Thesis presented November 10, 2022
Abstract:
Currently, III-Nitrides semiconductor based micro-LEDs (µLEDs) are a source of constant interest from industry, since they should become the new components which will be integrated in the next generation of display and augmented reality applications. By comparison with the competing OLEDs (Organic LEDs) and LCDs (Liquid Crystal Displays), the µLED technology should make possible displays exhibiting lower power consumption, higher lifetime and better image resolution. However, manufacturing of such devices is extremely expensive, and their light emission efficiency is so far very limited, in particular for red-light emission. Thus, this technology is still immature and non-marketable. In this context, nanowires based InGaN µLEDs allowing one to emit RGB pixel light (Red, Green, Blue), appear promising to improve the performances of this technology. In order to optimize this target, this PhD work focuses on the identification, the limitation and the quantification of non-radiative defects responsible of the nanowires µLEDs droop efficiency. To this end, a full study was made on nanowires based InGaN/GaN heterostructures, combining the material elaboration by Molecular Beam Epitaxy (MBE) and its optical, chemical, structural and electrical properties characterization. The first chapter of this manuscript consists of a state of the art of the nitrides materials properties as well as of the devices used for the visible light emission. The second chapter introduces the nanowires elaboration and characterization methods used during this PhD. The third chapter is devoted to the identification and removal of non-radiative recombination centers limiting the nanowires luminescence efficiency. Using several optical characterization techniques, such as cathodoluminescence (CL) and micro-photoluminescence (µPL), we show that the addition of an (In)GaN UL inside the nanowires structure significantly improves their optical properties. The surface recombination impact is also studied for various visible emission wavelengths. The combination of these improvements is characterized by a significant increase of the Internal Quantum Efficiency (IQE) of our structures. Using time-correlated cathodoluminescence (TC-CL), a source of non-radiative defects, reflected in the luminescence inhomogeneity at the nanowire scale and for which the density is dependent of the nanowire diameter, is identified. Its origin is studied in the fourth chapter. A statistical analysis of CL intensities at the NW scale, combined with the development of a model allowed us to identify the nature of this defect responsible of the luminescence inhomogeneity previously observed, and to quantify it. A study based on the growth parameters variation confirmed the nature of these non-radiative defect identified with the model. This manuscript ends with a chapter presenting the electrical properties of the LEDs devices composed of the studied heterostructures. In order to remove the sources of electrical losses observed, some solutions are proposed.
Keywords:
Nanowires, nitride semiconductors, InGaN, GaN