Picosecond Laser Induced Melting: The Dielectric Function of Molten Silicon and Superheating in the Liquid Phase
- PDF / 309,576 Bytes
- 5 Pages / 420.48 x 639 pts Page_size
- 5 Downloads / 213 Views
PICOSECOND LASER INDUCED MELTING: THE DIELECTRIC FUNCTION OF MOLTEN SILICON AND SUPERHEATING IN THE LIQUID PHASE
P.M. FAUCHET* and K.D. LI**
*Princeton Laboratory for Ultrafast Spectroscopy (PLUS), Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 "**Present address: Department of Applied Physics, Stanford University, Stanford, CA 94305 ABSTRACT The complex dielectric function of molten silicon produced after picosecond illumination is found to be well described by a Drude model from 1.1 eV to 3.5 eV. Close to the melting temperature, we obtain wp - 2.50 1016rad/s and r = 212 10-1 8 s. Transiently, the liquid temperature can exceed the melting temperature or even the boiling temperature Tb. We observe this transient heating and model it with a relatively simple computer code which includes superheating of the liquid above Tb. These measurements are possible thanks to a novel pump and probe technique. Introduction The nonequilibrium electronic properties that exist before melting takes place in a picosecond laser annealing experiment are now well documented [1-3]. Interest has now shifted towards an understanding of the nonequilibrium phase transitions that occur during picosecond laser annealing. In particular, superheating during the solid-liquid phase transition has been investigated in metals [4], in silicon [5I and more recently in GaAs [6,7]. In Refs. 6 and 7 superheating in the solid phase was clearly demonstrated by comparing the velocity distribution of atoms ejected during nanosecond and picosecond irradiation. All-optical pump-and-probe experiments have inherently a better time-resolution than any other type of experiments; hence, it is desirable to study the phase transitions with alloptical techniques. Unfortunately, as shown by the results of Ref. 5, reflectivity measurements are relatively insensitive to the temperature of the solid or that of the liquid. In this report, we demonstrate that it is possible to obtain accurate measurements of the dielectric function of materials by appropriate choice of the probe angle of incidence in picosecond pump-and-probe reflectivity experiments. Using this technique, we first measure the real and imaginary parts of the dielectric function of molten Si at or close to the melting temperature Tm. We then examine the time evolution of the reflectivity just after melting and compare the data to the predictions of a computer code. Our results are a clear indication of a liquid layer with T > Tm; at the highest pump fluences, our results indicate that strong superheating at the liquid-vapor phase transition occurs. Experimental The source is an actively-passively modelocked Nd:YAG laser followed by a double-pass amplifier. It generates 30-60 ps pulses with a 10 Hz repetition rate. Frequency doubling and tripling is accomplished in KDP crystals. The pump pulse at 532 nm is near-normal incident on the sample. The attenuated probe pulse, at 1064 nm, 532 nm or 355 nm, is ppolarized and its reflectivity is measured for selected angles of incide
Data Loading...
