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1 The 1 th International Conerence o the Slovenian Society or Non-Detructive Teting»Application o Contemporary Non-Detructive Teting in Engineering«September 1-3, 29, Ljubljana, Slovenia, INVESTIGATION OF OPTICAL PROPERTIES OF SnSb 2 S 4 and Sn 2 Sb 2 S 5 THIN FILMS BY A NON DESTRUCTIVE TECHNIQUE BASED ON THE PHOTOTHERMAL DEFLECTION SPECTROSCOPY Imen GAIED 1, Abdelaziz GUASSOUMI 2, Mounir KANZARI 2 and Noureddine YACOUBI 1 1 Intitut Préparatoire aux Etude d Ingénieur de Nabeul 8 Merazka Nabeul Tuniia 2 Ecole Nationale d Ingénieur de Tuni BP 37 Belvédère 12 Tuni Tuniia Correponding author: imen.gaied@ipein.rnu.tn ABSTRACT In thi work we determine the optical aborption pectrum o SnSb 2 S 4 and Sn 2 Sb 2 S 5 thin emiconductor layer depoed on a gla ubtrate by uing the Photothermal Delection Spectrocopy which i non detructive technique. The optical aborption pectrum i obtained by comparing the experimental normalized amplitude curve o the Photothermal Delection ignal veru wavelength to the correponding theoretical one veru optical aborption coeicient. By uing the Tauc law one can deduce the energy gap o emiconductor layer. For SnSb 2 S 4 layer we obtain only one energy gap however or Sn 2 Sb 2 S 5 layer we obtain two energy gap revealing the exitence o two phae which i conirmed by another pectrocopic method uing the tranmiion and the relective curve o the layer. The energy gap value obtained by the two technique are imilar. Key word: Photothermal Delection Spectrocopy, optical aborption pectrum, energy gap. 1. Introduction The SnSb 2 S 4 and Sn 2 Sb 2 S 5 thin emiconductor layer appear to be eriou candidate a aborber material which may be ued in photovoltaic olar cell [1, 2]. So it i important to determine their optical aborption pectra and their energy gap. In thi work we propoe to tudy the optical propertie o SnSb 2 S 4 and Sn 2 Sb 2 S 5 thin emiconductor layer by uing a non detructive technique baed on the Photothermal Delection Spectrocopy (PDS) [3-5] which ha proved to be a ueul technique to invetigate the optical propertie o Bulk emiconductor ample [6] and thin emiconductor layer [7]. The advantage o thi technique lie in it great enitivity to the optical aborption coeicient in a narrow energy band near the energy gap. The ilm o SnSb 2 S 4 and Sn 2 Sb 2 S 5 are depoited on a gla ubtrate repectively at 25 C and at the room temperature by the vacuum evaporation method. 493

2 2. Principle o the Photothermal Delection Technique Briely, the Photothermal Delection Technique (e.g. the o-called Mirage Eect ) conit in heating a ample with a modulated light pump beam. The thermal wave generated by the optical aborption o the ample will propagate in the ample and in the urrounding luid (Parain oil in our cae) inducing a temperature gradient then a reractive index gradient. A Laer probe beam kimming the ample urace and croing the inhomogeneou reractive index region undergo a delection (Fig. 1). Thi delection may be related to the optical propertie o the ample. Z Pump beam z Probe beam Fluid () X Sample () Backing (b) Fig. 1: Schematic repreentation o the probe beam delection. 3. Theoretical model The theoretical model i built on the reolution o the one dimenion heat equation in the dierent media, luid, ample and backing and by auming the continuity o the temperature and the heat low at the dierent interace z = and z = -l (Fig. 2). The ample ixed on a backing material and urrounded by a luid i illuminated perpendicularly by a modulated light pump beam coming rom a halogen lamp (uniorm heating cae). We aume that both luid and backing are optically non aborbing media or the incident light. The obtained expreion o the periodic elevation temperature at the ample urace T [8] given by Eq (1) will permit the calculation o the probe beam delection [8] given by Eq (2). 494

3 Backing Sample z Fluid Probe beam -l b -l -l Fig. 2: Schematic repreentation or dierent media browed by the heat. l T E [(1 r )(1 b ) exp ( l [(1 g ) (1 b ) exp ( l S b ) (1 r )(1 b ) exp ( l ) (1 g )(1 b ) exp ( l 2 2 Where E A ( ), b K K, g K 1 2 i ( 1 j ) i, i ( Di ) b K ) 2( r b ) exp )] (1) and r ( l )] / i the optical aborption coeicient o the ample and i the modulation requency. K i, D i and µ i are repectively the thermal conductivity, the thermal diuivity and the thermal diuion length o the i medium. Here the index i take the ubcript, and b, repectively, or the ample, luid and backing. ( z, t ) L n dn dt 2 T exp ( z ) z exp [ j( ) ] exp( j t ) 4 where z i the ditance between the probe beam axi and the ample urace. A T and are complex number, their may be written a: (2) T T exp ( j ) and ( z ) exp( j ) L dn 2 Where ( z ) T exp ( z ) n dt and z 4 are the amplitude and phae o the Photothermal delection ignal wherea T and are repectively the amplitude and phae o the ample urace periodic elevation temperature. 4. Experimental et-up The experimental et-up i decribed in Fig. 3. The ample aborb the monochromatic light coming rom a halogen lamp o power 25W ater it paage through a monochromator o type Jobin Yvon HR25. The pump light beam i modulated thank to a mechanical chopper o type SR54 and then ocued on the ample urace. A He Ne Laer probe beam o wavelength nm kimming the ample urace at a ditance z o average 6m i delected. A photodetector o our quadrant (QD5T) connected to a lock-in ampliier (EG&G521) meaure the delection o the probe beam. Via the 495

4 intermediary o IEEE bu, a PC microcomputer et the monochromatic light wavelength and read the amplitude and phae o the ignal rom the lock-in and inally draw their variation according to wavelength at a ixed modulation requency. We note that in our cae the luid and the backing media are repectively parain oil and gla whoe thermal conductivity and thermal diuivity are, repectively, K =.16 W.m -1.K -1, D = m and K b =1.5Wm -1 K -1, D b =6 1-7 m 2 S Experimental reult In thi ection we plot the amplitude and phae variation o the photothermal ignal veru wavelength in experimental way and veru optical aborption coeicient in theoretical one at a ixed modulation requency value equal to 8 Hz in order to relate in econd time thee two parameter and determine the energy gap o SnSb 2 S 4 and Sn 2 Sb 2 S 5 thin ilm. photodetector Sample immered in parain oil illed cell monochromator z y x Chopper Halogen lamp Probe beam He-Ne laer Lock-in P.C Fig. 3: Experimental et-up 5.1 Determination o the optical aborption pectrum On ig. 4-a i repreented the experimental normalized amplitude o the photothermal delection ignal veru wavelength at a ixed modulation requency equal to 8 Hz or SnSb 2 S 4 thin emiconductor layer. One can notice rom thi curve that the normalized amplitude i only enitive to wavelength in the vicinity o the energy gap and aturate elewhere. 496

5 1, 1, Normalized Amplitude,8,6,4,2 (a) Experience Normalized Amplitude,8,6,4,2 Theory (b),, 1,8 1,9 2, 2,1 2,2 2,3 2, Wavelength (m) Aborption Coeicient m -1 ) Fig. 4: Experimental (a) curve giving the variation o the normalized amplitude according to wavelength and correponding theoretical one (b) according to aborption coeicient o SnSb 2 S 4 thin ilm depoed at 25 C ubtrate temperature. From Fig. 4-b we remark that the theoretical amplitude variation according to the optical aborption coeicient aturate repectively or high (>1 6 m -1 ) and low (<1 4 m -1 ) optical aborption coeicient value which explain the inenitivity o the Mirage Eect in thee two region. The region where the amplitude vary with the optical aborption coeicient will permit u to deduce the optical aborption pectrum by comparing point by point the normalied experimental amplitude and the correponding theoretical one a hown on ig. 4 ; o we determine or each wavelength value the correponding optical aborption coeicient. The obtained optical pectrum i hown on ig. 5. Aborption Coeicient cm -1 ) SnSb 2 S 4 depoited at 25 C,52,54,56,58,6,62,64,66,68 Energy E (ev) Fig. 5: Optical aborption pectrum veru photon energy E o SnSb 2 S 4 thin ilm depoed at 25 C ubtrate temperature. 497

6 Normalized Amplitude 1,,8,6,4,2 Experience,,5,6,7,8 W avelength (m) Fig. 6: Experimental curve giving the variation o the normalied amplitude according to wavelength o Sn 2 Sb 2 S 5 thin ilm On ig. 6 i repreented the experimental curve giving the variation o the normalized amplitude according to wavelength o Sn 2 Sb 2 S 5 thin ilm. From thi curve one can clearly remark the exitence o three aturated zone which may be attributed to the exitence o two phae o two optical tranition or the material. 1 5 Aborption Coeicient cm -1 ) Sn 2 Sb 2 S 5 (Phae 1) Sn 2 Sb 2 S 5 (Phae 2) 1,6 1,8 2, 2,2 2,4 2,6 Energy E (ev) Fig. 7: Optical aborption pectrum veru photon energy E o Sn 2 Sb 2 S 5 thin ilm. 498

7 In the ame way a above; we can determine the optical aborption pectra or the two phae o Sn 2 Sb 2 S 5 thin ilm. The deduced optical aborption pectra are hown on ig. 7. Thi obervation wa conirmed by another pectrocopic technique baed on the tudy o optical tranmittance and relectance [2]. The experimental phae o the photothermal ignal a wa expected i independent o wavelength becaue the thickne o the layer i much maller than the thermal diuion length. 5.2 Determination o the band gap hit The gap energy i obtained rom the aborption pectrum by uing the Tauc law or energie above the gap: ( E) n ( E Eg), where i a contant, Eg i the optical gap energy between bottom o the conduction band and top o the valence band, E=h i the photon energy, n = 2 or direct tranition and n=1/2 or indirect tranition. The variation o (E) n veru photon energy E or repectively SnSb 2 S 4 and Sn 2 Sb 2 S 5 thin ilm are hown on Fig. 8 and Fig. 9. The extrapolation o the traight line to zero aborption coeicient (=) led to an etimate o the band gap energy (Eg) value. The two ample gap energy value o deduced are reported in Table 1. 2,x1 8 ( E) 2 ( ev 2 cm -2 ) 1,5x1 8 1,x1 8 5,x1 7 SnSb 2 S 4 depoited at 25 C,,52,54,56,58,6,62,64,66 Energy E (ev) Fig. 8: (E) 2 veru photon energy E near the band gap o SnSb 2 S 4 thin ilm depoed at 25 C ubtrate temperature 499

8 4x1 9 Sn 2 Sb 2 S 5 (Phae 1) Sn 2 Sb 2 S 5 (Phae 2) ( E) 2 ( ev 2 cm -2 ) 3x1 9 2x1 9 1x1 9 1,6 1,8 2, 2,2 2,4 Energy E (ev) Fig. 9: (E) 2 veru photon energy E near the band gap o Sn 2 Sb 2 S 5 thin ilm 6. Concluion Table 1: Experimental Gap energy value o SnSb 2 S 4 and Sn 2 Sb 2 S 5 thin ilm. Gap energy (ev) Gap energy (ev) Sample Our tudy Literature value [2] SnSb 2 S Sn 2 Sb 2 S 5 (phae 1) Sn 2 Sb 2 S 5 (phae 2) In thi work, we have determined with good preciion, the optical aborption coeicient pectrum and the gap energy o two new aborber SnSb 2 S 4 and Sn 2 Sb 2 S 5 thin ilm uing the PDS technique. The deduced energy gap value are in good agreement with thoe obtained by tranmiion and relectivity method. 7. Reerence [1] A. GASSOUMI, M. KANZARI and B. REZIG: Thermally induced change in optical and electrical propertie o thin SnSb 2 S 4 ilm, Eur.Phy. J. Appl. Phy Vol 41, 28, [2] A. GASSOUMI, M. KANZARI: Sn 2 Sb 2 S 5 ilm or photovoltaic application, Journal o Optoelectronic and Advanced Material Vol 11 No 4, 29,

9 [3] A.C. Boccara, D. Fournier and J. Badoz, 198, Thermo-optical pectrocopy: detection by the mirage eect, Appl. Phy. Lett. 36, pp [4] J. C. Murphy and L. C. Aamodt, 198, Photothermal pectrocopy uing optical beam probing: mirage eect, J.Appl.Phy.51, pp [5] W. B. Jackon, N. M. Amer, A. C. Boccara, and D. Fournier, 1981, Photothermal delection pectrocopy and detection, Appl.Opt.2, pp [6] S. Abroug, F. Saadalah and N. Yacoubi, Determination o doping eect on Si and GaA bulk ample propertie by photothermal invetigation, Phyica B, 4, 27, [7] Faycel Saadallah, Noureddine Yacoubi, Frédéric Genty and Claude Alibert, Photothermal invetigation o thermal and optical propertie o GaAlASb and AlASb thin layer, J. Appl. Phy. 94, 23, 541. [8] Imen Gaied, Aymen Amara, Noureddine Yacoubi and Taher Ghrib, Eect o beam ize on the amplitude and phae o photothermal delection ignal or both uniorm and non uniorm heating, Applied Optic, Vol. 47, Iue 8, 28,