InP quantum dots: Electronic structure, surface effects, and the redshifted emission

We present pseudopotential plane-wave electronic-structure calculations on InP quantum dots in an effort to understand quantum confinement and surface effects and to identify the origin of the long-lived and redshifted luminescence. We find that (i) unlike the case in small GaAs dots, the lowest unoccupied state of InP dots is the ${\Gamma }_{1c}$-derived direct state rather than the $X_{1c}$-derived indirect state and (ii) unlike the prediction of $\mathbf{k}\cdot \mathbf{p}$ models, the highest occupied state in InP dots has a $1sd$-type envelope function rather than a (dipole-forbidden) $1pf$ envelope function. Thus explanations (i) and (ii) to the long-lived redshifted emission in terms of an orbitally forbidden character can be excluded. Furthermore, (iii) fully passivated InP dots have no surface states in the gap. However, (iv) removal of the anion-site passivation leads to a P dangling bond (DB) state just above the valence band, which will act as a trap for photogenerated holes. Similarly, (v) removal of the cation-site passivation leads to an In dangling-bond state below the conduction band. While the energy of the In DB state depends only weakly on quantum size, its radiative lifetime increases with quantum size. The calculated $\sim 300-\mathrm{meV}$ redshift and the $\sim 18$ times longer radiative lifetime relative to the dot-interior transition for the 26-Å dot with an In DB are in good agreement with the observations of full-luminescence experiments for unetched InP dots. Yet, (vi) this type of redshift due to surface defect is inconsistent with that measured in selective excitation for HF-etched InP dots. (vii) The latter type of (“resonant”) redshift is compatible with the calculated screened singlet-triplet splitting in InP dots, suggesting that the slow emitting state seen in selective excitation could be a triplet state.