Two-Color Picosecond Measurements on Electron-Hole Plasmas Close to the Melting Phase Transition
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TWO-COLOR PICOSECOND MEASUREMENTS ON ELECTRON-HOLE PLASMAS CLOSE TO THE MELTING PHASE TRANSITION
P.M. FAUCHET AND A.E. SIEGMAN Edward L. Ginzton Laboratory, Stanford University, Stanford,
California 94305
USA
ABSTRACT We demonstrate a novel technique to study in detail the picosecond dynamics of semiconductors close to the melting phase transition. A variable-energy IR pulse (1.06 pm) is used to melt Si or GaAs samples via free carrier absorption (FCA) in the dense and hot electron-hole plasma (EHP) produced by a preceding visible (532 nm) pulse. By varying the delay between the pulses and their relative intensities, we are able to verify the Lietoila-Gibbons model for pulsed laser heating, and to measure in detail important parameters of the EHP over a wide range of experimental conditions.
INTRODUCTION The physics of highly excited semiconductors has recently been the subject of many studies, especially in connection with the problem of pulsed laser annealing. Careful experiments [1-3] have now been performed with the temporal resolution required to test the various models. These results indicate that the non-thermal models are probably not applicable, especially since the carrier relaxation time Tr is very short [4]. In addition, the thermal model of Lietoila and Gibbons [5] predicts accurately the melting threshold of c-Si irradiated with a 20 ps pulse at 532 rm. A detailed experimental study of highly excited semiconductors near melting threshold is desirable to confirm our faith in the thermal model and to provide additional information in a regime of excitation never achieved before. Such studies have so far not been possible because of the very small effect of a dense electron-hole plasma, produced by a strong visible pulse, on the reflectivity of a probe beam (at 1.06 pm), up to 3 densities -1021 cm- . In this paper, we present some results obtained using a novel technique that has allowed us to study in more detail the plasma close to but below the melting phase transition in Si and GaAs.
PRINCIPLE OF THE TECHNIQUE We have already described the principle of the method elsewhere [6]. A visible picosecond pulse (at 532 nm) with intensity below melting threshold "prepares" the sample, i.e., creates a dense electron-hole plasma at the surface, while still only slightly increasing the lattice temperature. The intensity of an infrared picosecond pulse (at 1.06 pm) which is delayed with respect to the visible pulse by variable increments up to plus or minus several nanoseconds, is then adjusted to just melt the surface. The absorption of 1.06 Pm radiation in silicon occurs predominantly via free-carrier absorption (FCA), and therefore measurements of melting threshold using 1.06 pm pulses alone are very sensitive to surface preparation and doping level. In our experiment, however, the initial freecarrier concentration is set by the 532 nm pulse, so that we can control the FCA for the IR pulse. As the IR pulse delay is increased from 0 to Mat. Res.soc. symp. Proc. Vol. 23 (1984) @Elsevier Science Publishing
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