Shallow-deep transitions of impurities in semiconductor nanostructures

We study the hydrogenic impurity in a quantum dot (QD). We employ the effec tive mass theory with realistic barrier and variable effective mass. The mo del is simple, but it predicts features not previously observed. We observe that the shallow hydrogenic impurity becomes deeper as the dot size (R) is reduced and with further reduction of the dot size it becomes shallow and at times resonant with the conduction band. Such a shallow-deep (SHADE) tra nsition is investigated and a critical size in terms of the impurity Bohr r adius (a(I)*) is identified. A relevant aspect of a QD is reduction in the dielectric constant, epsilon, as its size decreases. Employing a size depen dent epsilon (R), we demonstrate that the impurity level gets exceptionally deep in systems for which a(I)* is small. Thus, carrier "freeze out" is a distinct possibility in a wide class of materials such as ZnS, CdS, etc. Th e behavior of the impurity level with dot size is understood on the basis o f simple scaling arguments. Calculations are presented for III-V (AlGaAs) a nd II-VI (ZnS, CdS) QDs. We speculate that the deepening of the impurity le vel is related to the high luminescence efficiency of QDs. It is suggested that quantum dots offer an opportunity for defect engineering. (C) 2001 Ame rican Institute of Physics.