Biomedical Engineering Reference
In-Depth Information
Fig. 11.6
Low resolution biasmap showing the QDM ground and excited state PL emission as
well as the broad WL emission. Note that at this resolution and intensity scale the interdot lines are
not visible
Measurements similar to this could be done using coupled QWs; however,
comparing the linewidth of the ground state PL in the QDMs to that of the WL
(Fig.
11.6
), we can see the advantage of using the interdot PL to measure small
changes. Even the relatively narrow QW-like emission of the WL is one to two
orders of magnitude broader than the interdot linewidth. Therefore, as seen in
Fig.
11.5
b, even very small changes in the electric field will result in a measurable
shift in the emission energy of the interdot lines. We could either measure the shift
of the interdot line PL energy at a given applied field or measure the shift of the field
at a given interdot PL energy. Either way, changes in the electric field down to a few
tenths of a kilovolt per cm can be measured. The interdot PL peaks in the spectra
for different applied fields were fitted to a Gaussian, which was found to provide
a moderately better fit than the expected Lorentzian possibly due to the sensitivity
to random charge fluctuations. These peaks were then fitted to a line which was
used to calculate the shift in electric field for an arbitrary PL energy. This was
done as a function of excitation energy, excitation power, and applied electric field.
All of the results, which will be discussed in the following sections, are consistent
with the effect of photovoltaic band flattening [
38
,
39
]. Here, the ionization of the
photogenerated e-h pairs within the Schottky diode can induce an opposing field
if the ionized electrons and holes separate and remain within the device region.
This trapping most likely occurs at impurity sites and material boundaries (e.g.,
the GaAs/AlGaAs interface, the WLs, or the doped/intrinsic GaAs interface), while
carriers not trapped will contribute to the measured photocurrent. The most likely
source of trapping is at the GaAs/AlGaAs interface. As can be seen in Fig.
11.5
a,
for this device structure this interface forms a two-dimensional potential well for the
holes.
