3D Atomic Scale Correlative microscopy and Electronic Simulation of light Emitters – ASCESE-3D

Current energy challenges lie not only in energy conversion and storage, but also in reducing energy consumption. The use of light emitting diodes (LEDs) is widely spread today in lighting, signaling, displays, computing, telecommunication or disinfection. The power conversion efficiency of blue LEDs can exceed 80%. However, the efficiency drops dramatically for emission wavelength below 350 nm for UV AlGaN and between 500 and 600 nm for InGaN based materials. Improving the LED efficiency is a global challenge when it comes to reducing the energy consumption. Sustainable LED devices are necessary for a better social impact.
In semiconductor LEDs, the localization of carriers in quantum confined systems, i.e. quantum dots (QDs) or quantum wells (QWs), plays a major role in the optical performance. Various structural factors (alloy fluctuations, interface roughness, interdiffusion, dislocations or strain) can reduce/enhance the confinement of carriers leading to a modified carrier lifetime, emission linewidth and Auger recombination rate, which is at the origin of the efficiency droop of LEDs. To better understand and drastically enhance the performance of LEDs, particularly in the UV and green-red wavelength ranges, it is important to identify and lift the technical barrier associated with structural/chemical inhomogeneities in the active region. This requires advance characterization methodologies that can provide structural/chemical information at the nanometer scale, and a correlation with the light emission properties.
In this project, we aim at developing an original methodology for correlating multi-spectroscopic and microscopic analysis of LEDs based on III-nitride semiconductors (GaN and its alloys), which will lead to improved physical simulations of the device band structure. The goal is to understand the performance of LEDs with regard to their structural and chemical parameters. Two different approaches will be implemented for the investigation of experimental features: (i) ex-situ cathodoluminescence (CL), transmission electron microscopy (TEM), atom probe tomography (APT) on the same specimen and (ii) in-situ micro-photoluminescence (µPL) and APT. The ultimate goal of ASCESE 3D is to obtain a three-dimensional (3D) compositionally-resolved image with sub-nanometer spatial resolution and the emission spectrum of the same light emitting structure, and correlate the structural and optical properties with theoretical calculations.
Within ASCESE 3D, the methodology will be applied first on state-of-the-art GaN nanowires containing single QD emitters, to validate and strengthen the analysis procedures. Simulations of single emitters are self-consistent as they do not depend on the surrounding system. Therefore, they can be considered as a model system for the validation of the methodology. In-situ measurements of the emission spectra will allow us to follow and understand the effect of the local environment. In a second step, the methodology will be applied to both UV and visible LEDs based on AlGaN and InGaN nanostructures, respectively.