Interfacial Engineering Increases the Photoluminescence Efficiency and the Biexciton Auger Lifetime in InP/ZnSe QDs

Quantum dots (QDs) are zero-dimensional materials suitable for advanced optical applications. Especially Cd-based QDs have shown rousing results, but are difficult to implement in end-user applications due to restrictions on toxic elements, such as Cd. The most widely used, unrestricted alternative for light-emission applications are probably InP QDs. However, while light emitting devices using luminescent color conversion or direct electroluminescence by InP QDs show great promise, progress has been glacial for applications relying on stimulated emission. Lasers using Cd-based QDs as the gain material proliferated after research demonstrating that biexcitons – the excitation giving rise to net stimulated emission – in CdSe/CdS core/shell QDs with an alloyed interface exhibited a markedly increased lifetime related to slower Auger recombination. This result suggests that understanding and adjusting the interfacial composition in InP-based core/shell QDs is an important step for obtaining optical gain suitable for laser application from these materials.<br/>In this study, we report on the relation between the Auger recombination rate in InP/ZnSe QDs and the composition of the InP/ZnSe interface, which we adjust through (1) oxidation of the InP core surface and (2) addition of InCl₃ during the ZnSe shell growth. The resulting 4 possible samples – the untreated reference, two single-treatment and one double treatment combinations – have similar sizes and morphologies, featuring an 3 nm in diameter InP core and a 1.8 nm thick shell. However, elemental analysis by Rutherford backscattering spectrometry and solid-state NMR spectroscopy, shows that treatment 1 leads to the formation of oxidized phosphorous at the interface, while treatment 2 results in the incorporation of In in the layers of ZnSe closest to the interface. Interestingly, both separate treatments increase the PLQY from 44% for the reference sample to 51 and 63%, while the double treatment results in an 83% PLQY. Moreover, while the transient absorption decay of the reference sample points towards a rapid loss of biexcitons with an ill-defined rate constant, sample with an oxidized or an In-enriched interface exhibit a well-defined biexciton lifetime of ~40 ps. Most promisingly, the combined sample shows an even slower Auger lifetime of ~64 ps. Building on this result, we analyzed a set of samples with this double interfacial treatment, and a gradually increasing ZnSe shell thickness. This led to a further lengthening of the Auger lifetime to ~108 ps. We conclude that such interfacial treatment provide a promising path for reducing the Auger lifetime of biexcitons in InP-based QDs, thereby opening the path towards lasers using these QDs as the optical gain material.