radio frequency magnetron sputtering of bi2te3 and sb2te3 thermoelectric thin films on glass substrates

Deposição de filmes finos Bi2Te3 e Sb2Te3 em substratos de vidro via pulverização catódica por RF

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 &#176;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 coefficient6,7,8, 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 BiTe antisite acceptor defects4. However, Sb2Te3 is always p-type due to comparatively low formation energy of SbTe 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 conductivity11,12, 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 structure13,14.
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 deposition16,17, pulsed laser deposition18,19,20, screen printing8,21, 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 uniformity23,24,25.
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 sputtering26,27. 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 doping4,29,30 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.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/66248/66248fig01.jpg" /> 
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. <a href="https://www.jove.com/files/ftp_upload/66248/66248fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Autor em destaque: Avanços em filmes finos termoelétricos de alto desempenho por meio de pulverização catódica de magnetron de radiofrequência

Fabricação de Filmes Finos Termoelétricos Bi2Te3 e Sb2Te3 utilizando a Técnica de Sputtering por Magnetron de Radiofrequência

Our research focuses on fabricating high-quality thin films for thermoelectric generators to recover waste heat and turn it into electricity. Various thin film deposition technique have been researched and developed, including radio frequency magnetron sputtering that is able to produce high-density, robust thin films with better adhesion and high uniformity. This method does not require melting and evaporation of the target materials, enabling most of materials to be deposited regardless of their melting temperature, making it superior than other physical vapor deposition technologies.

Sputtering de magnetron de radiofrequência de filmes finos termoelétricos Bi2Te3 e Sb2Te3 em substratos de vidro

Para começar, remova o escudo de alumínio da arma e coloque o material alvo no centro da tampa. Aperte com segurança a tampa no suporte do magnetron e reconecte o escudo de alumínio. Envolva o corpo da câmara, as pistolas e o porta-amostras com papel alumínio.Usando uma verificação multimétrica para um circuito de classificação entre os corpos da câmara, o corpo da câmara e a arma, e o corpo da câmara e o alvo. Em seguida, feche a porta e inicie o processo de aspiração na câmara por 15 a 30 minutos. No início da aspiração, pressione firmemente a porta e o corpo juntos para garantir uma vedação apertada.Monitore o manômetro para confirmar uma diminuição na pressão. Agora ative o sistema de resfriamento e configure-o para 15 graus Celsius. Ligue a bomba e os botões de refrigeração e abra a válvula conectada ao instrumento de sputtering.Agora ajuste o fluxo de gás argônio para quatro centímetros cúbicos padrão por minuto e ative o botão de alternância de gás. Defina a velocidade de rotação para 10 RPM e ligue o botão de alternância de rotação. Pressione o botão liga/desliga para ativar o controlador de rede de correspondência automática e a fonte de alimentação de radiofrequência.No controlador de rede de correspondência automática use o botão min/max, ajuste a carga e ajuste para 50% cada. Em seguida, mude o modo de manual para automático. Na fonte de alimentação de radiofrequência, defina a potência de radiofrequência para 50 watts e pressione o botão Iniciar.Defina o temporizador para 15 minutos. Em seguida, desligue a alimentação e a rotação de RF. Defina o fluxo de argonne para zero e desligue o botão de alternância.Depois disso, desligue o vácuo. Certifique-se de desligar a bomba molecular turbo antes de ventilar a câmara para evitar danos. Em seguida, abra a câmara e carregue os substratos preparados no canto externo do porta-amostras giratório para uma deposição ideal.Feche a porta da câmara e vácuo por um período mínimo de seis horas. Ative o sistema de resfriamento como mostrado anteriormente e abra a válvula ligada ao instrumento de sputtering. Defina a velocidade de rotação, o fluxo de argônio, o controlador de rede de correspondência automática e a frequência de rádio, conforme demonstrado anteriormente.Pressione o botão Iniciar e inspecione a câmara para verificar a presença de plasma. Aumente incrementalmente a potência de RF em cinco watts a cada 10 segundos até atingir 75 watts. Ajuste o temporizador para 60 minutos Depois, desligue a energia de radiofrequência e o sistema de rotação.Desligue também o controlador de rede de correspondência automática e a fonte de alimentação de radiofrequência. Ajuste o fluxo de gás argônio para zero e desligue o interruptor de alternância de gás. Em seguida, desligue o vácuo e ventile a câmara para abri-la.Usando pinças, retire todas as amostras da câmara e coloque-as em uma placa de Petri limpa. Limpe a câmara e aspira-a por 10 a 15 minutos para manter uma condição de vácuo livre de impurezas.

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Radio Frequency Magnetron Sputtering of Bi2Te3 and Sb2Te3 Thermoelectric Thin Films on Glass Substrates

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