Alteration of band structure in Si nanocrystals

The photoluminescence of Si nanocrystals with dimensions ranging from 70 to 250 nm was investigated. Grains were prepared by mechanical ball-milling and subsequent sedimentation steps in order to segregate them in size. This was followed by heat treatment in argon atmosphere at 1000°C to reduce the density of dislocations introduced during milling. At the same temperature, an oxidation process in air was carried out in order to create a stable SiO2/Si interface and reduce the dimensions of the grains. Crystallinity of the samples was checked by X-ray diffraction and transmission electron microscopy-related techniques. Two emission bands have been determined. One was identified with the D1 center usually assigned to dislocations. The second one is identified as being due to an excitonic recombination and shows phonon replicas characteristic for bulk silicon. This band is reported here for the first time. In the fraction of the smallest silicon grains, a gradual upshift of the excitonic line was observed with diminishing average grain diameter. This was associated with band structure perturbation due to size-confinement.