Hot Electrons Relaxation
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HOT ELECTRONS
RELAXATION
A.M.MAZZONE CNR - Istituto LAMEL, Via Castagnoli 1 - 40126 Bologna (Italy)
ABSTRACT This work represents the first attempt of a direct simulation of the processes of electron-phonon interaction and lattice excitation which follow the formation of an excited electron gas such as, for instance, is the one produced by high-energy implants. The simulation method is of the type molecular dynamics and is based on a quantum mechanical representation. The results suggest a lattice disorder similar to the one of a thermal spike. INTRODUCTION Anomalous damage generation is observed under conditions of prevailing electronic stopping in semiconductor, insulating and metallic targets bombarded with ions of energy in the range from the MeV to the GeV. From a theoretical point of view the problem has interest of its own and for its analogy with the case of laser. Furthermore, owing to the applications of implants at very high energy for electronic devices fabrication and in superconductor materials, a broad experimental effort is being carried out to characterize such effects. For such excitations mostly qualitative models have been developed. As in the case of laser, the concepts of first and second order phase transition are in contrast. In the common model of coulomb explosion it is assumed that positively charged ions, formed by the dense ionization, strongly repel one another and are ejected into interstitial position. In the thermal model it is assumed that the electrons directly transfer energy to the lattice atoms, the size of the affected region depending on the mobility of the charge carriers in the solid (a detailed analysis of this subject can be found in [1]). The evident complexity of the problem arises from the need of evaluating either the excitation of the electrons or the mechanisms that couple the electronic processes to the subsequent atomic motions. In this work a molecular dynamics simulation method, based on quantum mechanics, is used to describe the dynamics of hot electrons and the nuclear excitation in a simplified crystalline structure. The results of the simulations indicate a lattice disorder fairly similar to a thermal excitation. THE THEORETICAL MODEL Three main problems of the simulation are worth being underlined. In the first place in studies of electron-phonon interaction a complex aspect is the adoption of the Born-Oppenheimer approximation. In a second place a realistic Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
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lattice requires a correct description of the bonding mode, which is a problem of its own. In the third place there are problems of computer memory and times as three-dimensional (in real space) wavefunctions must be recorded for the entire duration of the transient. Furthermore demanding requirements of accuracy arise from the three-dimensional representation and from the different time scales of electrons and nuclei (in fact, the response of an electron plasma has a sub-fs scale whereas the typical duration of lattice vibr
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