Synthesis and Characterization of Al- and SnO₂-Doped ZnO Thermoelectric Thin Films

Abstract

The effect of SnO₂ addition (0, 1, 2, 4 wt.%) on thermoelectric properties of c-axis oriented Al-doped ZnO thin films (AZO) fabricated by pulsed laser deposition on silica and Al₂O₃ substrates was investigated. The best thermoelectric performance was obtained on the AZO + 2% SnO₂ thin film grown on silica, with a power factor (PF) of 211.8 μW/m·K² at 573 K and a room-temperature (300 K) thermal conductivity of 8.56 W/m·K. PF was of the same order of magnitude as the value reported for typical AZO bulk material at the same measurement conditions (340 μW/m·K²) while thermal conductivity κ was reduced about four times.

1. Introduction

The increasing worldwide demand for energy and the resultant depletion of fossil fuels have brought new challenges for the scientific community [1]. One of the major issues is to develop high-efficiency devices for capturing energy from abundant natural sources such as solar, wind and geothermal energy. Another surplus, but mostly unused, source of energy is wasted heat. There are huge waste heat sources in our environments covering a wide range of temperatures (300~1200 K): industrial processes, domestic stoves and radiators, electrical lighting, pipelines, electrical substations, subway networks, automotive exhaust tubes, but also geothermal heat, body heat, and so on: about 66% of the annual world energy consumption is lost as waste heat, and the loss corresponds to the stellar amount of 3·10²⁰ J per year, just considering the past 10 years [2,3]. A highly promising method for energy recovery from such heat sources is the utilization of thermoelectric (TE) materials that can convert various types of waste heat flow into electricity. The possibility of conversion of heat in electricity was discovered in 1821 by Thomas Seebeck [4]: a junction of two metals generates the thermoelectric voltage ΔV when a temperature gradient ΔT is created across it. In mathematical form it can be expressed as:

ΔV = S · ΔT

(1)

where S is the material-dependent Seebeck coefficient. The performance of any TE material is quantified by the figure of merit (ZT):

$$ZT=\frac{\sigma S^2}{\left({\kappa }_{el}+{\kappa }_{ph}\right)}T$$

(2)

where σ is the electrical conductivity, S the Seebeck coefficient, T the absolute temperature, κ_el the electrical thermal conductivity and κ_ph the phononic thermal conductivity being κ = κ_el + κ_ph the total thermal conductivity.

ZT is related to the efficiency η of heat/electrical energy conversion by the relation:

$$\eta =\frac{∆T}{T_H}\left(\frac{\sqrt{1+Z{T}_{ave}}-1}{\sqrt{1+Z{T}_{ave}}+\frac{T_C}{T_H}}\right)$$

(3)

State-of-the-art TE materials for heat conversion can operate at T = 300~1200 K, with ZT = 0.1~2.6, corresponding to η = 1~20%. Therefore, after two centuries, TE devices are seldom utilized in daily life. TE modules have obtained brilliant success, however, in niche applications, for example for powering the space probe Cassini [5], while high-conversion modules to harvest waste heat of car engines remained at the state of prototypes [6]. ZT and η must be strongly improved to make TE power competitive with the common thermodynamic cycles based on fossil fuel burning, solar conversion and nuclear power plants. The improvement of ZT can be obtained by enhancing σ and/or S, or the product σS² which is called “power factor” (PF), and/or by decreasing the total thermal conductivity. However, κ_el = LTσ, with L = 2.44 × 10⁻⁸ W·Ω·K⁻² is the Lorentz number while κ_ph = 1/3 CvЛ where C is the heat capacity, v is the speed of phonons, Л is the phonon mean free path. It is thus necessary to decouple σ and κ_el and work on the depression of κ_ph.

Since the discovery of the Seebeck effect, the benefit of TE harvesting has been understood and a wealth of TE materials has been discovered. After the discovery of BiTe with ZT = 0.5 [7] in 1954, the ZT of TE materials did not improve for a very long time. In 1993, a seminal paper [8] theoretically predicted the drastic depression of κ_ph due to reduction of the mean free path of phonons (Л) to a few nanometers, which resulted in substantial improvement of ZT. This concept was firstly validated in Bi₂Te₃/Sb₂Te₃ multilayer thin films incorporating natural nano-sized precipitates, showing the extremely small κ value (0.22 W·m⁻¹ K⁻¹) and a huge value of ZT = 2.5 at T = 300 K [9].

These results were due to the scattering of phonons by nano-defects spontaneously formed inside the thin films. However, TE harvesters based on BiTe and related materials cannot be produced on a large scale because they contain rare (Te) and poisonous (Sb, Bi, Pb) elements, and require high-cost processing. Though the last ten years a great progress has been made with ZT = 2.6 for SnSe single crystals [10] and 3.1 for polycrystalline SnSe [11], there is still room for finding even more efficient TE materials in a wide T region.

In particular, the attention of researchers has been focused on the development of stable, environmentally benign, abundant, and cost-effective TE materials based on oxides. In the 1990s, extensive research in the area of bulk oxides focused on the enhancement of the TE performance using atomic substitutions and improved grain connection [12,13,14,15]. To date, the best TE performance of the currently available oxide materials is ZT = 0.64 for n-type Zn_0.96Al_0.02Ga_0.02O [13] and 0.74 for p-type Ca_2.5Tb_0.5Co₄O₉ [14] at 1000 K. The ZT of oxides is not yet up to the level of the best conventional TE materials and needs to be drastically improved to be acceptable for practical applications. The bulk oxides also have the main disadvantages of requiring a long time for sintering and fabrication of the n- and p- elements and their assembly in modular shape, and mechanical fragility. All these drawbacks can be overcome by using oxide thin films. Thin-films have significant advantages, such as low dimensionality, rapid fabrication, control of strain at the interface with substrates, and the possibility to insert artificial nano-defects to improve the phonon scattering. Thermoelectric thin films are also attractive for their applicative potential for energy harvesting to power Internet of Things (IoT) sensors [15,16].

Recently, our group studied oxide-thin TE films and obtained encouraging preliminary results. First, epitaxial thin films of 2% Al-doped ZnO (AZO) were fabricated by pulsed laser deposition (PLD) on several single crystal (SrTiO₃, Al₂O₃) and amorphous (silica) substrates. Regardless of a particular substrate, the films always showed higher values of ZT in comparison with the corresponding bulk AZO: for example, at T = 600 K, ZT_AZO-on-STO = 0.03 while ZT_bulk = 0.014 [17,18]. The superior performance of films is due to their lower thermal conductivity: κ_AZO-on-STO (300 K) = 6.5 W/m·K [16,17] while κ_bulk (300 K) = 34 W/m·K. In these series, the grain boundaries can be considered as natural nanodefects for an enhanced scattering of phonons and consequent decrease of κ. As a demonstration of this effect, the film on fused silica, showing additional grain boundaries at the seed layer on the substrate, had even lower thermal conductivity: κ_silica (300 K) = 4.89 W/m·K and larger ZT: ZT_silica (600 K) = 0.045 [18].

The insertion of artificial nanodefects has been subsequently considered with the purpose of further reduction of κ and enhancing ZT. Several approaches have been tried by our group: insertion of hydroquinone nanolayers in AZO films prepared by atomic layer deposition (ALD): κ_ALD (300 K) = 3.56 W/m·K [19]; addition of polymethylmethacrylate (PMMA) particles to AZO films prepared by multi-beam multi-target matrix-assisted PLD (MBMT/MAPLE-PLD): κ_MAPLE (300 K) = 5.9 W/m·K and ZT_MAPLE (600 K) = 0.0061 [20]; formation of nanopores in AZO films prepared by mist-chemical vapor deposition (mist-CVD): κ_porous (300 K) = 0.60 W/m·K and ZT_porous (300 K) = 0.057 [21]; dispersion of Al₂O₃ nanoparticulate in AZO films prepared by surface-modified target PLD: κ_nanoAl2O3 (300 K) = 3.98 W/m·K and ZT_nanoAl2O3 (600 K) = 0.0007 [22]. Several groups worldwide have recently obtained excellent results with different kinds of dopant, like the κ (300 K) = 1.19 W/m·K and ZT (300 K) = 0.1 of magnetron sputtered AZO films by Loureiro et al. [23], κ (323 K) = 3.37 W/m·K and ZT (423 K) = 0.052 of CNT-added evaporated porous AZO films by Liu et al. [24], the κ (300 K) = 1.1 W/m·K and ZT (300 K) = 0.042 of amorphous ZnO_xN_y PLD films by Hirose et al. [25], the κ (300 K) = 1.8 W/m·K and ZT (383 K) = 0.019 of dual-doped AlGaZnO PLD films by Nguyen, et al. [26], just to cite the most impressive. All these successful examples highlight the promise of nanostructured doped ZnO films for future energy-harvesting applications.

In this work, we have focused on enhancing the thermoelectric performance of Al-doped ZnO films by the addition of a controlled amount of SnO₂ as a secondary dopant. AZO has the same structure of hexagonal ZnO: wurtzite, hexagonal, space group P6₃mc, cell parameters a = b = 0.3289 nm and c = 0.5307 nm, while SnO₂ is cassiterite, tetragonal, space group P4₂/mnm with cell parameters a = b = 0.4832 nm and c = 0.3243 nm. Since the mismatch between the a-axis of AZO and the c-axis of SnO₂ is 1.4%, we forecasted the growth of quasi-epitaxial nanostructures (nanoparticles) of SnO₂ with c_SnO2//a_AZO, dispersed in the AZO matrix.

The motivation of this work is to prove the presence of SnO₂ nanoparticulates in the AZO matrix and verify the optimal content of SnO₂ for the TE properties of the nanocomposite Al- and SnO₂-doped (AZO + SnO₂) thin films.

2. Materials and Methods

Two sets of four thin films were prepared using various compositions and substrate materials. All samples were deposited both on amorphous silica and c-axis oriented alumina by pulsed laser deposition (PLD) technique focusing a Nd:YAG (266 nm, 10 Hz) laser on dense pellets. Four commercial targets (Kurt Lesker Ltd., Jefferson Hills, PA, USA) used for film making had different contents of SnO₂ (0, 1, 2 and 4 wt.%) dispersed in the Al-ZnO (AZO) main phase where the Al content was kept as 2 wt.%. Prior to the deposition, the substrates were cleaned at 773 K for 2 h, then glued by conductive silver glue to a rectangular Inconel plate, which was put in direct thermal contact with a cartridge heater and finally inserted into the vacuum chamber. All films were grown with an energy density of about 4.2 J/cm² for 60 min at 573 K, in the atmosphere of 20 mtorr of oxygen. The samples deposited on silica are named AZO_xS and the ones deposited on alumina AZO_xA, with x = 0, 1, 2 and 4 referring to the concentration (wt.%) of the SnO₂ in the respective targets.

Electrical conductivity (σ) and Seebeck coefficient (S) were measured by means of the four-probe method using a ZEM-3 (ULVAC Advance-Riko, Yokohama, Kanagawa, Japan) apparatus under partial He pressure to assure thermal transport between the heater and the sample. Measurements with ZEM-3 were conducted performing three cycles between 353 and 573 K and measuring both heating and cooling processes. The σ and S curves reported in Section 3.2 for each sample were those obtained from the last cooling cycle, considered as the most stable and reliable. The uncertainties of the ZEM-3 measurements were ±4% for the electrical conductivity and ±3% for the Seebeck coefficient.

For comparison and validation of data, some selected samples were also characterized by a custom-made apparatus whose working principle and measurement protocol are reported elsewhere [27,28]. The error in the thermoelectric coefficients measurements is ±5% in the case of σ, and ±7% for S.

The thickness of the thin films (in the range 295–585 nm, as summarized in Table 1) was measured using a Dektak 6M Stylus Profiler (Bruker, Billerica, MA, USA) and a Filmetrics Profilm 3D (KLA, Milpitas, CA, USA).

The X-ray diffraction analysis was conducted using a Bragg-Brentano powder diffractometer (Smart Lab 3, Rigaku Corporation, Tokyo, Japan) using the Cu K_α radiation in the 10–100° angular range with an angular step of 0.02° (power settings: 40 mA, 40 kV).

The TEM images and EDS analysis were conducted using a JEM-2100F (JEOL, Akishima, Tokyo, Japan) with EDS spectroscopy. The STEM beam size was set to 0.5 nm.

The picosecond time-domain thermo-reflectance (TD-TR) technique using a customized system PicoTR (PicoTherm, Netzsch Japan KK, Tsukuba, Japan) was utilized to measure the thermal conductivity of the samples at room temperature (T = 300 K) in the cross-plane direction.

Measurement of Hall coefficient at room temperature was performed using a standard four-terminal method with a commercial PPMS instrument (Quantum Design, San Diego, CA, USA) at an AC current of 5 mA.

3. Results and Discussion

3.1. Structural and Morphological Characterization

The XRD spectra of the thin films fabricated on amorphous silica and single-crystal c-axis oriented Al₂O₃ are presented in Figure 1a,b, respectively.

On both substrates, the preferred orientation of AZO films was along the c-axis since only (00l) reflection appeared. The presence of SnO₂ in the films of both series was undetectable by X-rays even for the highest amount (x = 4).

The cross-sectional TEM images (magnification 150,000, Figure 2a and Figure 3a of two representative samples deposited on fused silica without (AZO_0S) and with the addition of SnO₂ (2 wt.%, AZO_2S) reveal a typical columnar growth along the c-axis of the films as often reported in the literature for AZO PLD thin films [16,17,19,21]. The addition of SnO₂ (AZO_2S) did not affect the morphology of the films. The higher magnification images (300,000×, Figure 2b and Figure 3b for AZO_0S and AZO_2S, respectively) show that the columns were well connected without the presence of pores or misoriented grains so that the density of the films could be considered close to the theoretical value.

The TEM-EDS mapping performed on the sample AZO_2S (Figure 4) reveals a uniform distribution not only of the elements Al, Zn, O, but also of Sn. Contrary to the predictions based on the 1.4% lattice mismatch, Sn did not form nanoscale aggregates in the AZO matrix, but presumably entered as substitutional or interstitial atoms in the AZO elementary cell. According with TEM-EDS analysis, the atomic percentage of Sn in the film prepared with 2 wt.% SnO₂-added AZO target is 0.49%. The atom % of oxygen is 33.91%. Since it is not possible to separate the contribution of oxygen from SnO₂ and from ZnO, we can only compare the percentage of Sn in the target and in the film. The percentage of Sn in SnO₂ is 78.77%, which in 2 wt.% SnO₂ corresponds to 1.58%. This means that in the film the amount of Sn is about 1/3 than in the target. Si is not considered in the discussion being part of the substrate only.

3.2. Transport and Thermoelectric Characterization

Room temperature Hall measurement was taken to find the carriers’ concentration n_H and mobility μ_H values for the AZO_xS and AZO_xA films. The results are in Figure 5, together with room temperature resistivity values from the ZEM-3 analysis.

The resistivity at room temperature has a clear ascending trend with the increase of the content of SnO₂ for both substrates. The films on Al₂O₃ had half the resistivity of those on silica. The range of n_H was almost the same for two substrates, and of the order of 10²⁰ cm⁻³, as expected for a typical semiconductor such as AZO. The maximum number of carriers occurred at x = 1 on silica and at x = 2 on Al₂O₃. The carrier mobility had the maximum at x = 2 on both substrates but was about 20 times larger for AZO_2A than for AZO_2S. AZO_0S had greater mobility than AZO_0A and this could be explained by the larger misfit dislocations generated on Al₂O₃, due to a lattice misfit of 15% [18,29]. Surprisingly, by increasing the content of SnO₂ doping, there was a reversal trend and the films grown on Al₂O₃ presented higher mobility, with the maximum value for AZO_2A-20 times larger than for AZO_0S.

The electrical conductivity (σ) for the AZO films is presented in Figure 6 as a function of temperature in the range 300–600 K. Figure 6 shows that σ increases with rising temperature, confirming the semiconductor behaviour of the samples. On both substrates, the film grown without additional SnO₂ had the largest σ followed by the film with x = 2.

The temperature dependence (300–600 K) of Seebeck coefficient (S) of the AZO films is plotted in Figure 7. S had a negative sign, confirming the n-type conductivity of AZO. Figure 7 shows that S decreases with the rising temperature, a reversal trend with respect to σ as commonly expected for semiconductor materials. On both substrates, the pure AZO film had the lowest S while the sample fabricated with a target containing 4% of SnO₂ presents the largest one.

The Seebeck coefficient can be expressed as [30]:

$$S=\left(\frac{8{\pi }^2{k}_B^2}{3e{h}^2}\right)m^{\ast }T{\left(\frac{}{3n}\right)}^{2/3}$$

(4)

where k_B is Boltzmann’s constant, h is Planck’s constant, T is the absolute temperature, m* is the effective mass and n is the carrier concentration. The effective mass is also expressed as m* = α m_e, where m_e is the mass of the electron and α is a positive rational number.

The room temperature (300 K) “Pisarenko plots” (S versus n_H) for both series of films are presented in Figure 8. According to the graphs, m* was (0.3–0.6) m_e and (0.2–0.7) m_e for the films grown on silica and alumina, respectively. The fractional me was consistent with the parabolic band approximation and relatively simple band structure reported for ZnO [31].

The calculated power factor (PF) as a function of temperature is shown in Figure 9. The maximum power factor value was found for sample AZO_2S at 573K as 211.78 mW/m·K². This value was of the same order of magnitude as that measured for bulk AZO at the same conditions (−340 mW/m·K²) [32] and was also comparable with the values reported for AZO thin films [17,18,23,24,25,26].

The PFs of all films at 353 K and 573 K are summarized in Figure 10, making clear that on both substrates, 2% of SnO₂ represents the optimal dopant concentration.

For the films AZO_0S and AZO_2S deposited on silica the measurements were repeated using two different apparatuses to verify the accuracy of the reproducibility of data. The comparison between the pair sets of data for σ, S and PF is reported in Appendix C.

The transport and thermoelectric properties of the films are summarized in

Table 1. Thickness, electrical and thermoelectric properties of the AZO films deposited on silica (series AZO_xS) and on Al₂O₃ (series AZO_xA) for different concentrations x (%) of SnO₂ dopant.

Sample	Thickness [nm]	nH [cm−3]	μH [cm2/V·s]	σ [S/cm]		S [μV/K]		PF [μW/m·K2]		$\mathit{k}$ [W/m·K]
		(300 K)	(300 K)	(353 K)	(573K)	(353 K)	(573K)	(353 K)	(573K)	(300 K)
AZO_0S	595	−7.812·1019	3.571	394.7	405.3	−40.41	−58.66	64.47	139.5	8.88
AZO_1S	505	−1.671·1020	4.503	231.2	245.0	−48.07	−68.02	53.43	113.4	-
AZO_2S	300	−1.061·1020	7.119	369.2	385.0	−48.13	−74.17	85.54	211.8	8.56
AZO_4S	295	−5.865·1019	0.8606	59.63	80.23	−68.57	−95.73	28.03	70.70	-
AZO_0A	535	−6.832·1019	2.576	547.8	561.4	−23.38	−29.61	29.94	49.21	-
AZO_1A	498	−1.124·1020	6.345	229.8	247.7	−47.66	−71.72	52.19	127.4	-
AZO_2A	535	−1.986·1020	125.1	301.4	307.6	−49.00	−71.87	72.38	158.9	-
AZO_4A	480	−1.394·1020	2.143	123.7	155.4	−44.06	−67.45	24.01	70.70	-

.

The room temperature (300 K) thermal conductivity (κ) of the sample with the highest PF (AZO_2S) and of a reference sample from the same batch (AZO_0S) was evaluated by the TD-TR method [32,33,34,35] according to the details described in Appendix A. The value of κ was 8.88 and 8.56 W/m·K for samples AZO_0S and AZO_2S, respectively. These values were in the same range as those reported for pure AZO films deposited by PLD [16,17,21]. Therefore, the four-fold decrease of κ with respect to bulk [32] must be attributed to the phonon scattering of grain boundaries, and not to the addition of SnO₂. The calculation of ZT (T = 353 K) using Equation (2) gave the following result: ZT (AZO_0S) = 0.003 and ZT (AZO_2S) = 0.004, of the same order as for AZO thin film deposited by PLD on different substrates. Conservative estimates of ZT′ (T′ = 573 K) were ZT′ (AZO_0S) = 0.009 and ZT′ (AZO_2S) = 0.014. Since for oxide thin films κ is expected to decrease with T [33], ZT was calculated using a conservative approach from the PF values at 353 K or 573 K, κ at 300 K, and T = 353 K or 573 K.

These values of ZT did not surpass the reported performance of bulk AZO at the same conditions and were also of the same or lower level as for other AZO films prepared by PLD. This indicated that SnO₂ could not be considered as an optimal dopant for AZO thin films for thermoelectric applications. On the other hand, it could be that the fine dispersion of SnO₂ into AZO matrix enhanced the photoelectric response of AZO, as reported for SnO₂/ZnO hierarchical nanostructures prepared by the electrospinning method [34], or could be used to fabricate compact gas sensors, as reported for ZnO–SnO₂ nanofibers [35].

4. Summary

In summary, we fabricated using pulsed laser deposition (PLD) two series of thermoelectric thin films of Al-doped ZnO (AZO) doped with various concentrations of SnO₂ (x = 0, 1, 2, 4 wt.%) on the substrates made of fused silica and Al₂O₃ (001) single crystals. The goal was to enhance the thermoelectric performance. All the films were c-axis oriented. The films deposited on silica showed the highest values of Seebeck coefficient (S) and power factor (PF) in comparison with the films on Al₂O₃ with the same content of SnO₂. The AZO film on silica with x = 2 showed the best performance, with σ = 385.0 S/cm, S = –74.17 mV/K, and PF = 211.8 μW/m·K² at the maximum operating temperature (573 K). The room temperature (300 K) thermal conductivity of this sample was evaluated as κ = 8.56 W/m·K, with a calculated figure of merit ZT (353 K) = 0.003 and a projected value of ZT (573 K) = 0.014, comparable with ZT of the nanostructured PLD AZO films grown on several substrates.