Pyrite FeS2 films obtained by sulphuration of iron pre-deposited films

Materials Chemistry and Physics 78 (2003) 591–601

Pyrite FeS2 films obtained by sulphuration of iron pre-deposited films

N. Hamdadoua,∗, A. Khelil a, J.C. Bernèdeba Laboratoire de Physique des Matériaux et Composants pour l’Electronique, Université d’Oran, BP 92208 El Mnaouer Oran, Algeria

b Equipe de Physique des Solides pour l’Electronique, Groupe Couches Minces et Matériaux Nouveaux, Université de Nantes,FSTN, 2 rue de la Houssinière, BP 9209 44322 Nantes Cedex 3, France

Received 23 May 2001; received in revised form 10 September 2001; accepted 21 September 2001

Abstract

FeS2 thin films crystallised in the cubic pyrite-type structure have been obtained by sulphuration of iron pre-deposited films. The ironfilms have been sulphurated atTa = 823 K for ta = 6 h. The lattice constant deduced from XRD measurements isa = 5.42 Å. Thebinding energy values measured by XPS of the S 2p3/2 and Fe 2p3/2 are 162.3 and 707.12 eV, respectively. All these results are in goodagreement with reference data. Scanning electron study shows that the grains are as thick as the films, however, the surface visualisationput in evidence that the crystallite sizes are roughly distributed in two distributions, the first with an average size of about 150 nm, theother one being about 500 nm. Such large distribution in the crystallite size justifies that the barrier height, which controls the conductivityof the films varies from one point to another one. Moreover, a high density of impurity states is present in the grains of the films. All thisjustifies the high carrier density and the small carrier mobility measured in the films.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Pyrite; Sulphuration; XPS

1. Introduction

Iron pyrite thin films (FeS2) are extensively studiedbecause of their interesting properties for applications aselectro-optical devices, photo-electrochemical solar cells,battery [1–5]. Moreover, it is built up by non-toxic andabundant elements, which justifies the pursue of the FeS2investigation in spite of the difficulties encountered to obtainreproducible performing films[6,7].

If many deposition techniques have been used from thevery simple sulphuration of Fe films[2,6] to the very sophis-ticated MOCVD[8] and MBE[4], no one has been reallyperforming to obtain highly performing device. Therefore, asystematical study of the properties of the films appears nec-essary to understand the difficulties encountered with FeS2thin films.

The main advantage in using FeS2 thin films in devicesis the non-toxicity and the low cost price, therefore, the useof a simple cheap technique to obtain the thin films seemsmore interesting.

∗ Corresponding author. Present address: Equipe de Physique des Solidespour l’Electronique, Groupe Couches Minces et Materiaux Nouveaux,Universite de Nantes, FSTN, 2 rue de la Houssiniere, BP 9209 44322Nantes Cedex 3, France

In the present paper, iron films have been sulphured ina closed reactor and then characterised physico-chemicallyand electrically.

2. Experimental techniques

2.1. FeS2 thin films preparation

The substrates used were soda lime glasses of dimensions(25 nm×8 nm×1 nm), they were cleaned firstly, by acetonefor eliminating any greasy track and secondly, by ultrasoundsin alcohol. Then they were dried by a nitrogen flow.

Iron thin layers of about 80 nm thickness have been de-posited, using an EDWARDS’s type electron gun, under avacuum of 10−5 Pa. During the deposition, a quartz crys-tal balance was used to control the deposition rate and thethickness.

Then the films of iron were placed in a vacuum(∼10−3 Pa) sealed Pyrex tube with a small amount of sul-phur, for a first annealing at 823 K for 6 h. As it will bediscussed below, during the cooling of the tube some sul-phur condensation takes place on the surface of the layers,therefore, we proceed to a second annealing at 330 K for3 h under dynamic vacuum to sublimate this sulphur excess.

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592 N. Hamdadou et al. / Materials Chemistry and Physics 78 (2003) 591–601

Fig. 1. X-ray diagram of a FeS2 film annealed atTa = 823 K for 6 h.

2.2. FeS2 thin films characterisation techniques

The structure of the films was examined using an ana-lytical X-ray system type DIFFRACT AT V3.1 Siemens,which uses a graphics program EVA. The wavelengthλ was1.5406 Å. The full-width at half-maximum (FWHM) of thediffraction peaks was given directly by the DIFFRACT ATprograms.1

The quantitative X-ray photoelectron spectroscopy(XPS)2 studies were based on the determination of Fe 2pand S 2p peak areas with 0.125 and 0.44, respectively,sensitivity factors, which were given by the manufacturer,Leybold. The depth profile was realised by recording suc-cessive XPS spectra obtained after ion etching for shortperiods (∼1 min). Using an ion gun, sputtering was ac-complished at pressures of<5 × 10−4 Pa with a 10 mAemission current and 5 kV beam energy. The Ar+ ion beamwas restored over the entire sample surface. At the surfaceof the films there is a carbon–carbon bond corresponding tosurface contamination. In the apparatus used, this C–C bondhas a well-defined position at 284.4 eV and the carbon peakwas used as a reference to estimate the electrical chargeeffect as the film samples were on insulating glass sheet.

Electronic microanalysis was performed using a Jeol 5800LV scanning electron microscope (SEM) equipped with aPGT X-ray microanalysis system, in which X-rays were de-tected by a germanium crystal. The surface topography andthe cross-section of the films were observed with a field ef-fect SEM Jeol F 6400.

For the electrical measurements, metal electrodes wereevaporated after FeS2 synthesis. Gold was selected becauseit gives good ohmic contacts. The majority carrier type wasstudied by the hot probe technique. Hall measurements were

1 Measurements have been carried out at the LCS-IMN.2 Measurements have been carried out at Nantes (University CNRS).

carried out at room temperature by using the Van der Pauwmethod. The d.c. conductivity of planar samples has beenmeasured in the dark and under vacuum (∼10−1 Pa), usingtwo electrometers, the first (Keithley 617) for the range oftemperature 300–400 K and the second (Keithley 2000) forthe range 15–300 K.

3. Experimental results

It has been shown in the literature that sulphuration ofthe iron films is achieved only when the annealing temper-atureTa is at leastTa = 620 K [6]. Moreover, the crystalli-sation quality increases withTa, however, at a temperatureequal or higher than 873 K[6], the films peeled off from thesubstrate, therefore, the annealing temperature used in thepresent work was situated in the optimum domain defined,i.e. Ta = 823 K.

The samples have been annealed in sulphur atmosphereat Ta = 823 K for 6 h. During the annealing, the sulphuratmosphere in the Pyrex tube has been estimated to be about10−3 Pa.

The crystalline properties of the samples have beenchecked by X-ray diffraction (XRD). While the iron filmswere highly disordered before annealing (no diffractionpeak visible), after annealing in a sulphur atmosphere thefilms were crystallised (Fig. 1). By comparison with refer-ence data (JCPD No. 42-1340) it can be seen that the filmsare crystallised in the expected pyrite structure of FeS2.

All the main peaks of the powder diagram are visible,moreover, the relative intensity of the different peaks ismore or less respected, which means that the crystallites inthe films are mainly randomly oriented. The films are crys-tallised in the cubic system. The lattice constant deducedfrom the XRD diagram is 5.42 Å.

N. Hamdadou et al. / Materials Chemistry and Physics 78 (2003) 591–601 593

Table 1Quantitative XPS and microprobe (∗) analysis

Atomic concentration (%)

Fe S

Reference powder 33∗ 67∗Thin film after annealingTa = 823 K, ta = 6 h

14, 32.5∗ 86, 67.5∗

Same film after annealingunder dynamic vacuum

31, 32.5∗ 69, 67.5∗

The FWHM of the XRD peaks is small and of the sameorder of magnitude as that obtained with reference pow-ders. Therefore, it can be concluded that the grain size islarge (≥100 nm) and cannot be measured accurately withthis technique.

Fig. 2. XPS spectra: (—) before etching; (- - -) after 1 min etching. (a) S 2p, (b) Fe 2p and (c) O 1s.

Table 2Qualitative XPS analysis

Bond Energy(eV)

S 2p3/2 Fe 2p3/2

MBE film [4] 162.5 707.2Single crystal[4] 162.5 707.1Thin film after annealingTa = 823 K, ta = 6 h

162.3 707.12

The quality of the films has been checked by XPS. Quan-titative studies have shown (Table 1) that just after anneal-ing there is some sulphur excess. However, when analysedby microprobe (EMPA) the results are quite different, thefilms, in the accuracy range of the apparatus, are nearly

594 N. Hamdadou et al. / Materials Chemistry and Physics 78 (2003) 591–601

Fig. 2. (Continued).

stoichiometrics (Table 1). Therefore, since XPS is a surfacetechnique, while microprobe analysis is a bulk technique, itcan be concluded that the films are stoichiometrics but thatthere is some surface condensation of sulphur during thecooling of the tube. This surface contamination is probablyvery thin and, moreover, this sulphur is probably amorphous,which justifies that it cannot be put on evidence in the XRDdiagrams.

The sulphur pressure vapour being very high we haveproceeded to a second annealing of the films but under dy-namic vacuum to remove the sulphur surface contaminationof the films. After an annealing of 3 h at 330 K, the XPS sur-face analysis is in good agreement with the microprobe bulkanalysis (Table 1). The binding energies of such films arereported inTable 2, also are reported some reference values.

Fig. 3. Decomposition of the S 2p doublet before etching.

It can be seen that the binding energy of the Fe 2p3/2 and S2p3/2 lines are in good agreement with the values measuredon the single crystal. Moreover, the good quality of the filmsis also demonstrated by the shape of the S 2p line (Fig. 2a),good doublet resolution, and FWHM of the Fe 2p3/2 (BE707.12 eV) (Fig. 2b).

After etching, the O 1s peak has disappeared (Fig. 2c),which shows that oxygen corresponds only to surface con-tamination. The shape of the S 2p and Fe 2p lines are stronglymodified after etching (Fig. 2).

There is chemical shift of about 1.2 eV of the S 2p line,while the ratio of the peak intensity in the doublet is quitemodified. In fact, since the etching yield of S is far higherthan that of Fe there is partial decomposition of FeS2 dur-ing the etching. After etching the signal corresponds to two

N. Hamdadou et al. / Materials Chemistry and Physics 78 (2003) 591–601 595

Fig. 4. Decomposition of the S 2p doublet after an etching of 1 min.

doublets (Figs. 3 and 4) while before etching only one dou-blet, the FeS2 doublet is visible (Fig. 3).

After etching the doublet situated at 162.5 eV correspondsto the FeS2 still present in the films, while the other one sit-uated at 162.26 eV corresponds to FeS[4] issued from thehighest etching speed of the sulphur. The broad FWHM ofFe 2p peak after etching should be attributed to a mixture ofdifferent contributions: FeS2, FeS and Fe. The main infor-mation obtained by this surface study is that oxygen contam-ination is only present at the surface of the samples, whileit has been shown by microprobe analysis that the films arenearly stoichiometrics.

These films have been visualised by SEM (Figs. 5 and6). It can be seen (Fig. 5) that if the crystallites are wellgeometrically faceted they are shared in two size distribu-tions. The large grains are 0.5–1�m broad. Smaller grains(about 100 nm diameter) surround them. It can be seen thatthe cross-section of the films is homogenous (Fig. 6). Thereis a good adherence of the films to the substrate, while thecrystallites are as thick as the films (≈250 nm). The sizeof the crystallites estimated by the FWHM of the XRD

Fig. 5. SEM of a FeS2 films obtained by annealing atTa = 823 K for 6 h.

peak corresponds to the size of the grains in the directionperpendicular to the plan of the substrate. Therefore, thevisualisation of the cross-section confirms that the grainsize in this direction is higher than 100 nm. The films being

Fig. 6. Cross-section of a FeS2 film synthesised atTa = 823 K for 6 h.

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Fig. 7. Evolution of the conductivity versus the reciprocal temperature.

Table 3Room temperature electrical values of a FeS2 film annealed atTa = 823 K for 6 h

Conductivity type Resistivity (� cm) Carrier mobility (cm2 V−1 s−1) Carrier concentration (cm−3)

Thin film after annealingTa = 823 K, ta = 6 h

p 1.7 1.9 4.7 × 1018

stoichiometric and crystallised in the expected cubic struc-ture, we have proceeded to electrical characterisation.

The variation of the conductivity with the reciprocal tem-perature is presented (Fig. 7), while the value of the conduc-tivity, carrier mobility and density at room temperature arereported,Table 3. It can be seen that if the carrier mobilityis quite small the carrier density is high, which justifies thequite large conductivity. Moreover, the increase of the con-ductivity with the reciprocal temperature does not followsArrhenius plot, these results will be discussed below.

4. Discussion

It has been shown that FeS2 films are obtained by sulphu-ration of iron-deposited films. The films are nearly stoichio-metrics and they crystallise in the expected cubic structure.However, it has been shown that the grains, if well-faceted,exhibit two distribution size domains: large and small grains.

The electrical properties of the films should be explainedin the light of this chemico-physical characterisation. Asshown (Fig. 7), the variation of the conductivity of the filmscannot be explained by the classical semiconductor theo-ries. Even in the high domain temperature the activationenergy extrapolated is only�E ≈ 0.1 eV, which cannotbe attributed to the energy gap between the valence bandand the conduction band[7]. Moreover, the activation en-ergy increases with the reciprocal of the temperature whichis typical of samples, which conductivity is controlled by

grain boundaries. However, the classical grain boundary the-ory firstly introduced by Seto[9] cannot simply be usedpresently since the samples exhibits marked variations in∂ ln(σ /T)/∂T with temperature, which should not be the casewith Seto model where

σ = ATexp

(−qΦB

kT

)

whereσ is the conductivity,A the constant,T the tempera-ture, q the elementary charge,ΦB the barrier height at thegrain boundary andk the Boltzman constant.

The Seto and others classical grain boundary models[10,11] assume that the films are built up of identical rect-angular grains, in each direction. As visualised (Fig. 5) thisis not the case here and, therefore, the barrier height issuedfrom the grain boundary should vary from one point of thesample to another one.

Werner [12] has introduced potential variations amongdifferent boundaries, which can corresponds to the presentcase.

The classical thermal emission across grain boundary canbe described by two diodes in opposite direction[13], whichgives

J =A∗T 2 exp

(−qζkT

)exp

(−qVgb

kT

)(1 − exp

(−qVd

kT

))

with A∗ is the effective Richardson constant,kT/q the thermalenergy,qζ the Fermi level position within the grain:qζ =EC −EF = kTln(Nc/n), whereNc effective density of state,

N. Hamdadou et al. / Materials Chemistry and Physics 78 (2003) 591–601 597

n carriers density,Vg barrier potential at the grain boundaryandVd bias voltage.

As shown by Werner[12] if we introduce potential varia-tions among different boundaries and model the fluctuatingbarrierφ by a Gaussian distribution:

P(φ) = 1

σφ√

2πexp

(−(Φ − φ)2

2σ 2φ

)

with Φ mean barrier andσφ standard deviation.We obtain

φeff(T ) = Φ(T )− σ 2φ

2kT/q

Fig. 8. (a) Temperature dependence of the conductivity in the temperature range 40–400 K, (b) slope of the curved plots from (a).

It can be seen thatφeff decreases upon cooling. The slopesof Arrhenius plots of conductivity are therefore curved up-wards.

Werner has shown that the temperature dependent activa-tion energyEact is given by

Eact(T ) = −k d

dT −1ln(σgb

T

)= q

(Φ(T = 0)− σ 2

φ

kT/q

)

It can be seen (Fig. 8), where the curves deduced from thederivative of parabola are reported, that a good agreementbetween the Werner theory and the experimental results isobtained between 40 and 400 K. The value deduced areΦ =25.33 meV, the barrier height andσφ = 8.317 meV, the

598 N. Hamdadou et al. / Materials Chemistry and Physics 78 (2003) 591–601

standard deviation. The barrier height can be discussed withthe help of Hall theory in the case of polycrystalline films. Ithas been shown[14] that even in the case of polycrystallinefilms the carriers density deduced from Hall measurementscorresponds to the carriers density in the grains, while themobility deduced from these Hall measurements and fromconductivity measurements describes the grain boundary ef-fect introduced by the conductivity.

Therefore, if the barrier height is quite small, this canbe explained by the high carriers density estimated by Hallmeasurements (n = 4.7 × 1018 cm−3). In the grain bound-ary models the depleted domains, induced by the trappingstates present at the grain boundary, decrease when the car-

Fig. 9. ln(σT1/2) versusT−1/4 (Mott model).

Fig. 10. ln(σT) versusT−1/2 (Efros model).

riers density increases. Therefore, there is a good correlationbetween the high carriers density measured by Hall mea-surements and the small barrier height deduced from theWerner theory used to describe the experimental variationof the conductivity with the temperature.

Moreover, if the barrier height is small, the standarddeviation is relatively large, which corresponds to someimportant inhomogeneity of the films. This corresponds tothe two-grain size distribution visualised by SEM. Suchbroad variation in the grain size should introduce largeinhomogeneity of the grain size properties.

In the low temperature domain, from 15 to 40 K, an-other model should be introduced. Classically in the small

N. Hamdadou et al. / Materials Chemistry and Physics 78 (2003) 591–601 599

Fig. 11. Polycrystalline hopping model: band bending and localised states. (δ) Grain boundary width, (W) depletion region, (L) grain size, (d) halfdepletion region, (Q) charge density, (Ec) conduction band, (Eio) intrinseque Fermi level, (Ei ) intrinseque Fermi level in depletion region, (Ef ) extrinsequeFermi level, (Ev) valence band, (Eg) band gap, (eT) energy of trap states in the grain boundary versus (Ei ), (ET) energy of trap states in the grainboundary versusEio, (Ea) acceptor level, (qVb) potential barrier, (a) process a (thermionic emission), (b) process b (hopping conduction).

temperature domain, with the small activation energy mea-sured (�E = 25.33 meV) hopping conduction phenomenais introduced[15–17]. Two mechanisms of hopping conduc-tion tied to localised states are possible:

• Either the variable range hopping conduction describedby Mott [18].

• Or the hopping conduction tied to a Coulomb gapdescribed by Efros[19].

With variable range hopping one finds for the electricalconductivity[18]:

σ = σ0 exp−(

16α3

kTN(E)

)1/4

= σ0 exp−(TM

T

)1/4

600 N. Hamdadou et al. / Materials Chemistry and Physics 78 (2003) 591–601

whereσ0 ∝ A/T 1/2 (A is a constant),α = 107 cm−1 asproposed by Mott[18], k is the Boltzman’s constant andN(E) is the density of localised states.

On the other hand, Efros has shown that the Coulomb in-teraction between localised electrons creates a soft gap∆ inthe density of states near the Fermi level. The most impor-tant manifestation of the gap is the following temperaturedependence of the hopping conductivity:

σ = σ0 exp−(β e2α

KkT

)1/2

= σ0 exp−(TE

T

)1/2

whereσ0 ∝ B/T , e is the elementary charge,K is the macro-scopic static dielectric constant andβ = 2.8 [19], which isthe temperature dependence of the percolation threshold atlow temperatures.

At lower temperature the conductivity obeys theT−1/2 law[19]. If TC is the transition temperature between the Efrosand Mott regimes, the influence of the Coulomb gap can beneglected ifT > TC, whereT is a temperature value in theinvestigated range of temperature. In the neighbourhood ofthe transition temperature we have[20]

TC = 16

β2

T 2E

TM

Indeed, plot of ln(σT1/2) versusT−1/4 (Fig. 9) were linearin the low temperature range [15–40 K] and seemed to givea better fit than ln(σT) versusT−1/2 (Fig. 10), indicating thevalidity of the variable range hopping mechanism (Fig. 9).Moreover, the temperatureTC is found to be 5.4 K, whichis below the temperature range investigated. Then, it maybe concluded that Mott’sT−1/4 low is the dominant pro-cess between 15 and 40 K. Therefore, the value ofN(E) forthe films is 2.86 × 1020 eV−1 cm−3. The existence of lo-calised states necessary for such a conduction process is aconsequence of imperfection associated with polycrystallinefilms.

As shown by the discussion above, the density of localisedstatesN(E), which is related to the imperfections associatedwith polycrystalline films is quite large. This high density oflocalised states present in the grains and the large inhomo-geneity in the barrier height at the grain boundaries justifythe difficulty to obtain performing and reproducible FeS2thin films.

The thermionic emission and the hopping mechanism op-erate simultaneously whatever be the temperature. However,the first is dominant at high temperatures (process a) and thesecond at low temperatures (process b).

As discussed above the FeS2 films have a great number ofdefects, which may create a narrow band of localised statesin the forbidden band. At low temperature, the charges cantransit by variable range hopping in the narrow default bandin the forbidden band gap (Fig. 11).

5. Conclusion

The present results confirm those described earlier, what-ever the deposition technique used, it is difficult to obtainoptimum and reproducible films. It is probably related to theinhomogeneity of the barrier at the grain boundary and tothe high density of localised states in the band gap. More-over, this default justify the high carrier density and smallcarrier mobility of the FeS2 films.

Since, for the applications proposed, it is necessary touse simple deposition techniques, it should be interestingto pursue the sulphuration process of Fe films. However, inorder to decrease the localised state density, annealing informing gas could be interesting.

The films could be obtained by tarnishing in form-ing gas using sulphur powder as sulphur source[21] orH2S diluted in the forming gas. Moreover, annealing con-duction (increase of the temperature) should be carefullychecked in order to avoid broad size distribution of thegrains.

Acknowledgements

The authors wish to thank Mrs. Barreau and Assmann forperforming MEB and X-ray measurements. This work wassupported by a contract between France and Algeria (CMEP00 MDU 510).

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