Semiconductor quantum rings:: Shallow-donor levels

A theoretical study of shallow-donor states in toroidal GaAs-(Ga,Al)As quantum rings, within the effective-mass approximation and following a variational calculation, is presented. The particular topology of quantum rings is shown to produce interesting confining effects on the impurity states, namely, the effective dimensionality of a quantum ring may increase as it shrinks. The density of impurity states is calculated, and quantum-well wire and quantum-dot regimes are discussed. Also, the effects of an axial magnetic field on free-electron and shallow-donor states are analyzed.