Appl. Phys. A54, 279-283 (1992) Applied s,,, Physics A "" Surfaces © Springer-Verlag 1992 Optical Contrast by Laser-Induced Real Time Optical Measurement of Fast Transformation Times J. Solis, C. Ortiz*, C.N. Afonso, and F. Catalina Instituto de Optica del CSIC, C/Serrano 121, E-28006 Madrid, Spain Received 14 June 1991/Accepted 6 November 1991 Phase Changes: Abstract. Real time optical measurements are used to analyse two different kinds of phase changes which generate optical contrast in (In43Sb57)87Ge13 thin films. Amorphous to crystalline and amor- phous to amorphous structural transformations are induced by pulsed laser irradiation in micron-sized regions. A two beam configuration is used to follow the evolution of the optical properties of the films in real time. It is shown that real time optical measurements provide a unique tool to analyse laser-induced fast structural transformations leading to optical contrast. Processes occurring via relax- ation, solid state crystallization or melting-solidification are clearly distinguished. From the analysis of the optical transients the minimum transformation times are directly determined. PACS: 42.30, 61.80B, 42.60H Different kinds of structural transformations like amorphous to crystalline [1-5], crystalline to crystalline [6] or amor- phous to amorphous [7] have been reported as responsible for optical contrast upon laser irradiation in thin film materi- als. The use of laser-induced phase changes as the recording mechanism either in write once or in erasable optical stor- age systems requires accurate measurements of the minimum transformation times since the maximum allowed data rate is inversely proportional to this parameter [3]. Real time optical measurements have been successfully applied to study laser- induced transformations in micron-sized regions including rapid crystallization processes in isothermal conditions [8], metastable phase formation in rapid solidification processes [9] or liquid phase nucleation phenomena [10]. Their appli- cation to study fast structural processes leading to optical contrast may provide a very accurate method to analyse the transformation kinetics and to determine minimum transfor- mation times. Different works have shown the potential application of III-V thin films for optical storage applications. InSb [4, 6-8], GaSb [4] and InSbGe [71 alloys have received spe- cial attention. It has been shown [7] that laser irradiation of In43Sb57 and (In43Sb57)87Ge13 amorphous alloys may in- duce the formation of two states with reflectivities different than the as-deposited values: Irradiation with nanosecond pulses causes the formation of a high-reflectivity amorphous ~' On Sabbatical leave from IBM Almaden Research Center, San Jose, CA, USA phase while microsecond pulses cause the formation of a low-reflectivity crystalline state. An estimation of 2-5 gs was given for the minimum crystallization time of the Ge com- pound. In this work we present a dynamic study of the laser- induced transformations in (In43Sbsv)gvGet3 thin films by means of real time optical measurements. Two different pulse lengths are used in order to produce the low-reflectivity crystalline state and the high-reflectivity amorphous phase. Under both pulse length conditions systematic series of irra- diations with increasing power are performed so as to eluci- date the differences in the material response when the final states are reached via solid state and melting processes. The obtained optical transients illustrate the mechanisms of the amorphous to amorphous and the amorphous to crys- talline structural transitions. Their analysis allows us to es- tablish that the high-reflectivity phase is caused by structural relaxation. The minimum time required for such processes, the minimum time necessary to induce an optically apprecia- ble degree of crystallization or the power threshold to induce a melt-after-crystallization process are also determined. 1 Experimental The samples are 520A thick amorphous (In43Sb57)87Ge13 films grown on carbon-coated mica substrates. They were prepared by coevaporation of In, Sb, and Ge from separate Knudsen cells as further explained in [7].