Electrical transport mechanisms in three dimensional ensembles of silicon quantum dots

Abstract

In this review, we try to derive a comprehensive understanding of the transport mechanisms in three dimensional ensembles of Si quantum dots (QDs) that are embedded in an insulating matrix. This understanding is based on our systematic electrical measurements as a function of the density of Si nanocrystallites as well as on a critical examination of the available literature. We conclude that in ensembles of low density QDs, the conduction is controlled by quantum confinement and Coulomb blockade effects while in the high density regime, the system behaves as a simple disordered semiconductor. In between these extremes, the transport is determined by the clustering of the QDs. In view of the clustering, two types of transitions in the electrical and optical properties of the system are identified. In order to understand them, we introduce the concept of “touching.” The application of this concept enables us to suggest that the first transition is a local carrier deconfinement transition, at which the concentration of the non “touching” QDs reaches its maximum, and that the other transition is associated with the onset of percolation in a continuous disordered network of “touching” QDs. It is hoped that our conclusions for the entire possible density range will provide guidance for the discussion and understanding of the transport in ensembles of semiconductor QDs in general and in ensembles of Si and Ge QDs in particular.