is nonetheless worthwhile to note that a very good approximation to the electric field can be obtained with a constant Δ ¯ P ≃ 0 . 077 C/m 2 (for given t$_{QD}$ and R ) in a wide range of t$_{inf}$ and t$_{sup}$ . The value of Δ ¯ P slightly increases with decreasing t$_{QD}$ , up to Δ ¯ P ≃ 0 . 082 C/m 2 for t$_{QD}$ = 1 nm. As discussed above, the QDs might not be empty if they are are too far from the surface 16 FIG. 7: (Color online) (a) The amplitude of the electric field | E$_{QD}$ | in the GaN QD as a function of t$_{inf}$ and t$_{sup}$ ( t$_{QD}$ = 4 nm). (b) The average polarization discontinuity Δ ¯ P deduced from (a) and Eq. (11). The two dots in parenthesis are charged with holes.