Sep 16, 2009
Free-carriers beat excitons in spin-injection contest
Quantum dots (QDs) are a promising building block for future spin-functional devices with applications in spintronics and quantum information processing. Essential to the success of these devices is the ability to create a desired spin orientation of charge carriers (electrons and holes) in QDs via the injection of spin-polarized carriers. Researchers have now shown that this can be done most efficiently using independent (free) carriers rather than electron-hole pairs (excitons).
Scientists are busy working on a new area of spintronics aimed at quantum computing and quantum communication. One promising building block for these devices is the quantum dot, which may be used for spin storage, detection and transmission due to its atomic-shell-like electronic structure and extremely long spin lifetime.
To obtain information on the best way to create desired spins in various InAs QD structures by spin injection, the team from Linköping University, Sweden, and UCSD, US, employed optical orientation spectroscopy in combination with tunable laser spectroscopy. By using circularly polarized light at certain energies, spins of either independent carriers (free electrons and holes) or correlated electron-hole pairs as tightly coupled quasi-particles (excitons) could be selectively generated in a surrounding wetting layer of InGaAs and injected into the QDs.
The degree of spin conservation during such a spin-injection process, or spin-injection efficiency, was closely monitored by spin polarization of the injected electrons reaching the lowest energy state of the QDs via circular polarization of the emitted light when electrons and holes recombine.
The group showed that spin-injection efficiency was significantly lower for exciton spin injection than free-carrier spin injection. This was attributed to an electron-hole exchange interaction of the excitons that accelerates spin flips, such that the injected spins quickly lost their intended orientation.
This finding is relevant for future quantum communication and quantum information storage because it points out that separate carrier injection can be advantageous for next-generation spintronic devices based on QD spins.
The researchers presented their work in Nanotechnology.
About the author
The characterization work was performed by PhD student Jan Beyer at Linköping University, Sweden, in the group of Prof. Weimin M Chen and Prof. Irina A Buyanova. Sample growth was done by PhD student Suwaree Suraprapapich in the group of Prof. Charles W Tu at UCSD, US.