Shallow impurities and delta-doping in quantum dot-quantum well systems

In the present work we study the defect states in recently synthesized semi conductor quantum dot-quantum well systems (QDQWs). We employ the effective -mass theory (EMT) with a realistic barrier and variable effective mass. Th e model is simple and all of our results are obtained by an exact numerical diagonalization of the Schrodinger equation. We study the ground state of the host system as a function of quantum well size. We demonstrate that the upshift with QDQW downsizing differs from reported upshifts in simple quan tum dots (QDs) and explain this by means of a perturbative analysis. We stu dy the binding energy of the hydrogenic impurity and its variation with QDQ W size. Next the binding energy is studied as the impurity is moved off-cen tre. We find that the binding energy goes through a maximum. An analysis of the wavefunction is carried out to obtain an understanding of this surpris ing effect. The impurity calculations are carried out on CdS/HgS/CdS QDQWs. Recently, experimental studies on a monolayer of HgS in a US dot were carr ied out. We model this system as a 'delta -defect' consisting of a thin sph erical shell of fixed potential depth in a spherical US dot. The ground sta te of this system is studied as the shell is dragged from the periphery to the Centre of the US dot. Our results are explained on the basis of qualita tive arguments and asymptotic analyses. Outstanding issues and future direc tions are suggested.