Luminescence study of III-nitride semiconductor nanostructures and LEDs

In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively.In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively

High resolution cathodoluminescence hyperspectral imaging of surface features in InGaN/GaN multiple quantum well structures

InGaN/GaN multiple quantum wells (MQWs) have been studied by using cathodoluminescence hyperspectral imaging with high spatial resolution. Variations in peak emission energies and intensities across trench-like features and V-pits on the surface of the MQWs are investigated. The MQW emission from the region inside trench-like features is red-shifted by approximately 45 meV and more intense than the surrounding planar regions of the sample, whereas emission from the V-pits is blue-shifted by about 20 meV and relatively weaker. By employing this technique to the studied nanostructures it is possible to investigate energy and intensity shifts on a 10 nm length scale.Comment: 3 pages, 3 figure

Influence of substrate miscut angle on surface morphology and luminescence properties of AlGaN

The influence of substrate miscut on Al0.5Ga0.5 N layers was investigated using cathodoluminescence (CL) hyperspectral imaging and secondary electron imaging in an environmental scanning electron microscope. The samples were also characterized using atomic force microscopy and high resolution X-ray diffraction. It was found that small changes in substrate miscut have a strong influence on the morphology and luminescence properties of the AlGaN layers. Two different types are resolved. For low miscut angle, a crack-free morphology consisting of randomly sized domains is observed, between which there are notable shifts in the AlGaN near band edge emission energy. For high miscut angle, a morphology with step bunches and compositional inhomogeneities along the step bunches, evidenced by an additional CL peak along the step bunches, are observed

High-resolution cathodoluminescence hyperspectral imaging of nitride nanostructures

Hyperspectral cathodoluminescence imaging provides spectrally and spatially resolved information on luminescent materials within a single dataset. Pushing the technique toward its ultimate nanoscale spatial limit, while at the same time spectrally dispersing the collected light before detection, increases the challenge of generating low-noise images. This article describes aspects of the instrumentation, and in particular data treatment methods, which address this problem. The methods are demonstrated by applying them to the analysis of nanoscale defect features and fabricated nanostructures in III-nitride-based materials

Cathodoluminescence hyperspectral imaging of trench-like defects in InGaN/GaN quantum well structures

Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (90 meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (10 meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes

Influence of polyvinylpyrrolidone (PVP) in the synthesis of luminescent NaYF4:Yb,Er upconversion nanoparticles

Polyvinylpyrrolidone (PVP) can be used to produce upconversion nanoparticles (UCNPs) in an advantageous manner, i.e. at modest temperatures in open-to-air conditions with simple hotplate and flask apparatus. However, the influence of PVP parameters on the formation of UCNPs has not been previously investigated. In this exploratory study, we establish that PVP molecular weight and relative amount of PVP can greatly influence the morphology and diameter of NaYF4:Yb,Er UCNPs produced via the PVP-assisted route. At nominal amounts of PVP, varying the molecular weight of PVP in synthesis between 10,000 g/mol (PVP10), 40,000 g/mol (PVP40), and 55,000 g/mol (PVP55), had minimal effect on UCNP morphology, whereas reducing the quantity of PVP10 and PVP40 in the reaction to 10% of the nominal amount resulted in two notable effects: (1) the generation of a greater range of UCNP diameters and (2) the production of an unexpected sub-population of rhombus-shaped UCNPs. Bulk and individual nanoparticle analysis indicates that all UCNP morphologies were cubic (α-phase) crystal structure and consisted of NaYF4:Yb,Er. Optical emission properties exhibited only modest green and red luminescence emission ratio when PVP parameters were varied. However, separately produced PVP40 NaYF4:Yb,Tm UCNPs exhibited a much more intense and dual-band blue /red emission. This exploratory work demonstrates that tailoring PVP content in synthesis of UCNPs can greatly alter morphology of UCNPs produced and should be carefully considered in experimental design. However, the underlying mechanisms of action of the role PVP plays in this synthesis remain unclear. Ultimately, significant further work is still required to fully elucidate the relevant chemistry to achieve full control of PVP-UCNP synthesis

Microscopy of defects in semiconductors

In this chapter, the authors discuss microscopy techniques that can be useful in addressing defects in semiconductors. They focus on three main families: scanning probe microscopy, scanning electron microscopy and transmission electron microscopy. They first address the basic principles of the selected microscopy techniques In discussions of image formation, they elucidate the mechanisms by which defects are typically imaged in each technique. Then, in the latter part of the chapter, they describe some key examples of the application of microscopy to semiconductor materials, addressing both point and extended defects and both two-dimensional (2D) and three-dimensional (3D) materials

Unravelling the chloride dopant induced film improvement in all-inorganic perovskite absorber

CsPbI2Br perovskite material has been the focus of much recent research, thanks to its improved stability over CsPbI3, useful bandgap of 1.9 eV and enhanced thermal stability over hybrid perovskite materials with volatile organic components. It has great potential for both single junction solar cells for indoor applications, and implementation in tandem cells. However, moisture stability has remained an issue. In order to overcome this roadblock towards commercialisation, metal chloride dopants have been widely investigated to improve film quality and reduce damage from humidity. The majority of the research to date on this topic has focussed on device performance and bulk film characteristics, with limited attention paid to grain-level crystallinity and whether the dopant is proportionally incorporated into the film. In the present work, cathodoluminescence (CL) and electron backscatter diffraction (EBSD) are utilised to investigate the effects of a lead chloride dopant, both on emission and crystal structure at a grain level, with the findings supported by X-ray diffraction (XRD). Confirmation of proportional incorporation of the dopant into the final prepared films is provided by wavelength dispersive X-ray (WDX) spectroscopy. This work provides a valuable insight into the impact dopants have on all-inorganic perovskite absorbers, helping to influence future dopant design

Implementing fluorescent MOFs as down-converting layers in hybrid light-emitting diodes

One of the most important non-radiative relaxation processes that limits the quantum yield of a fluorophore is related to aggregation of the molecules in the solid-state causing excimer quenching. To limit this quenching mechanism, the fluorophore can be contained within a well-ordered 3D system that minimises aggregation through rigid bonds and spatial separation in a defined topological construct. Herein, the synthesis, characterisation and application as a down-converter of a new luminescent 3D material (MOF-BTBMBA) that incorporates a building block based on a benzothiadiazole (BT) derivative (BTBMBA) in a metal-organic framework (MOF) is presented. Notably, photoluminescent quantum yield and hybrid LED performance are significantly improved for the MOF-based device compared to that prepared with the free ligand, highlighting the effectiveness of the rigid scaffold arrangement

Cool to warm white light emission from hybrid inorganic/organic light-emitting diodes

The synthesis and characterisation of two novel organic down-converting molecules is disclosed, together with their performance as functional colour-converters in combination with inorganic blue light-emitting diodes (LEDs). Each molecule contains two fluorene-triphenylamine arms, connected to either a benzothiadiazole or bisbenzothiadiazole core. These molecules have been selected on the basis that they are free from absorption bands in the green region of the visible spectrum to maximise their performance and offer improvements compared with previous BODIPY-containing analogues. The inorganic InGaN/GaN LED emits at 444 nm, overlying the absorption of each of the organic molecules. The combination of the blue (inorganic) and yellow (organic) emission is shown to produce reasonable quality, white light-emitting hybrid devices for both down-converter molecules. Cool to warm white light is achieved for both molecules by increasing the concentration. An optimum colour rendering index (CRI) value of 66 is obtained for the mono-benzothiadiazole molecule. Also a high blue-to-white efficacy (defined as white luminous flux (lm)/blue radiant flux (W)) of 368 lm/W is achieved, superseding the current phosphor converters of 200-300 lm/W. A comparison of these down-converting molecules to the older generation BODIPY-containing molecules is also provided