Surface Temperatures and Dissociation Loss during the Pulsed Laser Annealing of GaAs
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SURFACE TEMPERATURES AND DISSOCIATION LOSS DURING THE PULSED LASER ANNEALING OF GaAs.
JOHN T.A. POLLOCK AND ALEX ROSE. CSIRO, Division of Chemical Physics, Lucas Heights Research Laboratories, NSW,
2232,
Australia.
ABSTRACT From reported equilibrium partial and total dissociation pressure data for GaAs and melt times derived from reported time resolved reflectivity experiments, estimates have been made of the anticipated rate of As loss. Good agreement was found with experimentally determined As loss. A similar approach using experimentally determined Ga loss data allowed estimates of the maximum temperatures reached during pulsed laser annealing. These temperatures are considerably higher than suggested in thermal modelling studies. The boiling point of Ga gould be exceeded at incident laser energies >0.8 J cm-
INTRODUCTION Initial enthusiasm for pulsed laser annealing (PLA) based on the expectation that such methods would lead to reduced dissociation loss and allow uncapped processing, has been tempered by the knowledge that Ga and Consequently, the optimum conditions for successful As loss still occurs. annealing need to be much more closely defined than is the case for Si (1,2). Thermal melting models for GaAs similar to those developed for Si, have been hampered by the absence of physical data at the assumed PLA For example, surface temperatures calculated during PLA temperature range. which, together with melt times, determine dissociation loss, are strongly controlled by the value assigned to the absorption coefficient. The only value recorded for GaAs is at room temperature (3). Thermal modelling studies have used values in the range 2 x 10 to 4 x 105 cm(4,5,6). Aydinli et al. (7) have suggested a value of 5 x 105 cm- I at the onset of melting. Nevertheless, the surface temperatures of about 1700 K predicted by these studies are too low to account for the As and Ga dissociation loss measured by Rose et al. (8) and others (6,9). This is especially true for the Ga data. Because of these problems, and the likelihood that Ga and As loss is important in controlling electrical properties (2), we have looked in some detail at the kind of information that may be generated by the use of vapour pressure data available for the GaAs system (10,11).
ARSENIC AND GALLIUM LOSS MEASUREMENTS Estimates of Ga and As loss by Rutherford backscattering (RBS) analysis of the PLA surface have been reported by several groups (12,8). Other workers (8,9,13) have used RBS to analyse the dissociation loss collected on catcher plates located just above the crystal surface. However, owing to poor resolution, RBS is unsatisfactory for the detailed measurement of individual Ga and As loss. Rose et al. (8) found that resolution is not a problem with neutron activation analysis. Boerma et al. (6) used proton induced X-ray emission (PIXE) to determine dissociation loss collected on catcher plates; although they did not comment on sensitivity, reference to characteristic X-ray emission tables indicates that this technique should be better
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