Electrodeposition and Thermoelectric Characterization of (00L)-Oriented Bi₂Te₃ Thin Films on Silicon with Seed Layer

Silicon wafers with different resistance were used as the substrate. Low-resistance silicon wafers (0.8 × 10⁻² Ω cm) with the seed layer were used to investigate the electrochemical deposition parameters for preparing thin films with different preferential orientations. Samples deposited on high-resistance silicon wafers (1000 Ω cm) with seed layer allowed for the measurement of electrical conductivity and Seebeck coefficient due to the poor ability of charge transfer through substrate to the probe. Prior to the electrochemically deposition of the thin film, a Bi₂Te₃ seed layer was grown on the silicon substrate by MBE in an ultrahigh vacuum chamber with high purity Bi (99.99%) and Te (99.99%) source fluxes. The base pressure of the growth chamber was in the range of 10⁻¹⁰ Torr. The substrate was heated up to 300°C, following by the codeposition of Bi and Te. Bi₂Te₃ thin films were electrochemically deposited from an aqueous nitric acid on the seed layer at room temperature. The electrolyte contained 0.008 M Bi³⁺ and 0.01 M HTeO²⁺ made from Bi (NO₃)₃·5H₂O and TeO₂ in 1 M L⁻¹ nitric acid. A conventional three-electrode electrochemical cell was employed with a saturated calomel reference electrode and a Pt disk counter electrode. The seed layer was used as the working electrode. The deposition time of all samples is one hour. After the deposition, samples were rinsed with nitric acid solution, ethanol and distilled water before drying in air.

The surface morphology of the Bi₂Te₃ thin film was observed with Scanning Electron Microscope (SEM, ULTRA55-36-69) by using an acceleration voltage of 5 kV. Energy dispersive X-ray spectroscopy (EDS) was employed to analyze the chemical compositions of the films. An X-ray diffraction (XRD, 3KW D/MAX2200V PC) in conventional θ-2θ mode with Cu-Kα radiation was used to observe the crystal structure of the films. The applied current and accelerating voltage were 200 mA and 40 kV, respectively. The specimens were scanned from 10°to 60° with a step of 0.02°.

In-plane electrical conductivity was measured at room temperature by four-probe method. The Seebeck coefficient was also measured in-plane using home-made IR microscope measurement system at room temperature. The IR microscope was used to measure the temperature, which had temperature sensitivity up to 0.1 K and spatial resolution of up to 38 μm. Prior to measuring spatially-resolved temperature profile, the emissivity for the thin film was calibrated by heating the film to a uniform temperature and calibrating the radiance. By using the IR microscope instead of the thermocouple, the minimum influence caused by the contact between the thermocouple and thin film could be expected. After the calibration, one side of the film was connected to a heated source at a fixed temperature and the other side was exposed to room temperature. The temperature difference between both sides is about 2°C. ΔV was recorded by Agilent 34970A data acquisition and the Seebeck coefficient was derived from the slope of a ΔV vs ΔT.

Bi₂Te₃ thin films were deposited on the seed layer in potentiostatic electrodeposition process. The electrodeposition of Bi₂Te₃ was carried out from acidic nitric baths.¹⁶ The electrolyte solution consists of Bi(NO₃)₃·5H₂O and TeO₂ powders with deionized water and HNO₃. Adsorption of ions happened at the growth sites at the electrode surface and then the reduction of adsorbed ions at the cathode resulted in the deposits. The seed layer acted as the cathode in the electrochemical process. The chemically reaction can be represented as the following reduction reaction:¹⁷

Figure 1 is the SEM images of electrodeposited thin films with 40 nm seed layer on doped and intrinsic silicon. Thin films with three preferential orientations were observed. The (015)-oriented thin films comprise a dense arrangement of needle-like crystals (Figure 1a and 1b) which were also observed by other researchers.^18,19 When the deposition condition was further improved, the highest crystallographically textured films of stoichiometric Bi₂Te₃ were obtained (Figure 1c and 1d). These thin films deposited at the cathode potentials of −0.17 V and −0.20 V show good control of surface micrographs and highest (110) peaks (Figure 2a). Since Bi₂Te₃ consists of five atomic planes in the order Te⁽¹⁾-Bi-Te⁽²⁾-Bi-Te⁽¹⁾ along the c-axis, thermoelectric properties along the a-axis are superior to those along the c-axis, indicating strong anisotropy in physical properties. The (110) orientation is thought favorable to thermoelectric performance of Bi₂Te₃ thin films.^20,21 In our work, due to the relative low resistivity of doped silicon substrate (0.8 × 10⁻² Ω cm), the prepared thin films with compact structure shows the (110) orientation, which is consistent with previous reported studies on the electodeposition of Bi₂Te₃ thin films on metallic substrates.^6,7,22 The existance of seed layer helps to reduce the lattice mismatch and thus results in the improved (110) peaks.

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When the substrate was replaced with intrinsic silicon (1000 Ω cm), we find that at the cathode potentials of −0.15 V and −0.20 V, the thin film shows preferential orientation of (00L) consistent with the seed layer (Figure 2b), which means the (00L) crystal plane prefers to be parallel to the wafer surface. This leads to the more smooth surface and uniform crystal size distribution compared to the (015) or (110)-oriented thin films (Figure 1e, 1f).

Figure 3 shows the composition, orientation and deposition rate of as-prepared thin films related to the deposition potentials on both doped silicon and intrinsic silicon substrates. When the intrinsic silicon is used as the substrate, the atomic ratio of thin films deposited at low potentials (i.e., −0.15 V, −0.2 V) is a little far from 2:3 (60.0 at% Te), which is 2:3.78 (65.4 at% Te) and 2:3.88 (66.0 at% Te), respectively, indicating that a Te-rich composition of the (00L)-oriented thin films. When the cathode potential increases to −0.3 V, the obtained stoichiometric Bi₂Te₃ film shows the same (110) preferential orientations with those deposited at −0.15 V ∼ −0.2 V on doped silicon substrate. The more negative potential of the reduction of adsorbed ions at the cathode is attributed to the conductivity of the substrate since the intrinsic silicon with high resistivity reduces the charge transfer in the electrolyte. However, when the potential continues to increase, thin film shows random orientations. Te content exceeds to 73.36% at the applied potential of −0.5 V. It is probably the reason that the fast growth velocity at the electrolyte make it hard for the Bi₂Te₃ nuclei to adopt their crystallographic orientation to the seed layer so that they grow along random orientation such as (015) and (205) (Figure 2b). The morphology and composition of the Bi₂Te₃ thin film are thought strongly affected by the electrolyte and deposition potential when electrodeposited on the metallic substrates.^23,24 According to our results, the seed layer as well as the silicon substrate also plays an important role.

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The highest deposition rate of the thin film is around 3.0 μm/h on the doped silicon substrate, while 1.3 μm/h on the intrinsic silicon substrate. Both thin films show the highest (110) peak. The deposition rate is much higher when the thin film grows along the normal to the substrate in the (110) plane than in the other directions. The deposition rate of (00L)-oriented thin films is rather low due to the low conductivity of intrinsic silicon and the layer-by-layer growth mode, which is 0.52 μm/h (at the deposition potential of −0.15 V) and 0.58 μm/h (at the deposition potential of −0.2 V), respectively.

The thermoelectric characterization of electrodeposited Bi₂Te₃ thin films on metallic substrate was reported by several research groups.^17,22 Since metal has good conductivity, it will introduce artifacts to the measurement of electrical properties. Researchers have taken different methods to eliminate the error. Heo et al used the resin epoxy to separate the films from the Au substrate before measuring.¹⁷ Schumacher et al used the stainless steel substrate so that the film could lift off after deposition.²⁵ By electrodepositing Bi₂Te₃ thin films on intrinsic silicon substrate with the help of seed layer, the reduced influence of the substrate on measurement can be expected. Also, to eliminate the influence of the seed layer, the film is considered thick enough (several hundreds of nanometers to several mircons) compared to the Bi₂Te₃ seed layer (40 nm). Structural effects on electrical properties were investigated.

Figure 4 reveals the dependence of electrical conductivity of as-prepared thin films on applied deposition potentials and preferential growth orientations. When the thin films were deposited with 40 nm seed layer, the electrical conductivity is the highest for (00L)-oriented thin film (610 S cm⁻¹). It increased by ∼72% compared with the (110)-oriented thin film (354 S cm⁻¹). As reported in the literature, the (00L) orientation is favorable to the thermoelectric performance of the Bi₂Te₃ thin films since the structural anisotropy of Bi₂Te₃ leads to the electrical conductivity along the ab-plane superior to those along the c-axis.²⁶ The lower roughness of thin films with (00L) orientation is another reason for this improvement. As shown in Figure 5, the preferential orientation has an influence on the surface roughness of as-prepared thin films. The thin film with particle size of about 230 nm shows strong (015) preferential orientation that restrains the electron transfer due to the rough surface (Rms: 65.6 nm), while the (110)-oriented thin film with particle size of about 120 nm seems to be more compact (Rms: 28.9 nm). The thin film with (00L) preferential orientation has a smooth (Rms: 4.87 nm) and featureless particle size of about 24 nm which is similar with the seed layer (Particle Size: 22 nm, Rms: 4.61 nm). Although smaller particles have more grain boundaries which will scatter the electrons and hence decreases the conductivity, the compact microstructure has a positive effect on the electrical conductivity of the thin films.^27,28 The overall electrical conductivity is increased for the (00L)-oriented thin film due to the much lower roughness despite its small particle size. However, there is no such trend toward the thin films deposited with the 20 nm seed layer. Detailed discussions are given further below. The comparison of our results with the reported literature is also shown in Figure 4.

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The Seebeck coefficients and power factors of the thin films were also considered in terms of preferential orientations (Figure 6). The Seebeck coefficient is the ratio of the potential and temperature difference between the thin film and seed layer and the power factor, PF = σS², where σ is the electrical conductivity and S is the Seebeck coefficient. The highest Seebeck value 156 μV was obtained at the applied potential of −0.3 V, showing a strong (110) preferential orientation, and the thin film with (00L) preferential orientation has the comparable Seebeck coefficient of 143 μV. The similar result of these two kinds of thin films with different orientations is due to the relatively isotropic Seebeck coefficient of Bi₂Te_3.^29,30 Also, since the mismatch between the thin film and substrate was reduced with the presence of the seed layer, the Seebeck coefficient is enhanced owing to the decreasing number of defects which restrain the charge transport under the influence of temperature difference. The (00L)-oriented sample exhibits the best power factors of 1247 μW m⁻¹ K⁻² while maximum value of 861 μW m⁻¹ K⁻² is observed for thin films with (110) preferential orientation. Thus the power factor of the (00L)-oriented thin film is improved by ∼45% compared to the (110)-oriented thin film. It appears that power factors characteristics of as-prepared thin films are either similar or improved in comparison with other researches of electrodepositing Bi₂Te₃ films by pulsed deposition or post annealing.^12,18,27

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The main reason for the obtain of (00L)-oriented thin film is the conductivity of the substrate as well as the seed layer. In the electrochemical process, the nucleation and growth take place in an energetically favorable way determining the preferential growth. Since the intrinsic silicon substrate has resistivity of 1000 Ω cm, much larger than that of the metallic substrate (∼10⁻⁷ Ω cm), in the electrodeposition process the electron transfer would take place on the seed layer rather than on the substrate although the seed layer has low thickness (40 nm). Crystals prefer to grow in the direction with relative low resistivity to reduce the difficulty of charge transfer. Since the seed layer shows (00L) preferential orientation and the atoms of Bi₂Te₃ crystals in (00L) plane are connected by covalent bond, the in-plane electrical conductivity is much higher than that along the other direction, which makes it easy for the crystals grow along the (00L) direction in the electrodeposition process. After growing freely without intensive contact with each other, the thin film prepared by electrodeposition also shows the (00L) preferential orientation. In contrast, when the thin film was deposited on the metallic substrates, Bi₂Te₃ would nucleate on the substrate more easily than on the as-deposited crystals due to the lower electrical resistivity (∼10⁻⁷ Ω cm) of metal than Bi₂Te₃ thin films (∼10⁻³ Ω cm) that leads to the occupy of all the available nucleation sites on the substrate. Thin films prepared on metallic substrates mostly showed the (110) preferential orientation.

Furthermore, the seed layer prepared by MBE is the main factor to the uniform structure and improvement of thermoelectric performance. The (00L)-orientated thin films exhibiting high power factors were usually prepared by epitaxial growth methods such as MBE³¹ and MOCVD³² but seldom prepared by electrodeposition. In our experiment, the Bi₂Te₃ seed layer on the cathode also has a c-orientation. From the XRD spectra, only strong (006) peaks in addition to very weak (0015) peaks corresponding to the Bi₂Te₃ rhombohedral structure is observed, which agrees with the spectra of as-prepared thin films (Figure 2). Since the mismatch of lattice types and parameters is reduced, the electrodeposited thin films may experience a layer-by-layer growth mode which restrains defect formations and thus promises the better thermoelectric performance.

We also find that thickness of the seed layer has a significant effect on the morphology, roughness and thermoelectric performance of the prepared thin films. In Figure 4 and 6, the electrical conductivity, Seebeck coefficients and power factors of the samples deposited on the seed layer of 20 nm are compared with the samples deposited on the seed layer of 40 nm. Although the thin film has (006) or (0015) orientations as well, the (015) is the predominant one (Figure 7). The thin films exhibit poor thermoelectric performance due to the large particle size and inhomogeneous structure. It is probably attributed to the lower thickness of the seed layer and thus the intrinsic silicon will have more effect on the electrodeposited thin film which results in the insufficient charge transfer in the electrodeposition process. The same effect of the seed layer thickness on further deposition of nanostructured films is reported by Lockett et al who used the seed layer as the template for ZnO nanostructures.³³ Any influence of the substrate on crystallographic texture of the thin film will lead to subtle differences in peak intensity and diffraction angle, which determine the morphology and thermoelectric performance of the thin films. This result was also found in Mishra's work who and his coworkers electrodeposited Bi₂Te₃ films on carbon nanotubes arrays.³⁴

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In summary, Bi₂Te₃ thin films were synthesized by the potentiostatic electrodeposition with the seed layer on the silicon substrate. The seed layer reduced the mismatch between the thin film and substrate in the electrodeposition process. When the intrinsic silicon was used as the substrate, the optimized growth process of the Bi₂Te₃ thin film resulted in the thin film with (00L) orientation which is different with previous reports of (110)-oriented thin films electrodeposited on the metallic substrates. The (00L)-oriented Bi₂Te₃ thin films show low surface roughness and uniform structure consistent with the seed layer. The maximum electrical conductivity of the (00L)-oriented thin film increased by ∼72% compared to that of (110)-oriented thin film, while the power factor was improved by ∼45% (1247 μW m⁻¹ K⁻²). The seed-layer assisted electrodeposition method opens new possibilities to understand and optimize the influence of substrate on electrodeposited thin films and it could be extensively applied to the electrodeposite various functional materials on the substrate with poor conductivity.

This research was supported by Shanghai Science and Technology Funds (12ZR1444000, 10PJ1403800, 11DZ1111200), Yunnan Provincial Science and Technology Department (2010AD003), National Natural Science Foundation of China (61204129, 21103104), Innovation Foundation of Shanghai University, and the Special Fund for Selection and Cultivation Excellent Youth in the University of Shanghai city.