The energy bands of a quantum confined system, like a quantum dot, and its surrounding barrier determine the type I to type II transition in these materials. In a classical type I quantum dot structure, both the conduction band and the valence band of the quantum dot lie between the band gap of the barrier material. This confines both an electron and a hole in the quantum dot, so they are easily able to recombine, releasing a photon as they do so. In a type II structure, on the other hand, either the conduction band or the valence band of the quantum dot lies between the barrier's band gap, while the other lies outside it.

The researchers, led by Diana Huffaker of the University of California at Los Angles and Guillaume Huyet at the Cork Institute of Technology/Tyndall National Institute in Cork, together with Gregory Salamo from the University of Arkansas, studied InAs quantum dots (a type I system) on gallium arsenide (GaAs). They manipulated the conduction band by incorporating different Sb compositions into the InAsSb dots. This process raises the quantum dot's conduction band above the barrier's conduction band – an arrangement that traps holes in the dot and forces electrons into the barrier, so making it more difficult for electrons and holes to recombine. The system is now type II.

Photoluminescence spectroscopy
The team employed "power-dependent" and "temporally resolved" photoluminescence spectroscopy (PL) to characterize the materials. In PL, a high-energy photon is absorbed by the sample, leaving an electron behind in the conduction band. This electron either recombines in the barrier material or in the quantum dot and produces a photon spectrum that can be measured using a spectrometer. The technique can be used to analyse both type I and type II structures.

"Where the type II structures differs, however, is that, as the laser power increases, more holes and electrons collect in and around the quantum dot," explained team members Jun He and Charles Reyner. "The Coulombic attraction between the carriers alters the wavefunction shapes of the electrons and holes and changes the resulting photon spectrum. By carefully measuring the spectral shift with laser power, we can infer whether a type II bandstructure exists."

InAs quantum dots are used in a variety of applications, including short-pulse lasers, solar cells and photodetectors. These dots have a type I band alignment, but Huffaker and colleagues have shown that they can toggle this to a type II band alignment by incorporating Sb into the dots, so extending the possible applications these materials might be used in. For instance, type II quantum dots with long recombination lifetimes could be used to increase carrier extraction in quantum dot solar cells. This would enhance their power conversion efficiency compared to that possible in type I materials.

"We have already fabricated basic devices in memory, photovoltaics and lasers," Reyner told "We will now work on optimizing the band structures of the type II quantum dots for these specific applications through more carefully designed material compositions."

The work was reported in Nano Letters.