PROPERTIES OF THIN FILMS BASED ON PARAFFIN DOPED CHALCOGENIDES, PREPARED BY PULSE LASER DEPOSITION

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1 Chlcogenide Letters Vol. 1, No. 2, Ferury 24, p PROPERTIES OF THIN FILMS BASED ON PARAFFIN DOPED CHALCOGENIDES, PREPARED BY PULSE LASER DEPOSITION M. Popescu *, F. Sv, A. Lörinczi, I. N. Mih ilescu, G. Socol, E. Axente, I. Kn, W. Hoyer Ntionl Institute of Mterils Physics, Buchrest-Mgurele, P. O. Box MG. 7, Romni Ntionl Institute of Lsers, Plsm nd Rdition Physics, Buchrest - Mgurele, P. O. Box MG. 6, Romni Technische Universität Chemnitz, Institut für Physik, D-917 Chemnitz, Germny A chlcogenide mteril of composition Se 42 S 58, doped y 1 wt.% prffins hs een prepred s thin films y pulsed lser deposition. The structurl properties of the films hve een investigted y X-ry diffrction. Smple stiliztion ws crried out y nneling t tempertures round 1 o C. 1. Introduction The chlcogenide glsses (e.g. selenium, rsenic sulfides nd selenides) re mterils sensitive to light. As consequence they re suggested s convenient medi for sensors nd opticl memories. Vrious vector nd sclr photo-induced phenomen hs een oserved [1-13], which hve no prllel in oxide glsses. The photoinduced effects in selenium-sulphur glss nd thick films hve een investigted y severl reserch groups. It ws proved tht the photo-structurl effects induced in these mterils re ccompnied y significnt shift of the sorption edge situted in the red prt of the spectrum nd chnge in the refrction index [14]. Due to divlency of selenium nd sulphur the chlcogenide glsses exhiit polymer chrcter with long chin configurtions similr to the cse of orgnic polymers. The orgnic polymers re usully soft mterils nd, therefore, exhiit esily photoinduced tom displcements (PAD) with corresponding chnge of the properties of the mteril. The process is similr to the cse of chlcogenide polymeric glsses: the light cretes photoinduced excited stte in which the chrge is redistriuted. As result, there is new polriztion in the structure tht induces specific tomic displcements. Due to the poor stility of polymers, PAD my led to not only chnge in physicl structure t lso chnge in the chemicl structure nd composition. The mgnitude of PAD in inorgnic glsses is much smller thn in orgnic polymers. The flexiility of the structure round the excited tom is the crucil prmeter to e considered in designing new mterils with desired properties. While the orgnic polymers show high photo-induced effects nd low stility, the inorgnic polymers (chlcogenides) exhiit low photo-induced effects nd good stility. This oservtion leds Jin [15] to propose the development of novel chlcogenide glss orgnic polymer mterils, which would comine the dvntges of oth components on moleculr scle. Pulsed lser deposition is modern method to deposit thin homogeneous nd stoichiometric films strted from solid trget. This method hs een previously used to prepre thin films of oxides [16], semiconductors [17], superconductors nd opticl glsses [18]. Recently the method ws pplied to chlcogenide glsses [19]. The first report on the ppliction of the PLD method to the preprtion of Se-S films ws given in [2]. This pper show the results of study on preprtion of thin chlcogenide Se-S films doped y prffins nd on the structurl properties efore nd fter mild therml nneling, crried out * Corresponding uthor: mpopescu@lph1.infim.ro

2 18 with the purpose to stilize the composition, viewed the possile pplictions in the development of n opticl memory. 2. Experimentl Bulk smples of inorgnic polymer of the eutectic composition in the system Se-S (Se 42 S 58 ) doped y 1 % wt. prffins were prepred y mixing the regent grde components (Selenium Hooken , Sulphur 99.9 Romnin Trde Mrk nd technicl Prrffins, Petrochim Ploie ti Romni) in n evcuted mpoule. Gentle heting t 15 o C (the softening temperture of the eutectic composition is T g = 15 o C) ws crried out nd the smple ws mintined for hlf n hour with continuous gittion of the mpoule in order to get homogeneous mixing. The cooling of the mpoule contining the melt ws mde in ir. After cooling, the mpoule ws open nd conveniently chunks of mterils hve een used s trgets in pulsed lser deposition. The ulk mteril ws investigted y X-ry diffrction. The X-ry pttern shows n morphous ckground nd severl line superposed on it. The crystlline compound Se 3 S 5 ws identified. This result demonstrtes the stoichiometry of the ingot, regrding the eutectic composition prepred in the mpoule. The pulsed lser deposition ws performed with KrF * excimer lser. The lser source generted pulses hving the wvelength λ = 248 nm with pulse durtion τ FWHM >2 ns nd 2 Hz repetition rte. The pulses were focused on the trget through MgF 2 cylindricl lens with the focl length of 3 cm. The incidence ngle to the trget ws 45 o. The lser spot ws set within 4.2 nd 6.4 mm 2. The mximum output energy ws 85 mj/pulse. The fluence ws ~ 22 J/cm 2. During lser irrdition rightly colored plume plsm ws oserved. The shpe of this plume is forwrd directed nd slightly divergent. The experiments were performed in stinles steel vcuum chmer, which ws evcuted down to P. The trget ws rotted with the frequency of.4 Hz during PLD deposition. The sustrte for film deposition ws (111) oriented silicon wfer plced prllel to the trget surfce nd situted t distnce of 2-5 cm. The numer of pulses pplied during deposition process ws 15. The film otined y PLD looks homogeneous, right nd without defects. The photoinduced effect in the films nd the chrcteriztion on the opticl recording fcility, s well s the resolution of n opticl memory ws tested y using YAG-Nd lser with t = 1 ns pulse durtion (the working regime of the lser ws: nnopulse (14 mj/pulse) nd doule pulse (28 mj/pulse), λ = 164 nm). A Leitz-Orthopln-pol microscope ws used to this purpose. 3. Results The properties of the PLD films mentioned ove hs een mesured firstly y ellipsometric method. A thickness of 2.8 micrometers hs een determined nd refrctive index of 2.5 t 7 nm wvelength, in greement with the results known from literture. X-ry diffrction mesurements hve een performed in two diffrctometers: TUR M62 nd Seifert XRD-7 FPM diffrctometer provided with copper trget X-ry tues. A proportionl detector nd step scn mode hve een used during mesurements. Automtic recording of the intensity hs een mde. The time of ccumultion of the scttered qunt ws 2 seconds per mesuring ngle. The ngulr step ws set t.2 o (2θ). The X-ry digrm of the s-prepred thin film shows n morphous chrcter (see Fig. 1). The morphous phse is evidenced y three rod peks situted t ~25 o, ~54 o nd 87 o. No crystlline phse is presented in the film ty (.u.) 2 15 sity (.u.) 2

3 19 Fig. 1.. the X-ry diffrction pttern of the s-prepred thin film Se 42 S 58 doped y prffins.. the X-ry diffrction pttern of the nneled film Se 42 S 58 doped y prffins. The chlcogenide PLD films were susequently nneled in order to stilize the structure. Tking into ccount the low softening temperture of the film we hve performed the nneling t 95 ± 5 o C, in rgon tmosphere (+1 % H), for hlf n hour. The gs pressure ws ~5 mr. After nneling the films were cooled very slowly (~3 deg/min) nd finlly were exposed to the norml tmosphere. The X-ry diffrction pttern of the nneled film ws recorded in exctly the sme conditions s the fresh (unnneled) film. The X-ry diffrction pttern on the nneled film (Fig. 1) shows tht prt of the film hs crystllized in minor phse with very smll crystllites (nnometer size) of verge size ~2 nm. The phse ws not clerly identified. It is possile tht comintion of Se 47 S 53 phse (ASTM file ) nd some other phse Se-S ws formed. The dditionl pek oserved on the digrm is due to the (111) plne of the silicon wfer sustrte. It is possile tht, during the stiliztion tretment, prt of prffins e eliminted nd phse seprtion occurred. If this hypothesis is vlid, then the detiled study of the rdil distriution function of the morphous phse efore nd fter het tretment will llow for getting informtion on the phse chnge. The X-ry diffrction curves were processed in order to get the rdil distriution functions, useful for the structurl informtion regrding the tom scle rrngement in the films. Fig. 2 nd 3 show the rdil distriution functions nd the difference curves. 3 1 RDF 2 1 DRDF Fig. 2. Rdil distriution function (RDF) () nd difference rdil distriution function (DRDF) () in fresh PLD deposited thin films of hyrid chlcogenide-prffin morphous mteril. 3 1 RDF 2 1 DRDF

4 2 Fig. 3. Rdil distriution function (RDF) () nd difference rdil distriution function (DRDF) () in nneled PLD deposited thin films of hyrid chlcogenide-prffin morphous mteril. From the rdil distriution functions nd difference rdil distriution functions clculted efore nd fter nneling it ws possile to extrct the min structurl chrcteristics of the films. Tle 1 shows the min prmeters of the films efore nd fter nneling. Tle 1. Structurl prmeters of the prffinted selenium-sulphur films. Structurl prmeter Fresh Se-S prff. film Anneled Se-S prff. film r 1 (Å) rms(r1) (Å) re under r 1 pek (e.u.) r 2 (Å) r 3 (Å) r 4 (Å) r 5 (Å) The res under the first pek in RDF shows tht in oth fresh nd nneled films the structurl onding specific to lternting di-vlent toms of selenium nd sulphur is preserved (Fig. 4). Nevertheless some differences re pprent. In the fresh films the coordintion of the chlcogen is situted ner 2, corresponding to the re of 114 in electron units. This is somewht lower thn the cse of the model with Se nd S lternting chins (996) nd much lower thn in the cse of phse seprted stte of the mteril (1268). This lower vlue cn e ttriuted to the presence of prrfins tht decrese the length of the chins selenium-sulphur. As concerning the nneled films, the re under the first pek decreses up to 875 e.u. nd this indictes tht the Se-S long chins rek, while the finl configurtion is determined, t lest prtilly y the interposition of the prrffin chins, nd long lternting Se-S. Are under first pek in RDF (e.u.) fresh nneled phse seprtion Se + S lternting Se nd S toms Se (%) Fig. 4. The re under the first pek of the thin film of hyrid chlcogenide-prffin morphous mteril. In the sme time the chlcogen orgniztion gives rise to minute mount of crystlline Se 47 S 53 (ASTM file ). The vrition of the opticl film properties during light irrdition gives the possiility to record the informtion on thin hyrid chlcogenide-prffin films. Some experiments on the possile chnge induced y light in the morphous hyrid films hve een performed. Persistent chnge of light sorption under the influence of lser pulses wrrnts the use of the films s opticl memory elements.

5 21 4. Conclusions For the first time thin films of chlcogenide/prffin morphous solid hve een prepred y pulsed lser deposition. The films re homogenous nd they ecome stle in time fter short tretment under T g. The stilized films contin minor nnometric size crystlline phses of composition Se 47 S 53. Acknowledgement The prtil support of the Ministry of Eduction, Reserch nd Youth under Contrct No. 23/21 CERES is kindly cknowledged. Thnks re due to Dr. I. Himcinski (Technicl University, Chemnitz) for the ellipsometric mesurements. One of us (M.P.) grtefully cknowledges the finncil support of the Sxonin Ministry of Science nd Arts for performing reserch in the Technicl University Chemnitz (23). References [1] M. Popescu, Non-Crystlline Chlcogenides, Kluwer, Dordrecht, 2. [2] K. Shimkw, A. Gnjoo, J. Optoelectron. Adv. Mter. 3(2), 167 (21). [3] V. Lyuin, M. Klenov, J. Optoelectron. Adv. Mter. 3(2), 167 (21). [4] R. Todorov, K. Petkov, J. Optoelectron. Adv. Mter. 3(2), 311 (21). [5] A. Kovlskiy, J. Optoelectron. Adv. Mter. 3(2), 323 (21). [6] M. Stuchlik, P. Krecmer, S. R. Elliott, J. Optoelectron. Adv. Mter. 3(2), 361 (21). [7] A. Kikineshi J. Optoelectron. Adv. Mter. 3(2), 377 (21). [8] A. Arsh, N. Froumin, M. Klenov, V. Lyuin, J. Optoelectron. Adv. Mter. 4(1), 27 (22). [9] Physics of Disordered Mterils, Ed. M. Popescu, INOE Pul. House, Buchrrest, 22. [1] A. V. Stronski, M. Vlcek, J. Optoelectron. Adv. Mter. 4(3), 699 (22). [11] T. Wgner, J. Optoelectron. Adv. Mter. 4(3), 717 (22). [12] I. Ivn, A. Kikineshi, J. Optoelectron. Adv. Mter. 4(3), 743 (22). [13] L. Tichy, H. Tich, P. Ngels, R. Mertens, J. Optoelectron. Adv. Mter. 4(3), 785 (22). [14] D. Jecu, J. Jklovszky, A. Truti, I. Apostol, M. Dinescu, I. N. Mihilescu, G. Aldic, M. Popescu, N. Vlhovici, S. Zmfir, E. Indre, J. Non-Cryst. Solids 9, 319 (1987). [15] H. Jin, J. Optoelectron. Adv. Mter. 5(1), 5 (23). [16] H. Snkur, Mt. Res. Soc. Symposium, Proc. 29, 373 (1984). [17] J. T. Cheung, Mt. Res. Soc. Symposium, Proc. 29, 31 (1984). [18] E. M. Vogel, E. W. Chse, J. L. Jckel, B. J. Wilkens, Applied Optics, 28(4), 649 (1989). [19] P. Nemec, M. Frumr, J. Optoelectron. Adv. Mter. 5(5), 147 (23) [2] A. Lorinczi, J. Optoelectron. Adv. Mter. 5(5), 181 (23).