Surprisingly high in-plane thermoelectric performance in a-axis-oriented epitaxial SnSe thin films
Graphical abstract
a-axis-oriented SnSe epitaxial films are prepared via pulsed laser deposition technology. The thin films exhibit relatively high power factor (PF) of ∼472 μW⋅m−1 K−2 at 600 K along the in-plane direction after the optimization of Sn vacancy concentration. The out-of-plane thermal conductivities of the thin films decrease about 33% compared with that of the single-crystalline bulk. An ultrahigh estimated-zT value (approximately 1.2 at 600 K) is achieved along the in-plane direction of SnSe thin films.
Introduction
Thermoelectric (TE) technology is becoming increasingly important with the growing global energy crisis [1,2]. TE materials enable the direct conversion between thermal energy and electricity; accordingly, these materials have become attractive in various fields, such as semiconductor power generation that harvests energy from the waste heat, solid state refrigeration that requires no maintenance, microdevice thin film coolers that exploit slight temperature gradients, and power wireless sensors used in medicine, defense, Internet of Things, fashion, and sport [3,4]. The TE performance can be evaluated by the dimensionless figure of merit (zT), which is defined as , where , , T, and and stand for the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivities contributed from charge carriers and phonons, respectively [[5], [6], [7], [8]]. However, a sophisticated interrelated coupling is discovered among parameters , , and [3], in which the optimization of one parameter conflicts with the other two. In the semiconductor with a low order of magnitude carrier concentration (n or p for an N- or P-type semiconductor), the contribution from ( where and are Lorenz number and carrier mobility, respectively) is limited to the total thermal conductivity (). Variable mainly depends on the lattice thermal conductivity (), which can be substantially reduced via nanotechnology engineering, including forming nanoparticles to build blocks, manufacturing 2D thin films, and synthesizing 1D nanowires or nanotubes and quasi-zero-dimensional quantum dots [9,10]. Few fluctuations occur on the electrical transport parameters while decreasing , which has become an effective strategy to enhance the TE performance [11]. At present, workhorse TE materials with remarkable zT values are widely explored, including semiconductors of chalcogenides, skutterudites, clathrates, and half-Heusler alloys [12,13]. Among these compounds, SnSe single crystal [[14], [15], [16]] is a typically efficient TE material at medium temperature with a record zT value of 2.6 ± 0.3 at 923 K along the b-axis.
SnSe is an IV–VI group indirect transition P-type semiconductor with a band gap in the range of 0.6–0.9 eV. The corresponding crystal structure is layered orthorhombic (space group Pnma No. 62), which similar to the distorted NaCl structure with anomaly long a lattice parameter (∼11.6 ). Accordingly, a strong Sn–Se covalent bonding forms in the b–c axis constructed plane, and weak van der Waals forces among the bilayers are found in the a-axis direction, which results in distinct anisotropic TE properties. The remarkable zT values are obtained in the b- and c-axis (zT ≥ 2.3 ± 0.3) due to the ultralow intrinsic that aroused from the anharmonicity in bonds [14]. However, the zT value along the a-axis is lower than 0.8 ± 0.2 because of the poor that suffered from the layered barriers [14]. Consequently, the control of the crystalline orientation is necessary for the single crystals, textured polycrystals, and thin films [17]. In contrast to the SnSe single-crystalline bulks with poor mechanical properties (cleaving property) and thermal stability (the monocrystal feature is easily destructed upon repeated heating and cooling), polycrystals and thin films have their own advantages in the practical applications. Thin films are more conducive to device integration than the 3D bulk materials. An ultralow can be obtained due to the proportionally increased surface and interface phonon scattering, and certainly leads to a comparatively high zT value. also generally reduces by several orders of magnitude due to the excessive crystalline defects in polycrystalline bulks and polycrystalline films; their zT values are lower than 1.4 [[18], [19], [20], [21]] and 0.3 [[22], [23], [24]]. Thus, the epitaxial thin films with a desired orientation and relatively few defects are expected to acquire high TE properties. In recent years, various methods for preparing SnSe thin films have been developed. These methods include chemical vapor deposition [25], reactive evaporation [26], atomic layer deposition [27], and molecular beam epitaxy [28]. Most TE performance of the SnSe films (the along the in-plane in particular), intrinsic mechanism, thermal stability, and preparation technology are yet to be systematically investigated.
In this study, a-axis-oriented SnSe epitaxial thin films with high thermal quality were successfully prepared on the single-crystalline MgO substrates by pulsed laser deposition (PLD) technique. The morphology, crystal structure, and element ratio of the as-prepared SnSe thin films were investigated via atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The record high estimated-zT value (∼1.2) in SnSe thin films was characterized and presented. The remarkable thermal stability and the corresponding mechanism of electrical conduction of the SnSe samples were studied in detail by annealing in an O-free atmosphere.
Section snippets
Synthesis
Commercial single-crystal MgO (100) wafers with a thickness of 0.5 mm and an area of 10 10 mm2 were used as substrates. Similar processes for manufacturing thin films have been conducted in our previous studies [29,30]. The SnSe thin films were fabricated via the pulsed laser deposition (PLD) technique with a 308 nm XeCl excimer laser with a deposition frequency of 5.0 Hz. The energy densities of laser (EDL) on the target were adjusted from 1.5 J·cm−2 to 2.7 J·cm−2. The polycrystalline SnSe1.2
Morphology, crystal structure, and crystal orientation of the SnSe thin films
For obtaining an epitaxial crystal structure, cubic MgO (a = b = c = 4.217 ) was selected as substrate to prepare the orthorhombic SnSe thin film (a = 11.57 , b = 4.19 and c = 4.46 ) because of their similar b and c lattice parameters. SnSe films with shiny and smooth surface (inset of Fig. 1A) were obtained via the PLD technique. The corresponding AFM image of the film is depicted in Fig. 1A with a small root-mean-square surface roughness Rrms of approximately 1.60 nm. The crystal
Conclusions
The manufacturing of textured SnSe thin films with predetermined orientation and compositions is a worldwide challenge at present. In this study, the a-axis-oriented epitaxial SnSe thin films were fabricated on the MgO single-crystalline substrate via the controllable PLD strategy. The a-axis-oriented and high crystalline quality features were proven by the AFM, XRD, TEM, and XPS technologies. The TE properties dependent on the samples before and after annealing reveal that the prepared SnSe
Credit author statement
Shuaihang Hou: Methodology, Formal analysis, Software, Writing – original draft. Zhiliang Li: Data curation, Visualization, Software, Writing- Reviewing and Editing, Supervision. Yuli Xue: Investigation, Data curation. Xinkun Ning: Conceptualization, Reviewing and Editing. Jianglong Wang: Conceptualization, Writing – review & editing, Supervision. Shufang Wang: Data curation, Conceptualization, Writing – review & editing, Supervision, Validation, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51372064), Nature Science, Foundation of Hebei Province, China (Nos. E2017201227, A2017201088, A2018201241, A201801003, and ZD2014018), National Natural Science Foundation for Young Scientists of China (No. 51802071), Outstanding Youth Science Fund Project of Natural Science Foundation of Hebei Province (No. A2020201032), Advanced Talents Incubation Program of the Hebei University (No. 521000981162), and Local
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