Thermoelectric Properties of Thin Films of Germanium-Gold Alloy Obtained by Magnetron Sputtering

Article Thermoelectric Properties of Thin Films of Germanium-Gold Alloy Obtained by Magnetron Sputtering

Damian Nowak * , Marta Turkiewicz and Natalia Solnica

Faculty of Microsystem Electronics and Photonics, Wrocław University of Science and Technology, Janiszewskiego 11/17, 50-372 Wrocław, Poland; [email protected] (M.T.); [email protected] (N.S.) * Correspondence: [email protected]; Tel.: +48-71-320-4943

Received: 3 December 2018; Accepted: 13 February 2019; Published: 15 February 2019

Abstract: In this paper, the electric and thermoelectric properties of thin films of germanium–gold alloy (Ge–Au) are discussed in terms of choosing the optimal deposition process and post-processing conditions to obtain Ge–Au layers with the best thermoelectric parameters. Thin films were fabricated by magnetron sputtering using the Ge–Au alloy target onto glass substrates at two various conditions; during one of the sputtering processes, the external substrate bias voltage (Ub = −150 V) was used. After deposition thin films were annealed in the atmosphere of N2 at various temperatures (473, 523 and 573 K) to investigate the influence of annealing temperature on the electric and thermoelectric properties of films. Afterwards, the thermocouples were created by deposition of the NiCrSi/Ag contact pads onto Ge–Au films. In this work, particular attention has been paid to thermoelectric properties of fabricated thin films—the thermoelectric voltage, Seebeck coefficient, power factor PF and dimensionless figure of merit ZT were determined.

Keywords: thermoelectric; thin ﬁlms; magnetron sputtering; waste heat; microgenerators

1. Introduction The gradual depletion of non-renewable sources of energy with the simultaneous growing demand for electricity is becoming a huge problem of the modern word. It is necessary to look for new methods of direct conversion of energy from renewable sources to useful electricity [1–3]. One of the alternative ideas for sourcing electric energy is the use of waste heat, which is lost, among other things, in high-energy industrial and production processes. It is estimated, that about 66% of the generated heat is not used in any way, and the ability to convert even a fraction of this heat into electricity could be one of the solutions for the global energy crisis, or at least powering the microelectronic components, Micro-Electro-Mechanical System (MEMS) or Nano-Electro-Mechanical System (NEMS) in Internet of Things (IoT) networks [4–6]. Thermoelectricity is one of the simplest methods for thermal energy conversion—thermoelectric generators (TEGs) can be easily integrated into existing industrial equipment and processes; they have no movable parts, exhibit a direct thermal-to-electric energy conversion mechanism, and are easily scalable from milliwatts to kilowatts [7–9]. Also, thermoelectric generators can work in harsh environments (e.g., high temperature, mechanical shock, radiation or big pressure) while maintaining good degradation characteristics [10,11]. Moreover, TEGs can be considered as one of the methods for producing electricity from waste heat. Microgenerators are built from a series connection of several (or several dozen) thermocouples; single thermocouples consist of materials which should exhibit very good thermoelectric properties [12,13].

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Therefore, the search for alternative thermoelectric materials is crucial for the production of microgenerators and development of microelectronics and small-sized energy sources. The search for new materials is also supported by the development of material sciences and techniques for producing materials with desirable properties [14,15]. Good thermoelectric material should be characterized by three parameters: high electrical conductivity which leads to minimizing the Joule effect and limiting the heat evolved in the material, a high Seebeck coefficient which guarantees the generation of significant values of thermoelectric voltage, and low thermal conductivity that provides a large and stable temperature gradient in the material [16,17]. However, it has been proven that in most cases, increasing one of these quantities results in a reduction in the others. Besides, all of the electric and thermoelectric parameters of materials depend on the carrier concentration in the material [18]. Two parameters were defined for describing the efficiency of thermoelectric materials: the so-called power factor PF (Equation (1)) and the dimensionless figure of merit ZT (Equation (2)) [19].

PF = S2·σ (1)

S2·σ ZT = ·T (2) κ where PF depends on the Seebeck coefficient and electrical conductivity, and the ZT in addition on absolute temperature and thermal conductivity. Optimization of thermoelectric materials should lead to the achievement of maximum ZT values. The peak values of ZT are usually observed for a carrier concentration in the range between 1019 and 1021 cm−3. Such a carrier concentration is typical for heavily doped and degenerated semiconductors (between metals and common semiconductors) [20]. Modern research on thermoelectricity is devoted, among other things, to the miniaturization of energy conversion systems, hence many studies are focused on thin film (or thick film) thermoelectric materials [21]. Recent research should lead to fabrication of thin and thick TE films with thermoelectric parameters (power factor and ZT) similar to conventional bulk materials (ZT ≈ 1 ÷ 2). Then thin and thick films can undoubtedly be the main form of materials used for the production of microgenerators and autonomous power supplies mainly because they can be made very short and thin, which results in a great saving of material [19]. This paper presents research on the electric and thermoelectric properties of one of the most promising thermoelectric materials—semiconductive thin films of germanium–gold alloy (Au content: 5 wt.% obtained by magnetron sputtering. Germanium-based thermoelectric materials were previously investigated in several research papers [22,23]. Firstly, Hasegawa and Kitagawa [24] and Kobayashi [25] studied the influence of post-deposition annealing on localized states in Ge films and on their electric and thermoelectric properties. Also, Kang et al. [26] studied the influence of annealing on the structure of germanium films (authors proposed that the highest temperature that will not led to degradation of the germanium layer is 823 K). Taking into account the fact that the proper selection of material composition and doping of basis materials leads to achieving better thermoelectric properties of materials, further research is focused on the influence of dopant materials on properties of germanium thin films. For example, the influence of antimony [27,28], vanadium [29] and gold [30,31] was investigated. The presented results of studies on thin films of germanium–gold alloy (Ge–Au) are discussed in terms of choosing the optimal deposition process and post-processing conditions to obtain layers with the best thermoelectric properties.

2. Materials and Methods Thin films of germanium–gold alloy were fabricated by magnetron sputtering from circular alloy targets onto glass substrates (Corning 7059, Corning, NY, USA) using the Pfeiffer Classic 570 vacuum system (Wetzlar, Germany) with a WMK-100 magnetron. The gold content in the Coatings 2019, 9, 120 3 of 11 target was 5 wt.%. Investigated layers were fabricated in two processes: One with the addition Coatings 2018, 8, x FOR PEER REVIEW 3 of 11 of external bias voltage (Ub = −150 V) to the glass substrate and the other one without external −3 biasthin films voltage. were The fabricated tested thin under films an wereAr pressure fabricated of 5.8 under × 10−3 an mbar Ar pressureand an applied of 5.8 constant× 10 mbar power and of an320 applied W. After constant the sputtering power ofprocess, 320 W. selected After the films sputtering were annealed process, at selected several filmstemperatures: were annealed 473, 523 at severaland 573 temperatures:K using the six-zone 473, 523 BTU and belt 573 furnace K using (BTU the six-zone International, BTU belt North furnace Billerica, (BTU MA, International, USA). To Northinvestigate Billerica, the thermoelectric MA, USA). To properties investigate of the german thermoelectricium doped properties with goldof layers, germanium it was necessary doped with to goldform layers,the thermocouples it was necessary containing to form thin the films, thermocouples so on the top containingof germanium-gold thin films, thin so films, on the the top two- of germanium-goldlayer NiCrSi/Ag contact thin films, pads the were two-layer fabricated NiCrSi/Ag (also, contactusing the pads magnetron were fabricated sputtering (also, under using the magnetronconstant power sputtering 350 W). under Figure the 1 shows constant a diagram power 350 of the W). fabrication Figure1 shows process a diagram of the Ge–Au/NiCrSi/Ag of the fabrication processthermocouple. of the Ge–Au/NiCrSi/Ag thermocouple.

FigureFigure 1 1.. TheThe Ge Ge–Au/NiCrSi/Ag–Au/NiCrSi/Ag thermocouplethermoco thermocoupleuple fabricationfabrication fabrication process:process: process: ((1 (11)) ) thinthin thin filmfilm ﬁlm depositiondeposition deposition using using thethe magnetronmagnetron sputtering sputtering;; ( (222))) thin thinthin film ﬁlmfilm post post-processpost-process-process annealing annealing;annealing;; ( (33)) the thethe NiCrSi/AgNiCrSi/Ag contact contact contact pad pad’s pad’s’s deposition deposition deposition usingusing magnetron magnetron sputtering. sputtering.

Figure 2 showsshows exemplary exemplary Ge–Au/NiCrSi/Ag Ge–Au/NiCrSi/Ag test structures test structures prepared prepared for thermoelectric for thermoelectric (Figure 2a) (Figureand electric2a) and (Figure electric 2b) (Figure measurements.2b) measurements. As shown As shownin Figure in Figure 2a, in2 a,the in thecase case of ofstructures structures for forthermoelectric thermoelectric measurements, measurements, five ﬁve separated separated stru structuresctures were were deposited deposited onto onto one one glass glass substrate, which was then divided into single testtest structures.structures.

(a) (b)

Figure 2.2. Ge–Au/NiCrSi/Ag test test structures: structures: (a (a) )structures structures for for thermoelectric measurements (ﬁve(five separated thermocouplesthermocouples deposited deposited onto onto one one substrate); substrate); (b )( structureb) structure for electricalfor electrical measurements measurements (van der(van Pauw der Pauw method). method).

The Talysurf Talysurf CCI CCI optical optical profilometer profilometer was was used used to measure to measure the thickness the thickness of deposited of deposited thin films thin filmsand NiCrSi/Ag and NiCrSi/Ag contacts. contacts. The average The thickness average thickness of the Ge–Au of the films Ge–Au was films1.4 µm was and 1.4 theµ mNiCrSi/Ag and the NiCrSi/Agbilayer was bilayer0.45 µm. was The 0.45 resultsµm. Theobtained results were obtained used wereto calculate used to the calculate resistivity the resistivityand electrical and electricalconductivity conductivity of the layers of theas well layers as to as determine well as to the determine deposition the rate deposition as a ratio rate of asfilm a ratiothickness of film to thicknesssputtering to time. sputtering The average time. Thegermanium average th germaniumin films deposition thin films rate deposition was 1.5 ratenm/s. was 1.5 nm/s.

3. Results 3. Results After the deposition and annealing process, four types of structures of germanium–gold thin After the deposition and annealing process, four types of structures of germanium–gold thin ﬁlms were obtained. In each group, there were three series of annealed thin ﬁlms and one series films were obtained. In each group, there were three series of annealed thin films and one series of of as-made layers. For all of the fabricated structures, the electric and thermoelectric properties as-made layers. For all of the fabricated structures, the electric and thermoelectric properties were investigated. Additionally, in order to investigate the influence of deposition and post-process parameters on structural properties of films, the layers’ microstructure was studied.

Coatings 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/coatings Coatings 2019, 9, 120 4 of 11 were investigated. Additionally, in order to investigate the influence of deposition and post-process parameters on structural properties of films, the layers’ microstructure was studied. Coatings 2018, 8, x FOR PEER REVIEW 4 of 11 3.1. Microstructure of Ge–Au Thin Films 3.1. Microstructure of Ge–Au Thin Films The structure of germanium–gold alloy thin films was investigated by scanning electron The structure of germanium–gold alloy thin films was investigated by scanning electron microscopymicroscopy (Schottky (Schottky Emission Emission VPVP FE-SEM,FE-SEM, SU6600,SU6600, Hitachi, Tokyo, Tokyo, Japan). Japan). Figure Figure 3 3shows shows SEM SEM imagesimages of of films films fabricated fabricated under under two two conditions:conditions: as-made film film sputtered sputtered without without additional additional bias bias voltagevoltage (Figure (Figure3a) 3a) and and film film deposited deposited with with externalexternal bias voltage and and then then annealed annealed at at573 573 K K(Figure (Figure 3b).3b).

(a) (b)

FigureFigure 3. 3.SEM SEM images images of of Ge-Au Ge-Au thin thin ﬁlms: films: ((aa)) unannealedunannealed layers layers sputtere sputteredd without without additional additional bias bias voltage;voltage; (b )(b layers) layers deposited deposited with with external external biasbias voltagevoltage and annealed at at 573 573 K. K.

SEMSEM images images of of the the surface surface of of bothboth as-madeas-made and annealed Ge–Au Ge–Au thin thin films films show show that that they they are are composedcomposed of of visible, visible, nanometer nanometer grains. grains. InIn the case of of as-made as-made films films deposited deposited without without external external bias bias voltagevoltage (Figure (Figure3a), 3a), the the size size of grainsof grains was was in thein the range range from from 60 60 to to 300 300 nm nm with with few few grains grains bigger bigger than than 400 nm.400 For nm. annealed For annealed films obtainedfilms obtained in the in sputtering the sputtering process process with with additional additional bias voltagebias voltage (Figure (Figure3b), grains3b), weregrains visibly were smaller, visibly andsmaller, their and size their was size in the was range in the from range 60 from to 160 60 nm to 160 with nm few with grains few biggergrains bigger than 200 nm.than No 200 cracks nm. and No defectscracks and on thedefects surface on the were surface observed, were which observed, indicates which the indicates good quality the good of films.quality of Furthermore,films. the element composition of films was analyzed using the EDS detector (Thermo ScientificFurthermore, NORAN System the element 7, Madison, composition WI, USA). of films The was microanalysis analyzed using showed the EDS an average detector gold (Thermo content inScientific germanium NORAN films System of 4.1% 7, by Madison, weight WI, and USA). negligible The microanalysis amounts of showed carbon andan average oxygen gold atoms. content Also, in germanium films of 4.1% by weight and negligible amounts of carbon and oxygen atoms. Also, obtaining clear emission lines associated with the signal from germanium and gold indicates the high obtaining clear emission lines associated with the signal from germanium and gold indicates the high purity of thin films. purity of thin films. 3.2. Electric Properties of Ge–Au/NiCrSi/Ag Test Structures 3.2. Electric Properties of Ge–Au/NiCrSi/Ag Test Structures In terms of the electric parameters of thermoelectric materials, the most important one is electrical In terms of the electric parameters of thermoelectric materials, the most important one is conductivity. In the presented work, the electrical conductivity was calculated using the van der Pauw electrical conductivity. In the presented work, the electrical conductivity was calculated using the method, based on measurements of films’ sheet resistance and resistivity. The described research were van der Pauw method, based on measurements of films’ sheet resistance and resistivity. The carrieddescribed out at research room temperature were carried (approx. out at room 300 temper K). In Tableature1 (approx., the resistivity 300 K). andIn Table electrical 1, the conductivity resistivity areand presented. electrical For conductivity thin films are fabricated presented. in For the thin process films without fabricated any in external the process bias without voltage, any the external electrical conductivitybias voltage, at the first electrical increases conductivity slowly for annealing at first increases temperatures slowly offor 473 annealing and 523 temperatures K, and then decreasesof 473 and for temperature523 K, and of then 573 decreases K. On the for other temperature hand, for layersof 573 sputtered K. On the in other the process hand, for with layers additional sputtered bias in voltage, the theprocess following with electrical additional conductivity bias voltage, values the behavior following was electrical observed: conductivity at first, a drop values of conductivitybehavior was was observedobserved: (compared at first, a todrop unannealed of conductivity films), was and obse thenrved the increase(compared for to annealing unannealed at 573 films), K. and then the increaseFor further for annealing characterization at 573 K. of films, the temperature dependence of films’ resistance was measuredFor (Figurefurther4 ).characterization For structures fabricatedof films, the in thetemperature basic sputtering dependence process of (without films’ resistance additional was bias voltage),measured it was (Figure observed 4). For that structures the resistance fabricated decreased in the basic within sputtering the annealing process (without temperature; additional however, bias it wasvoltage), larger thanit was for observed as-made that structures. the resistance Additional decreased bias within voltage the during annealing the temperature; deposition of however, thin films it was larger than for as-made structures. Additional bias voltage during the deposition of thin films resulted in an increase in the layers resistance and change in resistance behavior. In this case, the lowest resistance was obtained for film annealed at 523 K.

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Based on the R = f(T) characteristics, the temperature-dependence of electrical conductivity was Coatingsdetermined2019, 9, 120 (Figure 5). The electrical conductivity changed in a way typical for semiconductor5 of 11 materials—it increased within the temperature of the structure. The highest electrical conductivity was obtained for thin films sputtered in the process with additional bias voltage and then annealed resulted in an increase in the layers resistance and change in resistance behavior. In this case, the at Coatings573 K. 2018, 8, x FOR PEER REVIEW 5 of 11 lowest resistance was obtained for ﬁlm annealed at 523 K. Based on the R = f(T) characteristics, the temperature-dependence of electrical conductivity was TableTable 1. Electric1. Electric properties properties of of the the Ge–Au/NiCrSi/Ag Ge–Au/NiCrSi/Ag testtest structures:structures: resistivityresistivity ρρ andand conductivityconductivity σ at determinedat room temperature (Figure 5). ( TThem = 300 electrical K). conductivity changed in a way typical for semiconductor room temperature (Tm = 300 K). materials—it increased within the temperature of the structure. The highest electrical conductivity Deposition Process Type Annealing Temperature T (K) ρ (Ω∙m) σ (S/m) was obtainedDeposition for thin Process films Typesputtered Annealing in the proces Temperatures with additionalT (K) ρ bias(Ω·m) voltage σand(S/m) then annealed – 6.4 × 10−5 1.6 × 104 at 573 K. −5 4 – 473 6.4 6.0× 10 × 10−5 1.71.6 ×× 10104 no bias voltage 473 6.0 × 10−5 1.7 × 104 no bias voltage 523 5.7 × 10−5 1.8 × 104 × −5 × 4 Table 1. Electric properties of the Ge–Au/NiCrSi/Ag523 test structures: 5.7resistivity10 −ρ4 and1.8 conductivity103 σ 573 1.1 × −104 8.8 × 10 3 at room temperature (Tm = 300 K). 573 1.1 × 10 8.8 × 10 – 7.6 × 10−5 1.3 × 104 −5 4 – 7.6 × 10 −5 1.3 × 104 Deposition Process Type Annealing Temperature473 T (K) 8.1ρ (Ω∙ × −m)105 σ 1.2 (S/m) × 10 4 bias voltage Ub = −150 V 473 8.1 × 10 1.2 × 10 bias voltage Ub = −150 V 523 8 2 × 10−−55 1.2 × 104 4 523 – 8 2 6.4× 10× 10−5 1.61.2 × ×1010 4 573473 6.0 3.6 ×× 10−105−−55 1.7 2.8 × × 10 104 4 4 no bias voltage 573 3.6 × 10 2.8 × 10 523 5.7 × 10−5 1.8 × 104 573 1.1 × 10−4 8.8 × 103 – 7.6 × 10−5 1.3 × 104 473 8.1 × 10−5 1.2 × 104 bias voltage Ub = −150 V 523 8 2 × 10−5 1.2 × 104 573 3.6 × 10−5 2.8 × 104

(a) (b)

FigureFigure 4. 4.The The resistance resistanceR Ras as a functiona function of of the the structure’s structure’s temperature temperatureT Tbefore before and and after after annealing: annealing: (a) Ge–Au/NiCrSi/Ag(a) Ge–Au/NiCrSi/Ag test test structures structures deposited deposited in in the the sputtering sputtering process without without external external bias bias voltage; voltage; (b)(b Ge–Au/NiCrSi/Ag) Ge–Au/NiCrSi/Ag test structures structures deposited deposited in inthe the sputtering sputtering process process with with external external bias voltage. bias voltage.

Based on the R = f(T(a)) characteristics, the temperature-dependence(b) of electrical conductivity was determinedFigure 4. The (Figure resistance5). The R as electrical a function conductivityof the structure’s changed temperature in a T way before typical and after for annealing: semiconductor materials—it(a) Ge–Au/NiCrSi/Ag increased within test thestructures temperature deposited of in the the structure.sputtering process The highest without electrical external bias conductivity voltage; was obtained( forb) Ge–Au/NiCrSi/Ag thin films sputtered test structures in the process deposited with in the additional sputtering process bias voltage with external and then bias voltage. annealed at 573 K.

(a) (b)

Figure 5. The electrical conductivity σ as a function of the structure’s temperature T before and after annealing: (a) Ge–Au/NiCrSi/Ag test structures deposited in the sputtering process without external bias voltage; (b) Ge–Au/NiCrSi/Ag test structures deposited in the sputtering process with external bias voltage. (a) (b)

FigureFigure 5. The 5. The electrical electrical conductivity conductivityσ σas as a a function function of the structure’s structure’s temperature temperature T beforeT before and and after after annealing:annealing: (a) Ge–Au/NiCrSi/Ag(a) Ge–Au/NiCrSi/Ag test test structuresstructures deposited deposited in in the the sputtering sputtering proc processess without without external external biasbias voltage; voltage; (b) Ge–Au/NiCrSi/Ag(b) Ge–Au/NiCrSi/Ag testtest structuresstructures deposited deposited in in the the sputtering sputtering process process with with external external biasbias voltage. voltage.

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3.3. Thermoelectric Properties of Ge–Au/NiCrSi/Ag Test Structures 3.3. Thermoelectric Properties of Ge–Au/NiCrSi/Ag Test Structures To characterize thermoelectric properties of germanium thin films doped with gold, the To characterize thermoelectric properties of germanium thin films doped with gold, the following following parameters were determined: the thermoelectric voltage generated in the investigated parameters were determined: the thermoelectric voltage generated in the investigated thermocouples thermocouples Ge–Au/NiCrSi/Ag, Seebeck coefficient of the material, power factor PF, and PF dimensionlessGe–Au/NiCrSi/Ag, figure of Seebeck merit ZT coefficient. of the material, power factor , and dimensionless figure of ZT meritThe thermoelectric. voltage as a function of temperature difference between the hot and cold junctionsThe of thermoelectric thermocouples voltage was determined. as a function The of char temperatureacterization difference of the thermoelectric between the voltage hot and was cold carriedjunctions out ofusing thermocouples the measurement was determined. setup in which The the characterization investigated test of the structure thermoelectric was placed voltage on two was separatedcarried out plates, using so the that measurement the thermocouple setup in which junctions the investigated (so-called hot test structureand cold wasjunctions) placed onwere two positionedseparated at plates, two temperatures. so that the thermocouple In the presented junctions measurements, (so-called hot the and cold cold junction junctions) temperature were positioned was approximatelyat two temperatures. constant In the(T presented= 296 K) measurements, and the hot thejunction cold junction temperature temperature increased was approximatelyfrom room T temperatureconstant ( up= 296 to 490 K) andK. Furthermore, the hot junction two temperatureindependent increasedtemperature from sensors room were temperature used to updirectly to 490 measureK. Furthermore, the junctions’ two independenttemperatures. temperature The thermoel sensorsectric werevoltage used measurement to directly measureresults are the shown junctions’ in Figuretemperatures. 6. As shown The thermoelectricin Figure 6a, for voltage the thin measurement films fabricated results in are the shown sputtering in Figure process6. As without shown additionalin Figure 6biasa, for voltage, the thin a filmssignificant fabricated drop inin thethe sputteringvalues of the process thermoelectric without additional voltage generated bias voltage, in annealeda significant thin dropfilms inwas the observed. values of In the the thermoelectric case of thermocouples voltage generated with germanium in annealed layers thin sputtered films was withobserved. additional In the bias case ofvoltage, thermocouples such a withdecrease germanium was not layers as sputteredsignificant, with but additional still, the bias highest voltage, thermoelectricsuch a decrease voltage was notwas as generated significant, in the but as-made still, the structures highest thermoelectric (Figure 6b). All voltage of the wasU = f( generatedΔT) curves in hadthe as-madeslightly increasing structures (Figureslopes,6 b).which All ofindicates the U = fan(∆ Tincrease) curves hadin Seebeck slightly increasingcoefficient slopes,within which the temperatureindicates an of increase the hot injunction. Seebeck coefficient within the temperature of the hot junction. BasedBased on on the the UU = =f( fΔ(T∆)T curves,) curves, the the Seebeck Seebeck coefficient coefficient for for each each layer layer was was determined determined using using the the integralintegral method method and and Equation Equation (3). (3). Th Thee results results are are shown shown in in Figure Figure 7.7.

∆∆𝑈U 𝑈U2−𝑈− U1 S𝑆== == (3)(3) ∆∆𝑇T 𝑇T2 −𝑇− T1 wherewhere Δ∆UU isis as as thermoelectric thermoelectric voltage voltage difference difference within within temperature temperature difference difference Δ∆TT. . ForFor all all Ge–Au/NiCrSi/Ag Ge–Au/NiCrSi/Ag structures, structures, the the Seebeck Seebeck coef coefficientficient as as a function a function of of temperature temperature exhibits exhibits a anonlinear, nonlinear, growing growing within within temperature, temperature, character. character. For For structures structures fabricated fabricated in in the the basic basic process process (Figure(Figure 7a),7a), the the significant significant drop drop in in the the Seebeck Seebeck coef coefficientficient values values of of annealed annealed thin thin films films was was observed observed (resulting(resulting from from the the thermoelectric thermoelectric voltage voltage decrease). decrease). Also, Also, the the lowest lowest values values of of the the Seebeck Seebeck coefficient coefficient ofof annealed annealed films films were were calculated calculated for for structures structures sputtered sputtered with with additional additional voltage voltage (Figure (Figure 7b).7b).

(a) (b)

FigureFigure 6. 6.ThermoelectricThermoelectric voltage voltage UU asas a function a function of oftemperature temperature difference difference ΔT∆ Tbetweenbetween thermocouples thermocouples beforebefore and and after after annealing: annealing: (a ()a )Ge–Au/NiCrSi/Ag Ge–Au/NiCrSi/Ag test test structures structures de depositedposited in inthe the sputtering sputtering process process withoutwithout external external bias voltage;voltage; ( b()b Ge–Au/NiCrSi/Ag) Ge–Au/NiCrSi/Ag test test structures structures deposited deposited in the in sputtering the sputtering process processwith external with external bias voltage. bias voltage.

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(a) (b)

Figure 7. The Seebeck coefficient S as a function of the structure’s temperature T before and after annealing: (a) Ge–Au/NiCrSi/Ag test structures deposited in the sputtering process without external bias voltage; (b) Ge–Au/NiCrSi/Ag test structures deposited in the sputtering process with external bias voltage.

Based on the results of the Seebeck coefficient and electrical conductivity measurements, the power factor PF was determined (using the Equation (1)). Figure 8 shows the temperature dependence of PF for the layers sputtered in a basis process (Figure 8a) and in the process with additional bias voltage supplied to the substrate (Figure 8b). In addition, the value of the dimensionless figure of merit(a) was estimated. (b) The dimensionless figure of merit ZT and its temperature dependence was determined by the Figure 7. The Seebeck coefficient S as a function of the structure’s temperature T before and after indirectFigure method 7. The using Seebeck the resultscoefficient of power S as a factorfunction measurements. of the structure’s In thetemperature described T studies,before and the after thermal annealing:annealing: ( a()a) Ge–Au/NiCrSi/Ag Ge–Au/NiCrSi/Ag testtest structuresstructures depositeddeposited inin thethe sputteringsputtering process process without without external external conductivity value was assumed to be equal to 1.1 W/m·K. Such an assumption was made on the biasbias voltage; voltage; ( b()b Ge–Au/NiCrSi/Ag) Ge–Au/NiCrSi/Ag testtest structuresstructures depositeddeposited inin thethe sputtering sputtering process process with with external external basis of the previous results presented in the literature describing thin film thermoelectric materials, biasbias voltage. voltage. as well as the studies in which the thermal conductivity of the substrate was assumed for the thermal conductivity of the layer [32–35]. In the presented studies, germanium-based thin films were BasedBased on on the the results results of of the the Seebeck Seebeck coefficient coefficient and and electrical electrical conductivity conductivity measurements, measurements, the the sputtered onto glass substrates with a thermal conductivity of 1.1 W/m·K at 300 K. The results of ZT powerpower factor factorPF PFwas was determined determined (using (using the Equation the Equation (1)). Figure (1)).8 shows Figure the 8 temperatureshows the dependencetemperature measurements are shown in Figure 9. ofdependencePF for the layersof PF sputteredfor the layers in a basissputtered process in a (Figure basis 8processa) and in(Figure the process 8a) and with in the additional process bias with In Table 2, the electric and thermoelectric properties of investigated thin films of germanium– voltageadditional supplied bias tovoltage the substrate supplied (Figure to the8b). substrat In addition,e (Figure the value8b). ofIn theaddition, dimensionless the value figure of ofthe gold alloy are presented (determined at 300 K). meritdimensionless was estimated. figure of merit was estimated. The dimensionless figure of merit ZT and its temperature dependence was determined by the indirect method using the results of power factor measurements. In the described studies, the thermal conductivity value was assumed to be equal to 1.1 W/m·K. Such an assumption was made on the basis of the previous results presented in the literature describing thin film thermoelectric materials, as well as the studies in which the thermal conductivity of the substrate was assumed for the thermal conductivity of the layer [32–35]. In the presented studies, germanium-based thin films were sputtered onto glass substrates with a thermal conductivity of 1.1 W/m·K at 300 K. The results of ZT measurements are shown in Figure 9. In Table 2, the electric and thermoelectric properties of investigated thin films of germanium– gold alloy are presented (determined at 300 K).

(a) (b)

FigureFigure 8.8.The The power power factor factorPF asPF a as function a function of the of structure’s the structure’s temperature temperatureT before andT before after annealing:and after (annealing:a) Ge–Au/NiCrSi/Ag (a) Ge–Au/NiCrSi/Ag test structures test deposited structures in deposited the sputtering in the process sputtering without proc externaless without bias external voltage; (biasb) Ge–Au/NiCrSi/Ag voltage; (b) Ge–Au/NiCrSi/Ag test structures test deposited structures in thedeposited sputtering in the process sputtering with externalprocess biaswith voltage. external bias voltage. The dimensionless figure of merit ZT and its temperature dependence was determined by the indirect method using the results of power factor measurements. In the described studies, the thermal conductivity value was assumed to be equal to 1.1 W/m·K. Such an assumption was made on the basis of the previous results presented in the literature describing thin film thermoelectric materials, as well as the studies in which the thermal conductivity of the substrate was assumed for the thermal conductivity of the layer [32–35]. In the presented studies, germanium-based thin films (a) (b) were sputtered onto glass substrates with a thermal conductivity of 1.1 W/m·K at 300 K. The results of ZT measurementsFigure 8. The are power shown factor in FigurePF as 9a. function of the structure’s temperature T before and after annealing: (a) Ge–Au/NiCrSi/Ag test structures deposited in the sputtering process without external bias voltage; (b) Ge–Au/NiCrSi/Ag test structures deposited in the sputtering process with external bias voltage.

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(a) (b)

Figure 9. The dimensionless ﬁgurefigure of merit ZT as a functionfunction of thethe structure’sstructure’s temperaturetemperature T before and after annealing: (a)) Ge–Au/NiCrSi/AgGe–Au/NiCrSi/Ag test test structures structures deposi depositedted in in the the sputtering sputtering process process without external bias voltage;voltage; ((bb)) Ge–Au/NiCrSi/AgGe–Au/NiCrSi/Ag test test structures structures deposited deposited in in the the sputtering process with external bias voltage.

In Table2, the electric and thermoelectric properties of investigated thin ﬁlms of germanium–gold Table 2. Electric and thermoelectric properties of the Ge–Au/NiCrSi/Ag test structures at 300 K: alloyconductivity are presented σ, Seebeck (determined coefficient at 300 S, power K). factor PF, and ZT.

DepositionTable 2. Electric Process and thermoelectricAnnealing properties of the Ge–Au/NiCrSi/AgS test structuresPF at 300 K:ZT σ (S/m) conductivityType σ, Seebeck coefficientTemperatureS, power T (K) factor PF, and ZT. (µV/K) (µW/m∙K2) ×10−3 – 1.6 × 104 −51.5 41.9 11.4 Deposition Process Type Annealing Temperature T (K) σ (S/m) S (µV/K) PF µ · 2 ZT × −3 473 1.7 × 104 −25.7 ( W/m11.0K ) 10 3.0 no bias voltage 4 523– 1.6 1.8× × 10104 −51.5−25.0 41.911.1 11.4 3.0 473 1.7 × 104 −25.7 11.0 3.0 no bias voltage 573 8.8 × 103 −23.9 5.0 1.4 523 1.8 × 104 −25.0 11.1 3.0 4 573– 8.8 1.3× × 10103 −23.9−31.8 5.013.3 1.4 3.6 4 473 1.2 × 104 −28.2 9.8 2.7 b – 1.3 × 10 −31.8 13.3 3.6 bias voltage U = −150 V 4 523473 1.2 1.2× × 10104 −28.2−15.9 9.83.1 2.7 0.8 bias voltage U = −150 V b 573523 1.2 2.8× × 101044 −15.9−13.3 3.14.9 0.8 1.3 573 2.8 × 104 −13.3 4.9 1.3 4. Discussion 4. Discussion From the presented research, it is apparent that the deposition and post-processing processes parametersFrom the have presented a considerable research, influence it is apparent on the thatelectric the depositionand thermoelectric and post-processing properties ofprocesses thin film parametersthermoelectric have materials. a considerable influence on the electric and thermoelectric properties of thin film thermoelectricUndoubtedly, materials. the electric and thermoelectric parameters of thin films result directly from their microstructure.Undoubtedly, The the films’ electric microstructure and thermoelectric in turn is parameters related to the of thinsputtering films result process directly and mobility from their of microstructure.the adatoms during The films’growth. microstructure The total energy in turn supplied is related to the to the atoms sputtering and therefore process the and morphology mobility of theand adatomsmicrostructure during of growth. thin film, The depends total energy on: the supplied substrate to temperature, the atoms and final therefore working the pressure morphology in the andvacuum microstructure chamber, bias of thinvoltage film, applied depends to on:the substr the substrateate, and temperature,thermal characteristics final working of the pressure target. inIn the vacuumpresented chamber, research, bias the voltageadditional applied bias voltage to the substrate, applied to and the thermal substrate characteristics was used to ofimprove the target. the Indeposition the presented process research, by enhancing the additional the structural bias voltage ordering applied of the to thefilm. substrate was used to improve the depositionFabricated process thin by films enhancing of germanium–gold the structural ordering alloy exhibited of the film. high electrical conductivity, which furtherFabricated increased thin after films thermal of germanium–gold post-processing alloy(with exhibitedthe exception high of electrical thin films conductivity, sputtered without which furtheradditional increased bias voltage after thermalannealed post-processing at 573 K). It can (with be concluded the exception that the of thinthermal films post-processing sputtered without led additionalto the activation bias voltage of gold annealed dopant. For at 573 structures K). It can fabr beicated concluded in the that process the thermal with additional post-processing bias voltage, led to the activationgradual dopant of gold activation dopant. For process structures was fabricatedobserved, in which the process resulted with in additional a progressive bias voltage,increase the in gradualelectrical dopant conductivity. activation For process thin films was sputtered observed, with whichout resulted additional in a bias progressive voltage, increasesimilar behavior in electrical of conductivity.electrical conductivity For thin filmswas observed sputtered for without annealing additional temperatures bias voltage, of 473 similarand 523 behavior K. of electrical conductivityAdditional was bias observed voltage for led annealing to supplying temperatures growin ofg layers 473 and with 523 more K. energy and therefore to influencing microstructures as well as their physical properties. Accordingly, such films can be annealed at higher temperatures, which will not cause degradation of the layers and deterioration of

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Additional bias voltage led to supplying growing layers with more energy and therefore to influencing microstructures as well as their physical properties. Accordingly, such films can be annealed at higher temperatures, which will not cause degradation of the layers and deterioration of their parameters. However, higher annealing temperatures resulted in a decrease of the Seebeck coefficient which influenced the PF and ZT. Taking into account the results presented in Table2, the optimal thermoelectric parameters at room temperature were obtained for the thin films fabricated in the process without additional bias voltage and post-annealed at 473 and 523 K. For this layer, the Seebeck coefficient reached −25 µV/K, and the power factor reached 11.0 µW/m·K2. At the same time, this layer showed high electrical conductivity (1.7 × 104 S/m and 1.8 × 104 S/m). An interesting phenomenon was observed in the investigated Ge–Au/NiCrSi/Ag test structures. In the considered temperature range, the electrical conductivity of the germanium–gold alloy increased within the increasing temperature of the structure, which is typical for semiconductor materials. At the same time, the Seebeck coefficient also increased with the increase in temperature. Typically, in thermoelectric materials, the Seebeck coefficient increases in an inverse proportion to the electrical conductivity. The interrelation between the electrical conductivity and Seebeck coefficient can be seen from a relatively simple model of electron transport in metals and degenerated semiconductors. In such a model, the Seebeck coefficient is inversely proportional to the carrier concentration, which in turn affects in a proportional way the electrical conductivity. So, the higher the concentration of carriers, the higher the electrical conductivity, but the lower the Seebeck coefficient. However, in films with a thickness of the order of 1 micrometer and micro- and nanocrystalline microstructures, the variable range hopping (VRH) may be a dominant mechanism for d.c. charge transport [36]. This conductivity mechanism, in turn, can lead to a simultaneous increase in electrical conductivity and the Seebeck coefficient. Due to the simultaneous increase in electrical conductivity and the Seebeck coefficient, the power factor and dimensionless figure of merit increase within increasing temperature. For as-made structures fabricated in a process without additional bias voltage, the maximum PF reached 151.2 µW/m·K2.

5. Conclusions This paper presents research on thin films of germanium–gold alloy (Au content: 5 wt.%) obtained by magnetron sputtering using an alloy Ge–Au target. The electrical (resistivity, conductivity) and thermoelectric (thermoelectric voltage, the Seebeck coefficient, the PF, and ZT) properties of films were described. The results showed that the right selection of deposition process parameters is crucial to the fabrication of thin films with the desired thermoelectric properties. It is necessary to select and control the sputtering gas pressure, target, substrate type, additional bias voltage applied to the substrate, and current-voltage parameters of the plasma discharge. Also, the results clearly exhibit that for the optimization of thermoelectric properties of alternative materials, there must be a compromise between the high values of the generated thermoelectric voltage and the sufficiently large values of electrical conductivity and the Seebeck coefficient. In order to evaluate the examined Ge–Au/NiCrSi/Ag test structures in terms of practical applications, it is necessary to specify that some of the materials may be used in sensors, while others can be used to fabricate microgenerators. Thin films characterized by a high Seebeck coefficient can be used in sensors (for example in IoT networks), while those characterized by a high power factor can be successfully applied in microgenerators. For this reason, investigated layers may be considered in further research on thin film thermoelectric materials from the point of view of applications in both sensor and generator systems. In addition, it is possible to study complex material compositions based on germanium and gold alloys and optimize them in order to obtain the best thermoelectric parameters. Coatings 2019, 9, 120 10 of 11

Moreover, the subject of further studies of germanium–gold material composition can be the exact characterization of the conductivity mechanisms in Ge–Au films. The preliminary analyses carried out using impedance spectroscopy have initially confirmed the hopping conductivity in the investigated thin films.

Author Contributions: D.N. and N.S. conceptualized and designed the study; D.N. and N.S. performed technological process; N.S. collected experimental data; N.S., M.T. and D.N. analyzed and interpreted the data; M.T. and D.N. wrote the manuscript, prepared figures and approved it. Funding: This research received no external funding. Acknowledgments: This work was supported by the statutory activity of Wrocław University of Science and Technology. Conflicts of Interest: The authors declare no conflict of interest.

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