InGaN Single Quantum Wells Research Articles

High‐Performance and Temperature‐Stable InGaN Single‐Quantum‐Well Red Light‐Emitting Diodes via Selective Hydrogen Passivation

Herein, a selective passivation of p‐GaN via hydrogen plasma treatment for InGaN single‐quantum‐well (SQW) red light‐emitting diodes (LEDs) is reported. Insulating regions are formed on the p‐GaN top surface via hydrogen plasma treatment, suppressing current injection beneath the p‐pad and along the mesa perimeter to increase light output and mitigate non‐radiative recombination. The fabricated LEDs demonstrate a high on‐wafer light output power density of >88 mW cm−2, a peak on‐wafer external quantum efficiency of 0.65%, and on‐wafer wall‐plug efficiency of 0.41% with a 645 nm peak emission wavelength at 10 mA (7.2 A cm−2) current injection. Further, the temperature dependence of InGaN SQW red LEDs is compared with their AlGaInP counterparts. InGaN SQW red LEDs exhibit a high characteristic temperature of 208 K and a small redshift coefficient of 0.072 nm K−1 at 72 A cm−2 current injection, which are almost 3 and 2 times better than the characteristics of AlGaInP red LEDs, respectively.

Absolute evaluation of internal and external quantum efficiencies and light extraction efficiency in InGaN single quantum wells by simultaneous photoacoustic and photoluminescence measurements combined with integrating-sphere method

Absolute evaluation of internal and external quantum efficiencies and light extraction efficiency in InGaN single quantum wells by simultaneous photoacoustic and photoluminescence measurements combined with integrating-sphere method

Time-correlated luminescence blinking in InGaN single quantum wells

The blinking phenomenon in InGaN single quantum wells is a phenomenon where localized photoluminescence changes over time. Understanding its physics is important for the manufacture of more efficient light emission diodes. We present a study using two InGaN single quantum well samples, emitting at 460 and 510 nm wavelength, respectively. We confirmed that the luminescence intensity fluctuates in localized blinking regions, and we found that these optical variations are not random but are instead correlated in pairs, with either positive or negative coefficient, to a distant reference blinking point. Measurements were performed to obtain standard deviation and cross-correlation maps. Invoking the quantum confined Stark effect, we realized a simple phenomenological model that shows how charge carriers are exchanged among pairs of adjacent opposite correlation regions. As a result, it is suggested that the phenomenon is caused by fluctuations in the number of these exchanged carriers. Our model gives an explanation for the blinking phenomenon in InGaN single quantum wells, and it is important for a deeper understanding to InGaN-based materials.

Impact of nanoscale fluctuations and cap-layer thickness in buried InGaN single quantum wells probed by tip-enhanced Raman scattering

Ternary semiconductors such as InGaN thin films, quantum wells, and superlattices commonly exhibit alloy fluctuations that become increasingly pronounced with higher In-content. The thickness fluctuations of quantum wells and their thin cap-layers further introduce nanoscale inhomogeneities that alter the potential landscape. In this work, we present a combined theoretical and experimental study of InGaN single quantum wells with thin GaN cap-layers to unravel the influence of cap-layer thickness, compositional inhomogeneity, and thickness fluctuations on their electronic and optical properties. A pronounced spectral shift of quantum well emission for thin cap-layers between 1 and 10 nm is observed by micro-photoluminescence spectroscopy. The origin of this shift is explained by calculations of electronic band profiles and probability density overlap of carriers in the quantum well. The impact of alloy fluctuations and homogeneity for different cap-layer thicknesses is studied on both the microscale and nanoscale using UV micro-Raman scattering and tip-enhanced Raman spectroscopy (TERS). On the microscale, the alloy composition as determined by micro-Raman mapping appears very homogeneous except for the thinnest 1 nm cap-layer where small fluctuations are visible. On the nanoscale, TERS reveals local fluctuations on a 20–30 nm length scale. The influence of the cap-layer thickness on the TERS spectra is discussed regarding both the nanoscale homogeneity and the depth resolution of the near-field Raman scattering technique. Our results demonstrate the capabilities of TERS to resolve nanoscale thickness fluctuations and compositional inhomogeneities in ultra-thin semiconductor layers, even when they are buried by thin cap-layers with thicknesses below 10 nm.

High internal quantum efficiency of long wavelength InGaN quantum wells

Time-resolved and quasi-cw photoluminescence (PL) spectroscopy was applied to measure the internal quantum efficiency (IQE) of c-plane InGaN single quantum wells (QWs) grown on sapphire substrates using metal-organic chemical vapor deposition. The identical temperature dependence of the PL decay times and radiative recombination times at low temperatures confirmed that the low temperature IQE is 100%, which allowed evaluation of the absolute IQE at elevated temperatures. At 300 K, the IQE for QWs emitting in green and green–yellow spectral regions was more than 60%. The weak nonradiative recombination in QWs with a substantial concentration of threading dislocations and V-defects (∼2 × 108 cm−2) shows that these extended defects do not notably affect the carrier recombination.

Study on higher-energy emission observed locally around V-pits on InGaN/GaN quantum wells grown on moderate-temperature GaN

We investigated the origin of the multi-peak higher-energy (HE) emission observed in the local photoluminescence (PL) spectra of InGaN/GaN quantum wells (QWs) grown on moderate-temperature-grown GaN (MT-GaN) layers and evaluated the relationship among the energy difference between the HE emission and InGaN/GaN single QW (SQW) emission corresponding to the potential barrier height and the V-pit diameter. The distribution of HE emissions and dark spots suggested the formation of so-called potential barriers around dislocations in the InGaN SQWs on the MT-GaN layers. Multiple HE emissions were observed in the local PL spectra acquired from the InGaN SQWs on the MT-GaN layers as well in the case of InGaN multiple QW on MT-GaN previously reported. This observation suggested that the origin of multi-peak HE emissions is an in-plane variation in In composition and/or well thickness. The energy difference between InGaN emission from c-plane QW and HE emission from V-pit facet increased as the MT-GaN layer thickness or V-pit diameter increased. The origin of both the multiple HE emissions and the increase in the energy difference with V-pit diameter in InGaN/GaN SQW on MT-GaN was probably the diffusion of group III atoms between different facets of the InGaN SQWs in sub-micrometer scale V-pits.

Selective area growth of N-polar GaN nanorods by plasma-assisted MBE on micro-cone-patterned c-sapphire substrates

The site-controlled selective area growth of N-polar GaN nanorods (NR) was developed by plasma-assisted MBE (PA MBE) on micro-cone-patterned sapphire substrates (µ-CPSS) by using a two-stage growth process. A GaN nucleation layer grown by migration enhanced epitaxy provides the best selectivity for nucleation of NRs on the apexes of 3.5-µm-diameter cones, whereas the subsequent growth of 1-μm-high NRs with a constant diameter of about 100nm proceeds by standard high-temperature PA MBE at nitrogen-rich conditions. These results are explained by anisotropy of the surface energy for GaN of different polarity and crystal orientation. The InGaN single quantum wells inserted in the GaN NRs grown on the µ-CPSS demonstrate photoluminescence at 510nm with a spatially periodic variation of its intensity with a period of ∼6µm equal to that of the substrate patterning profile.

The relation between photoluminescence properties and gas pressure with [0001] InGaN single quantum well systems

We show for the first time that photoluminescence of InGaN single quantum wells (SQW) devices is related to the gas pressure in which the sample is immersed, also we give a model of the phenomena to suggest a possible cause. Our model shows a direct relation between experimental behavior and molecular coverage dynamics. This strongly suggests that the driving force of photoluminescence decrease is oxygen covering the surface of the device with a time dynamics that depends on the gas pressure. This aims to contribute to the understanding of the physical mechanism of the so-called optical memory effect and blinking phenomenon observed in these devices.

Nanoscopic Insights into InGaN/GaN Core-Shell Nanorods: Structure, Composition, and Luminescence.

Nitride-based three-dimensional core-shell nanorods (NRs) are promising candidates for the achievement of highly efficient optoelectronic devices. For a detailed understanding of the complex core-shell layer structure of InGaN/GaN NRs, a systematic determination and correlation of the structural, compositional, and optical properties on a nanometer-scale is essential. In particular, the combination of low-temperature cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM), and quantitative high-angle annular dark field imaging enables a comprehensive study of the nanoscopic attributes of the individual shell layers. The investigated InGaN/GaN core-shell NRs, which were grown by metal-organic vapor-phase epitaxy using selective-area growth exhibit an exceptionally low density of extended defects. Using highly spatially resolved CL mapping of single NRs performed in cross-section, we give a direct insight into the optical properties of the individual core-shell layers. Most interesting, we observe a red shift of the InGaN single quantum well from 410 to 471 nm along the nonpolar side wall. Quantitative STEM analysis of the active region reveals an increasing thickness of the single quantum well (SQW) from 6 to 13 nm, accompanied by a slight increase of the indium concentration along the nonpolar side wall from 11% to 13%. Both effects, the increased quantum-well thickness and the higher indium incorporation, are responsible for the observed energetic shift of the InGaN SQW luminescence. Furthermore, compositional mappings of the InGaN quantum well reveal the formation of locally indium rich regions with several nanometers in size, leading to potential fluctuations in the InGaN SQW energy landscape. This is directly evidenced by nanometer-scale resolved CL mappings that show strong localization effects of the excitonic SQW emission.

Structural and Optical Emission Uniformity of m-Plane InGaN Single Quantum Wells in Core–Shell Nanorods

Controlling the long-range homogeneity of core–shell InGaN/GaN layers is essential for their use in light-emitting devices. This paper demonstrates variations in optical emission energy as low as ∼7 meV·μm–1 along the m-plane facets from core–shell InGaN/GaN single quantum wells as measured through high-resolution cathodoluminescence hyperspectral imaging. The layers were grown by metal organic vapor phase epitaxy on etched GaN nanorod arrays with a pitch of 2 μm. High-resolution transmission electron microscopy and spatially resolved energy-dispersive X-ray spectroscopy measurements demonstrate a long-range InN-content and thickness homogeneity along the entire 1.2 μm length of the m-plane. Such homogeneous emission was found on the m-plane despite the observation of short-range compositional fluctuations in the InGaN single quantum well. The ability to achieve this uniform optical emission from InGaN/GaN core–shell layers is critical to enable them to compete with and replace conventional planar light-e...

Growth mechanisms of plasma-assisted molecular beam epitaxy of green emission InGaN/GaN single quantum wells at high growth temperatures

The results of the growth of thin (∼3 nm) InGaN/GaN single quantum wells (SQWs) with emission wavelengths in the green region by plasma-assisted molecular beam epitaxy are present. An improved two-step growth method using a high growth temperature up to 650 °C is developed to increase the In content of the InGaN SQW to 30% while maintaining a strong luminescence intensity near a wavelength of 506 nm. The indium composition in InGaN/GaN SQW grown under group-III-rich condition increases with increasing growth temperature following the growth model of liquid phase epitaxy. Further increase in the growth temperature to 670 °C does not improve the photoluminescence property of the material due to rapid loss of indium from the surface and, under certain growth conditions, the onset of phase separation.

Stacking faults and interface roughening in semipolar (202¯1¯) single InGaN quantum wells for long wavelength emission

The microstructure of InGaN single quantum wells (QWs) grown in semipolar (202¯1¯) orientation on GaN substrates was studied by transmission electron microscopy. Stress relaxation in the lattice mismatch InxGa1−xN layer was realized by forming partial misfit dislocations associated with basal plane stacking faults (BPSFs). For given composition x = 0.24, BPSFs formation was observed when the QW thickness exceeded 4 nm. The high density of partial threading dislocations that bound the BPSFs is detrimental to light-emitting device performance. Interface roughening (faceting) was observed for both upper and lower QW interfaces (more pronounced for upper interface) and was found to increase with the thickness of the QW. BPSFs had a tendency to nucleate at roughened interface valleys.

Atomic-scale nanofacet structure in semipolar and InGaN single quantum wells

Atomic-scale nanofacets were observed in semipolar and InGaN quantum wells (QWs)/GaN quantum barriers interfaces. Transmission electron microscopy studies showed that these nanofacets were mainly composed of , and planes, which led to significant fluctuations in QW thickness. Atom probe tomography studies were carried out to visualize the nanofacet structure. The In composition in the InxGa1−xN alloys followed a binominal distribution despite the formation of the nanofacet structure. One-dimensional (1D) Schrödinger–Poisson drift-diffusion simulation showed that these nanofacets and associated QW thickness fluctuations will lead to a large wavelength shift and a broadened spectral linewidth for semipolar QWs.

Defect recombination induced by density-activated carrier diffusion in nonpolar InGaN quantum wells

We report on the observation of carrier-diffusion-induced defect emission at high excitation density in a-plane InGaN single quantum wells. When increasing excitation density in a relatively high regime, we observed the emergence of defect-related emission together with a significant efficiency reduction of bandedge emission. The experimental results can be well explained with the density-activated carrier diffusion from localized states to defect states. Such a scenario of density-activated defect recombination, as confirmed by the dependences of photoluminescence on the excitation photon energy and temperature, is a plausible origin of efficiency droop in a-plane InGaN quantum-well light-emitting diodes.

Emission characteristics of single InGaN quantum wells on misoriented nonpolar m-plane bulk GaN substrates

InGaN single quantum wells (SQWs) grown on m-plane bulk GaN substrates show significant differences in peak emission wavelength when grown on substrates oriented nominally on-axis compared to substrates with small intentional misorientations (miscuts) towards the orthogonal −c-direction or a-direction. SQWs on substrates intentionally miscut toward the a-direction emit longer wavelengths than those with miscuts towards the −c-direction in a variety of identical growth conditions, while SQWs on nominally on-axis m-plane with pyramidal hillocks features display emission characteristics of both. These preliminary co-loaded growth studies may provide insight into broad or anomalous wavelength emission observed on nonpolar GaN-based visible light emitters and suggest opportunities for improving LED and laser diode device performance on this naturally occurring crystal plane.

Fabrication of GaN structures with embedded network of voids using pillar patterned GaN templates

In this paper we report on the MOCVD growth and characterization of GaN structures and InGaN single quantum wells grown on pillar patterned GaN/sapphire templates. During the regrowth a network of voids was intentionally formed at the interface of sapphire substrate and GaN epitaxial layer. The regrowth process was found to decrease the threading dislocation density of the overgrown layer. The quantum well sample grown on patterned template showed significantly higher optical output in photoluminescence measurements compared to the reference sample with identical internal quantum efficiency characteristics. We attribute the increase to enhanced light extraction efficiency caused by strong scattering and redirection of light from the scattering elements.

Spectrally and time‐resolved cathodoluminescence microscopy of semipolar InGaN SQW on (11$\overline {2} $2) and (10$\overline {1} $1) pyramid facets

AbstractSemipolar InGaN/GaN single quantum wells (SQWs) grown on {11$\overline {2} $2} planes of an inverted pyramid surface and {10$\overline {1} $1} facets of single self assembled pyramids have been studied by spatially, spectrally, and time‐resolved cathodoluminescence (CL) microscopy. Mappings of local spectra and local transients provide the distribution of spectral and time‐resolved luminescence properties by peak wavelength images, time delayed CL images (TDCLIs), and initial lifetime maps. The SQW on inverse pyramids exhibit strong local differences in recombination kinetics – two orders of magnitude change in initial lifetime – correlated with a giant shift in emission energy of ∼1 eV along a facet. For single pyramids a migration process of indium adatoms from the upper facet to the edges leads to an emission of longer wavelengths at the edges and shorter wavelengths at the upper facet with respect to the base.

Multicolour luminescence from InGaN quantum wells grown over GaN nanowire arrays bymolecular-beam epitaxy

The luminescence of InGaN single quantum wells grown bymolecular-beam epitaxy under fixed conditions over a series ofc-axis GaN nanowire arrays with different geometrical parameters was studied. For arrayswith variable GaN average wire diameters and fixed wire densities, the InGaN luminescencepeak shifted to higher energy with decreasing wire diameter. It is shown that this trendcannot be attributed to lateral quantum confinement or diameter-dependent InGaN strain.For arrays with variable wire densities and fixed average diameters, the InGaN emissionappeared as two distinct bands of different colours, the relative intensities of whichdepended on the wire density. By optimizing both the GaN wire density and InGaN growthconditions, the colours of the two different bands were combined to realize phosphor-freewhite light-emitting diodes. The mechanisms for the dependence of the InGaNluminescence on the geometrical parameters of the GaN nanowire array are discussed.

Growth and Characterization of High Quality a-Plane InGaN/GaN Single Quantum Well Structure Grown by Multibuffer Layer Technique

Nonpolar (1120) a-plane InGaN/GaN single quantum well (SQW) structure has been grown using a multi buffer layer on a (1102) r-plane sapphire substrate. The effects on the lattice constants of the a-plane GaN template caused by reactor pressure and V/III ratio of the first buffer layer were studied to improve the crystal quality. Under optimum growth conditions, the full widths at half maximum (FWHMs) of (1120) X-ray rocking curves along the c- and m-axis orientations were 430 and 530 arcsec, respectively. The optical characteristics of the nonpolar InGaN SQW determined from excitation-power-dependent photoluminescence and temperature-dependent photoluminescence spectra showed the absence of the quantum-confined Stark effect.

Transient memory effect in the photoluminescence of InGaN single quantum wells

The transition to maximum photoluminescence of InGaN single quantum wells is a phenomena that has time constants in the range of few seconds. Using a systematic illumination/darkening procedure we found that these characteristics are related to previous stimulations as if the sample has a memory of past illumination events. Choosing opportune time sequences, time constants were observed to vary more than 100%. These facts suggest the presence of carrier trapping/de-trapping processes that act beyond the single illumination event, accumulating over time in a complex effect.