Quantum dots are tiny pieces of semiconductor, each containing an electron or hole in a certain quantum spin state. Light-hole states have been little studied in the past, because most quantum dots made to date had a valence-band ground state with a predominantly heavy-hole character, explains team leader Armando Rastelli of the Institute of Semiconductor and Solid State Physics at Johannes Kepler University in Linz. “My colleague Yongheng Huo at the Institute for Integrative Nanosciences in IFW Dresden has now managed to make high-quality quantum dots with a valence band ground state that have almost pure light-hole character, something that will allow us to investigate the properties of light-hole excitons and light-hole spins. Light-hole spins may have several advantages over their heavy-hole counterparts when it come to spin manipulation,” he told nanotechweb.org.

Normally, and in the absence of any external strain, the edges of the two topmost valence bands (that is, the heavy-hole and light-hole bands) in a semiconductor, such as gallium arsenide, are degenerate. However, quantum confinement in a quantum dot lifts the degeneracy between heavy and light-hole states by shifting the light-hole states to lower energies more than the heavy-hole ones. “By applying tensile stress to the quantum dot, we can restore the degeneracy and even invert the order of states by shifting the band edge of the light-hole band to higher energies, and that of the heavy-hole to lower energies,” said Rastelli.

"Unambiguous signature"

The light-hole exciton emits light in a very different way to the heavy-hole exciton. While heavy-hole excitons only emit light polarized in the plane of the direction in which they were grown, light-hole excitons also emit light polarized along the growth direction. “We observed a vertically polarized light-emission line spectrally separated from the in-plane polarized line – an unambiguous signature of a light-hole exciton,” explained Rastelli.

“Now that light-hole states are finally accessible, we should be able to investigate their fundamental properties,” he added. “Particularly interesting would be to find out how long spins last in these states – that is, their spin coherent times – compared to in their heavy-hole cousins.”

The researchers, who report their results in Nature Physics doi:10.1038/nphys2799, now plan to look at how a heavy-hole transitions to a light-hole ground state. “With the approach employed in this work, we were not able to do this, but we are now designing a piezoelectric actuator that will allow us to smoothly follow how light emission changes as heavy- and light-hole states cross each other,” revealed Rastelli.

Further reading

Rare-Earth oxides for silicon-germanium quantum dot memories (Nov 2013)
Manipulating spins in magnetic quantum dots (Sep 2010)
Spin states endure in quantum dot (May 2008)