  Quantum dot heterostructures with excellent low-noise properties became
possible with high purity materials recently. We present a study on molecular
beam epitaxy grown quantum wells and quantum dots with a contaminated aluminum
evaporation cell, which introduced a high amount of impurities, perceivable in
anomalies in optical and electrical measurements. We describe a way of
addressing this problem and find that reconditioning the aluminum cell by
overheating can lead to a full recovery of the anomalies in photoluminescence
and capacitance-voltage measurements, leading to excellent low noise
heterostructures. Furthermore, we propose a method to sense photo-induced trap
charges using capacitance-voltage spectroscopy on self-assembled quantum dots.
Excitation energy-dependent ionization of defect centers leads to shifts in
capacitance-voltage spectra which can be used to determine the charge density
of photo-induced trap charges via 1D band structure simulations. This method
can be performed on frequently used quantum dot diode structures.
