Multi-Layer Metallization Structure Development for Highly Efficient

applied sciences

Article Multi-Layer Metallization Structure Development for Highly Efﬁcient Polycrystalline SnSe Thermoelectric Devices

Yeongseon Kim 1,2, Giwan Yoon 1, Byung Jin Cho 1 and Sang Hyun Park 2,*

1 School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea; [email protected] (Y.K.); [email protected] (G.Y.); [email protected] (B.J.C.) 2 Advanced Materials and Devices Laboratory, Korea Institute of Energy Research, 152 Gajung-ro, Yuseong-gu, Daejeon 305-343, Korea * Correspondence: [email protected]; Tel.: +82-42-860-3075; Fax: +82-42-860-3309

Received: 19 September 2017; Accepted: 23 October 2017; Published: 28 October 2017

Abstract: Recently, SnSe material with an outstanding high ZT (Figure of merit) of 2.6 has attracted much attention due to its strong applicability for highly efficient thermoelectric devices. Many studies following the first journal publication have been focused on SnSe materials, not on thermoelectric devices. Particularly, to realize highly efficient intermediate-temperature (600~1000 K) thermoelectric modules with this promising thermoelectric material, a more thermally and electrically reliable interface bonding technology needs to be developed so that the modules can stably perform their power generation in this temperature range. In this work, we demonstrate several approaches to develop metallization layers on SnSe thermoelectric legs. The single-layer metallization shows limitations in their electrical contact resistances and elemental diffusions. The Ag/Co/Ti multi-layer metallization results in lowering their electrical contact resistances, in addition to providing more robust interfaces. Moreover, it is found to maintain the interfacial characteristics without any significant degradation, even after heat treatment at 723 K for 20 h. These results can be effectively applied in the fabrication of thermoelectric devices or modules that are made of the SnSe thermoelectric materials.

Keywords: tin selenide; thermoelectric module; contact resistance; metallization

1. Introduction Thermoelectric generation is a well-known way of converting waste heat into electrical energy by the Seebeck effect. Thermoelectric generation has many advantages, mainly because thermoelectric systems have no moving parts and also are compact in size and easy to apply anywhere. However, thermoelectric materials are relatively lower in efﬁciency than other waste heat recovery methods such as organic rankine cycle (ORC) or alkali-metal thermal to electric converter (AMTEC). Recently, thermoelectric materials have shown great advances in the ﬁgure of merit (ZT), especially for intermediate-temperature (600~1000 K) thermoelectric materials, through nano-structured material, band structure engineering, and grain boundary engineering. Skutterudite, a well-known intermediate-temperature thermoelectric material, shows a relatively high ZT of 1.6 at 800 K for n-type case, and 1.3 at 775 K for p-type case by nano-structuring [1]. PbTe also shows peak ZT of 2.5 at 923 K, and average ZT 1.67 between 300 and 900 K which is due largely to its band structure engineering and nano-structuring [2]. SnSe also stands out for the record-high ZT of 2.6 at 923 K which is attributed to the ultra-low lattice thermal conductivity caused by a layered structure [3]. Many studies have been conducted to further investigate the thermoelectric properties of SnSe and

Appl. Sci. 2017, 7, 1116; doi:10.3390/app7111116 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 1116 2 of 8

Appl.and Sci.also2017 to, 7develop, 1116 the n-type SnSe, polycrystalline SnSe, and so on. Nonetheless, few studies 2have of 8 been reported on the SnSe thermoelectric devices or modules and their fabrication processing. The fabrication processes of thermoelectric devices or modules are typically composed of alsomaterial to develop sintering, the n-typemetallization, SnSe, polycrystalline slicing, cutti SnSe,ng, and and bonding so on. Nonetheless, to electrodes few and studies substrates. have been The reportedmetallization on the of SnSe thermoelectric thermoelectric materials devices is regard or modulesed as andone of their the fabrication most important processing. processes because the highThe fabricationelectrical and processes thermal of contact thermoelectric resistances devices at the or interface modules can are significantly typically composed degrade of the material power sintering,output. The metallization, formation of slicing, the metallization cutting, and layer bonding should to electrodes be made andto lower substrates. the contact The metallization resistance as ofwell thermoelectric as prevent inter-diffusion materials is regardedof composed as one elemen of thets. The most Wang important group processesinvestigated because SnSe and the metal high electricalcontacts andusing thermal the first-principle contact resistances calculation at the which interface discovered can significantly that Ag, Au, degrade and Ta the contacts power output. would Themake formation stable contact of the with metallization SnSe due layer to its should small belattice made mismatch to lower [4]. the Our contact group resistance reported as that well the as preventmetallization inter-diffusion of SnSe with of composed an Ag and elements. Ni bi-layer The co Wanguld lead group to the investigated formation SnSeof the and thick metal intermetallic contacts usingcompounds, the first-principle which act calculationas a diffusion which barrier, discovered but itthat could Ag, be Au, a reason and Ta contactsfor increasing would the make electrical stable contactcontact withresistance SnSe due[5]. to its small lattice mismatch [4]. Our group reported that the metallization of SnSe withIn this an work, Ag and the Niimplementation bi-layer could of lead the tosingle-layer the formation metallization of the thick on intermetallicthe p-type polycrystalline compounds, whichSnSe thermoelectric act as a diffusion material barrier, resulted but it could in some be a reason drawbacks for increasing such as undesirable the electrical interfacial contact resistance cracks and [5]. elementalIn this diffusion. work, the To implementation overcome its drawbacks, of the single-layer we proposed metallization a new metallization on the p-type technique polycrystalline to form SnSethe multi-layer thermoelectric structure material composed resulted of in Ag/Co/Ti some drawbacks layers. This such Ag/Co/ as undesirableTi multi-layered interfacial structure cracks andwas elementaldemonstrated diffusion. a relatively To overcome lower specific its drawbacks, contact weresistance proposed and a also new acted metallization as a good technique diffusion to barrier form thefor multi-layerthe SnSe thermoelectric structure composed material. of Ag/Co/Ti layers. This Ag/Co/Ti multi-layered structure was demonstrated a relatively lower specific contact resistance and also acted as a good diffusion barrier for2. Experiments the SnSe thermoelectric material.

2. ExperimentsFor the synthesis of pure SnSe ingots, a stoichiometric amount of the Sn (99.5%, Alfa Aesar, Haverhill, MA, USA) and Se (99.5%, Alfa Aesar) mixture powder was loaded into a fused silica tube For the synthesis of pure SnSe ingots, a stoichiometric amount of the Sn (99.5%, Alfa Aesar, and then it was heated up to 1220 K and held at the same temperature for 24 h under a vacuum Haverhill, MA, USA) and Se (99.5%, Alfa Aesar) mixture powder was loaded into a fused silica tube condition. The pure SnSe ingots were pulverized manually with a mortar and pestle and then the and then it was heated up to 1220 K and held at the same temperature for 24 h under a vacuum pulverized powder was passed through a sieve to obtain a mixture powder of less than 45 μm. For condition. The pure SnSe ingots were pulverized manually with a mortar and pestle and then the the single-layer metallization on SnSe, a set of Ag powder (325 mesh, 99.99%, Sigma-Aldrich, City of pulverized powder was passed through a sieve to obtain a mixture powder of less than 45 µm. For the St. Louis, MO, USA), Ni powder (325 mesh, 99.8%, Alfa Aesar), Ti powder (325 mesh, 99.99%, Alfa single-layer metallization on SnSe, a set of Ag powder (325 mesh, 99.99%, Sigma-Aldrich, St. Louis, Aesar) was prepared. The spark plasma sintering (SPS, Fuji Electronic Industrial Co., Ltd., Kyoto, MO, USA), Ni powder (325 mesh, 99.8%, Alfa Aesar), Ti powder (325 mesh, 99.99%, Alfa Aesar) was Japan) system was used to perform the SnSe sintering and metallization at the same time as shown prepared. The spark plasma sintering (SPS, Fuji Electronic Industrial Co., Ltd., Kyoto, Japan) system in Figure 1. was used to perform the SnSe sintering and metallization at the same time as shown in Figure1.

FigureFigure 1.1. SchematicSchematic ofof co-sinteringco-sintering processprocess ofof thermoelectricthermoelectric materialmaterial andand metalmetal layerlayer usingusing SPSSPS (Spark(Spark Plasma Plasma Sintering). Sintering).

A previous study [5] reported that both the metal powder and foil metallization methods showed no big differences on their interface bonding and electrical properties. Thus, for the multi-layer Appl. Sci. 2017, 7, 1116 3 of 8

A previous study [5] reported that both the metal powder and foil metallization methods showedAppl. Sci. 2017no ,big7, 1116 differences on their interface bonding and electrical properties. Thus, for the multi-3 of 8 layer metallization, we prepared different metal foils such as Ag foil (25 μm, 99.99%, Alfa Aesar), Co foil (50 μm, 99.99%, Alfa Aesar), and Ti foil (50 μm, 99.99%, Alfa Aesar) with the intention of making metallization, we prepared different metal foils such as Ag foil (25 µm, 99.99%, Alfa Aesar), Co foil regular interfaces over the entire region which are easily distinguishable from one another. The SnSe (50 µm, 99.99%, Alfa Aesar), and Ti foil (50 µm, 99.99%, Alfa Aesar) with the intention of making regular powder was sintered together with metallic layers at 550 °C for 10 min with 40 MPa pressure. This interfaces over the entire region which are easily distinguishable from one another. The SnSe powder sintering step completed the formation of metallized SnSe ingots. In order to investigate their was sintered together with metallic layers at 550 ◦C for 10 min with 40 MPa pressure. This sintering step interfaces, SnSe ingots were cut into several small thermoelectric legs using a diamond wire saw. The completed the formation of metallized SnSe ingots. In order to investigate their interfaces, SnSe ingots microstructures of the interfaces were analyzed using a scanning electron microscope (SEM, Hitachi, were cut into several small thermoelectric legs using a diamond wire saw. The microstructures Tokyo, Japan). The composition distributions of SnSe and metal elements were investigated using an of the interfaces were analyzed using a scanning electron microscope (SEM, Hitachi, Tokyo, Japan). energy dispersive spectroscopy (EDS) line-scan (Hitachi, Tokyo, Japan). The EDS line scan was The composition distributions of SnSe and metal elements were investigated using an energy dispersive conducted several times to verify that interface characteristics are almost the same over the entire spectroscopy (EDS) line-scan (Hitachi, Tokyo, Japan). The EDS line scan was conducted several times region. Heat treatments were conducted at 723 K in an Ar atmosphere to verify the thermal stability to verify that interface characteristics are almost the same over the entire region. Heat treatments were of metallization interfaces [6,7]. To understand the effects of heat treatments, the electrical contact conducted at 723 K in an Ar atmosphere to verify the thermal stability of metallization interfaces [6,7]. resistances were measured before and after heat treatments by home-made resistance scanning To understand the effects of heat treatments, the electrical contact resistances were measured before measurement equipment [8]. and after heat treatments by home-made resistance scanning measurement equipment [8]. 3. Results and Discussion 3. Results and Discussion A metallization process is performed to form a metallic layer on the SnSe thermoelectric material A metallization process is performed to form a metallic layer on the SnSe thermoelectric material and the formed metallic layer should act as a high electrical and thermal conductive layer as well as and the formed metallic layer should act as a high electrical and thermal conductive layer as well as an inter-diffusion barrier layer. The Ag, Ni, or Ti metal layers were selected to make a single metallic an inter-diffusion barrier layer. The Ag, Ni, or Ti metal layers were selected to make a single metallic layer each on SnSe because Ag has high electrical conductivity, and both Ni and Ti are well-known layer each on SnSe because Ag has high electrical conductivity, and both Ni and Ti are well-known diffusion barrier materials [9–13]. Because the sintering temperature of SnSe was already determined diffusion barrier materials [9–13]. Because the sintering temperature of SnSe was already determined according to the optimized material properties, the metallization in this work was also conducted according to the optimized material properties, the metallization in this work was also conducted using SPS at the similar sintering condition. The bonding interfaces and element distributions were using SPS at the similar sintering condition. The bonding interfaces and element distributions were analyzed, as shown in Figure 2. analyzed, as shown in Figure2.

FigureFigure 2. SEM (Scanning(Scanning ElectronElectron Microscopy) Microscopy) images images and and EDS EDS (Energy (Energy Dispersive Dispersive Spectrometer) Spectrometer) line linescanning scanning of single-layer of single-layer metallization: metallization: (a) Ni; (a) (Ni;b) Ag;(b) Ag; and and (c) Ti. (c) Ti.

TheThe Ni single-layer metallization formed formed a th thickick intermetallic compound (IMC) composed of NiNi5.635.63SnSeSnSe2 2andand Ni Ni3Sn3Sn alloys, alloys, shown shown in in Figure Figure 2a.2a. These These alloys alloys are are different different in their crystal structures andand cellcell sizes,sizes, causing causing severe severe cracks cracks across across the interfacesthe interfaces between between the Ni the and Ni IMC and layers. IMC This layers. indicates This indicatesclearly that clearly Ni is that not suitableNi is not for suitable SnSe metallization. for SnSe metallization. On the other On hand, the other both hand, Ag and both Ti layersAg and were Ti layersbonded were robustly bonded with robustly SnSe without with anySnSe cracks without or delaminationany cracks or at delamination the interfaces. at However, the interfaces. the Sn However,element of the SnSe Sn was element diffused of SnSe into thewas Ag diffused layer, as into shown the Ag in Figure layer,2 asb. shown This means in Figure that Sn 2b. could This diffusemeans continuously into the Ag layer at the operating temperature, possibly degrading the thermoelectric property of SnSe. The Ti layer could prevent the Sn diffusion effectively, as shown in Figure2c. Appl. Sci. 2017, 7, 1116 4 of 8 that Sn could diffuse continuously into the Ag layer at the operating temperature, possibly degrading the thermoelectric property of SnSe. The Ti layer could prevent the Sn diffusion effectively, as shown inAppl. Figure Sci. 2017 2c., 7, 1116 4 of 8 The effect of the metallization on electrical contact property was also investigated by the specific contact resistance (SCR). In general, the SCR is defined by Rsc = Rc·A where A is an interfacial area and RThec is effectthe resistance of the metallization difference onbetween electrical thermoel contactectric property materials was also and investigated metallization by layers. the speciﬁc The metallizedcontact resistance SnSe ingots (SCR). were In cut general, into rectangular- the SCR isshaped deﬁned segments by Rsc = to R measurec·A where their A isinterfacial an interfacial areas easily.area and Figure Rc is 3a,b the resistancepresent the difference resistance between versus distance thermoelectric relations materials for the andAg and metallization Ti metallization layers. layers.The metallized SnSe ingots were cut into rectangular-shaped segments to measure their interfacial areas easily. Figure3a,b present the resistance versus distance relations for the Ag and Ti metallization layers.

Figure 3. The graphs of SCR (Specific Contact Resistance) of (a) Ag and (b) Ti single-layer metallization. Figure 3. The graphs of SCR (Specific Contact Resistance) of (a) Ag and (b) Ti single-layer metallization. The SCR of the Ni metallization layer could not be measured mainly due to the severe cracks. · 2 TheThe SCR SCR measurement of the Ni metallization results for Ag layer and could Ti were not be 1.19 measured and 927 mainly mΩ cm due, respectively.to the severe cracks. The Ti Themetallization SCR measurement layer was results bonded for mechanically Ag and Ti wellwere with 1.19 SnSe, and but927 themΩ electrical·cm2, respectively. contact property The Ti metallizationwas poor, causing layer a was degradation bonded mechanically of the thermoelectric well with power SnSe,output. but the electrical contact property was poor,In causing general, a degradation the SCRs of theof the intermediate-temperature thermoelectric power output. thermoelectric legs need to be lower than −5 · 2 10 InΩ general,cm in orderthe SCRs to prevent of the intermediate-temperatu any significant degradationre thermoelectric of power output legs need by anyto be contact lower than loss. 10However,−5 Ω·cm2 thisin order should to beprevent considered any significant as the ratio degrada of the resistancetion of power of thermoelectric output by any materials contact to loss. the However,contact resistance. this should The be SnSeconsidered material as showsthe ratio a highof the ZT resistance value due of thermoelectric to its high Seebeck materials coefficient to the contactand low resistance. thermal conductivity,The SnSe materi butal itsshows electrical a high conductivity ZT value due is to much its high lower Seebeck than coefficient those of other and lowintermediate-temperature thermal conductivity, materials, but its aselectrical shown incond Tableuctivity1. is much lower than those of other intermediate-temperature materials, as shown in Table 1.

Appl. Sci. 2017, 7, 1116 5 of 8

Table 1. Thermoelectric property comparison between well-known thermoelectric materials which are utilized in an intermediate temperature range

Appl. Sci. 2017, 7, 1116 Skutterudite [1] Half-heusler [14] Polycrystalline5 of 8 SnSe Thermoelectric properties (776K) (776K) [15] (773K) Table 1. Thermoelectric property comparison between well-known thermoelectric materials which are Seebeck utilized in an intermediate temperature range.144 161 425 coefficient (μV/K) Skutterudite [1] Half-heusler [14] Polycrystalline SnSe [15] Thermoelectric properties Electrical (776K) (776K) (773K) Seebeck 1600 1700 18.1 conductivitycoefﬁcient (S/cm) (µV/K) 144 161 425

Electrical Thermal 1600 1700 18.1 conductivity (S/cm) 3.33 5.1 0.23 conductivity (W/mK) Thermal 3.33 5.1 0.23 conductivity (W/mK) ZT (Figure of merit) 0.77 0.67 0.96 ZT (Figure of merit) 0.77 0.67 0.96 The electrical conductivities of skutterudite and half-Heusler, both well-known intermediate- temperatureThe electrical thermoelectric conductivities materials, are of above skutterudite 1000 S/cm and in overall half-Heusler, temperature both ranges well-known [1,14]. The intermediate-temperatureelectrical conductivity of SnSe thermoelectric is only 18.1 materials,S/cm, indicating are above that it 1000 is not S/cm necessary in overall to have temperature low SCR rangesas in other [1,14 materials]. The electrical [15]. As conductivity a result, several of SnSe 10− is3 Ω only·cm2 18.1 SCR S/cm, of SnSe indicating thermoelectric that it is material not necessary could tobe haveapproximated low SCR as to in otherbe the materials same as [15 the]. As 10 a−4 result,~10−5 Ω several·cm2 SCR 10− 3 ofΩ ·cmskutterudite2 SCR of SnSe and thermoelectric half-Heusler materialthermoelectric could materials. be approximated The relation to be between the same the as SCR the and 10− 4power~10−5 outputΩ·cm2 forSCR the of SnSe skutterudite is calculated and half-Heuslerby the finite element thermoelectric method, materials. as shown The in relationFigure 4. between the SCR and power output for the SnSe is calculated by the ﬁnite element method, as shown in Figure4.

Figure 4. The simulation result of relation between power output and SCR depending on Figure 4. The simulation result of relation between power output and SCR depending on the the metallization. metallization.

2 The 8-couple SnSe thermoelectric modulemodule waswas designeddesigned toto bebe 2020× × 20 mm2 inin size. The hot and −7 2 cold side temperatures were were 873 873 K K and and 293 293 K, K, respectively. respectively. When When we we define define the the SCR SCR of of 10 10−7 Ω·cmΩ·cm2 as asmaking making no nosignificant significant electrical electrical contact contact loss, loss, th thee power power output output is is decreased decreased about about 1.9% 1.9% for for Ag and 86.6% for Ti metallizationmetallization layer for the respective SCRs. Therefore, the Ag metallization, where the 2 SCR is 1.15 mΩ·cm·cm2,, is is acceptable acceptable in in contact contact resistance resistance for for SnSe SnSe legs legs and and devices. devices. The analysis results show that it is difficultdifficult to apply a single metallization layer on the SnSe thermoelectric materialmaterial due due to itsto interfaceits interface cracks, cracks element, element diffusion, diffusion, or high electricalor high electrical contact resistivity. contact Inresistivity. order to complementIn order to compleme these drawbacks,nt these drawbacks, a multi-layer a metallizationmulti-layer metallization technique was technique used in thiswas work.used Basedin this onwork. the Based single-layer on the metallizationsingle-layer metallization experiments, experiments, it is found thatit is Agfound can that be usedAg can as be a metallicused as contact layer and Ti can be used as a diffusion barrier. The Ag/Ti bi-layer was formed on SnSe and the interfacial cracks were found to exist at the interface between Ag and Ti layers, as shown in Figure5. Appl.Appl. Sci. Sci. 2017 2017, ,7 7, ,1116 1116 66 of of 8 8 aa metallic metallic contact contact layer layer and and Ti Ti can can be be used used as as a a diffusion diffusion barrier. barrier. The The Ag/Ti Ag/Ti bi-layer bi-layer was was formed formed on on SnSeSnSe and and the the interfacial interfacial cracks cracks were were found found to to exist exist at at the the interface interface between between Ag Ag and and Ti Ti layers, layers, as as shown shown Appl. Sci. 2017, 7, 1116 6 of 8 inin Figure Figure 5. 5.

FigureFigure 5. 5. SEM SEM image image of of Ag/Ti Ag/Ti bi-lay bi-lay bi-layererer metallization metallization metallization on on on SnSe. SnSe. SnSe.

The bi-layer metallization seems unsuitable mainly because the difference in the coefficient of The bi-layer metallization seems unsuitable mainly because the difference in the coefficient coefﬁcient of thermal expansion (CTE) between Ag (18.9 × 10−6 K−1) and Ti (8.6 × 10−6 K−1) is so large that an thermal expansionexpansion (CTE)(CTE) betweenbetween AgAg (18.9 (18.9× ×10 10−−66 KK−−11)) and and Ti (8.6(8.6 ×× 1010−66 KK−−1)1 )is is so so large large that that an interfacial thermal stress is induced. To resolve this issue, as a buffer layer, we used the Co layer interfacial thermalthermal stressstress is is induced. induced. To To resolve resolve this this issue, issue, as aas buffer a buffer layer, layer, we used we theused Co the layer Co where layer where CTE is 13 × 10−6 K−1. Finally, the interface characteristics of the Ag/Co/Ti multi-layer whereCTE is 13CTE× 10is− 613K −×1 .10 Finally,−6 K−1. theFinally, interface the characteristics interface characteristics of the Ag/Co/Ti of the multi-layer Ag/Co/Ti metallization multi-layer metallization were analyzed by EDS line scanning and SCR, as shown in Figure 6a,b. weremetallization analyzed were by EDS analyzed line scanning by EDS andline SCR,scanning as shown and SCR, in Figure as shown6a,b. in Figure 6a,b.

Figure 6. (a) SEM image and EDS line scanning of the Ag/Co/Ti multi-layer metallization; FigureFigure 6. 6. ( a(a) )SEM SEM image image and and EDS EDS line line scanning scanning of of the the Ag/Co/Ti Ag/Co/Ti multi-layer multi-layer metallization; metallization; and and ( b(b) )the the and (b) the graph of SCR before heat treatment. graphgraph of of SCR SCR before before heat heat treatment. treatment.

TheThe metallization metallization layerslayers andand SnSeSnSe areare bonded bonded robustlyrobustly andand theirtheir interfacesinterfaces areare clearly clearly distinguisheddistinguished by by by EDS EDS EDS line line line scanning. scanning. scanning. The The The Sn Sn and and Sn andSe Se el elements Seements elements are are not not are diffused diffused not diffused into into the the into Co Co theand and CoTi Ti layers. andlayers. Ti 2 2 TheThelayers. SCR SCR Theof of the the SCR Ag/Co/Ti Ag/Co/Ti of the Ag/Co/Timulti-layer multi-layer multi-layeris is 1.53 1.53 m mΩΩ·cm is·cm 1.53 2which which mΩ is ·iscm similar similarwhich to to that isthat similar of of the the toAg Ag that single-layer. single-layer. of the Ag TheThesingle-layer. multi-layermulti-layer The metallization multi-layermetallization metallization cancan simultaneouslysimultaneously can simultaneously achieveachieve bothboth achieve highhigh both electricalelectrical high electrical contactcontact contact andand goodgood and diffusiondiffusiongood diffusion barrier barrier barrier properties. properties. properties. In order to verify the thermal stability of the multi-layer metallization, heat treatments were conducted at 723 K for 20 h under an Ar atmosphere. The Ag/Co/Ti multi-layer structure was Appl. Sci. 2017, 7, 1116 7 of 8

Appl.In Sci. order2017, 7 ,to 1116 verify the thermal stability of the multi-layer metallization, heat treatments were7 of 8 conducted at 723 K for 20 h under an Ar atmosphere. The Ag/Co/Ti multi-layer structure was maintainedmaintained robustly andand thethe IMC IMC of of each each interface interface did did not not increase increase even even after after heat treatment,heat treatment, as shown as shownin Figure in 7Figurea. 7a.

FigureFigure 7. 7. (a) SEM(a) SEM image image and EDS and line EDS scanning line scanning of the Ag/Co/Ti of the Ag/Co/Timulti-layer multi-layermetallization; metallization; and (b) the graphand (b of) theSCR graph after ofheat SCR treatment after heat at treatment723 K for 20 at 723h. K for 20 h.

However,However, the EDS lineline scanningscanning analysis analysis revealed revealed that that the the Sn Sn and and Se Se elements elements were were diffused diffused into intothe Agthe layerAg layer which which could could lead lead to a to slight a slight increase increase in SCR in SCR of2.27 of 2.27 mΩ m·cmΩ·cm2 in2 in Figure Figure7b. 7b. As As shown shown in inFigure Figure4, the4, the 20 20 h heath heat treatments treatments may may cause cause a negligiblea negligible reduction reduction in in power power output output of of about about 0.5%. 0.5%.

4.4. Conclusions Conclusions TheThe SnSe SnSe thermoelectric thermoelectric materials materials should should have have highly highly reliable reliable metallization metallization layers layers which which could could decreasedecrease contact resistancesresistances atat the the interfaces interfaces with with electrodes electrodes and and protect protect element element diffusion diffusion which which were werehardly hardly solved solved with awith single-layer a single-layer metallization metallization technique. technique. As a solution,As a solution, the Ag/Co/Ti the Ag/Co/Ti multi-layer multi- layermetallization metallization was was applied applied on SnSeon SnSe thermoelectric thermoelectric materials materials where where each each component component layer has has its its ownown role role such such as as Ag Ag layer layer for for low low electrical electrical contact contact resistance, resistance, Co Co layer layer for for thermal thermal coefficient coefﬁcient matchingmatching between between Ag Ag and and Ti Ti layers, layers, and and Ti Ti layer layer fo forr diffusion diffusion barrier. barrier. The The multi-layer multi-layer metallization metallization resultedresulted in in a arobust robust interfacial interfacial bonding bonding property property as well as well as a aslow a SCR low of SCR 1.53 of m 1.53Ω·cm m2 Ωat· cmthe2 multi-at the layer/SnSemulti-layer/SnSe interfaces, interfaces, becausebecause the electrical the electrical resistivity resistivity of SnSe ofis SnSerelatively is relatively higher highercompared compared to the otherto the intermediate-thermoelectric other intermediate-thermoelectric materials. materials. Moreover, Moreover, it maintained it maintained its microstructures its microstructures even after even theafter rigorous the rigorous heat heattreatments treatments at 723 at 723K for K for 20 20 h. h. The The Ag/Co/Ti Ag/Co/Ti multi-layer multi-layer metallization metallization was was demonstrateddemonstrated to to successfully successfully fabricate fabricate the the thermoel thermoelectricectric devices devices or or modules modules with with high high ZT ZT SnSe SnSe material,material, due due to to its its multi-functionality. multi-functionality. Nonetheless, Nonetheless, in in case case of of the the practical practical thermoelectric thermoelectric devices devices underunder operation, operation, the the metallized metallized layers layers should should withst withstandand rigorous rigorous environments environments for for a a long long time time at at the the operationoperation temperature. temperature. Thus, Thus, in in following following studies, studies, long-term long-term stability stability and and thermal thermal cycling cycling tests tests need need toto be be conducted conducted for for more more practical practical use use of of Ag/Co/Ti Ag/Co/Ti multi-layer multi-layer metallization metallization on on SnSe SnSe materials. materials.

Acknowledgment:Acknowledgments: ThisThis research research was was supported supported by by the the National National Research Research Foundation Foundation of of Korea Korea (NRF) (NRF) Grant Grant funded by the Korean Government (MSIP) (NRF-2015R1A5A1036133). This research was also supported by Basic funded by the Korean Government (MSIP) (NRF-2015R1A5A1036133). This research was also supported by Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of BasicEducation Science (Grant Research No. 2016R1D1A1B01007074). Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. 2016R1D1A1B01007074). Author Contributions: All the authors have equally contributed to the development of multi-layer metallization Authorand its Contributions: analysis. All the authors have equally contributed to the development of multi-layer metallization andConﬂicts its analysis. of Interest: The authors declare no conﬂict of interest. Appl. Sci. 2017, 7, 1116 8 of 8

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