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Growth mechanisms in laser crystallization and laser interference crystallization

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Growth mechanisms in laser crystallization and laser interference crystallization
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  Ž . Journal of Non-Crystalline Solids 227–230 1998 921–924 Growth mechanisms in laser crystallization and laser interferencecrystallization G. Aichmayr  a , D. Toet  a, ) , M. Mulato  a,b , P.V. Santos  a , A. Spangenberg  a ,R.B. Bergmann  a a  Max-Planck-Institut fur Festkorperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany ¨ ¨ b  Instituto de Fısica ‘Gleb Wataghin’, Uni Õ ersidade Estadual de Campinas, 13083-970 Campinas, Brazil ´ Abstract The processes involved in the pulsed laser crystallization of amorphous silicon thin films were studied using transientreflection measurements. A model of the melting and solidification induced by the laser exposure, based on a one-dimen-sional calculation of the heat flow, was used to simulate the time-dependent reflectivity, yielding agreement with the Ž . experiments. Two laser beams interfering on the sample surface lead to the growth of long grains up to 1.5  m m , with awell-defined orientation. We conclude that this lateral growth results from explosive crystallization combined with liquidphase growth.  q 1998 Elsevier Science B.V. All rights reserved. Keywords:  Crystallization; Transient effects; Computer simulations; Atomic force microscopy 1. Introduction Ž . Laser crystallization LC of amorphous silicon Ž .  w x a-Si 1 is a promising technique for the fabrication Ž . of polycrystalline silicon poly-Si -based thin filmtransistors used in active matrix liquid crystal dis- w x plays 2 . The interest in LC is mainly due to itscost-effectiveness, since it allows the use of inexpen- w x sive glass substrates. Kim and Im 3 have empha-sized the importance of decreasing the number of grain boundaries and controlling their locations forthe achievement of high performance, uniform de-vices. Grain sizes must therefore be larger than thefilm thickness, implying that lateral grain growth Ž . parallel to the substrate should play an important ) Corresponding author. Now at Lawrence Livermore NationalLaboratory, Livermore, USA. Tel.:  q 1 925 422 3112; fax:  q 1925 422 7309; e-mail: toet1@llnl.gov. role in the fabrication of the poly-Si films. In thiswork, we first study the kinetics of homogeneous LC Ž . using transient reflection TR measurements. Theresults of these experiments are analyzed using simu-lations based on a finite elements calculation of the w x heat flow 4 . Next, we show, using atomic force Ž . microscopy AFM , that laser interference crystal- Ž . lization LIC of a-Si, a procedure which combines w x LC with holographic techniques 5 , can lead to asubstantial lateral growth and oriented grain bound-aries. The insights provided by the simulations areused to interpret this result. 2. Experimental details The crystallization experiments were performedon 300-nm thick hydrogen-free a-Si films, grown on 0022-3093 r 98 r $19.00 q  1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0022-3093 98 00213-0  ( )G. Aichmayr et al. r  Journal of Non-Crystalline Solids 227–230 1998 921–924 922 glass by low-pressure chemical vapor deposition at Ž 450 8 C. A frequency-doubled Nd:Yag laser  l s 532 . nm, pulse width: 8 ns, beam diameter: 8 mm wasused to irradiate the samples. The LIC was carriedout by splitting the laser output into two equallyintense beams and bringing these beams to interfer-ence on the film surface. This pattern induces crys-tallization in the material around the interferencemaxima. The period  p  of the resulting poly-Si grat-ing depends on the angle of incidence of the beams;here, we used  p f 5.5  m m. The TR measurementswere performed by illuminating the center of thearea irradiated by the Nd:Yag laser with a diode Ž . laser  l s 635 nm, beam diameter: 1 mm and de- Ž tecting the reflected signal by a Si photodiode re- . sponse time  t  F 1 ns connected to a digital oscillo-scope. 3. Investigations of the grain growth in homoge - neously irradiated a-Si films 3.1. Transient reflecti Õ ity measurements Fig. 1 shows TR profiles measured at differentpulse energies. The most striking feature in thisfigure is the increase in reflectivity induced by thelaser pulse, followed by a more gradual decrease.This peak is caused by the melting and resolidifica-tion of the surface of the film due to the absorptionof the Nd:Yag pulse, liquid Si being characterized bya metallic-like reflectivity. The width,  G  , of thepeak, i.e., the melt duration, increases as a functionof the energy density,  E   : at 90 mJ r cm 2 ,  G  s 14 " 2 c ns, while at 370 mJ r cm 2 ,  G  s 23 " 2 ns. The melt-ing is accompanied by a permanent change in reflec- 2 Ž tivity for  E   G 100 mJ r cm the crystallization c . threshold energy  E   , implying that the material c,th has crystallized. This change was confirmed by Ra-man spectroscopy, since for  E   G 100 mJ r cm 2 , the c samples show a band at around 520 cm y 1 , character- Ž istic for light scattering by optical phonons in mi- .  1 2 cro- crystalline Si. For  E   s 90 mJ r cm , the re- c flectivities before and after laser exposure are identi-cal, but the peak in the TR profile can still be Ž . observed see Fig. 1a . This persistence indicates that Ž . Fig. 1. TR profiles circles measured during the irradiation of a Ž . 300-nm thick a-Si film with a Nd:Yag laser  l s 532 nm atdifferent pulse energies. The size of the circles corresponds to theerror margin of the TR experiments. The full line shows calcula-tions of the TR based on simulations of the melting and growthcaused by the laser pulse. Note that the curves for  E   s 370 c mJ r cm 2 have a different time scale. although the a-Si melts at this energy density, theduration of the liquid phase is too short for nucle-ation to occur. 3.2. Simulation of the laser crystallization process To further analyze the results of the TR experi-ments and obtain information about the growthmechanisms involved in LC, we performed simula-tions of the time-dependent reflectivity of the irradi-ated films. Our simulations are based on a model w x developed by Wood and Geist 4 , which uses afinite difference calculation of the heat transport inthe direction perpendicular to the film. In this model,the evolution of the state of any finite difference Ž . layer a subdivision of the film used in the programis governed by transitions between points on thephase diagram of Si, as well as by the state of theneighboring layers. This approach enables the simu-lation of the nonequilibrium phase transitions andnucleation events which occur in the undercooled Ž  w x liquid Si generated during short-pulsed LC see 4 . for more details .  ( )G. Aichmayr et al. r  Journal of Non-Crystalline Solids 227–230 1998 921–924  923 Fig. 2a and b show the simulated evolution of thestate of the finite difference layers for two differentenergy densities. The parameters of the simulatedsystems correspond to those of the investigated sam- 2 Ž . ples. At  E   s 120 mJ r cm Fig. 2a , only the top c portion of the film melts. As the melt cools down, at Ž . t  s 24 ns depth: 40 nm , a nucleation event occursat the liquid-amorphous interface, where the under-cooling is strongest, initiating an explosive crystal- Ž .  w x lization EC front 6 which moves with a velocityof 5 m r s towards the substrate. At  E   s 370 mJ r cm 2c Ž . Fig. 2b , the melt is deeper and its duration longer,confirming the results obtained by TR. An EC front Ž . appears at  t  s 30 ns depth: 165 nm . The fine-grained material srcinating from the EC serves as aseed for the crystallization of the molten layer aboveit, resulting in a layer containing large grains. Thegrowth fronts appearing at depths of 80 nm and 20nm are due to successive nucleation events occurringin adjacent layers; they propagate with a velocity of about 0.5 m r s towards the substrate. The simula-tions suggest that the average grain size and the Fig. 2. Simulation of the evolution of the state of the sublayers Ž . composing a 300-nm thick a-Si film grown on glass exposed to Ž .  2 Ž .  2 pulse energies of a 120 mJ r cm and b 370 mJ r cm . Thedifferent states of the Si are represented by different greyscales, asindicated in the legend.Fig. 3. AFM micrograph of lines fabricated by laser interferencecrystallization with an energy density of 380 mJ r cm 2 . Notice thelarge grains in region A, the small grains at the outer edge of the Ž . Ž line region B and the protrusions at the center of the line region . C . fraction of crystalline material increase as a functionof   E   ; this is confirmed by Raman spectroscopy.  1c Finally, we used the results of the heat-flow cal-culations to simulate the TR profiles. This was doneby converting the temperature and state depth pro-files generated by these calculations at each timestep into dielectric constants, which were in turnused to calculate the reflectivity using a transfermatrix procedure. Fig. 1 shows that the agreementbetween experiments and simulations is good, indi-cating that our model gives a realistic picture of thegrowth mechanisms involved in LC. The peak at 35ns in the TR profile for  E   s 120 mJ r cm 2 is due to c constructive interference between the reflection fromthe surface and that from the interface between thecrystallized material and the thin molten layer gener-ated during EC. The broad oscillation between 40and 240 ns in the profile for  E   s 370 mJ r cm 2 is c related to the upper front propagating by successivenucleation. The positions of these structures are wellreproduced by the simulations. 4. Lateral grain growth induced by laser interfer - ence crystallization In the case of LIC, the spatial intensity distribu-tion on the sample surface is sinusoidal, and hence 1 G. Aichmayr, D. Toet, M. Mulato, P.V. Santos, unpublished.  ( )G. Aichmayr et al. r  Journal of Non-Crystalline Solids 227–230 1998 921–924 924 heat flow and grain growth in the plane of the film isexpected to play a more important role than inhomogeneous LC. To study the crystallization mech-anisms involved in LIC, we carried out AFM experi- Ž . ments. These measurements show see Fig. 3 that Ž poly-Si lines fabricated with high-energy LIC  E   s c2 . Ž 380 mJ r cm consist of large grains region A in . Fig. 3, lengths up to 1.5  m m , oriented in the direc-tion perpendicular to the lines. Small grains can be Ž . resolved at the edge of the lines region B , while thecenter shows protrusions reaching heights of 200 nm Ž . region C . The spatial dependence of the grain sizeand the amorphous nature of the material betweenthe lines was confirmed by micro-Raman spec- w x troscopy 7 . 5. Discussion The growth model described in Section 4 can beused to understand the above result. Notice first thatthe energy densities in the region around the centerof the lines are, under the conditions given above,much larger than 500 mJ r cm 2 , the  E   for which, c according to the simulations, the a-Si film meltsthroughout its entire thickness  1 . In this region,which has a width of about 2.5  m m, the film meltscompletely. At larger distances from the center, theenergy density becomes insufficient to melt the wholefilm, and the undercooling of the liquid Si increases.Therefore, nucleation first occurs at the outer edgesof the line, at the point where  E   equals  E   . This c c,th condition triggers an EC front, explaining the pres- Ž ence of the small grains at the edges region B in . Fig. 3 . The liquid Si can solidify by growing off these grains. Nucleation in the molten material nearthe interference maxima is suppressed due to thesmaller cooling rate in that region. As a result,crystallites can grow laterally until they reach thecenter of the line, where they impinge with grainsgrowing in the opposite direction. The protrusions in Ž . region C Fig. 3 are probably due to the volumeexpansion accompanying the quenching of excess w x liquid Si, pushed along by the growth fronts 8 .The growth pattern induced by LIC is similar tothe one obtained by homogeneous exposure of a-Si w x films covered with a patterned SiO layer 8 , illus- 2 trating the role played by spatially selective totalmelting of the a-Si in both techniques. LIC has theadvantage of not requiring any lithography, and itssinusoidal intensity pattern ensures that the lateralgrowth can proceed up to the center of the linewhenever the whole a-Si film melts. 6. Conclusions Poly-Si lines fabricated by high-energy LIC of thin a-Si films contain large grains with preferen-tially oriented grain boundaries. Using a model srci-nally developed for the simulation of the melting andsolidification caused by homogeneous LC, weshowed that this lateral growth results from a combi-nation of explosive and liquid phase crystallization.Simulations of the time-dependent reflectivity of ahomogeneously irradiated a-Si film based on thismodel are in good agreement with experimental re-sults, indicating that it gives a realistic description of the growth processes involved in LC. Acknowledgements The authors thank G.A. Geist and R.F. Wood forproviding their Laser8 program, and A. Goebel for acareful reading of the manuscript. This work isfunded by the Bundesministerium fur Bildung, Wis- ¨ senschaft, Forschung und Technologie under projectno. 0329617. References w x 1 T. Sameshima, S. Usui, Mater. Res. Soc. Symp. Proc. 71 Ž . 1986 435. w x  Ž . 2 R.S. Sposili, J.S. Im, Appl. Phys. Lett. 69 1996 2864. w x  Ž . 3 H.J. Kim, J.S. Im, Appl. Phys. Lett. 68 1995 1513. w x  Ž . 4 R.F. Wood, G.A. Geist, Phys. Rev. B. 34 1986 2606. w x 5 M. Heintze, P.V. Santos, C.E. Nebel, M. Stutzmann, Appl. Ž . Phys. Lett. 64 1994 3148. w x 6 M.O. Thomson, G.J. Galvin, J.W. Mayer, P.S. Peercy, J.M.Poate, D.C. Jacobson, A.G. Cullis, N.G. Chew, Phys. Rev. Ž . Lett. 52 1984 2360. w x 7 D. Toet, G. Aichmayr, M. Mulato, P.V. Santos, A. Spangen-berg, R.B. Bergmann, Mater. Res. Soc. Symp. Proc. 467 Ž . 1997 337. w x  Ž . 8 H.J. Kim, J.S. Im, Mater. Res. Soc. Symp. Proc. 397 1996401.
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