Lasing in Indirect Gap Semiconductors
In earlier widely cited work, W.P. Dumke considered
optical transitions in indirect semiconductors for laser applications.
His results, published in the late ‘60s, have so far been
generally accepted as the theoretical proof that optical gain is
not possible in silicon. Recent theoretical work by centre researchers
however, now clarified that these conclusions are based on a flawed
theory [T. Trupke et al., J. Appl. Phys. 93, 9058, (2003)].
It could be shown that, in indirect semiconductors,
the absolute value of the negative absorption coefficient, which
describes optical gain, can be orders of magnitude larger than
the equilibrium absorption coefficient, contrary to the case for
direct semiconductors (lower curve in figure).
Normalised absorption coefficient for indirect
transitions as a function of the separation of the quasi-Fermi
energy Δµ at photon energies ħω = Eg ± ħΩ ,
with ħΩ the phonon-energy. Upper curve: photon absorption
with phonon emission (ħω=1.15eV). Lower curve: photon
absorption with phonon absorption (ħω= 1.05eV).
Note that these results do not necessarily mean that Dumke’s
conclusions are wrong, but that the question, whether fundamentally
optical gain via band-band transitions is possible in bulk silicon
or not should be checked using the corrected theory. This is currently
work in progress. |
