Interference laser crystallization of microcrystalline silicon using asymmetric beam intensities B. Rezek a, * , C.E. Nebel b,1 , M. Stutzmann b a Institute of Physics AS CR, Cukrovarnick a 10, 162 53 Prague, Czech Republic b Walter Schottky Institut, Technische Universit atM unchen, Am Coulombwall, 85748 Garching, Germany Abstract Periodic interference patterns formed by two laser beams of dierent intensities are used to crystallize ®lms of amorphous (a-Si) and crystalline silicon with nanometer sized crystallites (nc-Si). This asymmetric interference is ap- plied to generate variable temperature pro®les to optimize super lateral growth (SLG). The structural properties of the laser crystallized silicon layers have been determined by Secco etching and atomic force microscopy. The results show that, independently of the applied intensity pattern, 1.3±1.5 lm long and ca 0.4 lm wide grains can be grown by SLG. This growth lasts for about 100 ns, it is limited by spontaneous nucleation in the center of the liquid and is independent of the applied intensity pro®le. The use of a-Si or nc-Si as initial material makes no detectable dierence despite the fact that the total laser intensity has to be larger for nc-Si. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction To achieve large grain silicon by pulsed laser crystallization a very small energetic window has to be found, where super lateral growth (SLG) dominates the solidi®cation process [1,2]. Tech- nologically, this process is too dicult to be ap- plied in production lines so that up to now small grain silicon is used for thin ®lm electronic appli- cations. Seeded growth, which is required for large grain material, can be achieved by thermal gradi- ents in liquid silicon. By use of pulsed interference laser crystallization [3], where periodic sinusoidal variations in intensity are applied to melt and crystallize silicon, thermal gradients are inherently present [4]. The seeds are generated in the transi- tion regions from undercooled liquid to solid sili- con which are close to the intensity minima of the interference pattern. Here the nucleation rate is larger and therefore seeds are generated nearly instantaneously. From these seeds of SLG starts to dominate the solidi®cation process proceeding to- wards the hot center of the liquid. In this paper we discuss the application of laser interference crystallization where the interference pro®le is generated by two laser beams of dierent intensities (`asymmetric interference'). We will show that these experiments can be applied to vary the one-dimensional intensity pro®les and there- fore the thermal pro®les. Depending on the in- tensities, I 1 , I 2 , of the two laser beams, the spatial intensity modulation is given by I x I 1 I 2 2I 1 I 2 1=2 cos 2px=p; 1 p is the period given by Journal of Non-Crystalline Solids 266±269 (2000) 650±653 www.elsevier.com/locate/jnoncrysol * Corresponding author. Fax.: +420-2 312 3184. E-mail addresses: rezek@fzu.cz (B. Rezek), ne- bel@wsi.tum.de (C.E. Nebel). 1 Fax: +49-89 289 1737. 0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 7 6 6 - 8