Photoelectric Emission Studies from Crystalline Silicon at 266 NM
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PHOTOELECTRIC EMISSION STUDIES FROM CRYSTALLINE SILICON AT 266 NM
A. M. MALVEZZI, J. M. LIU,* AND N. BLOEMBERGEN Division of Applied Sciences, Harvard University, Cambridge, MA 02138; *Present address: Department of Electrical and Computer Engineering, Bell Hall, SUNY at Buffalo, Amherst, NY 14260
ABSTRACT Three different photoelectric regimes are observed in the interaction of 15 ps, 266 nm laser pulses with crystalline A superposition of silicon samples versus light fluence. linear and quadratic photoionization is followed by a space 4 charge limited regime up to the critical fluence F tn W for the surface amorphization where highly nonlinear ion emission is observed. Ion and electron emissions become equal in magnitude at a fluence - 2F~th The absence of observable thermionic effects indicates that thermal equilibrium of the electronhole plasma and the lattice is reached during the laser pulse duration.
INTRODUCTION The central question in the investigation of pulsed laser annealing of semiconductors in the picosecond range is related to the time scale of the energy transfer between the hot electron-hole plasma created by the laser pulse and the crystal lattice. In recent times much experimental effort has been devoted to the study of this energy transfer. This goal has been mostly pursued through optical diagnostic methods, either by measuring the optical constants of the semiconducotrs with pump and probe techniques [1] or by direct derivation of the lattice temperature from Stokes and anti-Stokes Raman scattering [2]. Measurements of the electron and ion emission from the semiconductor surface during the interaction with picosecond laser pulses offers an alternative approach to the problem. Here, one probes the material only in its outermost layers, and therefore the data are insensitive to the gradients in Thermionic emission the profiles of plasma density or lattice temperature. Conversely, the emission is extremely sensitive to the electron temperature. of positive ions is an indicator of a hot lattice. In this paper detailed measurements of the charges emitted by crystalline silicon samples irradiated with 266 nm, 15 ps laser pulses are reported. This work extends the measurements of Liu et al. (3] to laser fluences about four orders of magnitude below the critical value for a surface phase transOne of the main results of this analysis is the absence of obformation. servable thermionic effects throughout the whole range of laser fluence. Thus, an upper limit to the average electron temperature of m 3000 K within the laser pulse can be deduced. EXPERIMENTAL METHOD The source of excitation of the silicon samples is a 15 ps, 266 nm laser pulse obtained by frequency quadrupling the output of a Nd:YAG 30 ps laser The radiation is focused by quartz optics into a vacuum chamber in pulse. The angle of incidence is 480, and the which the silicon sample is located. reflected portion of the beam escapes from the chamber through a second Nat. Bfe.Soc. Symp. Proc. Vol. 23 (1984) @Elsevier science Publishing Co., In
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