Section 2. Growth of germanium and silicon–germanium alloys Pulsed laser crystallization and structuring of a-Ge on GaAs Paulo V. Santos a, * , A.R. Zanatta b , F. Dondeo c , A. Trampert a , U. Jahn a , D. Comedi c , M.A.A. Pudenzi c , I. Chambouleyron c a Paul-Drude-Institut f€ ur Festk€ orperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany b Instituto de F  ısica de S~ ao Carlos, USP, S~ ao Carlos 13560-250, S.P., Brazil c Instituto de F  ısica Gleb Wataghin, UNICAMP, Campinas 13083-970, S.P., Brazil Abstract We have investigated the crystallization and structuring of amorphous Ge (a-Ge) films deposited on crystalline GaAs ð100Þ substrates by nanosecond laser pulses. Epitaxial Ge films on GaAs are obtained for laser fluencies that completely melt the Ge film, but not the substrate. Higher fluencies lead to a partial melting of the substrate and to the formation of a ðGaAsÞ 1x Ge 2x epitaxial alloy at the interface with the substrate. We demonstrate the fabrication of line gratings of crystalline Ge on GaAs produced by laser interference structuring. The gratings display an unusual surface undulation with faceted surfaces, which is ascribed to a lateral solidification process. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.10.)h; 81.30.)t; 68.55.Jk 1. Introduction The ability to selectively melt an amorphous layer through the absorption of a short laser pulse has made pulsed laser crystallization (LC) of amorphous (a) films a standard technique for the fabrication of large area polycrystalline films on low cost substrates. LC has also been applied to crystallize (or re-crystallize) an amorphous (or amorphized) film on a crystalline (c) substrate. If the substrate is lattice matched to the crystalline phaseofthefilm,itmayprovideatemplateforthe solidification of the molten material in the form of an epitaxial layer. LC becomes, in this case, a liquid phase epitaxy (LPE) process operating on a time scale of a few ns. In this contribution, we investigate the hetero- epitaxial LC of an amorphous germanium (a-Ge) film on ð100Þ-oriented c-GaAs substrates. The a-Ge/GaAs system is ideal for LC studies since: (i) crystalline Ge and GaAs are almost ideally mat- ched with respect to their lattice constants (lattice mismatch less than 0.1%) and to the thermal ex- pansion coefficients and (ii) both materials are thermodynamically immiscible [1]. Material inter- mixing should, therefore, only occur under non- equilibrium solidification conditions. Finally, the lattice matching condition can be changed in a controlled way by the incorporation of silicon into the a-Ge network to form an a-SiGe alloy, as Journal of Non-Crystalline Solids 299–302 (2002) 137–142 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +49-30 2037 7221; fax: +49-30 2037 7515. E-mail address: santos@pdi-berlin.de (P.V. Santos). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(01)00998-X