Enormous thermoelectric power factor of ZrTe2/SrTiO3 heterostructure

Achieving high thermoelectric power factor in thin film heterostructures is essential for integrated and miniaturized thermoelectric device applications. In this work, we demonstrate a mechanism to enhance thermoelectric power factor through coupling the interfacial confined two-dimensional electron gas (2DEG) with thin film conductivity in a transition metal dichalcogenides-SrTiO3 heterostructure. Owing to the formed conductive interface with two-dimensional electron confinement effect and the elevated conductivity, the ZrTe2/SrTiO3 (STO) heterostructure presents enormous thermoelectric power factor as high as 4 × 10 μWcmK at 20 K and 4800 μWcmK at room temperature. Interfacial reaction induced degradation of Ti cations valence number from Ti to Ti is attributed to be responsible for the formation of the quasi-two-dimensional electrons at the interface which results in very large Seebeck coefficient; and the enhanced electrical conductivity is suggested to be originated from the charge transfer induced doping in the ZrTe2. By taking the thermal conductivity of STO substrate as a reference, the effective zT value of this heterostructure can reach 15 at 300 K. This superior thermoelectric property makes this heterostructure a promising candidate for future thermoelectric device, and more importantly, paves a new pathway to design promising highperformance thermoelectric systems.


Introduction
Seeking high performance thermoelectric thin films is a hot topic in condensed matter physics and materials science and is attracting growing interests recently [1][2][3][4][5][6] . The development of micro-cooling devices and miniaturized self-powered devices is technically important for improvement of large-scale integrated circuits, which drives the study of thermoelectric thin films. For example, in microprocessors, the "hot spot" regions with higher power density frequently cause the fatal thermal failure, and thin film-based thermoelectric device can be a potential solution for in-demand cooling of circuits. Furthermore, high-performance thermoelectric thin films are also essential to build micropower generators for self-powered devices in environments where a thermal gradient exists; this is significant for power engineering and management in future electronics.
The performance of a thermoelectric material can be evaluated by a dimensionless figure of merit = 2 /( + ) , where , , , , and represent Seebeck coefficient, electrical conductivity, absolute temperature, electronic thermal conductivity and lattice thermal conductivity, respectively 7,8 . To enhance the figure of merit, numerous efforts have been devoted to improve the power factor (power factor = 2 ) 9-13 or reduce the thermal conductivity [14][15][16][17][18] . Among those methods to realize large thermoelectric power factor in a thin film/substrate heterostructure, interface engineering has been proven to be an effective approach to enhance Seebeck effect of the heterostructure. Especially, the presence of quasi-two-dimensional electron gas (quasi-2DEGs) at the interface is a dominant factor to the enhancement of |S| 1, [19][20][21][22] . As an ideal substrate with unique physical and chemical properties, SrTiO3 (STO) has been widely applied in thin film growth as well as the exploration of novel low-dimensional physics 1,23-28 . For example, the unique electronic phenomena generated from the heterointerface between epitaxial oxide layer and the STO substrate such as the very intriguing LaAlO3/SrTiO3 system has raised much attention. However, although the LAO/STO interface generally shows enhanced |S|, itself has not been found to exhibit an outstanding thermoelectric power factor mainly due to the limitation of its low conductivity at room temperature.
For designing a viable high-performance thermoelectric system, it is necessary to couple both high conductivity and the enhanced thermoelectricity on a single hetero-interface.
Here, we demonstrate the realization of giant thermoelectric power factor in ZrTe2/SrTiO3 heterostructure, which mainly attribute to the coupling between interfacial quasi-2DEG generated on STO surface and the high conductivity of transition metal dichalcogenides (TMDs) film. An interfacial reaction between the pulsed-laser deposition (PLD) grown layered ZrTe2 thin film and STO substrate leads to the appearance of quasi-2DEG which results in a great enhancement of the Seebeck coefficient.
Meanwhile, the conductivity is also dramatically enhanced which may result from the doping effect of TMD film induced by charge transfer from the interface. Combining both the high Seebeck coefficient and electrical conductivity, this heterostructure system eventually presents an outstanding thermal power factor as well as thermoelectric figure of merit zT value. This work provides a promising pathway for designing of high-performance thermoelectric systems, and opens up new possibilities for exploration and development of intriguing thermoelectric materials, structures as well as thin filmbased thermoelectric devices.

Methods
In this study, PLD was used to grow ZrTe2 films using an alloy target with Zr:Te=1:5 on the (100) STO substrates. The substrate-target distance during the deposition was 5 cm, with the base pressure of around 5×10 -5 Pa, and the films were grown at optimized substrate temperature (Ts) of 550 ºC. To the Seebeck coefficient and electrical conductivity at a temperature range from 0.5 K to 300 K. The measurement of Seebeck coefficient and electrical conductivity in the range from 300 K to 600 K was carried out on Joule Yacht MRS-3 thin film thermoelectric parameter test system.

Results and discussion
The microstructure and composition of all samples were firstly investigated using various methods.  Figure 1(b). ZrTe2 has a hexagonal close-packed crystal structure with space group of 3 ̅ 1, and its 1T structure has an octahedral coordination of metal atoms 29 that can be illustrated as a CdI2-type structure 30 . A unit cell of ZrTe2 is denoted by a cyan rectangle in Figure 1(b) and can also be identified in the magnified HRTEM image. The crystal structure of our ZrTe2 film was also characterized by XRD. Figure 1 These results imply that the ZrTe2 film is preferentially grown along c-axis, which is consistent with the HRTEM observations. However, it should be pointed out that, besides the dominating c-axis orientation peaks, other orientation peaks are also found in XRD, suggesting that the growth of ZrTe2 film on STO is non-epitaxial. It is worth noting that between the layered ZrTe2 and the STO substrate, there exists a ~5 nm thick interfacial layer with a different lattice structure to ZrTe2. A detailed analysis of the lattice spacings and their angles suggests that this interfacial layer is ZrO2 (See Figure S1). This implies that interfacial reaction occurs between the deposited ZrTe2 film and the STO substrate, and the interfacial characteristic has remarkably influenced the physical properties of both the ZrTe2 film and the STO substrate. This can be revealed by the STEM energy disperse x-ray (EDX) spectroscopy element mapping and the electron energy loss spectroscopy (EELS) analysis of the interfacial region as shown in Figure 2. A sample with relatively thin ZrTe2 film (10 nm thick) was selected to reveal the overall elemental distribution of the whole sample. It is apparent in EDX mapping results shown in Figure   2(a), that the distributions of O and Te are not uniform along the thickness direction, i.e., the signal intensity of Te increases near the interface with a decrease of O signal. It is surprising to see that O even exists in the whole film, suggesting an out diffusion of O from STO which can be proven by EELS line-scan analysis across the interface as shown in Figure 2  We have carried out thermoelectric and electron transport properties measurements to different thicknesses of samples at temperatures ranging from 2 K to 600 K. The ZrTe2/STO heterostructures exhibit negative Seebeck coefficient throughout the whole temperature range, indicating a n-type conducting characteristic; and the general trend is that the thicker films show relatively lower Seebeck coefficient but with high electrical conductivity, while very thin films present highly fluctuated Seebeck coefficient and high resistance. Figure 3(a) shows the temperature-dependent Seebeck coefficients of the ZrTe2/STO heterostructures with 35 nm and 60 nm-thick ZrTe2 films from 2 K to 600 K. The Seebeck coefficient in the sample with a 35 nm-thick ZrTe2 generally increases with the temperature increases from 2 K and finally reaches the maximum value of about 530 V/K at 440 K, above which the Seebeck coefficient begins to decrease. The sample with 60 nm-thick ZrTe2 film shows a similar trend while the maximum Seebeck coefficient appears at 111 K with a value of 368 V/K. The temperature-dependent electrical conductivity of both samples are shown in Figure 3(b), which confirms that these heterostructures are highly conductive. The electrical conductivity can reach a remarkable value up to 10 7 S/cm at low temperature state (2 K). The coexistence of both very large Seebeck coefficient and very high electrical conductivity of the ZrTe2/STO heterostructure consequently leads to enormous thermoelectric power factor. As shown in Figure 3(c), the thermoelectric power factors monotonically increase with the decreasing of temperature in both samples and finally achieve a colossal value even exceeds 3 × 10 5 −1 −2 at 20 K. As well known, quasi-2DEG can exist in the conductive STO interface and its presence can be a prime factor for triggering intriguing thermoelectric properties. To clarify the contribution of the possible interfacial effect to the outstanding thermoelectric properties, we further conducted systematic electron transport measurements on the heterostructure. As a follow-up to our previous work 29 , in these high quality heterostructures, we observed obvious Shubnikov-de Haas (SdH) oscillations superimposed on the large magnetoresistance at low temperatures below 4 K. Figure 4(a) shows the oscillations after subtracting a smooth background from the measured magnetoresistance of a typical 60 nm-thick ZrTe2 film on STO. The periodicity in the reciprocal of magnetic field (1/B) demonstrates that the SdH effect is the origin. Fast Fourier Transform (FFT) results further reveal that the oscillating frequency peak is about 29.5 T as shown in Figure 4(b). By analyzing the temperature dependence of the oscillations using the Lifshitz-Kosevitch formula, we obtain an effective mass of 1.3m0 where m0 represents the rest mass of an electron (Figure 4(c)). In addition, the Dingle temperature is calculated as 1.4 K based on the measured quantum oscillations. It is interesting to find that the microscopic parameters of the quantum oscillations actually agree well with those acquired in conductive STO surfaces [31][32][33][34][35][36][37][38] . Thus, we believe that the interfacial effect between the ZrTe2 thin film and the STO substrate plays a significant role for presenting the very exotic transport properties in our observation 29 . Our previous study on the ZrTe2/STO heterostructures has revealed a quasi-2D transport characteristic of the hybrid heterostructure, within which the electrons exhibit a high mobility of about 1.8 × 10 4 2 −1 −1 at 2 K 29 . The quasi-2D transport feature is consistent to the layered structure of ZrTe2 and the property of quasi-2D electrons in the STO interface as well. These characteristics are supposed to be responsible for the observed enormous Seebeck coefficient and power factor. Similar findings also exist in TiO2/STO, LAO/STO and FeSe/STO heterostructures 1,39,40 . In the TiO2/STO heterostructure, with the presence of 2DEG, the room temperature (300 K) Seebeck coefficient is as large as 1050 / ; while our ZrTe2/STO heterostructures show a value of |S|300K smaller than 500 / (Figure 3(a)). It is worth noting that the Seebeck coefficient of the heterostructure shows obvious dependency with the thickness of the TMD film that a thicker ZrTe2 film leads to the decrease of |S|. Figure S2 shows comparison of our results to reported Seebeck coefficients from other 2D systems.In fact, a control sample with only the ZrO2/STO interfacial contribution shows much higher Seebeck coefficient with a value of about 750 μV/K at 300 K, well consistent with the result in the TiO2/STO heterostructure 1 (see Figure S3(b)) . Meanwhile, the electrical conductivity is also found to be thickness dependent; while this dependence is opposite to that of the Seebeck coefficient. The control sample with only ZrO2/STO shows a much higher resistance, making the Seebeck coefficient measurement fluctuate vigorously and also prevents the system achieving a higher power factor. As a conclusion, the deposition of TMD ZrTe2 thin films on the top of the quasi-2D system results in slight decrease of the Seebeck coefficient of the system due to its relatively lower Seebeck coefficient, but it contributes very high conductivity to the system. With the coupling between the thin ZrTe2 film and the STO-based interface, the largest thermoelectric power factor can be achieved in the ZrTe2/STO heterostructure.
However, it should be noted that for a much thinner thickness, the interface is not well conductive and EELS results show that the valence state of Ti cations at the interfacial region is still Ti 4+ instead of Ti 3+ (see Figure S4). Therefore, we come to the conclusion that the interfacial state for forming a confined 2D conductive interface is essential for achieving the outstanding thermoelectric properties in this system. On the other hand, the extremely high conductivity is another crucial factor for realization of the giant thermopower factor. We also prepared a ZrTe2 thin film on the insulating Al2O3 ( Figure S5) to clarify the origin. It can be found that the ZrTe2 thin film is indeed metallic with a high conductivity but still lower than the value achieved in the ZrTe2/STO system; this also suggests a modulation effect to the ZrTe2 film from the interface. As a reference, the FeSe/STO is a well-explored metal/STO heterostructure system due to the interface-enhanced high temperature superconductivity 41,42 . Strong evidences have shown that the interfacial electron-phonon coupling as well as the electron doping on the FeSe layer should be responsible for the high temperature superconductivity of the FeSe/STO system [43][44][45][46][47] . In particular, the electron doping to the film through the thermal annealing generally decreases the resistance of the system and thus enhances the conductivity 48 . We suggest that similar mechanism may also exist in the metallic ZrTe2/STO heterostructure that the electron doping from the substrate to the TMD film determines the very high conductivity of the system. More efforts are needed to further demonstrate the underlying physical origins.
It is noticeable that there exists a small peak in the Seebeck coefficient at about 20 K (Figure 3(a)), and this can be explained by the phonon-drag effect 49-51 originated from the STO (100) substrate. One study that related to the thermal transport of STO 52 exhibits a thermal conductivity peak close to 20 K.
When a thermal gradient is applied to the thin film and substrate, nonequilibrium phonons will be generated and transfer their momentum to the carriers; as a result, an additional electric field is formed in the film, thus enhance the Seebeck coefficient. The characteristic of phonon-drag effect is that, it is more effective at temperatures where the substrate attains its maximum thermal conductivity; this is the reason why the peak of Seebeck coefficient, as shown in Figure 3(a), is near the temperature where the substrate exhibits its peak thermal conductivity. ZrTe2/STO sample, the effective zT value at 300 K is about 15, which is extremely higher compared to most of the reported works according to the best our knowledge 3,54 . Even for the 60 nm thick ZrTe2 film sample, the effective zT is about 1.5 at 300 K and increases to 13 at 18 K. This superior thermoelectric property makes this heterostructure a promising candidate for future thermoelectric device.

Conclusion
In summary, we demonstrate that the ZrTe2/STO heterostructure possesses colossal thermoelectric power factor at both low temperature and room temperature. The excellent thermoelectric properties can be attributed to the formation of two-dimensional electrical transport property as well as the enhanced conductivity due to charge transfer occurs in the interface. This study reveals the promising thermoelectric properties of ZrTe2/STO heterostructure and provides a new pathway for development of thermoelectric materials and structures. Furthermore, the mechanism exploration is of fundamental importance for both thermoelectric physics and possible application on thin film-based electronic devices.