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MaterialsResearchBulletin, Vol. 32, No. 7, pp. 907-913,1997Copyright01997 Elsevier S&me LtdPrintedin the USA. All rightsreserved

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LASER-INDUCED CRYSTALLIZATION IN SbJ33FILMS

P. Arun, A.G. Vedeshwar*Departmentof PhysicsandAstrophysics,Universityof Delhi,Delhi 110007, India

N.C. MehraUniversityScience& InstrumentationCenter,Universityof Delhi,Delhi,India

(Refereed)(ReceivedDecember12, 1996;AcceptedFebruary24, 1997)

ABSTRACTCrystallizationin vacuumevaporatedamorphousSbX33films inducedby acontinuouswave argon laser is studiedsystematicallyas a fimctionof laserpower, irradiationtime, film thicknessetc. Results show that the observedamorphous-to-crystallinetransformationis irreversibleand is mainly due tothe thermalprocessduring the laser interaction.The threshold laser powerdensity required to induce crystallization is found to be 98 & 5 W/cm2.Analyses indicate Sb& films as a potential material for WORM kind ofStorageapplications.@1997ElsevierScienceLtd

KEYWORDS:A. chalcogenides,A. thin films,B. laser annealing,B. vapordeposition,C. electronmicroscopy,D. phasetransitions

INTRODUCTION

Laser-inducedmaterialprocessinghas becomequite importantand popular in recentyears,particularly in materials science. It has a prominent role in thin film fabrication andprocessing, device fabrication, optical data storage in films etc. Both reversible andirreversibleamorphous-to-crystallinetransformationsinducedby laserirradiationhavebeenstudiedin a varietyof thin films[1–5].Thesestudiesenableone to exploitthe differenceinphysicalpropertiesof amorphousand crystallinephasesfor theirpossibleapplications.As aresult,a searchis onto findnewmaterialsforuse as opticalstoragemedia.

*Towhomcotiespondenceshouldbeaddressed.

907

908 P.ARUNet al. vol. 32,No.7

Laser-inducedtransformationis believedto be eitherphoto-inducedor thermallyinduced,or induced by both in what is commonly known as photo-thermal process [6]. Inphotosensitivematerials, the photo-inducedtransformationis dominantand faster. In allother kind of materials,the transformationis believedto be due to heat generatedby laserirradiation.The temperaturewill be greaterat the irradiatedspot if the opticalabsorbanceofthe film is largerat the givenlaserwavelength,becauseof electronicexcitationand therebyinduced lattice vibrations. Several binary and ternary layered compounds both instoichiometricand non-stoichiometricforms have been investigatedin this direction [5].Therefore, we have tried to study in detail the laser-inducedcrystallization in vacuumevaporatedamorphousfilmsof Sb2Sj.

EXPERIMENTAL

Sb2S3films were grown on glass substrates at room temperature by evaporatingstoichiometriccrystallinebulksampleusinga molybdenumboat at a vacuumbetterthan 105torr. The films were amorphousin nature as confirmedby both X-ray diffraction(using aPhilips PW1840 diffractometer) and SEM (JOEL 840). The film thickness was about5300~, as measuredby a Dekteksurfaceprofiler,and was quiteuniformover the 3 cm x 3cm area. Filmsof the samethicknesswere used in all the measurementsfor the purposeofcomparison of various results. Films were found to be almost stoichiometricwithin theexperimental error as determined by ESCA (Shimadzu 750). A continuous Ar+ laser(Coherent’sINNOVA70-4)with 514nm wavelengthwas used for irradiationof the films.Films were also tested for amorphous-to-crystalline transformation using DTAmeasurements.We have also determinedthe opticalabsorbanceof the films in the visibleregionusinga Shimadzuspectrometer(UV260).

RESULTS AND DISCUSSION

The as-grownamorphousfilmswere irradiatedin air by a continuousAr+laserwith variousoutputpower for differentdurationsof time.The laserbeamradiuswas reducedby focusingand the final radius of the spot incidenton the film was about 200 pm, as determinedbymeasuringthe diameterof the spot on a laser-exposedphotographicplate. In Figure 1, weshow someresults of irradiatedspotson the film. The backgroundunirradiatedamorphousregion of the film can clearlybe seen in all the photographsin contrastwith the crystallinecircularspot. It can also be seen that the transformedcrystallinespotradius increasesfromabout 70 pm (Fig. la) to 140pm (Fig. lb) and to 160ym (Fig. Ic) when the laser powerwas increasedfrom 136mW (Fig. la) to 176mW (Fig. lb) and to 200 mW (Fig. Ic). Thereis an increased contrastwith laser power due to the improvedgrain size. Also, differentconcentricrings of contrastare clearly visiblein Figures lb and Ic, which are due to theradiallydecreasinggrain size. I his indicatesthe mechanismof transformationis due to thethermal process and the laser serves as heat source. This can be understoodeasily if weassumethe spatialpowerdensityof the laserbeamas Gaussian,which is quitereasonableinthe case of continuouswave lasers. The spatial distribution of temperature rise at the. - ..... ... --. .--r ---------------- - . . . -. .

907

Vol.32,No. 7 ANTIMONYSULFIDEFILMS 909

FIG. 1Crystallinespots inducedin as-grown5300 ~ thick amorphousSb& films by irradiatingAr+laser beam of 200 pm radiuswith outputpower of (a) 136mW,200mW. The irradiationtime(30s) was samefor all threecases.

~ , = Aoe-(rlr.)ze-w

{ }~–e-KAt/cv +~ e-KAt/cv

r.z.KA

0

where

and

Ao. @-

m td

~ = 5,784+ 1——r ~ d2

(b) 176mW, and (c)

(1)

Here A is the absorbanceof the filmat the laserwavelength,P is the laseroutputpower,ro is the laser radius at the irradiatedspot, d is the film thickness,z is the depth measuredfrom the surface of the film, ct is the absorptioncoetllcient, and Kand c, are the thermalconductivityand volumetricspecificheatof the film.Tois the temperatureof the filmbeforeirradiationwhichmay be takenas roomtemperature.The secondterm at the right-handsideof eq. 1 does not contribute significantlyto the temperatureprofile and, hence, can betreated as background.All these parameterswere determinedexperimentallyfor our films.However, we have taken the reportedvaluesof for K and c, for Sb& [8]. Here we haveneglected the influence of the glass substrate, even though the irradiation time in our

910 P.ARUNet al. Vol.32,No. 7

300t

\

I

!’oo~o -- —--- -----

A 200 mW● 176 mWo 136 mW

\k- ‘----+

100IwI II

o~100 200 300

f ( pm) —

FIG.2The calculated(usingeq. 1)radialdistributionof temperaturerise at the irradiatedspotsonthe film for threedifferentlaserpowersused in the experiment.Upwardarrows indicatetheexpectedradiiof the crystallinespotsat threedifferentlaserpowers.

experimentwas long. This is justified, since the temperatureat the film-glassinterface isquite low as comparedto the surfacetemperaturedue to the very low thermalconductivityof SbZStfilms and e-”=term in eq. 1. Using eq. 1, we can calculatethe radial distribution,depthprotile,and timeevolutionof temperaturerise at the irradiatedspot.

Figure 2 shows the calculatedradial distributionof temperaturerise for,three differentlaser powers used in the experiment.The horizontalbroken line represents 165°Con thetemperatureaxis abovewhich the film transformsfrom amorphousto crystallinestate, asdeterminedby our DTA analysisof the film. This has also been observed in amorphousSbi3J powders by Cervinkaand Hruby [9]. We have used upward arrows to indicatetheexpectedradii of the crystallinespotsat three differentlaserpowers.It can be seen that themeasuredradiiof crystallinespotsat the samelaserpowers(Fig. 1)agreevery wellwith theexpectedones (Fig. 2). Also, the radiallydecreasinggrain size can be understoodas due toradially falling temperature (Fig. 2). This fact strongly suggests the transformation isthermallyinducedin thisphoto-thermalprocess.

The increasinggrainsizewith laserpowerwhich is dueto the increasingtemperaturerisecan be seen explicitlyin Figure3. This figureshowsthe enlargedportionnear the centerofthe crystallinespotsformedat the variouslaserpowersirradiatedfor the sameduration.Thegraduallyincreasinggrainsizewith laserpowercan clearlybe seen in the figure.

The grain size also increaseswith irradiationtime, as can be seen in Figure4. Figure 5showsthe dependenceof the averagegrain size, as determinedfromthe SEMphotographs,on laser power and the irradiationtime. The grain size shows lineardependencewith bothlaserpower and irradiationtime. This is not very surprisingif we believethe dominanceof

Vol.32,No.7 ANTIMONYSULFIDEFILMS 911

FIG.3Enlargedcentralportionof the crystallinespotsinducedby laser irradiationwith power (a)136mW, (b) 154mW,(c) 200mW, and(d) 250mW forthe sameirradiationtime of 30s.

the thermal process. The grain size dependenceon laser power shows a growth of 12.5~m/W, as determinedby the slope and a threshold laser power of 124 mW (or a powerdensityof 98 * 5 W/cmz)is requiredto inducethe crystallizationin SbZS~films.The growthrate is determinedto be 0.03 pm/s from the dependenceof grain size on irradiationtime.Even thoughwe haveused filmsof variousthicknessesfor irradiationexperiment,we haveonlygiventhe resultsfor5300~ thickfilmin thispaper.However,thereis a slightvariation

FIG.4Developmentof grain in laser irradiated(power250 mW) amorphousSb45Jfilms for theirradiationtime of(a) 15s and(b) 60s.

912 F’.ARUNet al. vol. 32,No.7

t ( s)—15 30 45 60I I I

!

1‘/’,/A● 1

0130 150 170 190 210 230 250

P ( mW)—

FIG.5The dependenceof grainsizeon laserpowerandirradiationtimederivedtlom Figures3 and4, respectively.

in the threshold laser power for crystallizationwith tilm thickness. The thinner filmsrequired few mW of more power to inducecrystallization.This is because of the slightlylessertemperaturerise due to the decreasedabsorbanceof the film, as can be realizedfromeq. 1. We have not found any systematic dependence of threshold laser power forcrystallizationon film thickness.This may be due to the complex dependenceof criticaltemperatureof transformationon film thicknessfor low thicknesses.Thicker films showbetter contrast and more uniformgrain sizes. This can also be realized by eq. 1 as due todecreasing temperature along the depth of the film from its surface and the enhancedtemperaturerise at the surfacedueto increasedabsorbancein the film.

The overall results confirm the dominance of the thermal process in inducing thecrystallization.We have not observedany reversiblechanges,that is, from crystallinetoamorphousstate, despiteour effortsto irradiatethe crystallinespotsusing variouspowersand irradiation durations. Such changes have been observed in some materials whenirradiatedlongerat lowerpowerthan that requiredfor crystallization[4].Therefore,we feelthat the observed amorphous-to-crystallinetransformationis of irreversiblenature in thecase of SbJ3J films. The developmentof varyinggrain size (and thereby varying opticalcontrasts)with laserpowerand the thresholdpowerdensityfor crystallizationas low as 124mW and 98 * 5 W/cm*,respectively,indicateSbJ3Jfilmsm potentialopticalstoragemedia.Similarresultshavebeenreportedfor SbZSe@ns [10],a companioncompoundof Sb&

CONCLUSIONS

The crystallization in Sbi% as-grown amorphous films is mainly due to the thermalcomponentin the photo-thermalprocessof continuouswave laser irradiation.The threshold

Vol.32,No.7 ANTIMONYSULFIDEFILMS 913

laser power density required for crystallizationin SbZSjfiIms is estimated to be 98 + 5W/cmz,which is sufllcientlylow for opticalstorageapplications.The irreversiblenature ofamorphous-to-crystallinetransformationshowsthat these filmscan be used only for write-once-read-manytimes(WORM)kindof opticalstorageapplications.

ACKNOWLEDGMENT

We would Iiketo thankProf.A. Mansinghand Prof.G.K. Chadhafor their encouragement.The financial assistanceof UGC, India, in the form of SRF to P. Arun is also gratefullyacknowledged.

REFERENCES

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44,250 (1989).5. K. Schwartz, in The Physics of Optical Recording, Springer-Verlag,Berlin (1993) and the

referencetherein.6. P. Stradins,K. Skirts andJ. Teteris,J Non-Cryst.Solids114,79(1989).7. P. ArunandA.G.Vedeshwar,J. Appl.Phys.79,4029(1996).8. R.J.vonGutfeldandP. Chaudari,J. Appl.Phys.43,4688 (1972).9. L. CervinkaandA. Hmby,L Non-Ctyst.So[i&48,231 (1982).

10. K. KolevandM. Wautele6Appl.Phys.A 52,192(1991).