Quantum Dot Energy Levels

Susan Lehman

Quantum Dots

Scanning probe microscope image of quantum dots.

Quantum dots (QDs) are nanostructures that confine electrons to a three-dimensional potential well, much as individual atoms do. Like atoms, the optical and electronic properties of a QD depend on the strength of the potential well; unlike atoms, the depth of that well is tunable in a quantum dot. Much of the fundamental scientific understanding of the growth mechanism, morphology, and optical and electronic properties of the dots remains incomplete. Any realistic model for the electronic energy structure must take into account the shape and composition of the dot, properties which depend strongly on the technique used to prepare the dots.

We are using the technique of ballistic electron emission microscopy (BEEM) to investigate the electronic energy levels of individual InAs and InGaAs quantum dots and the effect that these dots have on the electronic bands of the surrounding GaAs. Ballistic electron emission microscopy is a scanning tunneling probe technique developed in the mid-1980s shortly after the development of the scanning tunneling microscope (STM). Scanning tunneling microscopy measures the total current that tunnels between the tip and sample and uses this current to provide an image of the sample. The BEEM technique separates this single current into two: one with all the electrons with enough energy to overcome an energy barrier in the sample, and one with the rest of the electrons without enough energy. The current of the energetic electrons is called the BEEM current and is only observed above a threshold energy that corresponds to the height of the energetic barrier within the sample. In sum, BEEM measures the electronic properties of buried interfaces with high lateral resolution, so that it is possible to measure the electronic structure of a single quantum dot.

We are currently modifying the College's scanning probe microscope into a ballistic electron emission microscope by modifying the electronics and probes that contact the sample. This summer we will continue this work, optimizing the scope in BEEM operation mode and using it to investigate InGaAs quantum dots of varying compositions.