Modelling of low power cw laser beam heating effects on a GaAs substrate

Abstract

A scanning laser beam is a common method used to characterise the optical response of GaAs devices. Laser heating of the substrate, however, can alter the local temperature and hence spuriously shift the values of the electrical parameters of interest. In order to assess the magnitude of this problem, we have solved the steady-state heat equation, with the aid of Kirchhoff's transformation. We show for practical dimensions, that correct temperature prediction does not depend on the lateral boundary conditions. We find that the variable that is most tightly coupled to any temperature increase is the power of the laser beam. Usual approximations for the power dissipation density, in the substrate, were found inadequate. A more complete model that considers power dissipation as an exponential function of substrate depth was found to be necessary. We conclude that for low power applications, i.e. using lasers less than 1 mW, heating effects can be considered negligible. For higher powers our results offer worst-case predictions of the local substrate temperature rise.