Epitaxial growth of high-quality Mg3Sb2 thin films on annealed c-plane Al2O3 substrates and their thermoelectric properties

High-quality epitaxial Mg3Sb2 thin films are promising thermoelectric materials to enable practical applications of compact and environmentally friendly thermoelectric conversion at RT. In this study, high-quality single-crystal Mg3Sb2 with high c-plane orientation was epitaxially grown directly on annealed c-Al2O3 substrates without passive layers. These thin films exhibited about three times higher thermoelectric power factor than any previously reported values due to high carrier mobility. The ultra-smooth surface of the annealed c-Al2O3 substrate facilitated the formation of high-quality Mg3Sb2 thin films without passive layers or polycrystalline interfaces that could be carrier scatters.

T hermoelectric conversion technology is a promising solution for converting wasted heat of thermal energy into electricity.Thermoelectric properties depend on the dimensionless figure of merit ZT (= S 2 σT/κ), where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively.In other words, materials with high electrical conductivity and Seebeck coefficient, and low thermal conductivity are suitable.3][4][5][6][7][8][9][10][11] To be widely adopted, thin-film thermoelectric materials must achieve high ZT at near RT.
][14][15][16][17] Mg 3 Sb 2 exhibits intermediate properties between covalent and ionic bonding due to its Zintl phase compound nature.Specifically, the material consists of a Mg 2+ layer and a [Mg 2 Sb 2 ] 2− layer, and this structure leads to low thermal conductivity, resulting in high thermoelectric properties. 18)In fact, Mg 3 Sb 2 -based bulk materials exhibit a high thermoelectric performance comparable to that of Bi 2 Te 3 -based materials, which were previously considered the best-performing materials at around RT. 19) Thin films of Mg 3 Sb 2 have also been extensively studied, 20,21) with a sputtered Mg 3 Sb 2 thin film exhibiting a power factor of about 0.11 μW cmK −2 . 22)It has also been reported that the power factor increases in thin films fabricated by MBE due to band convergence by strain control. 23,24)The report stated that a passive layer composed of not fully c-plane-oriented Mg 3 Sb 2 formed near the Al 2 O 3 substrate and a strained Mg 3 Sb 2 thin film grew on top of it.For strategies to obtain high thermoelectric performance by controlling nanostructures, such as distortion and anisotropy in thin films, as exemplified by artificial superlattice structures, 25) it is essential to obtain highly crystalline thin films with highly aligned crystal orientation, few defects, and homogeneity.We focused on substrate selection and pre-treatment, which are critical for epitaxial growth to obtain Mg 3 Sb 2 thin films that are relaxed and have no passive layers.The c-plane sapphire (c-Al 2 O 3 ) is an excellent substrate for Mg 3 Sb 2 thin films due to its low lattice mismatch.It has been extensively utilized as a substrate for growing various thin films, including GaN, 26) In 2 O 3 , 27) Ga 2 O 3 28) and ZnO, 29) and various studies have been thoroughly conducted including the evaluation of defects, strain, and crystallinity on the substrate.To control these crystal properties, annealing in air is effective as a pre-treatment of sapphire substrates.This is because atmospheric annealing is known to form clear steps on the surface of sapphire substrates. 30)In fact, it has been reported that annealing of c-Al 2 O 3 substrates improves the crystallinity of ZnO epitaxial thin films. 31,32)Therefore, c-Al 2 O 3 substrate annealing is considered to have an important impact on improving the crystallinity and orientation of thin films, and thus promoting direct growth without passive layers in Mg 3 Sb 2 .In this study, Mg 3 Sb 2 epitaxial thin films were prepared on annealed c-Al 2 O 3 substrates using MBE, and their crystal structure and thermoelectric properties were investigated.
Mg 3 Sb 2 thin films were epitaxially grown on c-Al 2 O 3 substrates using an MBE system (EIKO Co., EW-100, base pressure ∼10 −8 Pa).Before introduction into the main chamber, the c-Al 2 O 3 substrates underwent a sequence of treatments, with cleaning by piranha solution (a mixture of H 2 SO 4 and H 2 O 2 ) and annealing in air (1000 °C, 10 min).The growth of Mg 3 Sb 2 thin films involved the simultaneous evaporation of Mg (5 N) and Sb (6 N) from Knudsen cells at a flux ratio of 4:1 under ultra-high vacuum conditions while maintaining the substrate temperature at 550 °C.
The surface structure of the thin films, both before and after growth, was examined in situ using reflection high-energy electron diffraction (RHEED).An atomic force microscope (AFM) (Shimadzu, SPM9600) was utilized to assess the surface morphology of the Mg 3 Sb 2 thin film.The crystal structure was characterized through X-ray diffraction (XRD) with an X-ray diffractometer (Rigaku, Ultima IV) using Cu Kα radiation with Ni Kβ filters.High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with a TEM (JEOL, JEM-ARM200F) and energy dispersive X-ray spectroscopy (EDS) observed crystal structures and performed elemental analysis for the cross-section of the thin films, respectively.Sample preparation for the above cross-sectional observation was conducted using the focused ion beam (FIB) method (JEOL, JIB-4700F).The electrical conductivity and Seebeck coefficient were measured by a ZEM-3 (ADVANCE RIKO), and carrier concentration and mobility were measured by Hall effect measurements.
Figures 1(a The cross-sectional HAADF-STEM image of the Mg 3 Sb 2 thin film in Fig. 1(i) also confirms that Mg 3 Sb 2 thin films can be epitaxially grown directly onto the substrate.The wellordered Mg 3 Sb 2 crystal lattice close to the substrate is consistent with the results of electron diffraction pattern observations.The spacing of the white dots, which represent Sb atoms, was used to estimate the lattice constant a and is plotted in Fig. 1(j).Here, the lattice constants a of Al 2 O 3 and Mg 3 Sb 2 are approximately 4.76 and 4.57 Å, respectively. 33,34)learly, the lattice constant a calculated from the first Sb atomic layer is close to that of Al 2 O 3 , while the values from the second layer onwards are close to that of Mg 3 Sb 2 .This means that the tensile strain of the fabricated Mg 3 Sb 2 thin film relaxed at a few Å thick from the substrate, suggesting that a relaxed Mg 3 Sb 2 epitaxial thin film without passive layers was obtained.
Figures 2(a   ̅ diffraction peak of Mg 3 Sb 2 was also observed at 2θ ∼ 26.0°in samples grown on non-annealed c-Al 2 O 3 substrates.This growth in crystal planes other than c-plane is also observed in the RHEED pattern (see supplementary data).This result indicates that epitaxial growth of Mg 3 Sb 2 thin films on annealed c-Al 2 O 3 substrates results in the grown crystal planes being perfectly oriented to the cplane.When annealing c-Al 2 O 3 in air, surface reconstruction occurs and promotes the termination of the substrate by a single Al layer with a most stable surface state [35][36][37] resulting in very high flatness (see supplementary data).The resulting ultra-smooth surface with infinitesimal roughness is thought to have resulted in a perfect c-plane orientation in the direction of crystal growth.
Finally, the thermoelectric properties of Mg 3 Sb 2 thin films at RT are shown in Fig. 3. Figures 3(a) and 3(b) show the Seebeck coefficient (S) and power factor (S 2 σ) as a function of electrical conductivity (σ), where other reported values of Mg 3 Sb 2 thin films fabricated by MBE 24) or sputtering [20][21][22] are also plotted. T values in this study, indicated by the red circles on the graph, show notably higher S at the same σ compared to other Mg 3 Sb 2 thin films.The average S 2 σ of 0.27 μWcm −1 K −2 is about three times higher than the comparative data.To elucidate the origin of this high S 2 σ, the S and Hall mobility (μ) were plotted as a function of carrier concentration (p), which is shown in Figs.3(c) and 3(d).The S values follow the trend for the sputtered thin-film data and the calculated line (red line) assuming the single parabolic band (SPB) model using the following equations: where q is the elementary charge, T is the absolute temperature, E is the energy, D(E) is the density of states, f is the Fermi distribution function, v is the carrier velocity and E F is the Fermi energy, respectively.τ(E) is the relaxation time, defined by E E s 0 ( ) with the scattering parameter s

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd (=−1/2: acoustic phonon scattering).The density of states effective mass m * in D(E) was used as 0.6 m 0 , where m 0 is the electron mass, close to other literature that has applied and calculated the SPB model. 24,38)On the other hand, the μ values show an average of 38 cm 2 V −1 s −1 , which is very much higher than any other reports.Based on the above, the considerably high S 2 σ of the Mg 3 Sb 2 thin films in this study is due to the high μ, which can be attributed to the formation of high-quality Mg 3 Sb 2 thin films without passive layers or polycrystalline interfaces that could be carrier scatters.
In summary, high-quality Mg 3 Sb 2 thin films with a uniform growth orientation and no passive layers were epitaxially grown on an annealed c-Al 2 O 3 substrate using an MBE system.The epitaxial growth was confirmed by HAADF-STEM images and electron diffraction patterns.The strain of the thin films relaxed only at a few Å from the substrate.The RHEED and XRD patterns clearly indicate that c-plane Mg 3 Sb 2 thin films were epitaxially grown on the annealed c-Al 2 O 3 substrate without any mixture of crystalline planes.These thin films exhibited about three times higher S 2 σ than previously reported Mg 3 Sb 2 thin films.This is due to the extremely high μ, which can be attributed to the formation of high-quality Mg 3 Sb 2 thin films without passive layers or polycrystalline interfaces that could be carrier scatters.This study provides valuable insight into enhancing the performance and development of other Zintl phase compounds grown on c-Al 2 O 3 , and Mg 3 Sb 2 -based highperformance thin-film thermoelectric materials.Other reported values of Mg 3 Sb 2 thin films fabricated by MBE 24) or sputtering [20][21][22] are shown by open marks. Back dashed lines in (a) and (b) are guides to the eye.Red dashed line in (c) is calculated assuming use of the SPB model.
)-1(d) show a cross-sectional HAADF-STEM image of the Mg 3 Sb 2 thin film and the elemental mappings of Al, Mg and Sb by EDS, respectively.The distribution of Mg and Sb on the thin film is uniform.The interface between the Al 2 O 3 substrate and the thin film is clearly defined with a steep edge.Figure 1(e) presents the cross-sectional HRTEM image of the Mg 3 Sb 2 thin film.Electron diffraction patterns of the substrate, the interface region (thin film and substrate), and the thin film away from the substrate are displayed in Figs.1(f)-1(h), respectively.The pattern obtained from the interface region consists of a simple superposition of the patterns obtained from the substrate and thin-film regions, indicating that the Mg 3 Sb 2 thin film epitaxially grows on the substrate without passive layers.
) and 2(b) present RHEED patterns along the 11 direction before and after growth of Mg 3 Sb 2 thin films.The pattern of c-Al 2 O 3 appeared before growth and changed to a streak pattern after growth, indicating the successful epitaxial growth of Mg 3 Sb 2 on c-Al 2 O 3 substrates.The streak pattern observed in Fig. 2(b) confirms the formation of a flat thin film.There is no difference in this streak between annealed and non-annealed c-Al 2 O 3 substrates (see supplementary data).The line profiles corresponding to the dashed lines in each figure are presented in

Fig. 1 .
Fig. 1.(a) HAADF-STEM image and EDS mappings of (b) Al, (c) Mg and (d) Sb on a cross-section of Mg 3 Sb 2 thin film.(e) HRTEM image and electron diffraction patterns of the (f) substrate, (g) interface and (h) thin-film area of a cross-section of Mg 3 Sb 2 thin film.(i) HAADF-STEM image of cross-sections of Mg 3 Sb 2 thin film and (j) lattice constant a calculated from (i).

Fig. 2 (
Fig. 2(c).A comparison of the streak positions at both ends in Fig. 2(c) reveals a slight increase in spacing after growth.Therefore, after growth, the lattice constant a becomes smaller compared to that of Al 2 O 3 .The lattice constant after growth was calculated to be a = 4.58 Å based on Al 2 O 3 , which is in close agreement with the literature value for Mg 3 Sb 2 .This result indicates that the surface predominantly consists of relaxed Mg 3 Sb 2 , supporting the findings derived from the HAADF-STEM image.Figure 2(d) presents an AFM image of the Mg 3 Sb 2 thin-film surface.Clear steps and terraces can be seen, with an RMS roughness of 0.44 nm, confirming that a highly planar thin film has been achieved.Crystal structure and crystal planes of Mg 3 Sb 2 are shown in Fig. 2(e).In addition, Fig. 2(f) plots the out-of-plane XRD patterns of samples grown on annealed and non-annealed c-Al 2 O 3 substrates.The samples grown on annealed c-Al 2 O 3 substrate showed a 0006 diffraction peak of Al 2 O 3 , and 0002, 0003, 0004 and 0005 diffraction peaks of Mg 3 Sb 2 , indicating the formation of highly oriented Mg 3 Sb 2 thin films.However, a weak 011 1 ̅ or 10 1 1̅ diffraction peak of Mg 3 Sb 2 was also observed at 2θ ∼ 26.0°in samples grown on non-annealed c-Al 2 O 3 substrates.This growth in crystal planes other than c-plane is also observed in the RHEED pattern (see supplementary data).This result indicates that epitaxial growth of Mg 3 Sb 2 thin films on annealed c-Al 2 O 3 substrates results in the grown crystal planes being perfectly oriented to the cplane.When annealing c-Al 2 O 3 in air, surface reconstruction occurs and promotes the termination of the substrate by a single Al layer with a most stable surface state[35][36][37] resulting in very high flatness (see supplementary data).The resulting

Fig. 2 .
Fig. 2. RHEED patterns in the 1100 Al O 2 3 [ ̅ ] direction (a) before and (b) after growth and (c) line profiles at the dashed lines in the respective figures.(d) AFM image of Mg 3 Sb 2 thin film grown on the annealed c-Al 2 O 3 substrate.(e) Schematic diagram of the crystal structure and crystal planes of Mg 3 Sb 2 .(f) Out-ofplane XRD patterns of Mg 3 Sb 2 thin films on annealed and non-annealed c-Al 2 O 3 substrate.

Fig. 3 .
Fig. 3. Thermoelectric properties of Mg 3 Sb 2 thin films at room temperature.σ dependences of (a) S and (b) S 2 σ, and the p dependences of (c) S and (d) μ.Other reported values of Mg 3 Sb 2 thin films fabricated by MBE24) or sputtering[20][21][22] are shown by open marks. Back dashed lines in (a) and (b) are guides to the eye.Red dashed line in (c) is calculated assuming use of the SPB model.