Detection of the third transition of InAs/GaAsSb quantum dots

The optical properties of InAs quantum dots (QDs) embedded in GaAs_0.92Sb_0.08 barriers have been studied. The samples studied consist of 20 multiple layers of InAs QDs embedded in GaAs_0.92Sb_0.08, with each QD/barrier system separated by a 100 nm GaAs spacer. No appreciable changes in the QD properties, such as size, shape, and density, are observed by Scanning Transmission Electron Microscopy (STEM) images. The δ-doping plane beneath the InAs QDs allows the occupancy of the QD electronic sub-band states to be controlled. Low temperature (77 K) Fourier Transformation-Infrared Spectroscopy (FTIR) results, using a multiple internal reflection (MIR) technique to enhance the optical path length, show intersubband absorption in the InAs QD area. The broad peak observed around 240 meV corresponds to the energy separation between the electron ground state and the continuum state of the QDs. Another broad peak around 440 meV is ascribed to a transition between a deep level and shallow donor level due to δ-doping as the signal increases as the doping density increases. Band structure calculations using an eight band k·p method are used to confirm the experimental results observed here.