Chemistry Reference
In-Depth Information
I
2
>
I
1
>
I
0
I
2
I
1
I
0
E
g
Energy
Figure 5.13
Material gain,
g
, as a function of photon energy in a semiconductor
medium. Successive curves show the gain spectrum as the injected cur-
rent
I
and hence the carrier density is increased. In each case,
g
=
0atan
energy
E
=
F
c
−
F
v
, the quasi-Fermi level separation for the given drive
current.
energy range from
E
g
to
F
c
F
v
and is absorbing at higher energies as
illustrated in fig. 5.13. One of the major aims of band structure engineering
in semiconductor lasers is to obtain transparency (when
F
c
−
−
F
v
=
E
g
) and
gain for the lowest possible injected carrier and current densities.
As the current injected into the laser is increased, initially the electron
and hole densities also increase and so the separation of the quasi-Fermi
levels,
F
c
F
v
, increases (see fig. 5.13), leading to an increase in the peak
gain value. This continues until the maximum gain is equal to the total
losses from the laser cavity. Loss mechanisms within the cavity include
photon scattering due to imperfections and also reabsorption mechanisms
associated perhaps with defect states in the laser structure. In addition,
the gain must overcome the loss of photons from the end mirrors of the
laser.
The characteristics of III-V semiconductor lasers are in large part deter-
mined by the valence band structure, which is complicated even in the
unstrained case (fig. 5.14a). We saw in Chapter 4 how the Light- (LH) and
Heavy-Hole (HH) bands are degenerate at the zone centre,
−
, with the spin-
split-off band lying at an energy
E
so
belowthe two highest bands. Although
semiconductors are among the most efficient of laser materials there are
several drawbacks associated with the band structure of fig. (5.14a). First,
the conduction band effectivemass is small, while the highest (HH) valence
band always has a large effective mass, leading to a large valence band
density of states and hence requiring a large carrier density and a large
spread in electron energies at population inversion. If the conduction and
valence bands had more similar shapes, a lower density of both holes and
electrons, which must be equal to maintain charge neutrality, would be
required. Second, in bulk lasers, cubic symmetry means that the states at
the valence band maximum are made up in equal part of
p
x
,
p
y
and
p
z
-like
