Thermodynamics and Kinetics of Crystallization of Amorphous Si and Ge Produced by Ion Implantation
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THERMODYNAMICS AND KINETICS OF CRYSTALLIZATION OF AMORPHOUS Si AND Ge PRODUCED BY ION IMPLANTATION
E.P. DONOVAN,*+F. SPAEPEN,* D. TURNBULL,* J.M. POATE,** AND D.C. JACOBSON** *Division of Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138; **Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974
ABSTRACT Amorphous Si and Ge layers, produced by noble gas (Ar or Xe) implantation of single crystal substrates, have been crystallized in a differential scanning calorimeter (DSC). This technique allows determination of the growth velocity (which is proportional to the rate of heat evolution, AHac), and the total enthalpy of crystallization AHacAmorphous Ge was found to relax continuously to an amorphous state of lower free energy, with a total enthalpy of 1 before crystallization started. relaxation of 6.0 kJ.moleThe regrowth velocity on (100) substrates,measured to be 7 4.2x 101 exp (-2.17eV/kT)R/sec, is compared to other The value of AHac was found to be determinations. 11.66± 0.7 kJ.mole, in good agreement with AHac for For Si, AHac amorphous Ge produced by other methods. was determined to be 11.95± 0.7 kJ.mole without any The kinetics evidence of heat release due to relaxation. of crystallization measured by DSC are compared with those The effects of the determined by other techniques. implant profile on the regrowth velocity could also be observed directly in the DSC signal. The more accurate of value of AHac allowed a more precise determination 42 the melting temperature of amorphous Si: Tat= 1 0K.
INTRODUCTION Ion implantation offers some unique advantages in the study of the thermodynamics and kinetics of crystallization of amorphous elemental semiconductors: (i)
Since the impurity content can be controlled very precisely, the amorphous material and the crystalline-amorphous interface can be kept This results in an increased interface mobility over that very clean. in deposited samples, and rapid crystallization therefore occurs at lower temperatures [1,2].
(ii)
Since the amorphous-crystalline interface is planar, and remains so during regrowth, the kinetics of the regrowth process can be monitored H. This makes it possible to directly by the rate of heat evolution, identify impurity effects and to distinguish between the thermal effects of regrowth and structural relaxation.
Tat? Evidence is accumulating [3,4] that a-Si melts at a temperature, Tci, of crystalthat is considerably lower than the melting temperature, line Si. Given the higher chemical potential of the amorphous phase, this A quantitative check of Taz, however, requires more is to be expected. Our experiments [2] have led to a precise accurate thermodynamic data. determination of the heat of crystallization; additional measurements of the specific heat, Cp, of the amorphous phase would be desirable for a more Mat. Rs. Soc.Symp. Proc. Vol. 27 (1984) QGleevier science Publishing Co.,
Inc.
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accurate estimate of
Taz.
EXPERIMENTAL METHODS 0
Single crystal wafers of Si and Ge were implanted at 77 K with nobl
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