Christian Mounir

Role of substrate quality on the performance of semipolar (112¯2) InGaN light-emitting diodes

We compare the optical properties and device performance of unpackaged InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) emitting at ∼430 nm grown simultaneously on a high-cost small-size bulk semipolar ( 112¯2) GaN substrate (Bulk-GaN) and a low-cost large-size ( 112¯2) GaN template created on patterned ( 101¯2) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ∼105 cm−2–106 cm−2 and basal-plane stacking fault (BSF) density of 0 cm−1, while the PSS-GaN substrate has the TDD of ∼2 × 108 cm−2 and BSF density of ∼1 × 103 cm−1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density.

InGaN/GaN quantum wells for polariton laser diodes: Role of inhomogeneous broadening

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 origi...

Optical properties and internal quantum efficiency of InGaN/GaN core-shell microrods for solid state lighting

We investigate, via temperature and excitation density dependent quasi-resonant confocal micro-photoluminescence, the optical properties and internal quantum efficiency (IQE) of InGaN/GaN single quantum wells (QWs) on Ga-polar GaN microrods selectively grown by continuous flow metal organic vapor phase epitaxy on patterned SiO2/n-GaN/sapphire template. Seven samples were grown with different growth parameters for the InGaN/GaN QW. The homogeneity of their optical properties is analyzed by mappings along the m-plane facet of the microrods in order to get insight on the growth mechanisms of the shell. Excitation density dependent measurements show that the IQE is affected by the high doping level of the core, which is required to grow such high aspect-ratio structures. Local IQEs between 15±1 % near the tip and 44±5 % near the base of microrods are estimated from measurements at room and low temperature. By comparison with results reported on planar c-plane QWs, we conclude that the radiative recombination ...

Micro-pixel light emitting diodes: Impact of the chip process on microscopic electro- and photoluminescence

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.

Polarization-resolved micro-photoluminescence investigation of InGaN/GaN core-shell microrods

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 o...

Spatially dependent carrier dynamics in single InGaN/GaN core-shell microrod by time-resolved cathodoluminescence

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.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.

Determination of the radiative efficiency of GaN-based light-emitting diodes via bias dependent resonant photoluminescence

We report a method to determine the radiative efficiency (ηrad) of GaN-based light-emitting diodes using excitation density and bias dependent room temperature photoluminescence (PL) measurements selectively exciting the active region. Considering carrier escape by tunnelling out of the active region, we extrapolate the generation rate of charge carriers from photocurrent measurements under reverse bias. A model describing the recombination of carriers including phase-space filling is then fitted to excitation density dependent PL data obtained under forward bias to extract ηrad. Results show that ηrad vs. carrier density is asymmetric around its maximum due to phase-space filling.

On the optical polarization properties of semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells

In the framework of k · p-theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with calculations. We observe a strong correlation between DLPs and spectral widths, which indicates that inhomogeneous broadening affects the optical polarization properties. Considering indium content fluctuations, QW thickness fluctuations, and the localization length of charge carriers, we argue that different broadenings apply to each subband and introduce a formalism using effective masses to account for inhomogeneous broadening in the calculation of the DLP. We conclude that the different DLP of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) QWs might be related to different effective broadenings of their valence subbands induced by the rougher upper QW interface in ( 20 2 ¯ 1 ), by the larger sensitivity of holes to this upper interface due to the polarization field in ( 20 2 ¯ 1 ), and/or by the different degrees of localization of holes.In the framework of k · p-theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with...