https://doi.org/10.1007/s100510050415
Isotope effects on the electronic critical points of germanium: Ellipsometric investigation of the E1 and E1+Δ1 transitions
Max-Planck-Institut für Festkörperforschung,
Heisenbergstrasse 1, 70569 Stuttgart, Germany
Corresponding author: a ronnow@cardix.mpi-stuttgart.mpg.de
Received:
6
March
1998
Revised:
27
April
1998
Accepted:
15
May
1998
Published online: 15 September 1998
Within the past years the optical excitations of electrons
have been measured for semiconductor samples of different
isotope compositions. The isotope shift observed
have been compared with calculations of the effects of
electron-phonon interaction on the electronic band structure.
While qualitative agreement has been obtained, some
discrepancies remain especially concerning the
E1 and 
transitions.
We have remeasured the effect of isotope mass on the
E1 and transitions of
germanium with several isotopic compositions.
The results, obtained by means of spectroscopic
ellipsometry, confirm that the real part of the gap
self-energies induced by electron-phonon interaction is
larger than found from band structure calculations,
while the imaginary part agrees with those
calculations, which are based on a pseudopotential band structure and
a bond charge model for the lattice dynamics. Our results agree
with predictions based on the measured temperature
dependence of the gaps. We compare our data for
E1 and with
results for the lowest direct (E0) and indirect
(Eg) gaps. The measured values of
and 
increase noticeably with increasing
isotope mass. Similar effects have been observed in the
temperature dependence of in
and 
. A microscopic explanation for this effect
is not available.
PACS: 71.38.+i – Polarons and electron phonon interactions / 78.40.Fy – Semiconductors / 63.20.Kr – Phonon-electron and phonon-phonon interactions
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 1998
