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Hess, and Sigmon,

eds.

Laser and Electron-Beam Solid Interactions and Materials Processing3

39

PHASE TRANSITIONS IN AMORPHOUS SI PRODUCED BY PULSED LASER OR ELECTRON IRRADIATION

P. BAERI*, G. FOTI* AND J. M. POATE Bell Laboratories, Murray Hill, N.J. 07974 A. G. CULLIS Royal Signals and Radar Establishment Malvern, Worcs WR14 3PS England

ABSTRACT Amorphous, implanted, Si layers have been melted by pulsed electron irradiation. Implanted As has been used as a marker for determining melt duration. Systematic differences between As diffusion in initially amorphous or crystalline Si are interpreted in terms of different enthalpies of melting between amorphous (1220 J/g) and crystalline (1790 J/g) Si. The amorphous Si layers melt and crystallize at significantly lower electron energies than those required to melt crystalline Si, indicating that amorphous Si melts at 1170K compared to 1685 K for crystalline Si. We have used these thermodynamic parameters to successfully predict some of the phenomena associated with the laser induced melting and crystallization of amorphous Si.

INTRODUCTION At the first meeting of this series Bagley and Chen [11 and Spaepen and Turnbull [21 made some very interesting predictions about the thermodynamic properties of amorphous Si and Ge. They estimated the melting temperatures and enthalpies of melting of the amorphous semiconductors to be approximately 25% lower than the crystalline values. The advent of nsec pulsed heating techniques such as laser or electron beam irradiation, has made it possible to investigate this intriguing melting and recrystallization behavior. Conventional furnace annealing does not permit examination of these predicted phenomena as recrystallization of the amorphous phase will easily occur in the solid phase at elevated temperatures. For example, at a temperature of approximately 1000K, epitaxial recrystallization [1] of an amorphous Si layer 1000A in thickness can occur in a time of the order 10-1 sec. The amorphous layers have, therefore, to be raised to these elevated temperatures in very short times to avoid crystallization in the solid phase. ELECTRON IRRADIATION We have used pulses of electron [3] to heat the surface layers because the coupling between the incident beam and irradiated samples is independent of the physical state (amorphous, crystal or liquid) of the semiconductor. Our experimental configuration of an electron beam of average energy 10 KeV, incident at a glancing incidence to the Si surface ensures that nearly all the energy is deposited in the outermost I u of material. Heat flow calculations show that the temperature gradient over the first 2000A during the irradiation is less than 50K. This thermal behavior should be contrasted with laser irradiation [41 where gradients of 500 K can be generated over 2000A due to the enhancement of the absorption coefficient in the molten silicon. Moreover, unlike electron irradiation, the laser coupling changes strongly with the state of the semiconductor. * Permanent address: Istituto di Struttura