Biomedical Engineering Reference
In-Depth Information
Fig. 11.7
Dependence of the optically generated field (
F
o
) on laser excitation energy. A typical PL
spectrum is displayed in
gray
with intensity on a log scale. The
red circles
indicate the measured
strength of
F
o
and the
blue squares
are the measured photocurrent. Only when the laser energy
becomes resonant with, and then higher in, energy than the WL does
F
o
become measurable.
F
o
is
found to saturate at 3.25 kV/cm above the GaAs band edge
11.6.1
Energy Dependence
The dependence of the optically generated field (
F
o
) on laser power, laser energy,
and the applied field is a complicated function of the various tunneling, recombi-
nation, and generation rates, as well as the type and density of traps within the
device. We can begin to deduce some qualitative trends as we measure the effects
of
F
o
as a function of laser energy. Here we perform photoluminescence excitation
(PLE) experiments and monitor the shift of the interdot line as a function of laser
energy. To provide additional insight into the processes involved we also monitor
the photocurrent. The PLE intensity and the photocurrent are proportional to the
absorption of photons and tunneling of carriers out of the device and as the excitation
energy becomes resonant with the WL, a shift of the electric field-dependent spectra
is observed (Fig.
11.5
b). The shift increases with increasing laser energy, and once
the energy is above the GaAs band edge, no further shift is observed. Since these
measurements were done at a fixed power (2 W/cm
2
) this could be interpreted as
arising from an increased absorption probability with increasing laser energy. In the
next section we will discuss the dependence on power and what additional insight
we can gain. In Fig.
11.7
we see that at
880 nm there is a PLE signal but nearly
no photocurrent, suggesting that a fraction of the absorbed photons, those which do
not relax and recombine, are ionized and trapped, thereby contributing to
F
o
.As
the laser energy is increased we observe an increase in PLE, photocurrent, and the
optically generated field. Above the GaAs both the PLE and photocurrent increase
dramatically indicating efficient absorption, ionization, and tunneling.
λ
e
=
