/ZnO superlattice film can be explained by enhanced phonon scattering at the interface and minority carrier blocking at the interfacial barrier due to the AO layer, suggesting that the interfacial AO layer is important to enhance ZT in oxide-based films. These results open new applications for micro- or nanoscale thin film-based thermoelectric devices.

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INTRODUCTION Thermoelectric (TE) materials have wide potential for power generation, heat pump, and cooling or refrigeration applications. However, efficient device applications require materials with high Seebeck coefficients (S), low thermal conductivity (κ), and high electrical conductivity (σ), achieving large figure of merit, ZT = S2σT/κ, where T is absolute temperature.1−6 Superlattice films have attracted considerable attention toward TE devices due to their low dimensionality and lattice thermal conductivity, which can greatly improve ZT due to increased density of states (DOS) near Fermi level and enhanced phonon scattering at the superlattice film interlayer.5,7−9 Both electrons and phonons exhibit highly anisotropic behavior in superlattice films, with strong interface scattering in the crossplane direction compared to the in-plane direction. Venkatasubramanian et al. reported remarkable ZT ≈ 2.4 and 1.5 for pand n-type Bi2Te3/Sb2Te3 superlattice films, respectively.3,10,11 They argued that enhanced electrical conductivity together with suppressed lattice thermal conductivity enhances the ZT value because the superlattice structure reduces phonon heat conduction while maintaining or even enhancing electron transport. The Seebeck coefficient and thermal power factor (α = S2σ) can also be improved by energy filtering with a potential barrier in the films; cold (i.e., low energy) carrier filtering © 2019 American Chemical Society

occurred due to the potential barrier formed by Fermi level alignment, enhancing the Seebeck coefficient and power factor.12−14 Liang et al. showed significant TE power factor enhancement for organic−inorganic nanocomposites of poly(3-hexylthiophene)-tellurium nanowires by appropriately adjusting energetics at organic−inorganic interfaces.14 Thus, the superlattice film interlayer is important for Seebeck coefficient enhancement as well as suppressing lattice thermal conduction, hence increasing ZT. However, most previous studies focused on ZT improvement and rarely studied the interlayer role in superlattice films through detailed experiments, that is, how the barrier or interlayer influences thermal conductivity, Seebeck coefficient, power factor, and ZT in superlattice films. Oxide materials are promising alternatives to conventional lead-, bismuth-, antimony-, and tellurium-based TE materials because of their thermal and chemical stability, high natural abundance, lower toxicity, cost effectiveness, easy carrier controls, and high-temperature stability across a wide range of temperatures.15−19 Zinc oxide (ZnO), in particular, is promising for TE devices, providing a high-temperature heat to Received: February 14, 2019 Revised: May 23, 2019 Published: May 23, 2019 14187

DOI: 10.1021/acs.jpcc.9b01471 J. Phys. Chem. C 2019, 123, 14187−14194

Article

The Journal of Physical Chemistry C

Figure 1. (a) Schematic images of the four-point-probe 3-ω method for measuring cross-plane AO/ZnO superlattice film with (b) cross-sectional image on the area “A” in (a). (c,d) Schematic images of cross-plane Seebeck coefficient measurement for the samples. (e) Schematic image for measuring two-probe electrical conductivity of the samples.

resultant AO/ZnO superlattice film was ∼210 nm thick with ∼2 at. % AO insertion, comprising ∼0.82 nm-thick AO layers with adjacent ∼43 nm thick ZnO layers. The ZnO thin film was grown on the same substrate with similar thickness (∼200 nm). A detailed sample preparation description, particularly for the AO/ZnO superlattice film, can be found in our previous works.17,22 Surface morphologies, crystal structure, and crosssectional interface information with composition profile were studied using high resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray spectrometry (EDX). Figure 1a,b shows four point probe 3-ω measurement outcomes for cross-plane thermal conductivity.23−28 Detailed information with regard to heater and electrode fabrication on the samples can be found elsewhere.25 This technique is one of the best techniques to evaluate temperature-dependent thermal conductivity for thin and thick films with uncertainty ≤4%. Superlattice film thermal conductivity is generally determined from the 3-ω measurement as

electricity conversion environment due to high carrier mobility. Doping Zn sites can improve electrical properties by tuning carrier concentration and bandgap, and several dopants, including Al, Fe, In, Ti, Mn, and Ga have been evaluated, with doping efficiency for Al and Ga being much higher than other dopants.20,21 The current best TE performance for ZnO materials, ZT ≈ 0.65 at 1247 K, was achieved with dual Al and Ga doping of ZnO ceramics at the Zn site.15 The current authors recently showed that 2% Al-doped ZnO (AO/ZnO) superlattice was a promising candidate for high-temperature TE devices, suppressing thermal conductivity as well as enhancing S.18,22 However, further systematic studies are required with regard to interlayer and interface roles in the superlattice films by measuring cross-plane TE ZT, including thermal and electrical conductivity and S for the doped-ZnO superlattice films at various temperatures to clearly explain the underlying physics for the oxide based superlattice films. This study investigated cross-plane TE properties, including electrical and thermal conductivity and S for approximately 210 nm thick atomic layer deposition (ALD) Al2O3/ZnO superlattice film from 300 to 500 K and ZT dependence on the energy potential barrier. Undoped ALD-prepared ZnO films (approximately 200 nm thick) were also analyzed to clarify phonon-impurity scattering effects. The AO/ZnO superlattice film exhibited maximum power factor ≈ 276.2 μW/m K2 and thermal conductivity ≈ 0.31 W/m K, resulting in ZT ≥ 0.45 at 500 K, which is an approximately 2,650% improvement compared to undoped ZnO film (∼0.017 at 500 K).

κf =

Pdf 2b{ΔTs + f (ω) − ΔTs(ω)}

(1)

where P and b are the metal line power per unit length and width, respectively; κf is the superlattice film thermal conductivity; ΔTs+f(ω) and ΔTs(ω) are in-phase temperature oscillation components with and without the superlattice thin film, respectively. All measurements were performed in a vacuum probe station (MS Tech, Korea) with pressure < 10−3 Torr from 300 to 500 K. The measured thermal conductivities for both films, measured by the 3-ω method, are generally considered as total thermal conductivities including the electrical and lattice thermal conductivities, subsequently analyzing the difference in the effect of phonon scattering between the two samples through the theoretical calculation. Figure 1c,d shows the S measurement setup for the thin films.

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EXPERIMENTAL SECTION Two samples, 0.82 nm thick AO-inserted ZnO (AO/ZnO) superlattice and undoped ZnO film, were prepared on approximately 0.2 μm thick SiO2/Si(100) substrates by ALD. For the AO/ZnO superlattice film, we sequentially alternated six cycles of trimetylaluminum and diethylzinc in ALD. The 14188

DOI: 10.1021/acs.jpcc.9b01471 J. Phys. Chem. C 2019, 123, 14187−14194

Article

The Journal of Physical Chemistry C

Figure 2. (a) Schematic images of both samples deposited on the SiO2 (∼0.2 μm in thickness)/Si(100) substrates. (b,c) Cross-sectional TEM and HR-TEM images of undoped ZnO thin film and (d,e) AO/ZnO superlattice film (insets in (c,e) show FFT patterns for ZnO layers); crosssectional STEM images for AO/ZnO superlattice film (f) dark (DF) and (g) bright (BF) field; (h) STEM image for AO/ZnO thin film and corresponding elemental signal for Al across the superlattice film; EDX mappings for (i) Al (K line, green), (j) Zn (K line, purple), and (k) O (K line, blue) corresponding to (h). Scale bars in (h−k) represent 20 nm.

We recently developed a system to measure cross-plane S for superlattice films, sandwiched by two Pt heaters (Figure 1c). The Pt heaters were first calibrated in a vacuum probe station for 100−300 K and temperature coefficient ≈ 2.1 Ω/K (R2 = 0.999). To achieve better contact between the top Cu foil and film, a 200 nm thick Cu film was evaporated on the top of the films before fixing the top Cu foil on the film. Two K type thermocouples were soldered to the upper and lower Cu layer surfaces to measure temperature difference (ΔT) between the electrodes (Figure 1c). In particular, our ΔT measurement setup consisted of both Cu foils (top and bottom) with high thermal conductivity (∼400 W/m K at 300 K) as well as Cu films between the Cu foil and the AO/ZnO superlattice film, which in turn leads to minimize the effect of interface thermal resistance on the measured ΔT values. Applying an electric current to the lower Pt heater increases the lower Cu plate temperature, generating a temperature in the samples. Measured temperature differences between upper and lower Cu electrodes were transferred to the computer using CompactDAQ with LabVIEW software (National Instruments, USA). We continuously measured (ΔT) and voltage difference (ΔV) while increasing the lower Pt heater input power, and S was derived by fitting S = −ΔV/ΔT (Figure 1d). The measured S for the AO/ZnO superlattice and undoped ZnO films can be expressed as Sthin film = Smeasured − SCu

electrodes using a two-point-probe configuration (Figure 1e), using the same measurement setup as for sample S (Figure 1c).

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RESULTS AND DISCUSSION Material Characterization. Figure 2a shows both an undoped ZnO thin film and AO/ZnO superlattice consisting of periodic 6-pair superlattice layers in the deposition direction. To investigate the sample structural properties, we prepared cross-sectional TEM specimens with focused-ion beam ion milling. Figure 2b shows cross-sectional TEM image for a ZnO thin film, indicating growth of the uniform ∼200 nm thin film. Enlarged HR-TEM (Figure 2c) confirms a grain boundary between different oriented ZnO crystals, generated by columnar structure formation in the growth direction. Figure 2d shows cross-sectional TEM image for the AO/ZnO thin film, indicating formation of periodic ∼210 nm thick 6pair superlattice layers. The periodic appearance of bright and dark contrasts could be attributed to alternating AO and ZnO interlayers. This configuration is consistent with our previous results.17,18,22 The enlarged HR-TEM (Figure 2e) shows the crystalline ZnO layer (dark contrast) for the AO/ZnO superlattice film with a thickness of ∼43 nm. We also observed the AO insertion of ∼0.82 nm using spectroscopic ellipsometry. Interplanar d-spacing = 0.2609 and 0.2489 nm, from preferential fast Fourier transform (FFT) patterns of the ZnO layers (Figure 2c,e, insets, red circles), assigned to (002) and (101) lattice planes of hexagonal ZnO, respectively, according to the standard JCPDS card no. 36-1451. Thus, AO/ ZnO superlattice and undoped ZnO thin films both have polycrystalline nature corresponding to the wurtzite structure. Figure 2f,g shows dark field and bright field cross-sectional scanning-TEM (STEM) images, respectively, clearly exhibiting alternating 6-pair AO and ZnO interlayers in the superlattice structure. We used cross-sectional HAADF-STEM imaging with EDX line-scanning to further identify elemental

(2)

where Sthin film, Smeasured, and SCu are the Seebeck coefficients for the AO/ZnO superlattice or undoped ZnO films, Cu/thin film/Cu layers, and of plate layer only, respectively. Thus, SCu should be measured without thin films to evaluate Sthin film. We found SCu ≈ ∼+1.28 μV/K at 300 K. The cross-plane Seebeck coefficients were measured from 300 to 500 K in a vacuum probe station, with electrical conductivity derived from measured conductance between the upper and lower Cu 14189

DOI: 10.1021/acs.jpcc.9b01471 J. Phys. Chem. C 2019, 123, 14187−14194

Article

The Journal of Physical Chemistry C

Figure 3. AO/ZnO superlattice and undoped ZnO films (left and right, respectively) (a) atomic structures; (b) cross-plane electrical conductivity; (c) Seebeck coefficients, and (d) power factor. Uncertainties for electrical conductivity, Seebeck coefficient, and power factor ≤ 4, 8, and 12%, respectively.

configuration of the AO/ZnO superlattice film. Figure 2h shows the STEM image of the AO/ZnO superlattice, and the inset shows the elemental signal for Al. The Al signal periodic oscillation exhibits good agreement with the alternating AO layer thickness, hence allowing discrimination of two different material classes forming the superlattice. We also performed EDX mappings for Al (K line, green), Zn (K line, purple), and O (K line, blue) corresponding to the STEM image (Figure 2h). Yellow dashed lines indicate periodic AO layers between adjacent ZnO layers. As shown in Figure 2i, high-content Al signals were detected from the AO layers, which is mostly consistent with the STEM image and EDX line-scanning (Figure 2h). In contrast, low-content Al signals were also observed on the cross-sectional ZnO surface. This detection is caused by inevitable surface contaminants of sputtered AO particles during Ga-ion milling process. Figure 2j,k shows EDX mapping for Zn and O, indicating the presence of Zn and O all over the superlattice film, respectively corresponding to the ZnO and AO layers. These results provided sufficient information for confirming the presence of AO layers between adjacent ZnO layers in the AO/ZnO superlattice film. Electron Transport in Both AO/ZnO Superlattice and Undoped ZnO Films. Figure 3a−d shows cross-plane electrical conductivity, Seebeck coefficients, and power factors for AO/ZnO superlattice and undoped ZnO thin films from 300 to 500 K with the sample atomic structures (Figure 3a), respectively. Electrical conductivity uncertainty ≤4% over the whole temperature range (Figure 3b) and average cross-plane electrical conductivity for AO/ZnO superlattice film ≈ 83.1 ± 3.3 S/cm, which is approximately 1.7 times that of undoped ZnO film (≈50.0 ± 1.2 S/cm) over the entire temperature range. This electrical conductivity enhancement arises from AO insertion in the superlattice film and is consistent with previous Al-doped ZnO film studies.29−32 Figure 3b shows that the AO/ZnO superlattice film has weak electrical conductivity

temperature dependence 300−500 K, similar to quasi-metallic behavior due to the high carrier concentration.17,33 Figure 3c,d shows temperature-dependent S and power factors with uncertainties