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 a nd subsequent sedimentation steps in order to segregate them in size. This was followed by heat treatment in argon atmosphere at 1000 degreesC to redu ce the density of dislocations introduced during milling. At the same tempe rature, an oxidation process in air was carried out in order to create a st able SiO2/Si interface and reduce the dimensions of the grains. Crystallini ty of the samples was checked by X-ray diffraction and transmission electro n microscopy-related techniques. Two emission bands have been determined. O ne was identified with the DI center usually assigned to dislocations. The second one is identified as being due to an excitonic recombination and sho ws phonon replicas characteristic for bulk silicon. This band is reported h ere for the first time. In the fraction of the smallest silicon grains, a g radual upshift of the excitonic line was observed with diminishing average grain diameter. This was associated with band structure perturbation due to size -confinement. (C) 2001 Elsevier Science BN. All rights reserved.