Donor states in tunnel-coupled quantum wells

We study the energy spectrum of the impurity states in tunnel-coupled doubl e quantum wells for Coulomb and short-range donor potentials. We calculate the impurity contribution and the density of states and detect the transfor mation of a localized donor state into a resonant state when the binding en ergy of the donor in an isolated quantum well is less than the separation o f the energy levels of the double quantum wells. In the opposite case, wher e the binding energy is greater than the level separation, there is tunneli ng repulsion between adjacent impurity levels, with the degree of degenerac y of the levels changing when there is tunneling mixing of the ground and e xcited impurity states from different wells. Resonant states emerge in an a symmetric double quantum well, while in a symmetric double quantum well the impurity level at the barrier's center proves to be localized even against the background of the continuum. The calculations are based on a general e xpression for the impurity contribution to the density of states in terms o f a 2-by-2 matrix Green's function, i.e., only a pair of tunnel-coupled lev els of the double quantum wells is taken into account. For an impurity with a short-range potential, we derive a matrix generalization of the Koster-S later solution, while the impurity with a Coulomb potential is analyzed by using the approximation of a narrow resonance and close arrangement of the repulsive levels. (C) 1999 American Institute of Physics. [S1063-7761(99)01 304-9].