Using the optical phonons as an internal probe, the change in the dielectri
c permittivity has been analysed by Raman scattering in GaAs grown by MBE a
t low temperature (LT-GaAs). The screening effect is discussed in terms of
a solid solution whereas the strain one is treated in the perfect adhesion
hypothesis. Experimentally, the strain and the screening effects have been
discriminated by using various scattering geometries. As the LT-layers with
large non-stoichiometry have a poor thermal conductivity, it is shown that
local heating considerably affects the experimental data. This can be avoi
ded by surface convective exchange then leading to perfect agreement betwee
n the recorded phonon frequency shift and the expected ones. As a consequen
ce, this sustains a new method for a quantitative determination of the As e
xcess in LT-layers.