Electronic structure study of ion-implanted Si quantum dots in a SiO2 matrix: Analysis of quantum confinement theories

The electronic states and optical properties of Si quantum dots (QDs) with variable size prepared by ion implantation in a SiO2 matrix are studied by x-ray photoemission spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy. The results are compared with several theories of quantum confinement. Our Si 2p binding energies and the valence band energies do not change as a function of QD diameter nor compared to the bulk Si values. Raman spectra show no signs of stress on the Si-QDs. XPS data indicates the presence of a Si2O3 interfacial layer between the Si-QDs and the surrounding SiO2 matrix, which is understood to relieve stress in the QDs and to cause pinning of the valence level. Our XPS results for ion-beam implanted QDs are compared with other group's studies for Si-QDs prepared by alternative methods, and discrepancies in the interfacial compositions are discussed. These results call into question the fundamental predictions and assumptions of many quantum confinement models. It is concluded that the lack of a shift in the valence band is due to a symmetry-breaking process in the hole states, which is not currently accounted for by theory, demonstrating the importance of the hole states during radiative events. This work is intended as a first step in highlighting the features that should be present in a theoretical formalism for embedded Si-QDs, and cause is given to abandon particular formalisms.