Design of magnetron co-Sputtering configuration for preparing magnesium tin silicide-based thermoelectric alloy thin films

Science & Technology Development Journal, 22(4):385-390 Open Access Full Text Article Methodologies 1Laboratory of Advanced Materials, University of Science, Ho Chi Minh City, Vietnam 2Vietnam National University, Ho Chi Minh City, Vietnam 3Faculty of Materials Science and Technology, University of Science, Ho Chi Minh City, Vietnam 4Center for Innovative Materials and Architectures (INOMAR), Ho Chi Minh City, Vietnam History  Received: 2019-11-16  Accepted: 2019-12-17  Published: 2019-12-31 DOI : 10.32508/stdj.v22i4.1729 Copyright © VNU-HCM Press. This is an open- access article distributed under the terms of the Creative Commons Attribution 4.0 International license. Design of magnetron co-sputtering configuration for preparing magnesium tin silicide-based thermoelectric alloy thin films Anh Tuan Thanh Pham1,2, Cuong Nhat Le1,2, Dung Van Hoang1,2, Truong Huu Nguyen1,2, Phuong Thanh Ngoc Vo1,2,3, Thang Bach Phan1,2,4, Vinh Cao Tran1,2 Use your smartphone to scan this QR code and download this article ABSTRACT Introduction: Magnesium tin silicide (MgSiSn) is known as a good-thermoelectric-performance, safe and cost-efficient alloy material. The goal of this work is to design a magnetron co-sputtering configuration for depositing alloy thin films from three independent metal targets including mag- nesium (Mg), silicon (Si) and tin (Sn). Methods: By this solution, the elemental composition of the MgSiSn thin films can be effectively controlled through changing the sputtering power of the individual magnetron. The actual values of elemental composition in the as-deposited films were verified by using energy-dispersive X-ray spectroscopy. The as-deposited thin films were investi- gated carefully by using the X-ray diffraction to recognize crystalline structure characteristics. Most importantly, typically thermoelectric parameters including Seebeck coefficient, electrical conduc- tivity and power factor were indicated as functions of temperature. Results: The XRD analysis exhibits cubic anti-fluorite-type structure characteristics of the MgSiSn films; however, the pres- ence of the segregatedMg phase is still observed. The testing results for the representative MgSiSn thin film with good adherence show the power factor of PF ~20.5103 W/mK2 , as a result of See- beck coefficient of S ~159 mV/K and electrical conductivity of s ~8200 S/cm, at 325 K. At higher temperature than 473 K, the semiconducting behavior of the films tends to transform from p-type to n-type. Conclusion: The three-target co-sputtering configuration shows the possibility of suc- cessfully preparing alloy MgSiSn thin films with good adherence on Si substrate. Furthermore, the testing result suggests that the as-depositedMgSiSn thin films have some potential thermoelectric characteristics, which can be improved more significantly. Key words: Thermoelectrics, magnesium tin silicide, magnetron co-sputtering, thin films INTRODUCTION Magnesium tin silicide (MgSiSn) ternary alloy is one of the best lead-free thermoelectric materials in the medium temperature range (200 – 600oC). It has at- tracted much interest due to constituted composition from the rich-abundant and non-toxic elements1,2. According to the estimation expression of thermo- electric figure of merit (Z), ZT = S2s /k (where S is Seebeck coefficient, s and k are electrical and ther- mal conductivities, respectively), the increase of S, s values and the reduction of k value result in enhance- ment of ZT value. In the case of the MgSiSn alloy, Si4+ replacement of Sn4+ ion not only increases S value owing to the increasing density of state (DOS) in energy-band structure, but also reduces k value be- cause Sn atomhasmuch heaviermass than Si atom3,4. Another method to achieve high ZT value is produc- ing low-dimensional materials due to the quantum confinement, high s , and low k values5. Thin film is known as one of the low-dimensional materials, which doping effect and stoichiometry can be con- trolled. In literature, there have been limited works on the MgSiSn thin films, as compared to the bulk form. Typically, a study on the very thin MgSiSn film (50 – 90 nm) deposited on Si substrate was reported for optoelectronic and thermoelectric applications6. The used deposition technique, however, was a solid phase epitaxy (SPE), which is quite a complicated, ex- pensive and hard-to-control method. Recently, the Al- and Sn-dopedMg2Si thin films deposited by using low-cost and high-efficiency sputtering method were attracted7. The MgSiSn film was co-sputtered from Mg2Si and Sn targets. It facilitated to adjust the Sn content. However, the Mg and Si contents were not independent because their vapor pressure is very dif- ferent. To solve the above problems, in this work, a new co- sputtering configuration was set up. The magnetron sputtering system was used to prepare the MgSiSn thin films from three independent metal (Mg, Si and Sn) targets. Some electrical and thermoelectric char- acteristics of the as-deposited MgSiSn thin films were basically investigated. Cite this article : Tuan Thanh Pham A, Nhat Le C, Van Hoang D, Huu Nguyen T, Thanh Ngoc Vo P, Bach Phan T, Cao Tran V. Design of magnetron co-sputtering configuration for preparing magnesium tin silicide-based thermoelectric alloy thin films. Sci. Tech. Dev. J.; 22(4):385-390. 385 Science & Technology Development Journal, 22(4):385-390 MATERIALS - METHODS The 3-inch metal targets included Mg, Si, and Sn (99.99%, Gredmann, Taiwan) were used to co-sputter the MgSiSn thin films. Because of low conductivity, the Si target was connected to a 13.56 MHz radio- frequency (RF) source, while the Mg and Sn targets were controlled by direct-current (DC) sources. All theMgSiSn films were prepared on a LeyboldUnivex- 450 (Germany) sputtering system. Themagnetron co- sputtering configuration inside vacuum chamber can be modified to change conditional parameters, prop- erties and composition of the films. The 2x2 cm2 Si(200) wafer was used as substrate. The base vacuum pressure was set at 4106 torr, which was created by using a high-speed turbomolecular pump. The substrate temperature and working pressure in pure Ar gas atmosphere were maintained at 300oC and 3.5 mtorr, respectively. The distance from the target to the substrate was fixed at 7 cm for all the targets. Be- fore the deposition process, the three targets were pre- sputtered in 5 minutes to remove oxide layers and contamination on the target surface. Also, the sub- strate was cleaned by discharge in the high-pressure Ar gas atmosphere (102 torr). The deposition time was fixed at 5 minutes corre- sponding to the film thickness of~300 nm. The thick- ness was determined by using a Stylus profilometer (Veeco Dektak-6M, US) and cross-sectional scanning electronmicroscopy (FESEM,Hitachi S-4800, Japan). In the Stylus method, the Dektak-6M system was equipped a 12.5 mmdiamond tip. During themeasur- ing process, the Stylus tip contacted and scanned me- chanically on the film surface. A height deviation of the tip between the substrate and the film on the sub- strate was used to derive the film thickness. In the FE- SEM method, the MgSiSn films on Si substrate were observed horizontally. The obtained cross-sectional image gave information about the crystallization in- side the films and the interface between the film and the substrate. The crystalline structure of the films which was con- trolled through adjusting the power of the sputtering targets was investigated by using the X-ray diffrac- tion method (XRD, Bruker D8-Advance, US) with a monochromatic CuKa beam (l = 0.1541 nm) as an X-ray source. In the XRD method, the q – 2q scan- ning technique was employed, which q is the angle between incident beam and reflected plane, whereas 2q is the angle between transmitted beam and re- flected beam (detector). While the power of Mg tar- get was fixed at 30 W, the power of Si target increases from 0 to 100W corresponding to the decrease of the power of Sn target from 60 to 0 W, as listed in Ta- ble 1. The deposition rate from the different metal targets was measured by using a quartz crystal oscil- lator (Inficon XTM/2, US). In this method, a quartz crystal sensorwas applied parallel to the target surface with a similar target-substrate distance (7 cm). Dur- ing the sputtering process, the sputtered particle from the targets bombarded on the quartz surface. Ow- ing to piezoelectric property, the vibration resonance of quartz crystal created electrical signals. Based on these recorded signals, the deposition rate from each target was calculated. The temperature-dependent thermoelectric proper- ties (Seebeck coefficient and electrical conductivity) of the representative MgSiSn thin film was deter- mined by using a Seebeck measurement system (Ul- vac ZEM-3, Japan). The sample was cut into 15- mm long and 5-mm wide rectangular piece for the measurement. The investigated range and acceler- ating rate of temperature were 300 – 675 K and 50 K/min, respectively. At each temperature, the val- ues of electrical conductivity and the Seebeck coeffi- cient of theMgSiSn filmweremeasured three times to check the repeatability of the results. In addition, the elemental composition of the representative film was also checked through energy-dispersive X-ray spec- troscopy (EDS) which was an attachment of the FE- SEM technique. RESULTS Design of magnetron co-sputtering config- uration To prepare MgSiSn alloy thin films, we modified a co-sputtering configuration with three separate Mg, Si and Sn targets, which was based on the Leybold Univex-450 system (Figure 1a). The position of the targets was arranged as shown in Figure 1b and Fig- ure 1 c. The sputtering targets were located on the surface of magnetron guns which were continuously cooled at 20oC by using a water chiller. Because of the lowest vapor pressure, the sputtering yield of Mg tar- get is very high. To protect the substrate during the target pre-sputtering process, double shutters were designed. The lower shutter covered the Mg target surface, while the substrate was shielded by the up- per shutter. The substrate was attached on the holder which rotates around a centered axis with a rotational angle of ~270oC (from A to B and vice versa). The holder could rotate continuously with controllable angular velocity. The three magnetron guns were 15- cm equidistant from each other and 10-cm equidis- tant from the rotation axis. The sputtering power of 386 Science & Technology Development Journal, 22(4):385-390 Table 1: The variation of Si and Sn sputtering powers in depositing theMgSiSn thin films Samples Power of Si target (W) Power of Sn target (W) Mg-100Si 100 0 Mg-90Si-20Sn 90 20 Mg-80Si-25Sn 80 25 Mg-70Si-30Sn 70 30 Mg-60Si-35Sn 60 35 Mg-50Si-40Sn 50 40 Mg-40Si-45Sn 40 45 Mg-30Si-50Sn 30 50 Mg-60Sn 0 60 each target and the angular velocity of the substrate holder were the most important parameters which af- fected the uniformity and composition of the MgSiSn thin films. In this initial study, the investigation was focused on changing the sputtering power of each tar- get, thus the angular velocity was fixed at 0.375p rad/s during the deposition process. Initial results of theMgSiSn thin films Figure 2 shows the crystalline structure of the Mg- SiSn thin films. There are two peaks at 33.18o and 47.92o which belong to the (200) and (220) plane of the Si substrate, respectively. A clear peak located at ~34.50o is found to be the (002) plane of metal Mg phase (JCPDS 35-0821). The existence of a separate Mg phase reflects non-uniform stoichiometry or ex- cessive Mg content in the films. This phenomenon was also reported by Zhang’s work7. More impor- tantly, it is seen that almost the samples tend to form cubic anti-fluorite-type structure with characteristic crystalline planes, such as (111), (220), (311) and (222)8. Based on theXRD results, the good stoichiometry and low excessive Mg phase can be obtained in the Mg- SiSn thin films, if the sputtering power of Si and Sn targets are lower than 60W and higher than 35W, re- spectively. Among them, the representative Mg-50Si- 40Sn sample is chosen to investigate morphological and thermoelectric properties. Figure 3 shows the cross-sectional morphology and chemical composition analysis of the Mg-50Si-40Sn thin film. From the FESEM image, the thickness of the film is determined, approximately 300 nm. No layer separation is observed, which suggests good in- corporation of the Mg, Si and Sn contents in the al- loy structure. The elemental composition of the film is also checked and listed in the inset table. The EDS result indicates the successful deposition of the alloy MgSiSn film. Figure 4 shows some typical thermoelectric parame- ters (electrical conductivity, Seebeck coefficient and power factor) of the Mg-50Si-40Sn thin film. At a lower temperature than 473 K, the electrical conduc- tivity of the films is high, which is highly-degenerated semiconductor behavior. When temperature in- creases more than 473 K, the electrical conductivity of the films decreases strongly, simultaneously, the value of Seebeck coefficient tends to bemore negative. The thermoelectric power factor, PF = S2s , where S is the Seebeck coefficient and s is the electrical con- ductivity. The high PF value means the possibility of generating high voltage and power of thermoelectric materials when there is a temperature gradient. As a result, the highest power factor of PF ~20.5103 W/mK2 corresponding to the Seebeck coefficient of S ~159 mV/K and the electrical conductivity ofs ~8200 S/cm can be observed at ~325 K. DISCUSSION Another proof for the formation of MgSiSn alloy is the detection of Mg, Si and Sn contents in the films, as shown in Figure 3. A problem, however, is that the composition ratio of Si is very high. It can be due to the contribution of the signals from the Si sub- strate. Therefore, othermaterials will be used as a sub- strate in the future studies. In addition, the O content may come from residual gas in vacuum chamber or contamination. It is also a technique problem of this co-sputtering configuration for depositing alloy thin films, which is needed to be improved. From the measurement of thermoelectric properties in Figure 4, the Seebeck coefficient is positive, which 387 Science & Technology Development Journal, 22(4):385-390 Figure 1: Design ofmagnetron co-sputtering configuration: (a) Leybold Univex-450 (Germany) sputtering sys- tem with high-speed turbomolecular vacuum pump station; (b) and (c) magnetron configuration and targets ar- rangement in the vacuum chamber. The three targets are equidistant from each other and from the rotation axis of the substrate. reflects the p-type characteristic of the film. When temperature increases, the electrical conductivity de- creases strongly, simultaneously, the film transforms into n-type behavior due to a negative Seebeck co- efficient. It may be due to the decrease of car- rier concentration and mobility at high temperatures, which is suitable for the characteristic of the highly- degenerated semiconductor. However, the transfor- mation from p-type to n-type behavior of the film has not been understood yet. The obtained PF value is relatively high for the Mg2Si-based materials, but is still lower than the other reports 9–11. Consequently, from the above obtained results, the alloy MgSiSn thin films prepared by using the co-sputtering con- figuration exhibits some thermoelectric properties. Among them, relatively high electrical conductivity and temperature-dependent semiconductor behavior of Seebeck coefficient are interesting. It is believed that the thermoelectric properties of the MgSiSn thin films can be enhanced by optimizing conditional pa- rameters of the co-sputtering configuration. CONCLUSION In conclusion, the three-target co-sputtering config- uration shows the possibility of successfully prepar- ing alloy MgSiSn thin films with good adherence on Si substrate. The composition, stoichiometry, crys- talline structure and thermoelectric properties of the films can be controlled through adjusting the power sputtering of each target. The typical 300 nm-thick MgSiSn film deposited at 30 W of Mg target, 50 W of Si target and 40 W of Sn target exhibits the p- type semiconductor behavior with the Seebeck co- efficient of S ~159 mV/K, the electrical conductiv- ity of s ~8200 S/cm and the power factor of PF ~20.5103 W/mK2 at ~325 K. The result suggests that the as-deposited MgSiSn thin films have some potential thermoelectric characteristics, which can be improved more significantly in the next studies. LIST OF ABBREVIATIONS s : Electrical conductivity EDS: Energy-dispersive X-ray spectroscopy MgSiSn: Magnesium tin silicide PF: Power factor 388 Science & Technology Development Journal, 22(4):385-390 Figure 2: XRD patterns of the MgSiSn thin films deposited with different Si and Sn sputtering powers: (a) in large scale 2q = 20 – 50o , and (b) in small scale 2q = 22 – 24o . The sputtering power of Mg target is constant, whereas the power of Si target decreases from 100 W to 0 W, and the power of Sn increases from 0W to 60 W. Figure3: Themorphologyanalysis of theMg-50Si-40Sn thinfilm: (a) cross-sectional FESEM image, and (b) EDS elemental quantitative result. The obtained film thickness are about 300 nm, whereas the O composition might be from contamination. 389 Science & Technology Development Journal, 22(4):385-390 Figure 4: Thermoelectric parameters (electrical conductivity, Seebeck coefficient and power factor) of the Mg- 50Si-40Sn thin film in the temperature range of 300 – 675 K. 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