Inter-Conduction-Band Transitions
In an attempt to obtain laser light from silicon
(bulk silicon in this case) we examined the PL of bulk crystalline
silicon under UV-excitation. The idea behind these experiments
is schematically depicted below. Electrons are excited from the
valence-band into a higher conduction-band at the Gamma-Point in
the centre of the Brillouin-zone by absorption of a UV-photon (green
arrow).
Simplified schematic of the band-diagram of
silicon near the zone centre. Direct radiative transitions between
two conduction bands could be used for laser-action in bulk silicon.
From the 2nd conduction band state the electron then
recombines into the first conduction-band via another direct radiative
transition (red arrow). This is the targeted laser transition,
which should be very efficient due to the fact that it is a direct
transition. Also, more importantly, the final state of that transition
is always quasi-empty due to the fact that electrons quickly thermalise
from that state to the conduction band minimum via phonon interaction
(blue arrow), a very fast and thus efficient process. For this
reason even a small population of the second conduction band state
would be sufficient to yield optical gain, the basis of laser action.
Initial PL experiments in this context using a pulsed UV-laser
(259 nm wavelength) were carried out at the University of Wollongong.
The emission from the laser transition is expected in the spectral
range around 1eV in silicon, which makes it problematic to distinguish
this emission from the normal band-to-band luminescence. A tuneable
light source that is currently being set-up in our laser laboratory
will be used for further experiments. |
