Next generation white LEDs using hybrid inorganic/organic semiconductor nanostructures for general illumination and wireless communication

Lead Research Organisation: University of Strathclyde
Department Name: Physics

Abstract

There is a significantly increasing demand for sustainable energy-efficient technologies due to the world energy crisis and climate change. The energy consumed due to general illumination accounts for about 29% of the world's total energy consumption, currently using rather inefficient technologies often containing toxic elements. It is therefore necessary to develop ultra energy-efficient solid-state lighting sources to replace these incandescent and fluorescent lights, for which the leading candidates are mainly based on white light emitting diodes (LEDs). Such white LEDs can be fabricated from inorganic or organic semiconductors, with the former leading the way for high brightness and efficiency. These are constructed from III-nitride semiconductors, which have direct bandgaps across their entire composition range, covering the complete visible spectrum and a major part of the ultraviolet. Fast modulation of the white LEDs, at speeds undetectable to the eye, allows them to also be utilised as optical transmitters for wireless data communication. This opens up the exciting possibility of white LEDs serving as lighting sources for simultaneous illumination and wireless communication. This is the emerging technology of visible light communication (VLC) and has a number of major advantages over the present-day radio frequency (RF) communication technology, such as increasing security, eliminating any RF-induced health concern, etc

However, the performance and cost of current white LEDs is not sufficiently impressive to allow replacement of conventional lighting sources at the moment. Furthermore, in terms of VLC applications, the bandwidth is currently limited to the MHz level, which is well below the practical requirements of current broadband WiFi systems. This is due to the long carrier recombination lifetime of current III-nitride based LEDs, which are conventionally grown in a "polar" orientation containing intense piezoelectric fields. These fields result in a reduced overlap between the electron and hole wavefunctions in the active regions of the LEDs, which then suffer from long radiative recombination lifetimes (10-100 ns) and also low internal quantum efficiency. In addition, the conventional phosphors used to convert the emission to white light have even longer decay times and presents an additional limitation on the available bandwidth.

The project will employ non-polar III-nitrides and integrate the two major semiconductor families (organic and inorganic semiconductors) using a novel nanofabrication technology in order to achieve ultra energy efficient LEDs with ultrafast modulation speeds for next generation III-nitride based white lighting. Structuring on a nanometre scale will be used in the growth of the III-nitride layers to achieve high quality non-polar GaN, thereby eliminating the piezoelectric fields to give faster, more efficient devices. The nanostructures will also be used to introduce extra nanocavity effects, further reducing the radiative recombination lifetime and increasing the optical efficiency. The target of the project is a novel hybrid nanostructure to achieve prototype white-LEDs with a modulation speed on a level of 10 GHz and a step change in energy efficiency compared with the current state-of-the-art. The devices will be fabricated using metal-organic vapour phase epitaxy and cleanroom processing and fully characterised using optical and electrical measurements. Each stage in the process will be optimised and close working with industry will ensure that the resulting methods are practical and scalable to high volumes.

Publications

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Title Data for: "Luminescence behaviour of semi-polar (10-11) InGaN/GaN 'bow-tie' structures on patterned Si substrates" 
Description This dataset provides the experimental data used to generate the figures in the paper entitled "Luminescence behaviour of semi-polar (10-11) InGaN/GaN 'bow-tie' structures on patterned Si substrates". The room temperature cathodoluminescence (CL) data was recorded using a variable pressure field emission scanning electron microscope (SEM, FEI Quanta 250) which is equipped with a custom-built CL hyperspectral imaging system. The CL system collects the emitted light at an angle of 45° with respect to the incident electron beam using a Cassegrain reflecting objective. The light is then dispersed using a 125 mm focal length spectrograph (Oriel MS125) and detected using an electron-multiplying charge-coupled device (Andor Newton). Low temperature CL was performed in a field emission gun SEM (Zeiss LEO DSM 982) equipped with custom-built liquid helium flow cryostage (CryoVac). The light was collected using a UV-enhanced glass fibre placed in close contact with the sample, dispersed with a 90 cm focal length monochromator (SPEX 1702) and detected using a liquid nitrogen-cooled, UV-optimised CCD. As the electron beam scans across the sample surface, a whole CL spectrum is recorded per pixel building up the 3D hyperspectral data set. 2D CL images can then be extracted from the hyperspectral data set, such as peak energy, intensity or half width. The room temperature and low temperature (12 K) measurements were acquired with a beam voltage of 5 kV. Electron channelling contrast imaging is a non-destructive, diffraction technique performed in the SEM. ECC images are generally constructed by measuring the intensity of the backscattered electrons (BSEs) as the electron beam scans across the surface of a suitably-orientated sample. Any changes in crystallographic orientation and local strain can be monitored by the variation in the BSE intensity causing a change in contrast in an ECC image. This allows the imaging of low-angle tilt and rotation boundaries, atomic steps and extended defects (e.g. TDs). ECCI is carried out in a forward scattering geometry in a field emission SEM (FEI Sirion 200), equipped with an electron-sensitive diode and a custom-built signal amplifier. Electron backscatter diffraction (EBSD) measurements were performed using a Nordlys EBSD detector from Oxford Instruments attached to an FEI Quanta 250 variable pressure field emission SEM. The EBSD data was acquired at 20 kV and at a sample tilt of 70° with respect to the normal of the incident electron beam. For the analysis of the EBSD data, the electron backscatter pattern (EBSP) from each pixel was compared to simulated dynamical Kikuchi patterns using a Bloch wave approach. Abstract of the paper: In this work, we report on the innovative growth of semi-polar 'bow-tie'-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs), and on their structural and luminescence characterisation. We investigate the impact of growth on patterned (113) Si substrates which results in the bow-tie cross-section with upper surfaces having the (10-11) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission; and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channelling contrast imaging (ECCI) identifies the black spots as threading dislocations (TDs) propagating to the inclined (10-11) surfaces. Line defects in ECCI, propagating in the [1-210] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the centre of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region. 
Type Of Art Image 
Year Produced 2019 
URL https://pureportal.strath.ac.uk/en/datasets/c71c237d-a544-4bf5-b7d3-1432ea22608e
 
Description Improved LED materials, including use of non-standard crystal orientations for improvements
Exploitation Route To be added
Sectors Electronics,Energy,Manufacturing, including Industrial Biotechology,Other

 
Description To improve perfomance and understanding of white LEDs
First Year Of Impact 2015
Sector Electronics,Energy,Manufacturing, including Industrial Biotechology,Other
Impact Types Economic

 
Title Data for: "Cathodoluminescence studies of chevron features in semi-polar (11-22) InGaN/GaN multiple quantum well structures" 
Description "This dataset provides the experimental data used to generate the figures in the paper entitled ""Cathodoluminescence studies of chevron features in semi-polar (11-22) InGaN/GaN multiple quantum well structures"". The cathodoluminescence (CL) data discussed and presented in the paper was recorded using a variable pressure field emission scanning electron microscope (SEM, FEI Quanta 250) which is equipped with a custom-built CL hyperspectral imaging system. The CL system collects the emitted light at an angle of 45° with respect to the incident electron beam using a Cassegrain reflecting objective. The light is then dispersed using a 125 mm focal length spectrograph (Oriel MS125) and detected using an electron-multiplying charge-coupled device (Andor Newton). As the electron beam scans across the sample surface, a whole CL spectrum is recorded per pixel building up the 3D hyperspectral data set. 2D CL images can then be extracted from the hyperspectral data set, such as peak energy, intensity or half width. The electron beam-induced current (EBIC) is acquired using a Stanford pre-amplifier connected to the output of an LED. The pre-amp is connected to the computer via an analogue-to-digital converter, and a value for the voltage (from which the current can be calculated) is measured for every pixel in the map. The EBIC and CL are recorded simultaneously. Characterisation of the surface morphology was performed using atomic force microscopy (AFM, Bruker Dimension with Icon scanner) in PeakForce tapping mode with ScanAsyst Air probes. Abstract of the paper: Epitaxial overgrowth of semi-polar III-nitride layers and devices often leads to arrowhead-shaped surface features, referred to as chevrons. We report on a study into the optical, structural and electrical properties of these features occurring in two very different semi-polar structures, a blue-emitting multiple quantum well (MQW) structure and an amber-emitting light-emitting diode (LED). Cathodoluminescence (CL) hyperspectral imaging has highlighted shifts in their emission energy, occurring in the region of the chevron. These variations are due to different semi-polar planes introduced in the chevron arms resulting in a lack of uniformity in the InN incorporation across samples, and the disruption of the structure which could cause a narrowing of the QWs in this region. Atomic force microscopy has revealed that chevrons can penetrate over 150 nm into the sample, and quench light emission from the active layers. The dominance of non-radiative recombination in the chevron region was exposed by simultaneous measurement of CL and the electron beam-induced current (EBIC). Overall these results provide an overview of the nature and impact of chevrons on the luminescence of semi-polar devices." 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact
 
Title Data for: "Spatially-resolved optical and structural properties of semi-polar (11-22) Al_xGa_(1-x)N with x up to 0.56" 
Description This dataset provides the experimental data used to generate the figures in the paper entitled ""Spatially-resolved optical and structural properties of semi-polar (11-22) Al_xGa_(1-x)N with x up to 0.56"". The cathodoluminescence (CL) data discussed and presented in the paper was recorded using a variable pressure field emission scanning electron microscope (SEM, FEI Quanta 250) which is equipped with a custom-built CL hyperspectral imaging system. The CL system collects the emitted light at an angle of 45° with respect to the incident electron beam using a Cassegrain reflecting objective. The light is then dispersed using a 125 mm focal length spectrograph (Oriel MS125) and detected using an electron-multiplying charge-coupled device (Andor Newton). As the electron beam scans across the sample surface, a whole CL spectrum is recorded per pixel building up the 3D hyperspectral data set. 2D CL images can then be extracted from the hyperspectral data set, such as peak energy, intensity or half width. Electron channelling contrast imaging is a non-destructive, diffraction technique performed in the SEM. ECC images are generally constructed by measuring the intensity of the backscattered electrons (BSEs) as the electron beam scans across the surface of a suitably-orientated sample. Any changes in crystallographic orientation and local strain can be monitored by the variation in the BSE intensity causing a change in contrast in an ECC image. This allows the imaging of low-angle tilt and rotation boundaries, atomic steps and extended defects (e.g. TDs). ECCI is carried out in a forward scattering geometry in a field emission SEM (FEI Sirion 200), equipped with an electron-sensitive diode and a custom-built signal amplifier. Characterisation of the surface morphology was performed using atomic force microscopy (AFM, Bruker Dimension with Icon scanner) in PeakForce tapping mode with ScanAsyst Air probes. Abstract of the paper: Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar (11-22) Al_xGa_(1-x)N epilayers with AlN contents up to x=0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Impact Unknown 
 
Title Dataset for "Colour tuning in white hybrid inorganic/organic light-emitting diodes" 
Description This dataset provides the experimental data used to generate the figures in the paper entitled "Colour tuning in white hybrid inorganic/organic light-emitting diodes" by J. Bruckbauer et. al. published in Journal of Physics D: Applied Physics. Abstract of the paper: White hybrid inorganic/organic light-emitting diodes (LEDs) were fabricated by combining a novel organic colour converter with a blue inorganic LED. An organic small molecule was specifically synthesised to act as down-converter. The characteristics of the white colour were controlled by changing the concentration of the organic molecule based on the BODIPY unit, which was embedded in a transparent matrix, and volume of the molecule and encapsulant mixture. The concentration has a critical effect on the conversion efficiency, i.e. how much of the absorbed blue light is converted into yellow light. With increasing concentration the conversion efficiency decreases. This quenching effect is due to aggregation of the organic molecule at higher concentrations. Increasing the deposited amount of the converter does not increase the yellow emission despite more blue light being absorbed. Degradation of the organic converter was also observed during a period of 15 months from LED fabrication. Angular-dependent measurements revealed slight deviation from a Lambertian profile for the blue and yellow emission peaks leading to a small change in "whiteness" with emission angle. Warm white and cool white light with correlated colour temperatures of 2770 K and 7680 K, respectively, were achieved using different concentrations of the converter molecule. Although further work is needed to improve the lifetime and poor colour rendering, these hybrid LEDs show promising results as an alternative approach for generating white LEDs compared with phosphor-based white LEDs. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
Impact n/a 
 
Title Dataset for "Cool to Warm White Light Emission from Hybrid Inorganic/Organic Light-Emitting Diodes" 
Description This dataset provides the experimental data used to generate the figures in the paper entitled "Cool to Warm White Light Emission from Hybrid Inorganic/Organic Light-Emitting Diodes". The data discussed and presented in the paper was recorded using an integrating sphere (Labsphere Illumina 600/610). The resultant electroluminescence spectra are corrected for the system response and the intensity is measured in absolute values (W/nm). The LEDs were powered using a Keithley source measure unit, which provided the applied current and voltage values. Parameters such as colour rendering index, correlated colour temperature , chromaticity cordinates, temperature and various efficacy/efficienct values were calculated from the recorded response-corrected spectra and the electrical input power. Absorption spectra were recorded on a Shimadzu UV 2700 instrument. Photoluminescence measurements were recorded using a Perkin-Elmer LS 50 B fluorescence spectrometer in a quartz cuvette (path length 10 mm). Abstract of the paper: 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. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
Impact n/a 
 
Title Dataset for "Optical Investigation of Semi-polar (11-22) AlGaN with High Al Composition" 
Description This dataset provides the experimental data used to generate the figures in the paper entitled "Optical Investigation of Semi-polar (11-22) AlGaN with High Al Composition" The data discussed and presented in the paper was recorded using a variable pressure field emission scanning electron microscope (FEI Quanta 250) which is equipped with a custom-built cathodoluminescence (CL) hyperspectral imaging system. The CL system collects the emitted light at an angle of 45° with respect to the incident electron beam using a Cassegrain reflecting objective. The light is then dispersed using a 125 mm focal length spectrograph (Oriel MS125) and detected using an electron-multiplying charge-coupled device (Andor Newton). As the electron beam scans across the sample surface, a whole CL spectrum is recorded per pixel building up the a 3D hyperspectral data set. 2D CL images can then be extracted from the hyperspectral data set, such as peak energy, intensity or half width. Abstract of the paper: Exciton localization disturbs uniform population inversion, leading to an increase in threshold current for lasing. High Al content AlGaN is required for the fabrication of deep ultra-violet LDs, generating exciton localization. Photoluminescence and cathodoluminescence measurements have been performed on high quality semi-polar (11-22) AlxGa1-xN alloys with high Al composition in order to study the optical properties of both the near-band-edge (NBE) emission and the basal-plane stacking faults (BSFs) related emission, demonstrating different behaviours. Further comparison with the exciton localization of their c-plane counterparts exhibits that the exciton localization in semi-polar (11-22) AlGaN is much smaller than that in c-plane AlGaN. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Impact n/a