Enhanced electrical properties of stoichiometric Bi0.5Sb1.5Te3 film with high-crystallinity via layer-by-layer in-situ Growth

Author’s Accepted Manuscript Enhanced electrical properties of stoichiometric Bi0.5Sb1.5Te3 film with high-crystallinity via layerby-layer in-situ Growth Xin Mu, Hongyu Zhou, Danqi He, Wenyu Zhao, Ping Wei, Wanting Zhu, Xiaolei Nie, Huijun Liu, Qingjie Zhang www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(17)30013-7 http://dx.doi.org/10.1016/j.nanoen.2017.01.013 NANOEN1727

To appear in: Nano Energy Received date: 28 November 2016 Revised date: 21 December 2016 Accepted date: 6 January 2017 Cite this article as: Xin Mu, Hongyu Zhou, Danqi He, Wenyu Zhao, Ping Wei, Wanting Zhu, Xiaolei Nie, Huijun Liu and Qingjie Zhang, Enhanced electrical properties of stoichiometric Bi0.5Sb1.5Te3 film with high-crystallinity via layerby-layer in-situ Growth, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.01.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced electrical properties of stoichiometric Bi0.5Sb1.5Te3 film with high-crystallinity via layer-by-layer in-situ Growth Xin Mu1, Hongyu Zhou1, Danqi He1, Wenyu Zhao1*, Ping Wei1, Wanting Zhu1, Xiaolei Nie1, Huijun Liu2, Qingjie Zhang1* 1 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China. 2 School of Physics and Technology, Wuhan University, Wuhan 430072, China *E-mail: [email protected]; [email protected]

Abstract The preparation of high-performance Bi2Te3-based films is vitally important for the miniaturization of Bi2Te3 thermoelectric (TE) device. Herein, a series of stoichiometric Bi0.5Sb1.5Te3 films with different preferential orientations have been fabricated through in-situ crystallization during the co-sputtering process. We discover that the preferential orientation was transformed from (011̅ 5) to (101̅ 10) to (000l) orientation with increasing the substrate temperature. The (000l)-oriented films exhibit the best electrical transport properties, which the maximum electrical conductivity of 8.0104 S·m-1 and power factor of 3.8 mW·K-2·m-1 are much more than those of the bulk material. The excellent properties are attributed to the high-crystallinity, well-controlled preferential orientation, and minimized compositional deviation. A layer-by-layer in-situ growth model is proposed to understand the formation mechanism of the (000l)-oriented films. Our work demonstrates that the electrical transport performance of Bi2Te3-based films can be remarkably improved through finely controlling the crystallinity and preferential orientation under the condition of stoichiometric composition. Keywords: Thermoelectric material; Bi2Te3-based films; Electrical transport properties; Preferential orientation; Layer-by-layer in-situ growth

1. Introduction Thermoelectric (TE) materials enable power generation from various distributed waste-heat sources, such as automobiles, blast furnaces, thermal power plants, and domestic fireplaces [1]. The conversion efficiency is determined by the dimensionless figure of merit (ZT), defined as ZT=α2σT/(κE+κL), where T, α, σ, κE and κL are the absolute temperature, Seebeck coefficient, electrical conductivity, and the electronic and lattice components of the thermal conductivity (κ), respectively. To improve the thermal transport properties, various phonon engineering approaches have been used to enhance phonon scattering and decrease κL [2-10]. A series of band structure engineering approaches have been developed to improve the power factor (α2σ) [11-16]. Recently, we discovered that the electrical and thermal transport properties of TE materials could be simultaneously optimized through coexisting multi-localization transport behavior and magnetoelectric interaction [17,18]. These methods have played important roles in controlling electron and phonon transport and improving ZT of bulk TE materials. The TE films have created an important direction for developing various miniaturized TE application systems [19-25]. Bi2Te3 is one of the most important TE materials in cooling and power generation [22]. At present, all the best reported Bi2Te3-based bulk materials for n-type and p-type systems are both ternary, such as n-type Bi2Te2.7Se0.3 (ZT~1.0) [26] and ptype Bi0.5Sb1.5Te3 (ZT~1.5) [27]. The high-performance Bi2Te3-based films have attracted intense interest due to the growing concern on wearable and/or portable TE power generators for converting body heat into electrical energy and selfpowered microelectronic systems [23-25]. However, the electrical transport properties parameters σ, α, and α2σ of Bi2Te3based films is usually much less than those of the bulk materials [28-30]. A large number of approaches and strategies, such as superlattices, anisotropic effects, oriented and twinned nanoassemblies, antisite defect, interfacial lattice match, and stoichiometry, have been employed to improve the electrical transport properties of Bi2Te3 films in the past decades [23,2934]. However, one discovered that for the Bi2Te3-based films it is impossible to make the electrical transport properties a breakthrough unless we can simultaneously realize the (000l) orientation and stoichiometric composition [34], and remarkably improve the density [35] and crystallinity [36]. Up to now, the poor electrical transport properties of Bi2Te3based films are still a major bottleneck to limit the TE conversion applications in microelectronic systems. The preparation of high-performance Bi2Te3-based films is vitally important for the miniaturization of Bi2Te3 TE device. Herein, we have fabricated a series of (000l)-oriented as-prepared Bi0.5Sb1.5Te3 films with high-crystallinity, wellcontrolled preferential orientation, and minimized compositional deviation through in-situ crystallization during the 1

magnetron co-sputtering process. We discover that the preferential orientation was transformed from (011̅ 5) to (101̅ 10) to (000l) orientation with increasing the substrate temperature in the range 473-723 K. The highest σ and largest α2σ of (000l)-oriented as-prepared Bi0.5Sb1.5Te3 films are much more than those of the bulk materials. A layer-by-layer in-situ growth model is proposed to understand the formation mechanism of the (000l)-oriented films. Our work demonstrates that the electrical transport properties of Bi2Te3-based films can be remarkably improved through finely controlling the crystallinity and preferential orientation under the condition of stoichiometric composition.

2. Experimental Section Bi0.5Sb1.5Te3 films were deposited on glass substrate under Ar atmosphere using a magnetron co-sputtering system (JSD450, Anhui Jiashuo Vacuum Tech. Co., Ltd.). A self-made Bi0.5Sb1.5Te3 target with 50 mm in diameter and 5 mm in thickness was connected to a direct current power, and a Te target was connected to a radio frequency power to compensate for the Te deficiency. Before deposition, the glass substrate was ultrasonically cleaned in alcohol and treated using a plasma cleaner under Ar atmosphere. During the deposition process, the distance between the sputtering target and the glass substrate was fixed at 80 mm and the substrate holder was rotated at 150 r/min. The working pressure maintained at 3 Pa. The substrate temperature (Ts) was precisely controlled in the range 473-723 K. In order to achieve as-prepared stoichiometric Bi0.5Sb1.5Te3 films, the sputtering power applied on Bi0.5Sb1.5Te3 alloy target was set to 10 W, and the Te target was set to less than 4 W depending on the Ts. The film thicknesses were monitored with a quartz crystal sensor. The phase constituents of all the as-prepared films were determined by powder X-ray diffraction (XRD, Bruker D8Advance) using a Cu K radiation. The chemical compositions were quantitatively analyzed with electron probe microanalysis (EPMA, JEOL JXA-8230). The microstructures were examined with a field emission scanning electron microscope (FESEM, Zeiss ULTRA-PLUS-43-13). The surface roughness and thickness of all the as-prepared films was measured on a 500m×500m surface area by employing a 3D surface profilometer (NanoMap, 500LS) attaching scanning probe image processor image analysis software. The in-plane RH and electrical resistivity (ρ) of all the asprepared films at room temperature were measured with van der Pauw method using an Accent 5500 Hall measurement system. The in-plane H and n were calculated according to the equations n=1/(eRH) and H =RH/ρ, where e is the charge of an electron. The in-plane electrical conductivity (σ) and Seebeck coefficient (α) were measured with four-probe method (Ulvac Riko, ZEM-3). The measurement uncertainties for σ and α were both ±5%. To investigate the thermal stability of the electrical properties for the as-prepared films, the in-plane σ and α of the as-prepared films fabricated at Ts=523, 573, and 723 K had been repeatedly measured for five times. The theoretical calculations were carried out by the combination of first-principles calculations and Boltzmann transport theory by incorporating with the measured n. The structure was optimized using the projector augmented-wave method with the optB86b-vdW functional [37]. The theoretical σ and μ of Bi0.5Sb1.5Te3 along the (011̅ 5), (101̅ 10) and (000l) orientations were derived from semi-classical Boltzmann theory. The relaxation time (τ) was estimated to be 1.3 fs.

3. Results 3.1. Crystallinity and preferential orientation The XRD patterns of as-prepared Bi0.5Sb1.5Te3 films fabricated at different Ts of 473-723 K were shown in Figure 1a. It can be seen that all the diffraction peaks can be indexed based on JCPDS 49-1713 of Bi0.5Sb1.5Te3, indicating that these asprepared films are composed of single-phase Bi0.5Sb1.5Te3. The EPMA quantitative compositions (Figure 1b) show that the chemical compositions of these as-prepared films are very close to Bi:Sb:Te=10:30:60, indicating that these as-prepared films are stoichiometric Bi0.5Sb1.5Te3 with minimized compositional deviation. To investigate the crystallinities (xc) of the as-prepared films, the XRD patterns were fitted by Rietveld refinement to obtain the diffraction intensities of each reflection profiles for crystalline Bi0.5Sb1.5Te3 and the scattering intensities for amorphous Bi0.5Sb1.5Te3. All the diffraction peaks of crystalline Bi0.5Sb1.5Te3 can be indexed to those of Bi0.5Sb1.5Te3 based on JCPDS 49-1713. The xc were calculated based on the reference intensity ratio method [38]: xc = ΣIc/(ΣIc+ΣIa)×100%

(1)

where ΣIc is the sum of all the diffraction intensities and ΣIa is the sum of all the scattering intensities. The xc of all asprepared Bi0.5Sb1.5Te3 films were listed in Table 1. The as-prepared film fabricated at Ts=473 K has very low crystallinity (about 40%), indicating that the as-prepared film contains a great deal of noncrystalline compositions. However, the 2

crystallinity remarkably increased up to 88% when Ts=523 K, and gradually increased as increasing the Ts in the range 573-723K. The maximum crystallinity reached 99% for the as-prepared film fabricated at Ts=723 K. These results clearly indicated that the stoichiometric as-prepared Bi0.5Sb1.5Te3 films with high-crystallinity had been formed through in-situ crystallization during the magnetron co-sputtering process. In particular, we discovered that the orientations of these as-prepared Bi0.5Sb1.5Te3 films strongly depend on the Ts. The diffraction peak of (011̅ 5) reflection is the strongest for the as-prepared film fabricated at Ts=523 K. When Ts=573 K, the diffraction peak of (101̅ 10) reflection become the strongest one. However, for the as-prepared film fabricated at Ts≥623 K, the diffraction intensities of (000l) reflections dramatically increased. To further investigate the evolution feature of the preferential orientations for these as-prepared Bi0.5Sb1.5Te3 films with increasing the Ts, the orientation factors (F) were calculated with Lotgering’s method [39]: F(X) = (P-P0)/(1-P0) P = ΣI(X)/ΣI(hkil)

(2)

P0 = ΣI0(X)/ΣI0(hkil)

(3)

where ΣI(X), ΣI0(X), ΣI(hkil), and ΣI0(hkil) are the sum of the integrated intensities of the X and (hkil) reflections for the oriented sample and the non-oriented one, respectively; P and P0 is the ratio of the sums for the oriented sample and the non-oriented one, respectively. The X reflections stand for the (011̅ 5), (101̅ 10), and (000l) reflections in our work. The F values of all as-prepared Bi0.5Sb1.5Te3 films were listed in Table 1. It can be seen that at Ts=523 K the F of (011̅ 5) orientation is 0.52. For the as-prepared film fabricated at Ts=573 K, the F of (101̅ 10) orientation increased to 0.21 from 0.02 at Ts=523 K while the F of (011̅ 5) plane decreased to 0.10 from 0.52 at Ts=523 K. It is worth noting that with increasing the Ts in the range 623-723 K, the F of (000l) orientations dramatically increased to 0.26 at Ts=623 K and gradually increased to 0.96. These results clearly reveal that the preferential orientations of the as-prepared Bi0.5Sb1.5Te3 films have been transformed from the (011̅ 5) orientation at Ts=473-523 K to the (101̅ 10) orientation at Ts=573-623 K to the (000l) orientation at Ts=623-723 K. Table 1. Crystallinity and orientation factor of as-prepared Bi0.5Sb1.5Te3 films fabricated at different substrate temperatures.

Ts

Crystallinity

473 K 523 K 573 K 623 K 673 K 723 K

40% 88% 90% 92% 95% 99%

F(011̅ 5) 0.52 0.10 0.09 -0.07 -0.22

F(101̅ 10) 0.02 0.21 0.10 -0.04 -0.18

F(000l) -0.08 0.00 0.26 0.64 0.96

Figure 1. XRD patterns (a) and EPMA compositions (b) of as-prepared Bi0.5Sb1.5Te3 films fabricated at different substrate temperatures of 473-723 K.

3.2. Microstructure and surface roughness Figure 2 shows FESEM photographs of the surface (left side) and cross section (right side) morphologies of as-prepared Bi0.5Sb1.5Te3 films fabricated at Ts=523-723 K. These surface morphologies indicate that all the as-prepared films consist of hexagonal nanoflakes. It is worth noting that the hexagonal nanoflakes are almost perpendicular to the substrate at Ts=523 3

K (Figure 2a,b), then become tilted to the substrate when the Ts rose to 573 K (Figure 2c,d), and lastly are stacked into a layered structure parallel to the substrate when Ts ≥ 623 K (Figure 2e-g). The cross-sectional morphologies further reveal that the thickness is about 605, 635, 680 and 653 nm for the as-prepared film fabricated at 523 K, 573 K, 623 K and 723 K, respectively. Combining the evolution features of preferential orientations from XRD results, the (011̅ 5) preferential orientation results in the hexagonal nanoflakes perpendicular to the substrate, the (101̅ 10) preferential orientation causes the hexagonal nanoflakes tilted to the substrate, and the (000l) preferential orientations make the hexagonal nanoflakes stacked into a layered structure parallel to the substrate. It is easy to understand that the (000l)-oriented arrangement of hexagonal nanoflakes can dramatically reduce various structural defects such as interstitial pores and stacking faults, and remarkably increase the density of as-prepared Bi0.5Sb1.5Te3 films. The grain sizes of the hexagonal nanoflakes are calculated according to the statistic results of five 8 μm × 10 μm surface areas from FESEM photographs, which are 288 nm, 384 nm, 429 nm and 926 nm for the as-prepared films fabricated at 523 K, 573 K, 623 K and 723 K, respectively. These microstructural evolutions clearly indicate that increasing the Ts can not only help to form the (000l) preferential orientation but also control the growth of Bi0.5Sb1.5Te3 grains.

Figure 2. FESEM photographs of the surface and cross section of as-prepared Bi0.5Sb1.5Te3 films fabricated at different substrate temperatures: (a, b) 523 K; (c, d) 573 K; (e, f) 623 K; (g, h) 723 K.

Figure 3 shows the surface roughness of as-prepared Bi0.5Sb1.5Te3 films fabricated at Ts=523-723 K. It can be seen that the surface of the as-prepared films is extremely rough at Ts=523 K and become more and more flat with increasing the Ts. When Ts=623 K, the height and number of the bulges dramatically decreased. The surface roughness is 159, 61, 20, and 12 nm for the as-prepared films fabricated at 523 K, 573 K, 623 K, and 723 K, respectively. The significant decrease in the surface roughness with increasing the Ts is attributed to the transformation of preferential orientations from (011̅ 5) to (101̅ 10) to (0001) orientation, which is in good accordance with the results from XRD and FESEM analysis.

4

Figure 3. 3D images of the surfaces of as-prepared Bi0.5Sb1.5Te3 films fabricated at different substrate temperatures: (a) 523 K; (b) 573 K; (c) 623 K; (d) 723 K.

3.3. Electrical Transport Properties The room-temperature electrical transport properties of as-prepared Bi0.5Sb1.5Te3 films fabricated at different Ts from 473 K to 723 K were listed in Table 2. The positive Hall coefficient (RH) indicates that the majority carriers are holes in all the as-prepared films. The p-type conduction feature is consistent with the corresponding bulk materials. At Ts=473 K, the asprepared Bi0.5Sb1.5Te3 film shows very low carrier concentration (n) and Hall mobility (μH) due to extremely low crystallinity, hence having extraordinarily low σ and α2σ. For the as-prepared film fabricated at Ts=523 K, the n and μH significantly increased to 7.01019 cm-3 and 16.4 cm2·V-1·s-1, respectively. As a result, the σ and α2σ remarkably increased to 1.8104 S·m-1 and 0.6 mW·K-2·m-1 at Ts=523 K, respectively. With increasing the Ts in the range 523-723 K, the n, μH, and α gradually increased while the RH gradually decreased due to the transformation of preferential orientations from (011̅ 5) to (101̅ 10) to (0001) orientation, hence the remarkable increases in σ and α2σ. When Ts=723 K, the n and μH of the as-prepared film reached the maximums, which was 7.9×1019 cm-3 and 62.5 cm2·V-1·s-1, respectively. Compared with the as-prepared film fabricated at Ts=473 K, the largest n and μH increased by 52% and 1463%, respectively. Figure 4 displays the temperature dependences of electrical conductivity, Seebeck coefficient, and power factor for the as-prepared Bi0.5Sb1.5Te3 films fabricated at different Ts of 473-723 K. It is worth noting that the σ, α, and α2σ simultaneously increased with increasing the Ts. When Ts=723 K, the largest σ and α reached 8.0104 S·m-1 and 219 μV·K1

at 300 K, increased by about 2560% and 15% compared with those of as-prepared film fabricated at Ts=473 K,

respectively. As a result, the largest α2σ reached 3.8 mW·K-2·m-1 at Ts=723 K, increased by about 370% compared with that of as-prepared film fabricated at Ts=473 K. In particular, the largest σ and α2σ of as-prepared Bi0.5Sb1.5Te3 film are much more than the σ of 3.8 ×104 S·m-1 and α2σ of 2.7 mW·K-2·m-1 of the bulk material (Bi0.5Sb1.5Te3 target with high crystallinity nearby 99.9% and random orientation). The small increase in α is attributed to the minimized compositional deviation of as-prepared Bi0.5Sb1.5Te3 films in the Ts range 473-723 K. Based on the results described above, it can be concluded that the electrical transport performance of Bi2Te3 films can be remarkably improved through finely controlling the crystallinity and preferential orientation under the condition of stoichiometric composition. As shown in Figure 4a, for the films fabricated at low Ts, the electrical conductivity increases with temperature, while that of the films fabricated at high Ts decreases with increasing temperature. We consider that the different temperature dependences are mainly due to the crystallinity variation. For those films fabricated at low Ts, the crystallinity is low (only about 40% for the film fabricated at Ts=473K), the electron scattering from disorder structures including amorphous 5

regions and defects is much stronger, which leads to low mobility and weak temperature dependence of mobility (~T-0.5) [40]. As a consequence, the σ increases slightly with increasing temperature due to the slightly increased carrier concentration over temperature. For the films fabricated at high Ts, the crystallinity is high (up to 99% for the film fabricated at Ts=723K), herein the electron scattering from the disorder structures is much weaker and the lattice scattering becomes dominating, which leads to high mobility and stronger temperature dependence of mobility ( ~T-1.5). As a result, the σ decreases with increasing temperature. In addition, besides the crystallinity and orientation, the grain boundaries also play an important role in affecting the σ of these films. For the (011̅ 5) or (101̅ 10)-oriented films, the carriers pass through a large number of interlayer boundaries and are strongly scattered, as a result, these films show low mobility and σ. However, for the (000l)-oriented films, the carriers transport mainly along the in-plane direction. Therefore, the carriers are less scattered and these films exhibit high mobility and σ. As shown in Figure 4b, the α of all the as-prepared films shows a peak at a certain temperature. In particular, the peak temperature changes with the Ts. To investigate the origin of the α peaks, the measured α is expressed by α=(σpαp– σnαn)/(σp+σn) [41], where σp and αp is electrical conductivity and Seebeck coefficient for holes, σn and αn is for electrons. The α peaks are related to the onset of bipolar conduction, which involves thermal excitation of electrons and holes across the band gap. Herein, the maximum value αmax can be expressed as αmax=Eg/2eTmax, where Tmax is the temperature at which the α is the largest, Eg is the band gap. The Eg can be estimated with measured Tmax and αmax at peaks according to Eg=2eTmaxαmax [41], and the results are added in Table 2 in the revised manuscript. It is obvious that the Eg increases gradually with increasing Ts. Therefore, the shift of the α peaks to higher temperature as shown in Figure 4b can be attributed to the gradual increase in the Eg with increasing Ts. The further analysis shows that the shift of the α peaks is in fact due to the increased crystallinity. In the case of amorphous semiconductors, a series of localized energy levels exist in the band gap [42], which makes the bipolar conduction easier under lower temperature. Therefore, those films fabricated at low Ts show the α peaks at low temperatures because of the low crystallinities. With increasing Ts, the crystallinity of the films is gradually increased and the localized energy levels in the band gap gradually eliminate. As a result, the Eg is gradually increased and the α peaks gradually shift to higher temperatures. For majority of TE materials, the dependence between σ and α of TE materials is inversely correlated. For the asprepared stoichiometric Bi0.5Sb1.5Te3 films in our work, the simultaneous increases of all the electrical properties parameters σ, α, and α2σ are attributed to the following reasons: i) the crystallinity gradually improved with increasing the Ts in the range 473-723 K; ii) the transformation of preferential orientations from (011̅ 5) to (101̅ 10) to (0001) orientation results in the gradual increases in the densities and the gradual decreases in various structural defects. Therefore, the (000l)oriented as-prepared stoichiometric Bi0.5Sb1.5Te3 film fabricated at Ts=723 K exhibits the largest σ and α, which are much more than those of the bulk material. The excellent electrical transport properties are attributed to the comprehensive effects induced by the high-crystallinity, well-controlled preferential orientation, and minimized compositional deviation.

Figure 4. Temperature dependences of (a) electrical conductivity, (b) Seebeck coefficient, and (c) power factor for as-prepared Bi0.5Sb1.5Te3 films fabricated at different Ts of 473-723 K. Table 2. The room-temperature electrical properties and estimated band gaps of as-prepared Bi0.5Sb1.5Te3 films fabricated at different substrate temperatures.

Ts

RH (10-2cm3·C-1)

n (1019cm-3)

μH (cm2·V-1·s-1)

σ (104S·m-1) 6

α (μV·K-1)

α2σ (mWK-2m-1)

Eg (eV)

473 K 523 K 573 K 623 K 673 K 723 K

12.0 8.93 8.45 8.12 8.12 7.91

5.2 7.0 7.4 7.7 7.7 7.9

4.0 16.4 26.2 45.9 58.7 62.5

0.3 1.8 3.1 5.7 7.2 8.0

189 189 197 207 212 219

0.1 0.6 1.2 2.4 3.2 3.8

0.123 0.131 0.135 0.153 0.155 0.156

4. Discussion 4.1. Effects of preferential orientation on electrical transport properties The structure of Bi0.5Sb1.5Te3 is similar to that of Sb2Te3, which consists of Te1–Sb(Bi)–Te2–Sb(Bi)–Te1 quintuple layers along the c-direction, as shown in Figure 5. The quintuple layers are weakly coupled with each other by van der Waals interactions. In Bi0.5Sb1.5Te3 crystal, a quarter of Sb sites are occupied by Bi atoms. The (011̅ 5), (101̅ 10), and (000l) preferential orientations are depicted in the structure, and their tilt angles to the ab-plane are 54°, 33°, and 0°, respectively. In order to explore the beneficial roles of these preferential orientations in improving the electrical transport properties of the as-prepared Bi0.5Sb1.5Te3 films, the carrier mobility (μ) and σ along the (011̅ 5), (101̅ 10), and (000l) preferential orientations had been calculated based on the combination of first-principles calculations and Boltzmann transport theory by incorporating with the measured n. The theoretical μ and σ along the (011̅ 5), (101̅ 10), and (0001) preferential orientations at room temperature were listed in Table 3. It can be seen that the μ greatly increased due to the transformation of preferential orientations from (011̅ 5) to (101̅ 10) to (000l) orientation. The layered structure stacking along the (000l) orientation provides the fastest channels for carriers transport. Therefore, the (000l)- oriented film has the largest μ of 43.7 cm2·V-1·s-1. However, the carrier transport along the (011̅ 5) and (101̅ 10) orientations has been hindered by extra scattering from more atoms between the quintuple layers, as shown in Figure 5. Therefore, the μ of those films with (011̅ 5) and (101̅ 10) orientations are remarkably less than that of the (000l)-oriented film. As a result, the μ of (101̅ 10)-oriented film is only 36.4 cm2·V-1·s-1. The (011̅ 5)-oriented film exhibits the lowest μ of 28.9 cm2·V-1·s-1 since the number of atoms to scatter the carriers along the (011̅ 5) orientation is the most in three preferential orientations. The μ dependences of preferential orientations provide a reasonable explanation why the σ of Bi0.5Sb1.5Te3 films along (011̅ 5), (101̅ 10), (000l) preferential orientation gradually decreased, which is 3.2×104 S·m-1, 4.3×104 S·m-1, and 5.4×104 S·m-1, respectively. Based on the theoretical prediction results above, the gradual increases of measured σ and α for as-prepared stoichiometric Bi0.5Sb1.5Te3 films fabricated at different Ts of 473-723 K are attributed to the transformation of preferential orientations from (011̅ 5) to (101̅ 10) to (0001) orientation, which are rooted from the μ dependences of preferential orientations.

Figure 5. Crystal structure of Bi0.5Sb1.5Te3 for the first-principles calculations.

7

Table 3. Theoretical electrical transport properties of Bi0.5Sb1.5Te3 along the different orientations at room temperature.

Preferential orientation (011̅ 5) (101̅ 10) (0001)

μ (cm2·V-1·s-1) 28.9 36.4 43.7

Measured n (1019 cm-3) 7.0 7.4 7.7

σ (104S·m-1) 3.2 4.3 5.4

4.2. Layer-by-layer in-situ growth of as-prepared films The films growth generally obeys several classical growth models, such as Stranski-Krastanov model, Volmer-Weber model, and Frank-van der Merwe model [43-45]. In order to clarify the formation mechanism of the as-prepared (000l)oriented Bi0.5Sb1.5Te3 films, the morphology evolution features of the films during the growth process at Ts=723 K have been investigated using a series of FESEM photographs (Figure 6). At the beginning stage (about 5s), the discrete Bi0.5Sb1.5Te3 nuclei with ~15 nm in diameter rapidly formed and deposited on the substrate, as shown in Figure 6b. After having co-sputtered for 10s, the number and density of Bi0.5Sb1.5Te3 nuclei remarkably increased. The early formed nuclei preferentially occupied the bare surface rather than agglomerate into clusters or islands (Figure 6c). After having cosputtered for 15s, a uniform and dense monolayer, which is composed of Bi0.5Sb1.5Te3 nanoparticles, covered the entire substrate (Figure 6d). Therefore, the formation of a perfect monolayer at Ts=723K needs about 15s. Figure 6e-f show that the second layer starts to grow directly on the first layer in the same growth process. After having co-sputtered for 30s, the third layer starts to grow. The layer-by-layer in-situ growth process will proceed repeatedly with increasing the time. Obviously, the growth process of as-prepared Bi0.5Sb1.5Te3 films obeys the principle of Frank-van der Merwe growth model. Like the reported situation, the Frank-van der Merwe model dominates the film growth process when the surface atoms may obtain sufficient energy to overcome the diffusion barriers [46]. Therefore, in our layer-by-layer in-situ growth process, it is very crucial for the as-prepared (000l)-oriented Bi0.5Sb1.5Te3 film that the substrate was heated up to 723 K before sputtering, because this may provide sufficient energy for atom diffusion. To further reveal the layer-by-layer in-situ growth process, the morphology evolution during the sputtering process is schematically described as shown in Figure 7. At the beginning stage, the Bi0.5Sb1.5Te3 nuclei have been continuously diffusing on the substrate surface until the substrate is entirely covered. The nuclei further grow and form the hexagonal nanoflakes. To keep the minimum potential energy, the hexagonal nanoflakes will be stacked on the substrate horizontally. At the same time, some nuclei are filled in the gaps between hexagonal nanoflakes. It is easy to understand that the filled nuclei will subsequently grow in the following two forms. If the heated substrate can provide sufficient energy for atom diffusion, the nanoflakes and the filled nuclei will be gradually merged and formed a uniform and dense monolayer. Then, the second layer starts to grow on the first one in the same process, and then grow the third layer in the layer-by-layer growth. However, if there is no the sufficient energy for atom diffusion due to low Ts, the filled nuclei will preferentially grow along the (011̅ 5) and (101̅ 10) orientations with higher growth rates. As showed in Figure 2, the randomly inclined hexagonal nanoflakes in the (011̅ 5) and (101̅ 10)-oriented films must lead to the formation of interstitial pores and stacking faults, while the hexagonal nanoflakes in the (000l)-oriented film are densely stacked on the substrate. As a result, the densities of as-prepared Bi0.5Sb1.5Te3 films become higher and higher and the structural faults become less and less during the transformation process of preferential orientations from (011̅ 5) to (101̅ 10) to (000l) orientation. Therefore, the layer-bylayer in-situ growth model also provides a reasonable explanation why the σ and α of as-prepared stoichiometric Bi0.5Sb1.5Te3 films simultaneously increased with increasing the Ts in the range 473-723 K.

Figure 6. FESEM photographs of the (000l)-oriented Bi0.5Sb1.5Te3 films fabricated at Ts=723K for different times. (a) 0; (b) 5 s; (c) 10 s; (d) 15 s; (e) 20s;

8

(f) 30 s.

Figure 7. A schematic growth mechanism for the formation of (000l)-oriented Bi0.5Sb1.5Te3 film.

4.3. Thermal stability of as-prepared films The force between different layers is of Van der Waals force in the (000l)-oriented films. However, there is no effective method to determine the strength of this force for our samples. As can be seen from Figure 5, since the surface atom density of the (000l) plane is the largest, the total surface free energy of the (000l)-oriented films should be the smallest according to Gibbs-Wulff theorem. Theoretically, the (000l)-oriented films should be kinetically stable. To investigate the thermal stability of these as-prepared films, the in-plane σ and α of the as-prepared films fabricated at Ts=523, 573, and 723 K were repeatedly measured for five times in the range 300-425 K, and the results are shown in Figure 8. It can be seen that the deviations are very small and within the measurement uncertainties (about ±10%), substantiating the good thermal stability. The largest deviations in the a2σ are about 4.2%, 2.6%, and 4.0% for the films fabricated at 523 K, 573 K, and 723 K, respectively. Therefore, there is no correlation between the stability and the drop of a2σ with the measuring temperature in the range 300425 K for the (000l)-oriented films. We conclude that the drop of a2σ should be related to the decrease of electrical conductivity due to strong electron-phonon scattering at high temperatures.

Figure 8. Temperature dependences of power factor a2σ for as-prepared Bi0.5Sb1.5Te3 films fabricated at (a) 523 K, (b) 573 K, and (c) 723 K.

5. Conclusion Bi0.5Sb1.5Te3 films with high-crystallinity, well-controlled preferential orientation, and minimized compositional deviation were fabricated through in-situ crystallization during the magnetron co-sputtering process. It is discovered that the preferential orientations of the as-prepared stoichiometric Bi0.5Sb1.5Te3 films strongly depends on the substrate temperature Ts, and was transformed from the (011̅ 5) to (101̅ 10) to (000l) orientation with increasing the Ts in the range 473-723 K. The effects of preferential orientations on the electrical transport properties have been investigated through the combination of theoretical prediction and experimental measurement. The as-prepared (000l)-oriented Bi0.5Sb1.5Te3 film fabricated at Ts=723 K exhibits the best electrical properties due to the rapid transport of carriers along the (000l) orientation. The highest σ of 8.0104 S·m-1 and α2σ of 3.8 mW·K-2·m-1 are much more than those of the bulk material. A layer-by-layer in-situ growth model is proposed to understand the formation mechanism of the (000l)-oriented films. The transformation of preferential orientations from (011̅ 5) to (101̅ 10) to (000l) orientation provides a reasonable explanation why the σ and α of the as-prepared stoichiometric Bi0.5Sb1.5Te3 film simultaneously increased with increasing the Ts. This work demonstrates that the electrical transport performance of Bi2Te3 films can be remarkably improved through finely controlling the crystallinity and preferential orientation under the condition of stoichiometric composition.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51620105014, 11274248, 51572210, 51521001), National Basic Research Program of China (973-program) No. 2013CB632505. XRD, FESEM and EPMA experiments were performed at Center for Materials Research and Testing of Wuhan University of Technology. Hall measurements were performed at State Key Lab of Advanced Technology for Materials Synthesis and Processing of 9

Wuhan University of Technology. Authors thank to S. B. Mu, W. Y. Chen, M. J. Yang, and C. H. Shen for their help in the structure characterization.

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Personal portrait photos and biosketches

Xin Mu received his B.S. degree from Wuhan University of Technology (WUT) in 2012. Currently, he is pursuing his Ph.D. at State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (SKATMSP) of WUT, China. His research interest is focused on the low-dimensional thermoelectric materials.

Hongyu Zhou received his M.S. degree from WUT in 2012. He is pursuing his Ph.D. at SKATMSP of WUT, China. His main research interests are the thermoelectric devices.

Danqi He received her B.S. degree from WUT in 2012. She is currently pursuing her Ph.D. at SKATMSP of WUT, China. Her research interests mainly focus on thermoelectric bulk materials.

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Wenyu Zhao received his M.S. degree in 1996 at China University of Geosciences and Ph.D. in 2004 at WUT. He started to work at SKATMSP of WUT as a full professor since 2008. His research interests include thermoelectric materials, magnetic ferrite materials, magnetic thermoelectric materials, novel structure thermoelectric devices, and thermoelectric application technology.

Ping Wei is currently working at SKATMSP of WUT as an associate professor. He received his B.S. degree at Shandong University in 2005, M.S. degree in 2009 and Ph.D. in 2012 at WUT. His research focuses on the performance optimization and structural characterization of thermoelectric materials.

Wanting Zhu received her M. S. degree from WUT in 2013. Now she works as a laboratory technician at Center for Materials Research and Analysis of WUT, China. Her research focuses on the morphology and structural characterization of thermoelectric materials.

Xiaolei Nie received her M. S. degree from Tianjin University in 2012. Now she works as a laboratory technician at Center for Materials Research and Analysis of WUT, China. Her research focuses on the microanalysis and chemical composition analysis of thermoelectric materials.

Huijun Liu received Ph.D. in Hong Kong University of Science and Technology in 2003. Currently, he is a professor at School of Physical Science and Technology, Wuhan University. His research focuses on the theoretical calculations of transport properties of thermoelectric materials.

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Qingjie Zhang received his M.S. degree in 1984 and Ph.D. in 1990 from Huazhong University of Science and Technology. Currently, he is the president of WUT, director of SKATMSP, and chief scientist of National 973 Program in the field of thermoelectric materials. His research focuses on the thermoelectric materials and the related application technology.

Highlights     

Stoichiometric Bi0.5Sb1.5Te3 films with different preferential orientations were fabricated through in-situ crystallization. It is a breakthrough that the (000l) orientation, stoichiometric composition, improved density and crystallinity were simultaneously realized. All the electrical properties parameters (σ, α) are simultaneously increased in the (000l)-oriented films. The effects of preferential orientations on the electrical properties were investigated by the combination of theoretical prediction and experimental measurement. A layer-by-layer in-situ growth model is proposed to understand the formation mechanism of the (000l)-oriented films.

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