Cathodoluminescence studies of the optical properties of a zincblende InGaN/GaN single quantum well

Zincblende GaN has the potential to improve the efficiency of green- and amber-emitting nitride light emitting diodes due to the absence of internal polarisation fields. However, high densities of stacking faults are found in current zincblende GaN structures. This study presents a cathodoluminescence spectroscopy investigation into the low-temperature optical behaviour of a zincblende GaN/InGaN single quantum well structure. In panchromatic cathodoluminescence maps, stacking faults are observed as dark stripes, and are associated with non-radiative recombination centres. Furthermore, power dependent studies were performed to address whether the zincblende single quantum well exhibited a reduction in emission efficiency at higher carrier densities—the phenomenon known as efficiency droop. The single quantum well structure was observed to exhibit droop, and regions with high densities of stacking faults were seen to exacerbate this phenomenon. Overall, this study suggests that achieving efficient emission from zinc-blende GaN/InGaN quantum wells will require reduction in the stacking fault density

Investigation of wurtzite formation in MOVPE-grown zincblende GaN epilayers on AlxGa1−xN nucleation layers

The influence of AlGaN nucleation layers on zincblende GaN epilayers was studied to investigate the formation of wurtzite phase inclusions in the epilayer. GaN epilayers grown on AlGaN nucleation layers with varying aluminum contents suffer from the increasing presence of wurtzite inclusions as the aluminum content of the nucleation layer increases. High-resolution transmission electron microscopy along with four-dimensional scanning transmission electron microscopy is used to investigate the origin of the wurtzite inclusions in the nucleation layer and at the GaN/AlGaN interface. It was observed that a GaN nucleation layer and an Al0.95Ga0.05N nucleation layer grew in the zincblende and wurtzite phase, respectively. These phases were then adopted by the overgrown GaN epilayers. For a GaN epilayer on an Al0.29Ga0.71N nucleation layer, wurtzite inclusions tend to form at the GaN/ Al0.29Ga0.71N interface due to strong {111}-type faceting observed in the zincblende nucleation layer. This strong faceting is correlated with an enrichment of aluminum in the upper part of the nucleation layer, as observed in energy dispersive x-ray spectroscopy, which may influence the kinetics or thermodynamics controlling the surface morpholog

Unraveling the cause of degradation in Cu(In,Ga)Se ₂ photovoltaics under potential induced degradation

Abstract: Copper indium gallium diselenide (CIGS) based technology is actively competing in the global photovoltaic market with high conversion efficiency. Commercial CIGS modules are anticipated to perform on rated output in the field condition for 20 years. Potential induced degradation (PID) is considered as one of the critical concerns among all the current reliability assessment issues. PID accelerated tests have been performed on pre‐commercial CIGS modules to investigate reduction in electrical performance. We report the severe reduction in electrical performance after PID is correlated to the microstructural and chemical properties of the constituent materials. Under extreme PID stress, the cell surface reveals various defects including crater formation. The aim of this article is to explore the consequences of PID induced craters on the efficiency of CIGS solar cells by investigating material degradation kinetics. In this perspective, we present the root cause of PID in CIGS thin‐film modules in relation to microstructural defects by detailed investigation using J‐V analysis, field emission scanning electron microscope (FESEM), Raman spectroscopy, X‐Ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). This analysis can provide more effective and sustainable research strategies to cultivate more efficient and reliable CIGS technologies in the long run

Polarity determination of crystal defects in zincblende GaN by aberration-corrected electron microscopy

Aberration-corrected scanning transmission electron microscopy techniques are used to study the bonding configuration between gallium cations and nitrogen anions at defects in metalorganic vapor-phase epitaxy-grown cubic zincblende GaN on vicinal (001) 3C-SiC/Si. By combining high-angle annular dark-field and annular bright-field imaging, the orientation and bond polarity of planar defects, such as stacking faults and wurtzite inclusions, were identified. It is found that the substrate miscut direction toward one of the 3C-SiC ⟨110⟩ in-plane directions is correlated with the crystallographic [1–10] in-plane direction and that the {111} planes with a zone axis parallel to the miscut have a Ga-polar character, whereas the {111} planes in the zone perpendicular to the miscut direction have N-polarity. The polarity of {111}-type stacking faults is maintained in the former case by rotating the coordination of Ga atoms by 180° around the ⟨111⟩ polar axes and in the latter case by a similar rotation of the coordination of the N atoms. The presence of small amounts of the hexagonal wurtzite phase on Ga-polar {111} planes and their total absence on N-polar {111} planes is tentatively explained by the preferential growth of wurtzite GaN in the [0001] Ga-polar direction under non-optimized growth conditions. I. INTRODUCTIO

Sub-surface imaging of porous GaN distributed Bragg reflectors via backscattered electrons

In this article, porous GaN distributed Bragg reflectors (DBRs) were fabricated by epitaxy of undoped/doped multilayers followed by electrochemical etching. We present backscattered electron scanning electron microscopy (BSE-SEM) for sub-surface plan-view imaging, enabling efficient, non-destructive pore morphology characterization. In mesoporous GaN DBRs, BSE-SEM images the same branching pores and Voronoi-like domains as scanning transmission electron microscopy. In microporous GaN DBRs, micrographs were dominated by first porous layer features (45 nm to 108 nm sub-surface) with diffuse second layer (153 nm to 216 nm sub-surface) contributions. The optimum primary electron landing energy (LE) for image contrast and spatial resolution in a Zeiss GeminiSEM 300 was approximately 20 keV. BSE-SEM detects porosity ca. 295 nm sub-surface in an overgrown porous GaN DBR, yielding low contrast that is still first porous layer dominated. Imaging through a ca. 190 nm GaN cap improves contrast. We derived image contrast, spatial resolution, and information depth expectations from semi-empirical expressions. These theoretical studies echo our experiments as image contrast and spatial resolution can improve with higher LE, plateauing towards 30 keV. BSE-SEM is predicted to be dominated by the uppermost porous layer's uppermost region, congruent with experimental analysis. Most pertinently, information depth increases with LE, as observed

Development of zincblende GaN grown on 3C-SiC/Si substrates for LED applications

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Electronic structure and chemical state analysis of nanoflowers decorated GaN and AlGaN/GaN heterostructure

The present article reports electronic structure, chemical and defect states analysis of Quasi-continuous GaN film, nanoflowers decorated nanostructured GaN and nanoflowers decorated AlGaN/GaN hetero-structure. The nanostructured GaN and AlGaN surfaces were decorated with nanoflowers having a size variation between 200 and 400 nm. Extensive photoemission analysis was performed to analyse surface chemistry and electronic structure and their correlation with surface morphology. Indication of free electron accumulation was perceived by the observed downwards band bending at the interface of AlGaN/GaN heterostructure. The optical response inveterate defects minimization in nanoflower decorated GaN and AlGaN/GaN heterostructure and the presence of minimum residual stress

Research data supporting "Investigation of wurtzite formation in MOVPE-grown zincblende GaN epilayers on AlxGa1-xN nucleation layers"

The data consists of Scanning transmission electron microscopy based Energy-dispersive X-ray spectroscopy measurements. Data consists of the information from the Line profile obtained across the AlGaN (x=0.29) nucleation layer for different elements such as Si Kα, Pt Mα, Ga Lα, Al Kα. The second file (dm3 file) is a cross-sectional HRTEM image (zone axis = [110]) of the zb-GaN epilayer on GaN NL grown over 3C-SiC, shown in figure 1 of the associated publication (https://doi.org/10.1063/5.0077186).We would like to thank EPSRC for support through grant no. EP/M010589/1 and grant no. EP/R01146X/1. DJ Wallis would like to acknowledge support through EPSRC fellowship EP/N01202X/2

Influence of active layer thickness on electrical properties of P3HT/<em>n</em>-Si based hybrid heterostructure

468-474In the present study, we analyze the effect of active (organic) layer thickness on the optical and electrical properties of poly 3-hexylthiophene/n-silicon hybrid hetero-structure. The organic/inorganic sandwiched heterojunction have been prepared via spin-coating of poly 3-hexylthiophene film onto an oxide passivated Si substrate at room temperature. The device structure has been fabricated via depositing silver and aluminum contacts on Poly 3-hexylthiophene and n-silicon layers, respectively. The optical and electrical properties of the fabricated heterostructures have been examined by varying the active layer thickness from 50 to 120 nm. Photoluminescence measurements displayed a sharp intense peak at 578 nm corresponding to characteristic poly 3-hexylthiophene band-to-band transition. Enhancement in forward current and reduction in leakage current was observed with increased active layer thickness. It has been observed that employing an active layer thickness of 100 nm, the device produces enhanced forward currents with low leakage currents which leads to the formation of high quality heterojunction and demonstrates better performance of the device

Cathodoluminescence studies of the optical properties of a zincblende InGaN/GaN single quantum well.

Zincblende GaN has the potential to improve the efficiency of green- and amber-emitting nitride light emitting diodes due to the absence of internal polarisation fields. However, high densities of stacking faults are found in current zincblende GaN structures. This study presents a cathodoluminescence spectroscopy investigation into the low-temperature optical behaviour of a zincblende GaN/InGaN single quantum well structure. In panchromatic cathodoluminescence maps, stacking faults are observed as dark stripes, and are associated with non-radiative recombination centres. Furthermore, power dependent studies were performed to address whether the zincblende single quantum well exhibited a reduction in emission efficiency at higher carrier densities - the phenomenon known as efficiency droop. The single quantum well structure was observed to exhibit droop, and regions with high densities of stacking faults were seen to exacerbate this phenomenon. Overall, this study suggests that achieving efficient emission from zinc-blende GaN/InGaN quantum wells will require reduction in the stacking fault density.&#xD