Photoluminescence of buried InGaAs/GaAs quantum dots spectrally imaged by scanning near-field optical microscopy

We present scanning near-field optical microscopy (SNOM) investigations of buried quantum dots. InGaAs/GaAs quantum dots are grown in the Stranski-Krastanov growth mode and covered with a 50 nm thick GaAs-cap by metal-organic vapour phase epitaxy (MOVPE). For near field investigations with high spatial resolution, uncoated glass fiber tips are used in internal reflection mode to illuminate the sample with 2.54 eV argon ion laser at 300 K. Overall photoluminescence (PL) intensity images as well as images taken at one wavelength are compared. The overall PL intensity image shows individual spots with a resolution of about 300 nm. The corresponding shear force image (topography) shows small height corrugation of 3 nm in the surface topography at the centers of the emitting spots. These high corrugations are possibly correlated to a surface topography above the quantum dots as revealed by transmission electron microscopy (TEM). Different contributions of the near field and far field in the PL intensity are separated and the emission spectra of individual emission centers are shown to have a spectral half with larger than 15 meV.