The Influence of Layer Structure on the Synthesis of AlSb by Laser Irradiation of Al/Sb Thin Film Couples
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THE INFLUENCE OF LAYER STRUCTURE ON THE SYNTHESIS OF AlSb BY LASER IRRADIATION OF Al/Sb THIN FILM COUPLES
LUC BAUFAY, ANDRE PIGEOLET, AND ROD ANDREW Universit6 de l'Etat, 23 Avenue Maistriau, 7000 Mons,
BELGIUM
ABSTRACT We have previously described the formation of thin AlSb films by laser irradiation of sandwich-like structures of the components, and have shown that under quasi-adiabatic conditions, the transformation can be triggered by furnishing sufficient laser energy to just melt a part of the components; subsequent progressive liberation of the heat of formation of the compound drives the reaction to completion. In this paper, we clarify some details of the reaction and we investigate the influence of initial layer structure,thermal history, and total film thickness on the required laser threshold energy
INTRODUCTION We have previously shown that exothermic reactions of the type Cd+Te= CdTe, AI+Sb=AlSb, may, under certain circumstances, be initiated by laser irradiation and driven to completion by the liberation of the heat of formation of the product. For this to occur, the reaction environment must be at least quasi-adiabatic, a condition satisfied for thin films on poorly conducting substrates. Here the thermal capacity of the film is negligible and hence the thermal behaviour is dominated by the substrate whose thermal relaxation time is long compared to the laser pulse duration or spot dwell time. In a model [1) derived for such laser-induced synthesis of compound semiconductor films, we show that the reaction will be abruptly triggered when a sufficient part of the constituents is melted; the rapid liquid mixing of this part providing sufficient heat evolution to continue the process. An implication of this synthesis model is that,due to the internal heat liberation during the compound formation,local temperature gradients will be present in the film and, since we know from experiments using inhomogeneous or optically structured laser beams [2] that temperature gradients can promote grain growth in preferred orientations, we believe these gradients to be at least partly responsible for the relatively large grain 1 6 sizes and low impurity content (,u range; 410 /cm3 ) that we have observed in our films, particularly when compared to those resulting from laser annealing of already formed compound semiconductor films. In this paper, we describe some experiments intented to clarify details of the formation process and to indicate the optimum film structure that should be aimed for prior to laser treatment.
INVESTIGATION OF THE REACTION THRESHOLD BY RBS DEPTH PROFILING In our previous work, we have established laser-induced transformation energy thresholds, etc. simply by observing the optical transparency of the completely transformed film. At sub-threshold energies, we have therefore no information as to what degree, if any, the reaction has proceeded at the internal interfaces of the film. Since this is an important test of our model, we have now performed a series of experiments using depth profil
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