is sometimes also called
Reflectance Difference Spectroscopy (RDS). Since there is another
technique with the same RDS abbreviation, in our group we prefer to
use RAS.
Principle RAS setup with two polarizers and photoelastic modulator
The basic setup is quite
simple. A linear polarized light source is shown perpendicular on the
sample. The light is reflected from the sample. The change of
polarisation due the reflection is analysed by a photoelastic
modulator (PEM) and an anlyser prim. The only requirement for the
system is a transparent ambient and a window above the sample. This
is easily fulfilled for MOVPE as form MBE, and even for etching
cells.
Not every sample is
suitable for RAS. The sample needs exactly two distict eigenaxis on
the surface. Hexagonal surfaces (like GaAs (111) or GaN (0001))
cannot change the polarization due to symmetry reasons. The same is
true for Si (001), where both axis are equal. Luckily, most III-V
semiconductors (like GaAs, InP, ... ) have a fcc bulk lattice
structure and can be investigated by RAS.
Further reading about RAS
can be found on this
korean webseite.
