Biomedical Engineering Reference
In-Depth Information
Fig. 11.14
Decay of the optically generated electric field with laser excitation above the WL for
two different powers of P (0.3 mW) and 2P (0.6 mW).
Inset
shows the semi-logarithmic plot of the
power dependence
11.7
Summary
In this chapter we have briefly discussed the Stark effect observed in exciton
emission in QDMs. The observed energy shifts with electric field are a result of
carrier separation, the formation of molecular wavefunctions, Coulomb interactions,
and the differing effective masses for the electron and hole which results in a
measureable effect. The intradot exciton can be engineered to display Stark shifts
of up to
0.13 meV/kV/cm, an order of magnitude larger than their intrinsic
linewidths. The interdot exciton Stark shift is predominantly due to the barrier
separation between the QDs and can therefore result in extremely large field
dependencies of up to
∼
0.97 meV/kV/cm, an order of magnitude larger than the
intradot shifts. It is worth noting that if one could resolve the lifetime limited
linewidth of the interdot exciton that, with the Stark shifts measured in these devices,
it would be theoretically possible to detect the field due to a single electron on the
order of microns away.
We then used the large interdot Stark shift, from a QDM with a 4 nm barrier, to
study the optically generated electric field, which was found to have a maximum
value of
∼
3.25 kV/cm, corresponding to 5.04% of the total field. The observed
effects are consistent with photovoltaic band flattening, which is produced by the
ionization of the photogenerated e-h pairs within the Schottky diode. Basically, the
shift of the PL in electric field indicates a local electric field which is the result
of creating electron-hole pairs by means of optical absorption by the sample and
consequent tunneling of charges to opposite sides of the device. This will create
charge accumulation which will generate a local electric field opposing the applied
∼
