Fabrication of Bi₂Te₃ and Sb₂Te₃ Thermoelectric Thin Films using Radio Frequency Magnetron Sputtering Technique

Thermoelectric (TE) materials have been attracting a considerable amount of research interest regarding their ability to convert thermal energy into electricity via the Seebeck effect¹ and refrigeration via Peltier cooling². The conversion efficiency of TE material is determined by the temperature difference between the hot end of the TE leg and the cold end. Generally, the higher the temperature difference, the higher the TE figure of merit and the higher its efficiency³. TE works with no requirement for additional mechanical parts involving gas or liquid in its process, producing no waste or pollution, making it environmentally safe and considered a green energy harvesting system.

Bismuth telluride, Bi₂Te₃, and its alloys remain the most important class of TE material. Even in thermoelectric power generation, such as the recovery of waste heat, Bi₂Te₃ alloys are most commonly used due to their superior efficiency up to 200 °C⁴ and remain an excellent TE material at ambient temperature despite the zT value of more than 2 in various TE materials⁵. Several published papers have studied the TE properties of this material, which shows that the stoichiometric Bi₂Te₃ has a negative Seebeck coefficient^6,7,8, indicating n-type properties. However, this compound can be adjusted to p- and n-type by alloying with antimony telluride (Sb₂Te₃) and bismuth selenide (Bi₂Se₃), respectively, which can increase their bandgap and reduce bipolar effects⁹.

Antimony telluride, Sb₂Te₃ is another well-established TE material with high figure of merit at low temperature. While stoichiometric Bi₂Te₃ is a great TE with n-type properties, Sb₂Te₃ has p-type properties. In some cases, the properties of TE materials often depend on the atomic composition of the material such as the n-type Te-rich Bi₂Te₃, but a p-type Bi-rich Bi₂Te₃ due to Bi_Te antisite acceptor defects⁴. However, Sb₂Te₃ is always p-type due to comparatively low formation energy of Sb_Te antisite defects, even in Te-rich Sb₂Te₃⁴_. Thus, these two materials become suitable candidates to fabricate p-n module of thermoelectric generator for various applications.

The current conventional TEGs are made of diced ingots of n-type and p-type semiconductors connected vertically in series¹⁰. They have only been used in niche fields due to their low efficiency and bulky, rigid nature. Over time, researchers have started to explore thin film structures for better performance and application. It is reported that thin film TE have advantages over their bulky counterpart such as higher zT due to their low thermal conductivity^11,12, less amount of material and easier integration with integrated circuit¹². As a result, niche TE research on thin film thermoelectric devices has been on the rise benefitting from the advantages of nanomaterial structure^13,14.

Microfabrication of thin film is important to attain high performance TE materials. Various deposition approaches have been researched and developed including chemical vapor deposition¹⁵, atomic layer deposition^16,17, pulsed laser deposition^18,19,20, screen printing^8,21, and molecular beam epitaxy²² to serve this purpose. However, the majority of these techniques suffer from high operation cost, complex growth process or complicated material preparation. On the contrary, magnetron sputtering is a cost-effective approach for producing high-quality thin films that are denser, exhibit smaller grain size, have better adhesion, and high uniformity^23,24,25.

Magnetron sputtering is one of the plasma-based physical vapor deposition (PVD) processes which is widely used in various industrial applications. Sputtering process works when sufficient voltage is applied to a target (cathode), ions from the glow discharge plasma bombards the target and release not only secondary electrons, but also atoms of the cathode materials which eventually impact the surface of the substrate and condense as a thin film. Sputtering process was first commercialized in 1930s and improved in 1960s, gaining significant interest due to its ability to deposit wide range of materials using direct current (DC) and RF sputtering^26,27. The magnetron sputtering overcomes low deposition rate and high substrate heating impact by utilizing magnetic field. The strong magnet confines the electrons in the plasma at or near the surface of the target and prevent damage to the formed thin film. This configuration preserves the stoichiometry and thickness uniformity of the deposited thin film²⁸.

The preparation of Bi₂Te₃ and Sb₂Te₃ thermoelectric thin films using magnetron sputtering method has also been extensively studied, incorporating technique such as doping^4,29,30 and annealing³¹ in the procedures, leading to different performance and quality. Study by Zheng et al.³² uses thermally induced diffusion method to diffuse Ag-doped Bi and Te layer which were sputtered separately. This method enables precise control on the composition of the thin films and the diffusion of Te by thermal induction protects the Te from being volatilized. The properties of the thin films can also be enhanced by pre-coating process³³ before sputtering which results in better electrical conductivity due to high carrier mobility, consequently enhancing the power factor. Other than that, study by Chen et al.³⁴ improved the thermoelectric performance of sputtered Bi₂Te₃ by doping Se via post-selenization diffusion reaction method. During the process, Se vaporizes and diffuses into the Bi-Te thin films to form Bi-Te-Se films, which results in 8-fold higher power factor than undoped Bi₂Te₃.

This paper describes our experimental setup and procedure for the RF magnetron sputtering technique to deposit Bi₂Te₃ and Sb₂Te₃ thin films on glass substrates. Sputtering was performed in a top-down configuration as shown in the schematic diagram in Figure 1, cathode was mounted at an angle to the substrate normal, leading to a more concentrated and convergent plasma to the substrate. The films were systematically characterized using FESEM, EDX, Hall effect and Seebeck coefficient measurement to study their surface morphology, thickness, composition, and thermoelectric properties.

Figure 1: A schematic of the top-down configuration sputter. The diagram was designed according, but not to scale, to the actual sputtering configuration available for this study including the arrangement of glass substrates to be sputtered viewed from the top. Please click here to view a larger version of this figure.

1. Substrate preparation

1. Wipe the glass substrates with lint-free cloth to remove loose dirt or debris. Wash glass substrates with water and soap, use brush to scrub any dirt on the glass.
2. Prepare all solvents listed below in beakers, submerge the glass substrates in the solvent and sonicate accordingly at 37 kHz. Prepare methanol at 80 °C for 10 min; acetone at 80 °C for 10 min, ethanol at 80 °C for 10 min, distilled (DI) water at 80 °C for 20 min.
   CAUTION: Handle highly volatile chemicals in a fume hood.
3. Take out the substrates from the beaker one by one using a tweezer, put on a clean flat surface, hold down the substrate with tweezer and blow with nitrogen gas until dry.
4. Put substrates on a hot plate at 120 °C for 5-10 min to vaporize any residue. Put substrates in UV-ozone cleaner for 10 min.

2. Sputtering method

1. Chamber preparation
   1. Take off the aluminum shield from the gun and put the target material at the center of the cover. Tightly screw the cover on the magnetron holder and put back the aluminum shield. Cover the body of the chamber, guns, and sample holder with aluminum foil.
   2. Run short-circuit inspection by touching the probes of a multimeter between the chamber bodies (short), followed by the chamber body and the gun (short), and finally the chamber body and the target (open). This test is necessary to ensure there is no current leakage between the body (anode) and the target (cathode), which can hinder the formation of plasma.
2. Pre-sputtering
   1. Close the door and vacuum the chamber for 15 - 30 min. Press the door and the body together at the beginning of the vacuuming to ensure the door is tightly closed. Make sure the reading of the pressure gauge is decreasing.
   2. Switch ON the cooler system and set to 15 °C. Turn ON the pump and refrigeration button, and open the valve connected to the sputtering instrument.
      NOTE: RF sputtering does not function without a cooling system. The formation of plasma will not happen.
   3. Set Argon flow to 4 sccm and switch ON the gas toggle switch. Wait until the flow reaches the set value.
   4. Set rotation to 10 rpm and switch ON the rotation toggle switch. Push the Power button to switch ON the automatic matching network controller and radio frequency power supply.
   5. On the automatic matching network controller, set load and tune to 50 W each by pushing the Min/Max button and push the button from Manual to Auto.
   6. On the radio frequency power supply, set RF power to 50 W and push the Start button. Set the timer to 15 min.
   7. Switch OFF the RF power and rotation. Set Argon flow to 0 and switch OFF the toggle switch. Switch OFF the vacuum.
      NOTE: Wait until the Argon flow reaches 0.1 sccm before switching off the vacuum.
   8. Vent to open the chamber. Make sure the turbomolecular pump (TMP) is OFF before venting. Venting while TMP is running will damage the system.
   9. Open the chamber and load substrates. Place the substrates at the outer corner of the rotating sample holder for better deposition as shown in Figure 1.
      CAUTION: Wear mask and glove when handling the inside of the chamber to avoid inhaling small particles of materials.
   10. Close the door as shown in Figure 2 and vacuum for at least 6 h. Lower base pressure gives better deposition. The optimum base pressure for a high vacuum system such as sputtering process is 1 x 10^-5 Torr.
3. Sputtering
   1. Switch ON the cooler system and set to 15 °C. Turn ON the pump and refrigeration button, and open the valve connected to the sputtering instrument.
   2. Set rotation to 10 rpm and switch ON the rotation toggle switch. Set Argon flow to 4 sccm and switch ON the gas toggle switch. Wait until the flow reaches the set value.
   3. Push the Power button to switch ON the automatic matching network controller and radio frequency power supply.
   4. On the automatic matching network controller, set load and tune to 50 W each by pushing the Min/Max button and push the button from Manual to Auto.
   5. On the radio frequency power supply, set RF power to 50 W and push the Start button.
      NOTE: Wait until the Argon flow reaches the set value and becomes stable before turning on the RF power.
   6. Check for presence of plasma in the chamber. The formation of plasma is indicated by a glowing purple light in the chamber. If the plasma is not present once the RF power is turned ON, switch OFF Argon for 10 s, and turn it back ON. Repeat until plasma forms in the chamber.
   7. Gradually increase the RF power with 5 W per 10 s interval until it reaches 75 W. Set the timer to 60 min.
4. Post-sputtering
   1. Switch OFF the RF power and rotation. Turn OFF the automatic matching network controller and radio frequency power supply.
   2. Set Argon flow to 0 and switch OFF the gas toggle switch. Switch OFF the vacuum.
      NOTE: Wait until the Argon flow reaches 0.1 sccm before switching OFF the vacuum.
   3. Vent to open the chamber. Make sure the TMP is OFF before venting. Venting while TMP is running will damage the system.
   4. Take out all samples using tweezer and put in a clean Petri dish.
      ​CAUTION: Wear mask and glove when handling the inside of the chamber to avoid inhaling small particles of materials.
   5. Clean the chamber and vacuum for 10 - 15 min to keep the chamber under vacuum condition (free from impurities).

Figure 2: Experimental setup. Photograph of the sputtering machine used in this study. Please click here to view a larger version of this figure.

3. Characterization

1. Perform topographic and cross-sectional scanning using Field Emission Scanning Electron Microscope (FESEM, under 3.0 kV operating voltage) to obtain the surface microstructural details and thickness of the sputtered films.
2. Perform calculation on composition of the films using data of the energy dispersive X-ray spectra (EDX), attached with the FESEM. Measure Hall voltage in a permanent magnetic field of 0.57 T and probe currents of 0.8 mA and 10 mA for Sb₂Te₃ and Bi₂Te₃,, respectively to obtain the carrier concentration and conductivity of the films³⁵.
3. Perform in-plane measurement of Seebeck coefficient using a similar instrument used by Isotta et al.⁵. Mount samples with a rectangular geometry of approximately 2 cm x 1.25 cm on the setup. Measure the absolute Seebeck coefficient in 2-contact configuration against a Pt standard, with a temperature gradient of ≈25 °C.

Cross-sectional micrographs of as-deposited Bi₂Te₃ and Sb₂Te₃ thin films were recorded using FESEM as shown in Figure 3A and Figure 3B, respectively. The surface of the overall film appears uniform and smooth. It is apparent that the crystal grains of the Bi₂Te₃ thin film were hexagonal, conforming the crystal structure of Bi₂Te₃ while the crystal grains of the Sb₂Te₃ thin film were composed of fine circular grains, similar to reported by Amirghasemi et al³⁶. The cross-sectional images of both samples reveal densely packed particles growing on top of the substrate. The films had uniform thickness of approximately 1.429 ± 0.01 µm and 0.424 ± 0.01 µm for Bi₂Te₃ and Sb₂Te₃ thin films, respectively. The composition of the films was calculated from the EDX spectra in Supplementary File 1 and Supplementary File 2, and the values are tabulated in Table 1. The estimated values show that both thin films have stoichiometric ratios.

Carrier concentration and conductivity of the as-deposited thin films were determined at ambient temperature, while the absolute Seebeck coefficient was measured at temperature of approximately 50 °C. These results are presented in Table 2. The Bi₂Te₃ thin film shows negative absolute Seebeck coefficient and carrier concentration values confirming that the film was n-type and the Sb₂Te₃ film shows positive values for both absolute Seebeck coefficient and carrier concentration confirming its p-type conductivity.

Figure 3: FESEM cross-sectional images. (A) Cross-section image of Bi₂Te₃ with the film thickness.(B) Cross-section image of Sb₂Te₃ with the film thickness. Please click here to view a larger version of this figure.

Table 1: EDX composition analysis. The table consists of the weight percent acquired from the EDX spectra, calculated atomic percent of each element, composition ratio, thickness, and deposition rate of both Bi₂Te₃ and Sb₂Te₃ samples.

Table 2: Thermoelectric properties of microfabricated thin films. The table consists of the target materials, RF powers used, absolute Seebeck coefficients, Hall coefficients, and conductivity values of both Bi₂Te₃ and Sb₂Te₃ samples.

Supplementary File 1: Planar FESEM and EDX spectrum of Bi₂Te₃ with weight percent of each element. Please click here to download this File.

Supplementary File 2: Planar FESEM and EDX spectrum of Sb₂Te₃ with weight percent of each element. Please click here to download this File.

The technique presented in this paper presents no significant difficulty in setting up the equipment and implementation. However, several critical steps need to be highlighted. As mentioned in step 2.2.10 of the protocol, optimum vacuum condition is key to produce high quality thin films with less contamination as vacuum removes residual oxygen in the chamber³⁷. The presence of oxygen can cause cracks in the films called stress cracking indicating the importance of high vacuum system in sputtering process³⁸. This also reduces collisions with residual gas molecule in the movement of atoms³⁹ from the target to substrate, producing highly uniform thin films. Other than that, step 2.2.2 in the protocol is important to ensure continuous sputtering by dissipating heat through heat transfer with water from the cooling system. This method employs high voltage and electric current which ultimately manifests as target heating. Poor heat dissipation may lead to overheating beyond Curie temperature, resulting in failure of the entire sputtering process⁴⁰. On top of that, it is suggested to gradually increase the RF power during sputtering by 5 W per 10 s intervals until it reaches the desired power before starting the timer. This step is important to avoid cracks on the target due to thermal shock when too much power is supplied in a very short period of time⁴¹.

Sputtering is mainly affected by its parameter including sputtering power, deposition time, working pressure, substrate temperature, and target to substrate distance^42,43,44,45. Sputtering power influences the deposition rate and thickness of the film. Increasing voltage causes greater deposition rate, consequently increases the thickness of the film⁴⁴. Study by Sahu et al.⁴⁶ shows variation in deposition rate resulting from co-sputtering process of Ni and Zr, with different DC power applied to the Zr target. The results indicate that the deposition rate of the Ni-Zr films increases as the DC power supply for Zr increases. Their later study⁴⁷ investigated the effect of negative substrate bias voltage on the deposition rate. The result shows that the deposition rate gradually decreases with an increase in the substrate bias voltage. This phenomenon can also be seen in the results of this study where Bi₂Te₃ sputtered with 75 W produces thicker films than Sb₂Te₃, which was sputtered with much lower RF power at the same deposition time. However, both films were successfully deposited indicating the RF power exceeds the threshold voltage of each target and can be used in other studies depending on desired thickness.

According to the protocol, this method does not require melting and evaporation of the target material. This leads to nearly all materials can be deposited regardless of their melting temperature, making it superior to other PVD technologies. RF sputtering used in this study is also more advantageous than DC sputtering in terms of stability. Study by Yaqub et al.⁴⁸ shows that charge accumulation on the surface of the target causes plasma instabilities which hindered the DC sputtering process. In contrast, RF sputtering plasma tends to defuse throughout the chamber rather than concentrating around the target material creating better stability during deposition. Other than that, RF sputtering prevents charge build up which reduce arcing on the target surface, consequently resulting in better films than DC sputtering⁴⁹.

Despite having various attractive advantages, RF sputtering requires significantly higher voltage compared to DC sputtering with lower deposition rates⁴⁸. It is also exposed to the risk of overheating due to high voltage which demands advanced circuitry and additional cooling system as stated in step 2.2.2 in the protocol. Apart from that, RF sputtering sustained plasma at much lower pressure by the alternating current, yet the lack of secondary target causes slower deposition rate. This problem can be mitigated by adding secondary discharge between the target and the substrate in order to increase the ionization fraction of sputtered species²⁴. Yet, all these factors contribute to higher cost that leads to RF sputtering only been used in smaller scale and substrates.

Magnetron sputtering is the core of semiconductor industry, where creating highly insulating oxide films (barrier layer), conductive layer and metal grid are significant for integrated circuit fabrication. Various development have been done intended for energy related application such as energy conversion⁵⁰, widening the implementation of the presented technique, not only for thermoelectric materials, but also for thin film sensors and photovoltaic thin films. Recently, Lenis et al.⁵¹ studied the potential of this technique in the biomedical field by depositing the biocompatible and antibacterial coating of HA-Ag/TiN-Ti which is widely used in surgical prostheses. Study by Wang et al.⁵² also shows the implementation of this technique in depositing nanostructure tungsten trioxide thin films which has important potential application in smart windows field. In conclusion, this method represents a robust platform for thin film deposition and wide range of applications with the potential of further development.