Thomas Fjord Kjaersgaard Weatherley
Today's world depends on optoelectronic devices: light-emitting diodes illuminate our houses and backlight the displays on our gadgets, while laser diodes underpin fibre-optic communication. Such optoelectronic devices rely on crystalline semiconductor heterostructures which almost universally possess quantum wells (QWs) as the core light-emitting region. Point defects (PDs) in the QW crystal can drastically reduce device efficiencies and lifetimes; consequently, detecting and classifying PDs is essential to semiconductor industry. Nevertheless, it remains a serious challenge to isolate and characterise these nonradiative PDs due to the difficulty of pinpointing atomic-scale defects in bulk material. This thesis individually resolves nonradiative PDs buried in QWs and elucidates their nanoscale impact on the efficiency of the well, employing InGaN/GaN single QWs as a model system.Initially, we optimise our cathodoluminescence (CL) microscopy parameters and sample design to achieve sub-100 nm spatial resolution. Additionally, including an In-containing layer under the InGaN QW allows us to tune the PD density in the well from sample to sample. CL hyperspectral mapping of the lowest PD density specimen reveals anisotropic carrier transport in the QW due to step-edge-induced thickness fluctuations; the same dataset also demonstrates substantial carrier diffusion lengths at 300 K, overturning the common assumption of room-temperature carrier localisation in InGaN. Having understood the inherent carrier dynamics in the absence of PDs, we then investigate how rising PD densities perturb QW properties.We individually spatially resolve two different types of PD in the well, one of which acts as a critical nonradiative recombination centre with a strong impact on the CL intensity. A reproducible counting procedure is applied to discern the density of this PD directly from QW CL images, successfully tracking the concentration as it increases by over a factor of 30 with decreasing In-containing underlayer thickness. Analysing QW CL peak energy images clearly shows that this rising PD density leads to a severe shortening of the carrier diffusion length in the well, overwhelming the intrinsic carrier transport. Meanwhile, we evidence that increasing the CL carrier density in the QW saturates the nonradiative recombination at each PD until they are barely detectable in the intensity images.In our final study, we harness time-resolved CL to acquire a luminescence decay map which displays how carrier diffusion and nonradiative recombination evolve around individual PDs on the nanometre- and nanosecond-scale. Developing a complete diffusion-recombination model allows us to derive the inherent properties of a single isolated PD in the map. We extract a significant PD capture cross-section of ~ 6 nm^2, among the largest reported for PDs in the III-V semiconductor system. Furthermore, our results indicate that nonradiative recombination at this PD becomes phonon-limited when capture times fall below around 17 ps; to our knowledge, this is the first experimental evidence of such a phenomenon. Overall, this thesis serves as a proof-of-concept for nanoscale detection of PDs in QWs and sheds light on their nonradiative-recombination physics.
EPFL2023
