Imaging non-radiative point defects buried in quantum wells using cathodoluminescence

Crystallographic point defects (PDs) can dramatically decrease the efficiency of optoelectronic semiconductor devices, many of which are based on quantum well (QW) heterostructures. However, spatially resolving individual non-radiative PDs buried in such QWs has so far not been demonstrated. Here, using high-resolution cathodoluminescence (CL) and a specific sample design, we spatially resolve, image, and analyse non-radiative PDs in InGaN/GaN QWs. We identify two different types of PD by their contrasting behaviour with temperature, and measure their densities from $10^{14}$ cm$^{-3}$ to as high as $10^{16}$ cm$^{-3}$. Our CL images clearly illustrate the interplay between PDs and carrier dynamics in the well: increasing PD concentration severely limits carrier diffusion lengths, while a higher carrier density suppresses the non-radiative behaviour of PDs. The results in this study are readily interpreted directly from CL images, and represent a significant advancement in nanoscale PD analysis.Comment: Main text: 8 pages, 6 figures. Supplementary: 11 pages, 8 figure

Coarse and fine-tuning of lasing transverse electromagnetic modes in coupled all-inorganic perovskite quantum dots

10.1007/s12274-020-3051-yNano Research141108-11

Enhanced photoluminescence quantum yield of MAPbBr(3) nanocrystals by passivation using graphene

Diminishing surface defect states in perovskite nanocrystals is a highly challenging subject for enhancing optoelectronic device performance. We synthesized organic/inorganic lead-halide perovskite MAPbBr(3) (MA = methylammonium) clusters comprising nanocrystals with diameters ranging between 20-30 nm and characterized an enhanced photoluminescence (PL) quantum yield (as much as ~ 7 times) by encapsulating the MAPbBr(3) with graphene (Gr). The optical properties of MAPbBr(3) and Gr/MAPbBr(3) were investigated by temperature-dependent micro-PL and time-resolved PL measurements. Density functional theory calculations show that the surface defect states in MAPbBr(3) are removed and the optical band gap is reduced by a 0.15 eV by encapsulation with graphene due to partial restoration of lattice distortions

Resonantly Pumped Bright-Triplet Exciton Lasing in Cesium Lead Bromide Perovskites

The surprising recent observation of highly emissive triplet-states in lead halide perovskites accounts for their orders-of-magnitude brighter optical signals and high quantum efficiencies compared to other semiconductors. This makes them attractive for future optoelectronic applications, especially in bright low-threshold nanolasers. While nonresonantly pumped lasing from all-inorganic lead-halide perovskites is now well-established as an attractive pathway to scalable low-power laser sources for nano-optoelectronics, here we showcase a resonant optical pumping scheme on a fast triplet-state in CsPbBr3 nanocrystals. The scheme allows us to realize a polarized triplet-laser source that dramatically enhances the coherent signal by 1 order of magnitude while suppressing noncoherent contributions. The result is a source with highly attractive technological characteristics, including a bright and polarized signal and a high stimulated-to-spontaneous emission signal contrast that can be filtered to enhance spectral purity. The emission is generated by pumping selectively on a weakly confined excitonic state with a Bohr radius similar to 10 nm in the nanocrystals. The exciton fine-structure is revealed by the energy-splitting resulting from confinement in nanocrystals with tetragonal symmetry. We use a linear polarizer to resolve 2-fold nondegenerate sublevels in the triplet exciton and use photoluminescence excitation spectroscopy to determine the energy of the state before pumping it resonantly