The optical properties of InGaN/GaN core-shell microrods are studied by time-resolved cathodoluminescence. Probing the carrier dynamics along the length of the rod from 4 to 300 K enables us to decompose radiative (τr) and non-radiative (τnr) lifetimes. At 300 K, τnr decreases from 500 at the bottom of the rod to 150 ps at its top. This variation results from an increased In-content in the upper part of the rod that causes a higher density of point defects. We further observe that thanks to the use of nonpolar m-plane growth, τr remains below 1.5 ns up to room temperature even with a thick active layer, which is promising for pushing the onset of the efficiency droop to higher current densities.
We investigate the optical emission properties of the active InGaN shell of high aspect-ratio InGaN/GaN core-shell microrods (μRods) by confocal quasi-resonant polarization-resolved and excitation density dependent micro-photoluminescence (μPL). The active shell, multiple thin InGaN/GaN quantum wells (MQWs), was deposited on GaN μRods selectively grown by metal organic vapor phase epitaxy on patterned SiO2/n-GaN/sapphire template. High spatial resolution mappings reveal a very homogeneous emission intensity along the whole μRods including the tip despite a red-shift of 30 nm from the base to the tip along the 8.6 μm-long m-plane sidewalls. Looking at the Fabry-Perot interference fringes superimposed on the μPL spectra, we get structural information on the μRods. A high degree of linear polarization (DLP) of 0.6–0.66 is measured on the lower half of the m-plane side facets with a slight decrease toward the tip. We observe the typical drop of the DLP with an excitation density caused by degenerate filling of valence bands (Fermi regime). Local internal quantum efficiencies (IQEs) of 55±11 % up to 73±7 % are estimated on the m-plane facet from measurements at low temperature. Finally, simultaneously fitting the DLP and IQE as a function of the excitation density, we determine the carrier density inside the active region and the recombination rate coefficients of the m-plane MQWs. We show that phase-space filling and the background carrier density have to be included in the recombination rate model.
Contrary to the case of III-nitride based visible light-emitting diodes for which the inhomogeneous linewidth broadening characteristic of InGaN-based multiple quantum well (MQW) heterostructures does not appear as a detrimental parameter, such a broadening issue can prevent a microcavity (MC) system entering into the strong light-matter coupling regime (SCR). The impact of excitonic disorder in low indium content (x ∼ 0.1) InxGa1–xN/GaN MQW active regions is therefore investigated for the subsequent realization of polariton laser diodes by considering both simulations and optical characterizations. It allows deriving the requirements for such MQWs in terms of absorption, emission linewidth, and Stokes shift. Systematic absorption-like and photoluminescence (PL) spectroscopy experiments are performed on single and multiple In0.1Ga0.9N/GaN quantum wells (QWs). Micro-PL mappings reveal a low temperature PL linewidth of ∼30 meV, compatible with SCR requirements, for single QWs for which the microscopic origin responsible for this broadening is qualitatively discussed. When stacking several InGaN/GaN QWs, a departure from such a narrow linewidth value and an increase in the Stokes shift are observed. Various possible reasons for this degradation such as inhomogeneous built-in field distribution among the QWs are then identified. An alternative solution for the MC design to achieve the SCR with the InGaN alloy is briefly discussed.
We investigated the influence of a μ-pixelated chip process on the photoluminescence (PL) and electroluminescence (EL) of a monolithic InGaN/GaN based blue light emitting diode with a continuous n-GaN layer. Particularly, we observed the impact of the metallic p-contact on the PL emission wavelength. A PL wavelength shift in the order of 10 nm between contacted and isolated areas was assigned to screening of internal piezoelectric fields due to charge carrier accumulation. μPL and μEL mappings revealed correlated emission wavelength and intensity inhomogeneities, caused by the epitaxial growth process. The edges of single pixels were investigated in detail via resonant confocal bias-dependent μPL. No influence on the intensity was observed beyond 300 nm away from the edge, which indicated a good working edge passivation. Due to the low lateral p-GaN conductivity, the μPL intensity was enhanced at isolated areas.
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