Density Functional Theory Investigation of Trap States in Indium Phosphide Quantum Dots

Cadmium selenide, one of the premier quantum dot (QD) materials for light emission, solar cells, and biomedical imaging, is highly toxic to both humans and the environment. Alternative, nontoxic III-V QDs are held back by a high density of surface-localized mid-gap states, which trap photoexcited charge carriers and reduce device efficiency. Despite the prevalence of density functional theory (DFT) in past investigations of surface trapping in the II-VI family of QDs, very few studies have applied DFT to the substantially different III-V family of QDs. In this work, we show that trap states can form in indium phosphide QDs as the result of both 2- and 3-coordinate surface indium and phosphorous atoms. However, not all 3-coordinate surface atoms lead to localized midgap states; in particular, structures with many 3-coordinate surface atoms often form delocalized surface states near the band edge. Moreover, we observe the formation of both trap-inducing phosphorous dimers and trap-inert indium dimers on the surface of our model dots. Ultimately, our results suggest that a single-atom picture is insufficient to explain the surface trapping in indium phosphide QDs, hinting at new passivation strategies for improving nontoxic III-V QD devices.