Recent technologies aim to supply high-purity synthetic crystals that turn out to be excellent semiconductors when doped with impurities as electron donors or acceptors of other elements. Researchers led by Kyoto University has now determined the magnitude of the spin-orbit interaction in acceptor-bound excitons in a semiconductor. They broke through the energy resolution limit of conventional luminescence measurements by directly observing the high quality structure of certain excitons in boron-doped blue diamond, using optical absorption.
Besides being “a woman’s best friend,” diamonds have broad industrial applications, resembling in solid-state electronics. Recent technologies aim to supply high-purity synthetic crystals that turn out to be excellent semiconductors when doped with impurities as electron donors or acceptors of other elements.
These extra electrons — or holes — don’t take part in atomic bonding but sometimes bind to excitons — quasi-particles consisting of an electron and an electron hole — in semiconductors and other condensed matter. Doping may cause physical changes, but how the exciton complex — a certain state of two positively-charged holes and one negatively-charged electron — manifests in diamonds doped with boron has remained unconfirmed. Two conflicting interpretations exist of the exciton’s structure.
A world team of researchers led by Kyoto University has now determined the magnitude of the spin-orbit interaction in acceptor-bound excitons in a semiconductor.
“We broke through the energy resolution limit of conventional luminescence measurements by directly observing the high quality structure of certain excitons in boron-doped blue diamond, using optical absorption,” says team leader Nobuko Naka of KyotoU’s Graduate School of Science.
“We hypothesized that, in an exciton, two positively charged holes are more strongly certain than an electron-and-hole pair,” adds first writer Shinya Takahashi. “This acceptor-bound exciton structure yielded two triplets separated by a spin-orbit splitting of 14.3 meV, supporting the hypothesis.”
Luminescence resulting from thermal excitation could be used to look at high-energy states, but this current measurement method broadens spectral lines and blurs ultra-fine splitting.
As an alternative, Naka’s team cooled the diamond crystal to cryogenic temperatures, obtaining nine peaks on the deep-ultraviolet absorption spectrum, in comparison with the standard 4 using luminescence. As well as, the researchers developed an analytical model including the spin-orbit effect to predict the energy positions and absorption intensities.
“In future studies, we’re considering the opportunity of measuring absorption under external fields, resulting in further line splitting and validation attributable to changes in symmetry,” says Université Paris-Saclay’s Julien Barjon.
“Our results provide useful insights into spin-orbit interactions in systems beyond solid-state materials, resembling atomic and nuclear physics. A deeper understanding of materials may improve the performance of diamond devices, resembling light-emitting diodes, quantum emitters, and radiation detectors,” notes Naka.
