Carrier multiplication in germanium nanocrystals

Abstract

Carrier multiplication is demonstrated in a solid-state dispersion of germanium nanocrystals in a silicon–dioxide matrix. This is performed by comparing ultrafast photo-induced absorption transients at different pump photon energies below and above the threshold energy for this process. The average germanium nanocrystal size is approximately 5–6 nm, as inferred from photoluminescence and Raman spectra. A carrier multiplication efficiency of approximately 190% is measured for photo-excitation at 2.8 times the optical bandgap of germanium nanocrystals, deduced from their photoluminescence spectra. Germanium nanocrystals can efficiently generate more than one electron–hole pair following absorption of a single photon of light. That is the finding of a study by scientists from the Netherlands and Portugal. Carrier multiplication — the ability of a semiconductor to create several electron–hole pairs from the absorption of a single photon whose energy exceeds the material’s bandgap — is potentially important for improving the efficiencies of devices like solar cells and photodetectors. In the present study, 5–6 nm diameter germanium nanocrystals inside a silicon-dioxide matrix offer carrier multiplication with an efficiency of 190% for a photon energy of 3.5 eV, which implies that 1.9 electron electron–hole pairs are created for each absorbed photon with 2.8 times the bandgap energy. Importantly, this efficiency is significantly better than that of bulk germanium.