Pressure and geometrical shape effects on donor impurity self-polarization in a $$\:{\mathbf{G}\mathbf{a}\mathbf{A}\mathbf{s}/\mathbf{G}\mathbf{a}}_{1-\varvec{\upmu\:}}{\mathbf{A}\mathbf{l}}_{\varvec{\upmu\:}}\mathbf{A}\mathbf{s}$$ pillbox surrounded by $$\:{\mathbf{G}\mathbf{a}}_{1-\varvec{\upupsilon\:}}{\mathbf{A}\mathbf{l}}_{\varvec{\upupsilon\:}}\mathbf{A}\mathbf{s}$$