Shallow states of donor impurity at the surface of isotropic semiconductors

Abstract
The perturbative variational method is applied to the surface impurity states in semiconductors. It is found that this method can be used to calculate any state, no matter how high it may be, and the surface impurity levels are obtained from those of the hydrogen atom simply by a redefinition of the charge. As a concrete example of the application numerical results are obtained for surface impurity states in GaAs using this method.