STM-based luminescence spectroscopy on single quantum dots

The ability to fabricate quantum dots using the Stranski-Krastanow growth technique has improved dramatically during the last number of years. Due to the large number of dots formed (typically 108–109 cm−3) the emission linewidth is inhomogeneously broadened. We have grown quantum dots of InP in between barriers of GaInP, as well as on top of GaInP, having a low density of dots in order to perform single-dot spectroscopy. These dots have been studied by photoluminescence and scanning tunneling luminescence. We find in photoluminescence that the fully formed dots have well-defined, sharp (0.04–1 meV) emission lines, which are very similar from dot to dot. In scanning tunneling luminescence we find that we can very locally excite only a few partially formed dots which have sharp emission lines (0.1 meV at 77 K). These emission lines display a quantum confined Stark effect when the applied tip to sample bias is varied. We can carefully determine the onset of exciton formation as a function of applied bias. The applied bias which is necessary for impact ionization is found to agree well with simple theory.