Precision measurements of the effect of implanted boron on silicon solid phase epitaxial regrowth
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M.F. Becker and R.M. Walser"1 143 Engineering Science Building, Electrical and Computer Engineering Department and Materials Science and Engineering, University of Texas, Austin, Texas 78712 (Received 20 June 1987; accepted 30 October 1987) The epitaxial recrystallization rates of self-ion amorphitized layers in silicon wafers with (100) substrate orientation were measured by in situ, high precision, isothermal cw laser interferometry. With this one-sample technique the changes produced by implanted boron impurity concentrations (A^) in the activation energy Ea and preexponential Vo of solid phase epitaxy were measured for concentrations in the range 5 X10 18 cm" 3 < NB < 3 X 1020 cm" 3 and for temperatures from 450 to 550 °C. The differential changes in Ea produced were measured to within + 23 meV when systematic errors were eliminated. Changes in activation energy and entropy [Ea and log (Vo) ] were found to be linearly correlated for all concentrations. This observation is consistent with the idea that electronically active impurities alter regrowth velocities by reducing the critical temperature for disordering at some of the interfacial sites at which elementary reconstructive processes are driven by thermal fluctuations. For small NB, the critical temperature of the impurity-modified reconstruction is estimated at 1200 K, approximately 200 K below the melting temperature of amorphous silicon. The Ea decreased exponentially with NB to a concentration Nina, larger than the estimated equilibrium solubility limit, where there was an inflection point in the FvsiVB curve. The Ea increased for values of NB larger than JVinfl, showing that the differential increase in V for higher concentrations was due to a differential increase in the activation entropy. A change in the correlation between Ea and log(Fo) at./Vinfl indicated that larger NB produced an additional reduction of the critical temperature of the reconstruction. For small JVB, the data support a simple Fermi level shifting model for the "electronic effect" of impurities on SPE (solid phase epitaxial) regrowth.
I. INTRODUCTION The solid phase epitaxial (SPE) recrystallization of amorphous silicon layers formed by self-ion implantation is of great interest and has been widely studied by many techniques including Rutherford backscattering spectroscopy (RBS), 1 ' 2 electron microscopy,3 ellipsometry,4 and transient interferometry.5 Silicon SPE experiments have revealed that the process is thermally activated and that the substrate orientation,6 and the concentration and types of impurities present,2'7"13 have significant effects on the kinetics. Most of the available data suggest that the effect of the substrate orientation is primarily geometric 1415 and that the thermal activation of the SPE regrowth velocity V can be described by an Arrhenius equation: V = Vo exp( — Ea/kT). The activation energy Ea appears to be nearly independent of the substrate orientation 614 but is sensitive to impurities.2'7'8 Some impurities like boron and phosphorus are promoters,
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