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Characterization of sintered inkjet-printed silicon nanoparticle thin filmsfor thermoelectric devicesEtienne Drahi*,1, Anshul Gupta1, Sylvain Blayac**,1, Sébastien Saunier2, and Patrick Benaben1

1 Centre Microélectronique de Provence, ENSM-SE, 880 Avenue de Mimet, 13541 Gardanne, France2 Science des Matériaux et des Structures, ENSM-SE, 158 cours Fauriel, 42023 Saint Etienne, France

Received 1 June 2013, revised 23 December 2013, accepted 4 March 2014Published online 15 April 2014

Keywords Raman spectroscopy, Si nanoparticles, sintering, thermoelectrics, thin film

*Corresponding author: e-mail drahi@emse.fr, Phone: þ33 (0)4 42 61 67 55, Fax: þ33(0)4 42 61 65 93** e-mail blayac@emse.fr, Phone: þ33 (0)4 42 61 67 47, Fax: þ33(0)4 42 61 65 93

Nanostructured silicon-based materials are good candidatesfor thermoelectric (TE) devices due to their low thermalconductivity, customizable electrical conductivity, and reducedcost. Generally, nanostructured TE bulk materials are obtainedthrough compaction and sintering at high temperature(>1000 8C) of silicon nanoparticles (NPs). In order to introduceTE generators in flexible electronic devices, development ofthin film TE is needed. Inkjet-printing of silicon NPs-based inkis an interesting technology for this targeted application dueto its low cost and additive process. This paper presents theimplementation of inkjet-printing of a silicon NPs-based ink

toward the fabrication of TE material on flexible substrate andthe development of a characterization method for this material.After printing, recovering of electrical properties throughsintering is mandatory. Nevertheless, special care must be takenin order to keep thermal conductivity low and reduce theannealing temperature to allow the use of flexible substrates.The functional properties: electrical and thermal (measured byRaman spectroscopy), are studied as a function of the annealingprocess. Two types of annealing: rapid thermal annealingand microwave annealing, are investigated as well as twoatmospheres: inert (N2) and reducing (N2–H2 5%).

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1 Introduction Thermoelectric (TE) materials areable to convert wasted heat into electrical power andtherefore contribute to the generation of sustainable energy.Their ability to produce TE power is expressed by the figureof merit ZT. It is a dimensionless parameter determined by

ZT ¼ ðS2sel TÞðkel þ klatticeÞ

; ð1Þ

where S is the Seebeck coefficient, T is the absolutetemperature, sel is the electrical conductivity, and k is thethermal conductivity (k¼ klatticeþ kel, where klattice and kel arethe lattice and electronic contributions, respectively). Forconducting materials (metals or degenerated semiconductors),electronic contribution of thermal conductivity kel is linked toelectrical conductivity sel through the Wiedemann–Franz law

kel ¼ LTsel; ð2Þ

where L is the Lorentz number depending on the scatteringprocess of the carriers. Therefore, by improving the electricalconductivity, kel increases and ZT is impacted. The best TEmaterial would have the electrical properties of a crystallinematerial with the thermal properties of an amorphous one. Thisis the “phonon-glass-electron crystal” [1].

In the past decades, introduction of nanostructuredmaterials created a breakthrough allowing the improvementof TE materials figure of merit ZT [2]. By using a film madeof nanosized domains, k and sel are decoupled to a certaindegree and the thermal conductivity can be reduced aroundsome Wm%1K%1 or even lower while the electricalconductivity can be maintained. Therefore, ZT> 1 couldbe obtained using nanostructured materials.

Silicon is an interesting material because of its relativemoderate price, in comparison with other TE materials [3],and also due to know-how of the complete fabricationprocess developed in microelectronics throughout the years.

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However, as a bulk it is not a suitable material for TEapplications because of its high thermal conductivity(k& 140Wm%1K%1 at room temperature). Nevertheless,thermal conductivity of sintered Si nanoparticles (NPs)compacts has been strongly reduced without degrading theelectrical conductivity. This led to an increase of ZT from0.008 to 0.023 at room temperature and from&0.2 to&0.7 ataround 1280K [3]. This is attributed to strong diminution ofthe phonon contribution of the thermal conductivity klattice(Eq. 1). It results in the integration of barriers at the grainboundaries therefore reducing the phonon dissipation.

Recently, nanostructured silicon-based TE materials areobtained through compaction of doped silicon NPs intocompacts and high temperature sintering (T> 1000 8C) inorder to recover the electrical conductivity of the material.Two sintering methods are mainly used and give goodresults: spark plasma sintering [4, 5] and hot pressing [3, 6].Nanostructured bulk materials are therefore obtained. UsualTE generators with thickness around somemm are fabricatedfrom these materials.

Lately, power supplies such as photovoltaic devices orTE generators are being introduced into electronic packagesin order to make them autonomous [7, 8]. To make themportable and easy to integrate in various applications: smartcards, textiles…, it is important to improve their flexibility.Therefore, fabrication of thin film TE generator appears to beessential.

In this objective, solution-based deposition of Si NPs isattractive due to its low cost and the specific properties of SiNPs for TE applications. Some proofs of concept have beendone by spin coating a suspension of doped Si NPs onpolyimide substrate followed by laser annealing [9, 10].

This paper studies the fabrication of inkjet Si NPs thinfilms for TE application and their characterization. Indeed,inkjet printing is a non-vacuum and maskless process thathas a strong potential for cost reduction by keeping all theadvantages of solution-based processes with improvedhomogeneity of the fabricated thin film and reduced materialwaste. Nevertheless, it also needs customized characteriza-tion methods.

2 Fabrication of silicon thin film by inkjet-printing

2.1 Si nanoparticles and ink composition Sodiumpolymethacrylate (NaPMA) was added to commercial(Meliorum Technologies, Inc.) undoped crystalline SiNPs. The latter were obtained via chemical synthesis(colloidal precipitation of silicon chloride precursors by areducing agent, most commonly NaBH4) in a reagent gradeorganic solvent in order to avoid oxidation and guaranty theirstability [11]. Synthesized NPs have sizes ranging between20 and 150 nm (Fig. 1).

In order to guaranty good printability of the ink, the NPs(around 1–2wt%) were dispersed in reagent-grade ethyleneglycol. Some water could be observed. It was probablyincorporated into NaPMA during the synthesis. The obtainedsuspensions have a viscosity varying between &13mPa s at

23 8C and 5.5mPa s at 50 8C and a superficial tension around47mNm%1 at 23 8C.

2.2 Substrates preparation and inkjet-printingprocess Si ink was printed on commercial polished quartzsubstrates (ACM JGS1) with average roughness Ra¼ 0.62nm (measured with a Veeco SP-II atomic force microscope).Substrates were cleaned in an acetone ultrasonic bath for5min and dried under N2. Substrates surface energy aftercleaning was &47mNm%1 (measured by goniometry).

A Dimatix DMP 2800 printer was used to deposit the inkon clean quartz substrates. Specific printing parameters wereoptimized to guaranty repeatable and stable printing. Dropspacing of 20mm between two consecutive drop centers wasobserved to be optimal. Two layers were printed withoutintermediate drying in order to increase both layers thicknessand connectivity.

After printing, drying of the samples was performed intwo steps: a vacuum drying at room temperature followed bydrying at 200 8C for 5min under N2.

2.3 Sintering of inkjet-printed Si nanoparticlethin films Two different sintering methods have beenstudied: rapid thermal annealing (RTA) and microwaveannealing (mW).

RTA equipment is Jipelec Jetfirst equipped with 12Halogen lamps (144V and 1200W). Heating is performedby three mechanisms: infrared to ultraviolet radiation,heat convection, and conduction [12]. Annealing can beperformed under two different atmospheres: under purenitrogen (99.999%) and under forming gas (N2–H2 5%).

The mW system and the temperature measurement setuphave been described extensively elsewhere [13]. Heating isperformed by direct absorption of the mW (2.45GHzmultimode) by the Si NPs but also by adding a SiC (home-made ring) susceptor in the oven cavity. It absorbs the mWand reemits infrared radiations. Thermally activated chargecarriers in the Si NPs are thus created, which enhances the

Figure 1 Scanning electron microscopy (SEM) picture of used SiNPs.

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absorption of mW [14]. Annealing is carried under pure N2

(99.999%).

3 Raman spectroscopy applied to monitoring Sinanoparticle sintering Raman spectroscopy is oftenused to study chemical composition of a material fromthe peak position and its intensity. In the case of Si, it isgenerally used to control the crystallinity or crystallinefraction of silicon through Raman peak shift and full width athalf maximum (FWHM) measurements [15–17].

Micro-Raman spectroscopy measurements were per-formed on various samples with two different laserwavelengths: 488 nm Ar and 633 nm HeNe lasers. It allowsvarious penetration depth of the laser beam and thereforeinvestigation of a bigger volume of material.

After a calibration step with a crystalline silicon (c-Si)reference, the position of the peaks has been estimatedthrough fitting of the experimental data with: a linearbackground and a Lorentzian peak (Fig. 2).

Raman spectroscopy is also used in order to determinatethe stress state for crystalline silicon (c-Si) and polycrystal-line silicon (pc-Si) [18]. Under stress, the peak shifts to thelower wavenumbers (Dv< 0) for a tensile stress and tohigher wavenumbers for a compressive stress (Dv> 0) [19].For c-Si thin film under bi-axial stress, the followingequation is used to link the peak shift Dv (in cm%1) and thestress s in Pa:

Dv ¼ v% v0 ¼ %4' 10%9 sxx þ syy

2

! "; ð3Þ

where v is the position of the Raman peak and v0 is thereference position (both in cm%1), sxx and syy are the stresscomponents (in Pa) in the x–y plane (thin film plane).

To measure the stress, the probing laser power has beenkept under 50mW (below 8811Wcm%2 for the usedobjective) in order to avoid crystallization, melting, or evenheating of the silicon NPs. A D2 neutral density filter hastherefore been applied for all stress measurements in order toavoid parasitic behavior such as heating of the sample.

Under higher probing laser power, Si Raman peak shiftsto lower frequencies and broadens due to local heating [20,21]. Using this property, Raman spectroscopy allows alsothe estimation of thermal conductivity of both thin films andbulk materials [9, 22–28]. The following linear relationshipexists between the peak shift Dv and the temperature T forc-Si:

Dv ¼ TdvdT

; ð4Þ

where dv/dT¼%0.0242 cm%1K%1 for c-Si [17].The temperature of the thin film can therefore be

obtained from Eq. (4) by measuring the peak shift whiledifferent probing laser powers are applied. Various neutraldensity filters (D2, D1, D0.6, D0.3, and none) are thus used.A confocal Raman microscope (Jobin-Yvon Labram HR800) with 100mm pinhole has been used for the measure-ments. It results in a Gaussian laser beam, which 50% ofintensity is concentrated in a spot diameter &1mm for theused objective.

With this setting, laser beams are narrow. Therefore,laser heating is considered homogeneous on the laser spotdiameter (&1mm). Given the penetration depth of the lasers,100–500 nm [17], the maximum temperature will bedissipated into the remaining thickness of the layer and ina hemispherical way around the laser spot. It gives birth toa temperature gradient DT along the layer thickness.Therefore, along the thickness, a 1D model developed byLechner [9] can be applied to measure thermal conductivity kof thin film samples. It was obtained by modifying thegeneral equation developed by Nonnenmacher and Wickra-masinghe [27] and applied to Raman spectroscopy byPérichon et al. [26, 29]

k ¼ ððd % 1=2aÞPabsÞDT

; ð5Þ

where d is the thickness of the thin film, a its absorptioncoefficient, Pabs the laser absorbed power, and DT thethermal gradient.

4 Morphological investigation and oxidation ofsintered Si thin films

4.1 Thin films surface morphology andoxidation Structural observations of the samples wereled using a Carl Zeiss Ultra 55 scanning electron microscopy(SEM) equipped with an Oxford energy dispersive X-rayspectroscopy (EDS) detector useful for chemical analysis(Fig. 3).Figure 2 Fit examples of Raman measurements.

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SEM pictures (Fig. 3) confirm a clear change ofmicrostructure of layers depending on annealing temperatureand to a smaller extent, the annealing method. Layerthicknesses around 1–2mm were measured by bothmechanical profilometry and SEM observation on cross-sections (Fig. 4, here two times the thickness of usualsamples).

At 600 8C, the Si NPs coating is eliminated [30] and thefirst sintering stages occur, which leads to the formation of aneck between the particles. Nevertheless, at this temperatureSi thin films are still highly porous and composed ofnanosized grains.

By increasing the annealing temperature, neck growthand grain coarsening and/or densification occurs. At 800 8C,the tiniest Si NPs melt and solidify around biggest NPs, asstated by Schierning et al. [31]. Therefore, around thistemperature the microstructure evolves strongly, whichresults in the formation of fibrous structures at the surface ofthese samples. This could be a proof of grain coarsening dueto the domination of non-densifying sintering mechanisms

such as gaseous transport. This mechanism could beenhanced because of preferred and faster oxidation[32–34] at the surface of the layer.

At higher temperature, cracks are observed in SEMpictures revealing high stress state in the samples. There areno clear differences in SEM pictures between the samplesannealed at a same temperature with different method.Nevertheless, grains seem lightly bigger by mW annealing.

Attenuated total reflectance-Fourier transform infraredspectroscopy (ATR-FTIR) was led on some thin filmsusing a Bruker Vertex 70 spectrometer (Fig. 5). It givesinformation about their chemical composition.

Before sintering, d (Si-CH2) mode confirms that aportion of Si NPs are functionalized with organic chainsand not oxidized. After sintering, this peak vanishes andn(Si–O–Si) modes peaks intensity increases demonstratingoxidation of the sample while mode peaks from inkcomponents disappear: n(O–H), n(C–H), n(C%%%%O), andd(C–CH3). Concerning samples sintered at 1000 8C a peak&610 cm%1 appears. Literature refers to possible n(Si–Si)mode linked to Si–Si bonds in a cubic crystal [35].

4.2 Stress state of sintered thin films Afterannealing, the surface stress state of the silicon thin filmsFigure 3 SEM pictures of sintered inkjet-printed thin films.

Figure 4 SEM cross-section of a sample printed (four layers) andannealed at 800 8C for 1 h.

Figure 5 ATR FTIR experiments on dried and sintered thin filmsunder N2 (H2 – 5%).

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was estimated by Raman spectroscopy (cf. Eq. (3)).Evolution of the stress follows the same behavior regardlessof the annealing method (Fig. 6). Up to 800 8C, the siliconappears to be in tension with linear increase of the stress andsecondly, above 800 8CRaman shift indicates that the silicongrains are in compression.

Measured stress values correlate very well with thetheoretical tension sf (Fig. 6) provoked during cooling by thedifference in dilatation coefficients [36] between the quartz(aquartz) and the silicon thin film (aSi),

sf ¼ESiðaSi % aquartzÞDT

ð1% vSiÞ; ð6Þ

where DT is the temperature gradient, ESi and nSi Si Youngmodulus and Poisson coefficient, respectively.

Above an annealing temperature of 800 8C, Si NP thinfilms are in compression. Above this temperature, Sioxidation rate is increased as ATR-FTIR experimentsconfirmed (Fig. 5). By oxidizing faster and therefore gainingin volume, surrounding grain boundaries put the Si grainsinto compression [37]. Measurements with a probing laser ofhigher wavelength (633 nm vs. 488 nm), which has a deeperprobing depth [17], demonstrate that the oxidation is moreimportant at the surface [30]. The annealing atmospherebeing pure (99.999%), the oxidizing elements are attributedto originate from the ink components.

Lower stress is observed for RTA under N2 (H2–5%) andfor mW annealing. Concerning RTA under N2 (H2 – 5%)hydrogen being a reducing agent, deoxidizes or at least limitssilicon oxidation. This gas is generally used in microelec-tronics in back end processes in order to reduce metallicoxides or in silicon in order to passive the danglingbonds [38]. Concerning mW, it will be discussed in theSection 5.2.

5 Properties of sintered Si nanoparticle thinfilms This part aims at showing the potential of thismaterials and processing method for TE application.Nevertheless, since used material is undoped, electricalconductivity, and thermal conductivity are not optimized.

5.1 Electrical properties Electrical characterizationof the annealed samples (1 cm2 and 1–2mm thick) wascarried out by I–Vmeasurements performed using a Keithley4200 semiconductor parameter analyzer. Large probes(round shape of about some hundreds of micrometers indiameter) were used in order to limit degradation of the softlayers. Contacts have been taken directly at the corners ofthe printed pattern, which results in a probing length ofaround 10mm. Equivalent conductance of the layers werecalculated from linear I–V curves and plotted as a function ofthe sintering temperature to allow a comparison betweenthe different annealing methods (Fig. 7).

The conductance of the layers evolves as a function ofthe annealing temperature. Concerning RTA in N2 atmo-sphere, first it increases up to almost 2 orders of magnitudeand above 850 8C, the conductance decreases. At one hand,the increased conductance is attributed to grain coarseningand densification of the layers. It therefore improves thepercolation of the charge carriers. At the other hand, thedecreased conductance is attributed to oxidation at the grainboundaries (Figs. 5 and 6) and appearance of cracks (Fig. 3)both hindering the transport of charge carriers.

Concerning RTA in N2 (H2 – 5%) atmosphere, the samebehavior is observed. Nonetheless, the highest conductanceis measured for a sintering temperature of 800 8C and isalmost 1 order of magnitude lower than in N2 atmosphere. Itcould be explained by lower grain coarsening in the caseof RTA under N2 (H2 – 5%), the carriers would thereforecollide with many grain boundaries and electrical

Figure 6 Stress estimation from Raman measurements on sinteredsamples.

Figure 7 Equivalent layer conductance calculated from I–Vmeasurement with &1 cm probing length.

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conductivity would be strongly reduced [39, 40]. Thermalconductivity measurements in the next section will tend toconfirm this hypothesis.

Concerning mW annealing, the measured electricalconductivity is close to the one obtained by RTA underN2 (H2 – 5%) atmosphere for temperature up to 800 8C butstill increases for sintering temperature of 900 8C. Sampleannealed at 1000 8C gave no electrical response probablybecause of too many and too broad cracks in the layer(see Fig. 3).

Significant electrical conductivity has been measured;nevertheless this is incoherent with undoped nature of the SiNPs. Furthermore, both 4-points probe and Hall measure-ments have been performed on annealed samples but theycould not give useful information on conductivity, carrierconcentration, and type. This is probably due to presence ofcracks, defects but also lack of homogeneity of the thin filmson a large scale. Nonetheless, given the measured electricalconductivity, it is likely that an unintentional doping ispresent or that the electrical conduction is assisted bydefects, traps, and contamination included in the layer duringannealing.

Nevertheless, previous works with doped (both n andp-type) NPs sintered by hot-press [3], laser [9, 10, 41], orflash annealing [4] demonstrated Seebeck coefficients Swith absolute values around 100–300mVK%1 and conduc-tance s values ranging from 1.1' 10%3 to 900V%1 cm%1

depending on doping level as well as annealing method andtemperature or laser energy. Especially, results obtainedfrom solution-based processes followed by laser anneal-ing [10] are expected to be reached with the presentedprocessing method for doping level around 1019 cm%3:s¼ 0.4V%1 cm%1 and S¼ 220mVK%1.

5.2 Thermal conductivity Precautions have to betaken when analyzing thermal conductivity results becauseof various assumptions that had to be taken in order tocalculate thermal conductivity of the samples applyingEq. (5):

(1) Reflectivity (&30%, needed for Pabs determination) andabsorption coefficient a (&5' 104 cm%1) have beenestimated from literature [9] and supposed constant.

(2) A constant thickness of 1mm has been assumed.

Taking into account these assumptions, the thermalconductivity of the annealed layers has been estimated byRaman spectroscopy, as described above, and represented asa function of the sintering temperature (Fig. 8).

Estimated thermal conductivities from Eq. (5) areranging from some Wm%1 K%1 to almost bulk values(140Wm%1 K%1). Thermal conductivity can therefore betailored both depending on sintering temperature andmethod. Strong increase is observed (Fig. 8) as a functionof sintering temperature. These values are in the range ofvalues found in literature and obtained through differentsintering methods: <1Wm%1 K%1 for laser annealing [9],

10–50Wm%1 K%1 for hot press annealing [3], and 10–20Wm%1 K%1 for flash annealing [4].

Concerning RTA and mW annealing, both under N2, astrong evolution of thermal conductivity is observed. This isattributed to the reduction by grain coarsening of the numberof barriers in the charge carrier path. The higher increase ofthermal conductivity for mW-annealed samples could beexplained by formation of coarser grains than for RTAannealing under N2. This would also explain the lower stressmeasured by Raman spectroscopy (Fig. 6). Grain sizemeasurements would be necessary in order to validate thishypothesis. Nonetheless, neither electron backscatteringdiffraction nor X-ray diffraction gave satisfying results.

Finally, slower increase of thermal conductivity isobserved for RTA annealing under N2 (H2 – 5%). This tendsto confirm the lower grain coarsening for this annealingatmosphere as supposed from electrical measurements.

6 Summary To conclude, inkjet-printing of siliconNPs is a good candidate to fabricate thin film thermogen-erators on flexible substrates due to its low implementationprice. Furthermore, being non-vacuum and maskless processit is easy to integrate a printed device onto a smart-package.Nevertheless, strong reduction of sintering temperature of SiNPs would be needed in order to implement these thin filmson flexible substrates. High doping levels (1019 cm%3) of theSi NPs during synthesis will also be needed in order to reachSeebeck coefficient approaching values of c-Si [9, 10].

In this paper, development of specific Si ink with coatedNPs allowed sufficient stability of the suspension for goodjetability as well as improved protection against oxidation.Therefore, sintering has been reduced from temperatureabove 1000 8C to temperature around 600 8C. Optimalannealing in order to limit the oxidation of the Si thin filmsshould be kept under 850 8C. These thin films should now betransferred to flexible substrates since authors do not knowavailable temperature resistant flexible substrates.

Figure 8 Thermal conductivity estimation from Raman shiftmeasurements.

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Electrical and thermal properties could be tailored bymodifying the annealing conditions: annealing method andatmosphere. In order to keep the thermal conductivitylow RTA under N2 (H2 – 5%) has been defined as the bestprocess. Nevertheless, the electrical conductivity was not asgood as with another annealing (rapid thermal or microwaveannealing under N2). A hypothetical explanation couldresult of lower grain coarsening in the case of RTA underN2 (H2 – 5%). The charge carriers would therefore collidewith many grain boundaries and electrical conductivitywould be strongly reduced [39, 40]. Nevertheless, by dopinginitially the Si NPs, electrical conduction could be greatlyimproved allowing the fabrication of a Si nanoparticle-basedinkjet-printed TE generator.

Acknowledgements This work was financially supportedby the French National Research Agency (ANR) through theInxilicium project. The authors are particularly thankful to J.Mazuir, M. Saadaoui, T. Camilloni, and D. _Zymełka (ENSM-SE)for helpful discussions and experimental assistance. The siliconnanoparticles were procured from Meliorum TechnologiesIncorporation.

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