We observe topography induced spectral shifts using high resolution grating
spectrometers which we attribute to the fact that the focal spot has a finite
size. The topography induced spectral shifts depend on spectrometer grating
orientation and numerical aperture of the microscope objective. This is
demonstrated by spectroscopic imaging trenches in GaAs in directions parallel
and perpendicular the spectrometer entrance slit. Differences along the two
directions of the LO phonon band show that the spectral shift is due to the
variation of the grating angle across the non uniform illuminated focal spot
caused by topography. Alignment errors of the optical axis lead to additional
spectral shifts. Topography induced spectral shifts can be detected by
recording spectra by scanning the sample in two perpendicular orientations with
respect to the spectrometer entrance slit.Comment: 9 pages, 3 figure