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Structural and optical properties of sprayed In2−2xAl2xS3−3yO3y alloys

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Page 1: Structural and optical properties of sprayed In2−2xAl2xS3−3yO3y alloys

Materials Chemistry and Physics 72 (2001) 320–325

Structural and optical properties of sprayed In2−2xAl 2xS3−3yO3y alloys

L. Bhiraa, T. Ben Nasrallaha, J.C. Bernèdeb, S. Belgacema,∗a Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Campus Universitaire, 2092 Tunis, Tunisia

b Equipe de Physique des Solides pour l’Electronique, Faculté des Sciences et des Techniques, Université de Nantes,2 Rue de la Houssinière, BP 92208, 44322 Nantes, France

Received 17 May 2000; received in revised form 21 July 2000; accepted 16 September 2000

Abstract

Structural and optical properties of In2−2xAl2xS3−3yO3y alloys obtained by the spray pyrolysis technique have been studied. X-raydiffraction (XRD) shows for low compositions(x ≤ 0.2) well crystallized films preferentially oriented towards (4 0 0) direction cor-responding to�-In2S3 phase; forx > 0.2, the structure becomes amorphous as confirmed by scanning electron microscopy (SEM).Moreover, microanalysis and X-ray photoelectron spectroscopy (XPS) measurements have detected oxygen in the films present in Al2O3

and In2O3 oxides and Al(OH)3 hydroxides form. On the other hand, from the optical transmissionT(λ) and reflectionR(λ) curves, thestudy of the absorption coefficient of thin layers versus incident light energy revealed that the value of the band gap energy increases withthe compositionx according to a parabolic profile. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: In2−2xAl2xS3−3yO3y ; Spray pyrolysis; X-ray photoelectron spectroscopy (XPS); Optical properties

1. Introduction

In2−2xAl2xS3−3yO3y alloys are attractive materials as ac-tive semiconductors in optoelectronic applications. Indeedthese compounds offer the possibility to cover a large bandgap and were not been studied up to now.

In this work, the spray pyrolysis technique is selected toprepare In2−2xAl2xS3−3yO3y thin layers [1] which were an-alyzed by X-ray diffraction (XRD) and scanning electronmicroscopy (SEM). In the same way, thin layers of this ma-terial have been examined by microprobe analysis and X-rayphotoelectron spectroscopy (XPS). We have also deducedthe band gap energies for different compositions from spec-trophotometric measurements.

2. Experimental

In2−2xAl2xS3−3yO3y thin films were obtained by sprayingan aqueous solution containing indium chloride (10−3 M),aluminum chloride and thiourea 2× 10−3 M as starting ma-terials. The value of the ratio concentrationx = [Al] /[In]

∗ Corresponding author. Tel.:+216-1-872600, ext: 496;fax: +216-1-885073.E-mail address: [email protected] (S. Belgacem).

was varied in solution from 0 to 1. Solution and gas flowrates were kept constant at 2 cm3 min−1 and 4 l min−1, re-spectively, corresponding to a mini spray pyrolysis. Nitro-gen was used as carrier gas. The substrate temperatureTswas adjusted to 340◦C.

The crystallinity and the orientation of the studied com-pounds were obtained by means of a Philips (PW 1729)system using Cu K� monochromatic radiation (wavelengthλ = 1.5405 Å). The morphology of the film surface wasvisualized by SEM using a 6400 F JEOL field emission ap-paratus. The optical transmission and reflection of the filmswere determined in the wavelength range 0.3–2.4�m by aShumadu UV 3100 F spectrophotometer equipped with anLISR 3200 integrating sphere.

The samples have been characterized by electron mi-croprobe analysis (EMPA). A software program (PGI —IMIX PTS) did the background correction and gave thepercentage of the elements. XPS measurements were per-formed with a magnesium X-ray source (1253 eV) oper-ating at 10 kV and 10 mA. An in-depth study was doneby recording XPS spectra after ion etching using an iongun. Sputtering was accomplished at pressures of less than5× 10−4 Pa with a 10 mA emission current and 5 kV beamenergy. The quantitative studies have been based on the de-termination of the In 3d5/2, Al 2p, Cl 2p, C 1s and O 1speak areas.

0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0254-0584(01)00333-9

Page 2: Structural and optical properties of sprayed In2−2xAl2xS3−3yO3y alloys

L. Bhira et al. / Materials Chemistry and Physics 72 (2001) 320–325 321

Fig. 1. XRD pattern of sprayed In2−2xAl2xS3−3yO3y alloys deposited onpyrex glass at 340◦C.

3. Results and discussion

3.1. Structure

For compositionsx ≤ 0.15, the X-ray diffraction spectraof In2−2xAl2xS3−3yO3y alloys show well-defined peaks of(3 1 1), (4 0 0) and (5 1 1) principal orientations correspond-ing to �-In2S3 (Fig. 1). The (4 0 0) orientation is privilegedin all cases (Table 1).

On the other hand, we establish a decrease in the orien-tation coefficientFo defined by

Fo(h k l) = I(h k l)/I0(h k l)∑

I(h k l)/I0(h k l)

indicating that the films having weak compositions are rel-atively better crystallized than the others (Table 2). These

Table 1Orientation of the (3 1 1) and (4 0 0) planes of In2−2xAl2xS3−3yO3y

sprayed alloys

Compositionx Orientation

I4 0 0/I5 1 1

(ASTMa: 0.66)I3 1 1/I5 1 1

(ASTMa: 0.86)

0.00 7.20 1.260.05 6.65 1.810.10 8.80 1.400.15 4.60 3.25

a Values corresponding to�-In2S3 phase.

Table 2Structural parameters of In2−2xAl2xS3−3yO3y thin layers

x e (�m) ∆ (◦) D (nm) Fo

0.00 0.8 0.34 40 0.820.05 1.0 0.46 28 0.760.10 1.2 0.33 30 0.810.15 1.2 0.63 18 0.60

observations are coherent with the decrease of the averagegrain sizeD (Table 2) deduced from Scherrer formula [2]

D = kλ

cosθ(∆2 − ∆20)

1/2

wherek = 1.05 and∆0 = 0.22◦. Whenx increases(x ≥0.2), the structure becomes amorphous.

3.2. Morphology

Fig. 2 shows the surface micrographs of thin films corre-sponding tox = 0, 0.1, 0.3, 0.5 and 0.8. It can be seen fromthese images that the samples corresponding to low compo-sitions are continuous, fairly homogeneous and with littlesurface roughness. Forx > 0.1, the films exhibit the pres-ence of rounded small clusters less than 1�m onto a homo-geneous fond. In the same line, we observe an increase ofthe density of circled filaments located around these clusters.

3.3. Microanalysis

The microprobe analysis results of the thin layers are re-grouped in Fig. 3. We point out the presence of oxygen inthe samples besides In, Al, S and C elements. Its proportionincreases from 11 to 65% with the detriment of the sulfurwhenx varies from 0 to 1. Indeed, this rate is complemen-tary to that of the sulfur and is necessary for a stoichio-metric composition (theoretically 60% of sulfur and 40%of In–Al). The increase of the oxygen proportion with thecompositionx in the films can be attributed to the aluminumoxide Al2O3 phase which becomes abundant and gives anamorphous structure for the film having high composition.

3.4. XPS measurements

The XPS analysis of the alloys for 0≤ x ≤ 0.4 compo-sitions have been done at the surface and in the volume inorder to identify the different phases present in the films.Surface analysis shows that all the films are contaminatedwith carbon. After etching, the intensity of the carbon peaksdecrease strongly (Fig. 4) which shows that the main partof the contamination is only a surface effect. Chlorine inthe films is mainly present in the form of chlorine anionwhich corresponds to the starting solution (InCl3, AlCl3),this means that there is some contamination of the films bythe starting solution.

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322 L. Bhira et al. / Materials Chemistry and Physics 72 (2001) 320–325

Fig. 2. SEM micrographs of In2−2xAl2xS3−3yO3y thin layers: (a)x = 0; (b) x = 0.1; (c) x = 0.3; (d) x = 0.5; (e) x = 0.8.

On the other hand, the S 2p and In 3d peaks (Figs. 5 and6) decrease after etching for low compositions showing thatthe films are sulfur and indium deficient at the surface. Thisdeficiency increases progressively whenx increases.

Moreover, these analysis pointed out the oxygen contam-ination for all the films, especially for high compositions.For x = 0, before etching, the O 1s peak shows that thereare two components (Fig. 7). The first one situated at about532 eV can be attributed to air contamination during sam-ple elaboration. The second contribution situated at about530.4 eV corresponds to the binding energy of oxygen in in-dium oxide In2O3 [3]. Since first minute of etching, the lastcontribution becomes dominant indicating that the totalityof the oxygen present in the film is bounded to indium.

For higher compositions(x = 0.4), the oxygen peak hasbeen decomposed in order to determine its nature. Fig. 8shows that the oxygen peak exhibits clearly three max-ima. The first O 1s peak is situated at a binding energy of530.4 eV, the second at 532.4 eV and the third at 534.1 eVwhich can be attributed, respectively, to indium oxide In2O3,aluminum oxide Al2O3 and hydroxyl aluminum Al(OH)3compounds with 5, 76 and 19%, respectively.

The decomposition of the Al 2s peak corresponding tox = 0.4 shows two peaks situated at 121 and 125.2 eV(Fig. 9), which corresponds to the phases Al2O3 andAl(OH)3 with approximate percentages of 84 and 16%,respectively. This result confirms the O 1s peak decomposi-tion described below. Forx = 0.2, the Al 2s peak situated

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L. Bhira et al. / Materials Chemistry and Physics 72 (2001) 320–325 323

Fig. 3. Variation of the atomic percentages of Al, O, S and In elementsas a function of the compositionx of the In2−2xAl2xS3−3yO3y alloys.

Fig. 4. XPS spectra of C 1s line of sprayed In2−2xAl2xS3−3yO3y thinfilms: (a) before etching and after etching of (b) 1 min; (c) 3 min; (d)6 min.

Fig. 5. XPS spectra of S 2p line of sprayed In2−2xAl2xS3−3yO3y thinfilms: (a) before etching and after etching of (b) 1 min; (c) 3 min; (d)6 min.

Fig. 6. XPS spectra of In 3d line of sprayed In2−2xAl2xS3−3yO3y thinfilms: (a) before etching and after etching of (b) 1 min; (c) 3 min; (d)6 min.

Fig. 7. XPS spectrum of O 1s line of sprayed In2−2xAl2xS3−3yO3y thinfilm for x = 0: (a) after etching of 1 min; (b) before etching; (c) afteretching of 3 min.

Fig. 8. Decomposition of the O 1s peak of In2−2xAl2xS3−3yO3y thinfilm (x = 0.4) after etching of 1 min: (– – –) experimental spectrum; (—)decomposed spectrum.

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324 L. Bhira et al. / Materials Chemistry and Physics 72 (2001) 320–325

Fig. 9. Decomposition of the Al 2s peak of In2−2xAl2xS3−3yO3y thinfilm (x = 0.4) after etching of 6 min: (– – –) experimental spectrum; (—)decomposed spectrum.

Fig. 10. Decomposition of the Al 2s peak of In2−2xAl2xS3−3yO3y thinfilm (x = 0.2) after etching of 6 min.

Fig. 11. Transmission spectra of sprayed In2−2xAl2xS3−3yO3y thin films:(a) x = 0; (b) x = 0.05; (c) x = 0.10; (d) x = 0.15; (e) x = 0.30; (f)x = 0.70.

Fig. 12. Reflection spectra of sprayed In2−2xAl2xS3−3yO3y thin films:(a) x = 0; (b) x = 0.05; (c) x = 0.10; (d) x = 0.15; (e) x = 0.30; (f)x = 0.70.

at 119.8 eV shows the existence of pure aluminum, whilethe other one situated at 124.3 eV can be attributed to theAl2O3 phase (Fig. 10).

3.5. Optical measurements

The optical transmissionT(λ) and reflectionR(λ) spectrafor films deposited on pyrex substrate are shown in Figs. 11and 12, respectively. For low compositions, we point outthe presence of the interference fringes due to the multitude

Fig. 13. Variation of the absorption (αhν)2 as a function of the lightenergyhν of In2−2xAl2xS3−3yO3y thin films: (a)x = 0.30; (b) x = 0.40;(c) x = 0.50; (d) x = 0.70; (e) x = 0.80.

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L. Bhira et al. / Materials Chemistry and Physics 72 (2001) 320–325 325

Fig. 14. Evolution of the band gap energyEg of the In2−2xAl2xS3−3yO3y

alloys versus the composition.

reflection phenomenon showing fairly homogeneous films.Moreover, these films exhibit a good transparency in thevisible and infrared regions. For higher compositions, theseinterferences disappear; this is probably due to the inhomo-geneities present in the volume and the perturbed relief atthe surface as shown in the SEM micrographs. On the otherhand, these spectra shows that for an approximately samethickness(e ≈ 1�m) (Table 1), the transmission decreasesas a function of the composition.

The exploitation of theT(λ) andR(λ) spectra in the fun-damental absorption zone allowed us to determine the ab-sorption coefficientα according to the approximate relation

T = (1 − R)2 exp(−αe)

We have thus studied the absorption (αhν)2 as a function ofthe photon energyhν (Fig. 13). These quasi-linear variations

show in fact that the transitions are direct as seen in therelation

(αhν)2 = A(hν − Eg), A = cte

The intersection with thehν axis gives the band gap energyvalues 2.08 ≤ Eg ≤ 2.95 eV for 0≤ x ≤ 0.8 according toa parabolic profile (Fig. 14).

4. Conclusions

In2−2xAl2xS3−3yO3y alloys have been deposited by thespray technique, showing well-crystallized films for lowcompositions as confirmed by XRD and SEM analysis. Adetailed study of the composition of these layers by means ofmicroprobe and XPS analysis permitted to identify�-In2S3,Al2O3, In2O3 and Al(OH)3 phases present in the layers.The optical study shows an increase in the band gap energywhen the composition increases as hoped, giving for suchmaterials the possibility to be used in several optoelectronicapplications.

Acknowledgements

This work was supported by a cooperation contractbetween LPMC (Tunis–Tunisia) and EPSE (Nantes–France)laboratories (contract CMCU No. 98/F 1304).

References

[1] L. Bhira, M. Amlouk, S. Belgacem, R. Bennaceur, D. Barjon, Phys.Stat. Sol. (a) 165 (1998) 141.

[2] E.F. Kaeble, Handbook of X-ray, McGraw-Hill, New York, 1967.[3] L. Bhira, H. Essaidi, S. Belgacem, G. Couturier, J. Salardenne, N.

Barreaux, J.C. Bernède, Phy. Stat. Sol. (a) 181 (2000) 427.