Quantum dots - semiconductor structures in which electrons are confined in all three dimensions - are of interest to physicists for both fundamental and technological reasons. In particular, quantum dots might form the basis of a new generation of semiconductor lasers with lower threshold currents and higher efficiencies than existing devices. However, the development of new devices will depend on a detailed understanding of how the quantum dots actually emit light. Two recent experiments have shed new light on the process.
Manfred Bayer of the University of Wurzburg in Germany, and co-workers in Wurzburg and the National Research Council of Canada in Ottawa, studied indium gallium arsenide quantum dots grown on a gallium arsenide surface (M Bayer et al. 2000 Nature 405 923). The dots measured about 20 nanometres across. Meanwhile, a group lead by Khaled Karrai at the Ludwig Maximilians University in Munich, working with Pierre Petroff’s group at the University of California at Santa Barbara, grew nanometre-sized quantum rings of indium arsenide on gallium arsenide (R Warburton et al. 2000 Nature 405 926).
Electrons become trapped in the dots and rings because both indium gallium arsenide and indium arsenide have small energy band gaps than gallium arsenide. Moreover, the confinement of the electrons inside the dots and rings leads to electronic structure similar to that found in atoms. In bulk semiconductors, on the other hand, the electrons can occupy a band of energies.
Both groups then used lasers to excite electrons to higher energy levels within the dots and rings. The combination of the excited electron and the “hole” it leaves behind is called an exciton. Photons are emitted when the electron and hole recombine. Both groups studied how the emission wavelength depended on the number of electrons and excitons within the quantum dot or ring. A major challenge was to isolate the light from just one dot or ring.
While there were many similarities with the behaviour observed in atoms – indeed quantum dots are often called artificial atoms – there were also differences. Bayer and colleagues, for instance, observed that “hidden symmetries” within the dots lead to unusual quantum interference effects and emission properties.