We have explored the formation by molecular-beam epitaxy (MBE) of CdSe quantum dots on ZnSe, a wide band gap semiconductor. The growth of CdSe on ZnSe, similar to the now well-studied InAs quantum dots in a GaAs matrix, under specific growth conditions undergoes a transition from layer-by-layer to three-dimensional Stransky-Krastanov island growth, pictured below.

The formation of the CdSe islands is verified by plan-view transmission electron microscopy (TEM) and atomic force microscopy (AFM), both shown below. The broad peak in the photoluminescence, immediately below, reflects the non-uniform size distribution of the dots over the large (d~100 μm) area of the luminescence. In contrast, small numbers of dots can be probed using near-field scanning optical microscopy (NSOM) to reveal sharp peaks in the photoluminescence spectra originating from carrier recombination within a single quantum dot.

Photoluminescence Spectra in the Near-Field

To study a small number of quantum dots, the area optically probed has to be reduced below that achievable by classical optics (d~1 μm). Due to the high density of quantum dots (~200 μm-2), submicron optical resolution is required and can be obtained by utilizing near-field scanning optical microscopy. In the spectra shown below, clear evidence of the reduction of the probed area is seen as the tip enters the near-field regime. At large distances between the probe and sample (far-field regime) the probed area is large, and the spectrum is a wide featureless peak. In contrast, the sharp peaks originating from individual quantum dots are clearly resolved as smaller numbers of dots are included by probing in the near-field regime (tip-sample separation on the order of 0.02 μm). By systematically collecting near-field spectra over the surface of the sample, maps of the photoluminescence at a wide range of detection energies can be formed.

Luminescence Images of CdSe Quantum Dots

By collecting spectra in the near-field on a regularly spaced grid over a 2 μm x 4 μm area of the surface of the sample, we can form maps of the luminescence at any desired energy. Four such images taken at the energies indicated by the green arrows are shown below. As you can see, luminescence originates from localized islands whose distribution changes with the detection energy. These islands are associated with the radiative recombination of carriers in quantum dots at the selected energy levels. The sizes of the islands are larger than the sizes of the dots as a result of carriers diffusing in the ZnSe matrix before being trapped in the quantum dots. As expected the average of all the spectra over the 2 µm x 4 μm area gives a spectrum resembling the far field.