Semiconductor nanocrystals are of enormous interest due to their tunable properties and wide variety of promising applications, from quantum computing to bio-labelling and non-linear optics. The effect of size and shape on their characteristics has been subject to intense investigation. The introduction of dopants can also be explored, and a relevant question is whether doping in nanocrystals influences their properties when the size leads to strong quantum confinement.
Hydrogen can act as a dopant in many semiconductors, often forming a shallow-donor state in bulk material with an extended electron wavefunction. Muons are used to model hydrogen atoms, and can be used to study shallow-donor behaviour and characteristics – in the present case, to explore the effects of reducing the semiconductor crystal size to the nanoscale.
Temperature dependence of the muonium shallow donor fraction in nanocrystalline CdTe compared with bulk material. Inset – nanocrystalline CdTe.
In nanocrystalline samples of II-VI semiconductors the shallow-donor state is still observed, but is significantly affected by the variation of muon sites within the nanocrystal structure. The shallow donor state now persists to higher temperature than in the bulk material due to the higher binding-energy of the donor state limited by the onset of inter-nanocrystal conductivity.
AJ Houtepen, P Liljeroth, D Vanmaekelbergh (University of Utrecht, The Netherlands), JM Gil, HV Alberto (University of Coimbra, Portugal), J Gavartin (University College London), D Visser (ISIS-NWO), JS Lord, SFJ Cox (ISIS)
Research date: December 2006
Dr JM Gil, [firstname.lastname@example.org] JM Gil et al., Phys. Rev. B 64 (2001) 075205.