Electronic structure of defects in semiconductor quantum dots

The strong industrial drive towards smaller and ever smaller devices has fueled interest in semiconductor nanostructures, also known as quantum dots (QDs). The electronic structure of these systems has been the subject of intense investigation over the last decade. Little work, however, has been done to investigate the role of defects in these systems. We have employed three distinct methodologies to understand the electronic structure of defects in QDs : (i) the effective mass theory (EMT); (ii) the tight-binding theory (TB); (iii) and the first principles local density approximation (LDA). The behaviour of defects in QDs is surprisingly different from that in the bull. The hydrogenic level undergoes a shallow-deep instability as the size of the quantum dot is decreased. The deep level on the other hand is relatively insensitive to the size when the quantum ddt is large. It exhibits random variations as the dot size is lowered below a threshold value. We find that there is an optimum size for which the quantum dot of a given semiconducting material is most efficient. Our work should have a direct bearing on efforts to make devices from these QDs.