Luminescence Spectroscopy on Highly Excited Si - Experimental Determination of the Plasma Drift Velocity
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LUMINESCENCE SPECTROSCOPY ON HIGHLY EXCITED SI - EXPERIMENTAL DETERMINATION OF THE PLASMA DRIFT VELOCITY
B. LAURICH, A. FORCHEL Physikalisches Institut, Teil 4, Universit2t Stuttgart Pfaffenwaldring 57, D-7000 Stuttgart 80, Germany
ABSTRACT The emission spectra of the electron-hole plasma created at the surface of a Si sample by intense laser pulses (65 ns or 400 ps) are investigated. It is shown that the plasma drift is necessary for a consistent interpretation of the luminescence line shape and the energetic position of the line. 19 3 We determine average densities up to 2.10 cmfor 6 300K and typical drift velocities up to 4-10 cm/s.
INTRODUCTION In high excitation experiments such as pulsed laser annealing(PLA), electron-hole-plasmas of high density are created at the surface of a semiconductor by strong laser pulses. The influence of this plasma on the melting process has been considered in a "Nonthermal Model". A significant reduction of the melting temperature requires plasma 2 1 densities of the order of 10 cm-3[1]. In order to reach these high densities, one has to assume that diffusion and (Auger)recombination are inhibited[2]. In contrast to the assumption of a plasma induced phase transition the melting is usually interpreted as a simple thermal process[3]. In this "Thermal Model" one assumes an immediate transfer of the total excitation energy to the lattice, and so in general does not take into account the energy transport of the plasma. A lot of experimental work has been done to determine the plasma properties and the lattice temperature [4-8] and to decide whether an influence of the plasma on the melting process is possible. Host of the experiments are evaluated by use of classical models. So in the case of studies of the surface carrier density by reflectivity[5] which led to a discussion about the importance of the Augereffect and the value of the effective masses[9]. Further methods to determine the plasma density are transmission and luminescence. In contrast to reflectivity measurements which are sensitive to the surface density, luminescence experiments yield densities averaged over the volume occupied by the plasma. Although the carrier densities obtained by these different methods cannot be compared directly emission spectroscopy in Si is suitable to study many-body and non-equilibrium transport properties of the plasma for the following reasons: - High excitation experiments at liquid helium temperature show the formation of electron-hole droplets (EHD). The spectral shape and the band renormalization by many-body effects are in quantitative agreement with theory[10]. - Above the critical point for the droplet condensation (23K) a nonequlibrium electron-hole plasma (EHP) is observed[Il]. The non-equilibrium EHP system can be compared to the quasi-equilibrium fermi system (EHD) using the same experimental procedure and the same sample.
mat. RS.
soc. Symp. Proc. Vol. 23 (1984) DElsevier Science Publishing Co., Inc.
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The subject of the present study is to apply the methods of measur
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