In this paper we present a detailed analysis of the structural, electronic, and optical propertie... more In this paper we present a detailed analysis of the structural, electronic, and optical properties of an m-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.
Non-polar (11–20) InGaN quantum dots (QDs) have been grown using a modified droplet epitaxy metho... more Non-polar (11–20) InGaN quantum dots (QDs) have been grown using a modified droplet epitaxy method by metal-organic vapour phase epitaxy on top of a 15-period AlN/GaN distributed Bragg reflector (DBR) on a-plane GaN pseudo-substrate prepared by epitaxial lateral overgrowth (ELOG), in which the QDs are located at the centre of a ca. 180 nm GaN layer. The AlN/GaN DBR has shown a peak reflectivity of ∼80% at a wavelength of ∼454 nm with a 49 nm wide, flat stop-band. Variations in layer thicknesses observed by cross-sectional scanning transmission electron microscopy have been identified as the main source of degradation of the DBR reflectivity. The presence of trenches due to incomplete coalescence of the ELOG template and the formation of cracks due to relaxation of tensile strain during the DBR growth may distort the DBR and further reduce the reflectivity. The DBR top surface is very smooth and does not have a detrimental effect on the subsequent growth of QDs. Enhanced single QD emission at 20 K was observed in cathodoluminescence.
A significant fraction of global electricity demand is for lighting.
Enabled by the realization a... more A significant fraction of global electricity demand is for lighting. Enabled by the realization and development of efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent and fluorescent lighting, which are being broadly implemented. However, a key challenge for this industry is to achieve the right photometric ranges and application-specific emission spectra via cost effective means. Here, we synthesize organic−inorganic lead halide-based perovskite crystals with broad spectral tuneability. By tailoring the composition of methyl and octlyammonium cations in the colloidal synthesis, meso- to nanoscale 3D crystals (5−50 nm) can be formed with enhanced photoluminescence efficiency. By increasing the octlyammonium cations content, we observe platelet formation of 2D layered perovskite sheets; however, these platelets appear to be less emissive than the 3D crystals. We further manipulate the halide composition of the perovskite crystals to achieve emission covering the entire visible spectrum. By blending perovskite crystals with different emission wavelengths in a polymer host, we demonstrate the potential to replace conventional phosphors and provide the means to replicate natural white light when excited by a blue GaN LED.
InGaN structures epitaxially grown on a-plane or m-plane GaN exhibit in-plane optical polarizatio... more InGaN structures epitaxially grown on a-plane or m-plane GaN exhibit in-plane optical polarization. Linear elasticity theory treats the two planes equivalently and is hence unable to explain the experimentally observed higher degree of linear polarization for m-plane than a-plane InGaN. Using density functional theory, we study the response of InGaN random alloys to finite biaxial strains on both nonpolar planes. The calculated m-plane InGaN valence band splitting is larger than that of the a-plane, due to a greater degree of structural relaxation in a-plane InGaN. We provide a parametrization of the valence band splitting of InGaN strained to a-plane and m-plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between a-plane and m-plane polarization.
Droplets grown by modified droplet epitaxy on non-polar (11-20) surfaces of InGaN epilayers on GaN... more Droplets grown by modified droplet epitaxy on non-polar (11-20) surfaces of InGaN epilayers on GaN have been seen to be associated with underlying ring-like structures.This work discusses droplet etching as a possible mechanism for ring formation, and droplet creeping as a possible explanation for the droplets sitting askew of the ring centre. Transmission electron microscopy (TEM) analysis shows the droplets to move along the (0001) c-axis, and indicates that they have a very high indium content.
Nano-cathodoluminescence reveals the spectral properties of individual InGaN quantum wells in hig... more Nano-cathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nano-cathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.
In this paper we describe the implementation and the characterisation of five different in-situ d... more In this paper we describe the implementation and the characterisation of five different in-situ defect reduction techniques for non-polar (a -plane) GaN growth on r -plane sapphire. Sample 1 (3D/2D) employs a methodology frequently applied on the c -plane, involving a low temperature nucleation layer (LTNL) followed by 3D GaN island formation and lateral coalescence. For Sample 2 (d3D) GaN islands are grown directly onto the sapphire with no LTNL, followed by lateral growth. Sample 3 (d3D Si) follows a similar procedure, but with high silicon doping in order to adjust the 3D GaN island shape. Sample 4 (SiNx) utilises a silicon nitride interlayer between a LTNL and subsequent growth of a GaN layer. Sample 5 is grown by epitaxial lateral overgrowth (ELOG) coupled with a SiNx interlayer. X-ray diffraction, scanning electron microscopy and cathodoluminescence are used to identify defects, and determined the threading defect density to vary from 1 x 10^10–1 x 10^9 cm^–2 and basal-plane stacking fault (BSF) density to vary from 5 x 10^5 – 5 x 10^3 cm-1. The improvement in crystal quality is reflected in the photoluminescence spectra by a comparison of the ratio of the GaN near band edge (NBE) emission to the BSF associated emission. It was determined that the ELOG method was most successful in blocking BSFs, with a density reduction of 2 orders of magnitude resulting in a fifteen-fold increase in the NBE:BSF emission ratio increase.
We report on the optical properties of non-polar m-plane InGaN/GaN multi-quantum wells (MQWs) gro... more We report on the optical properties of non-polar m-plane InGaN/GaN multi-quantum wells (MQWs) grown on ammonothermal bulk GaN substrates. The low temperature continuous wave (CW) photoluminescence spectra are broad with a characteristic low energy tail. The majority of the emission bands decay with a time constant ~300 ps, but detailed photoluminescence time decay and time resolved spectroscopy measurements revealed the existence of a distinct slowly decaying emission band. This slowly decaying component is responsible for the low energy tails observed in the CW spectra. Scanning electron microscopy–cathodoluminescence (SEM-CL) studies show that the low energy emission band originates from regions across step-bunches, which are associated to the GaN substrate miscut. Subsequent scanning transmission electron microscopy imaging demonstrates that semi-polar QWs had formed continuous layers on the step bunches between the m-plane QWs and were responsible for the slower decaying, low energy emission band. Thus we assign the asymmetric low energy emission tails observed in photoluminescence studies to the formation of semi-polar facet QWs across the step bunches associated with the GaN miscut.
Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non... more Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non-polar a-plane InGaN quantum wells (QWs) grown on a GaN pseudo-substrate produced using epitaxial lateral overgrowth. Application of the focused ion beam microscope enabled APT needles to be prepared from the low defect density regions of the grown sample. A complementary analysis was also undertaken on QWs having comparable In contents grown on polar c-plane sample pseudo-substrates. Both frequency distribution and modified nearest neighbor analyses indicate a statistically non-randomized In distribution in the a-plane QWs, but a random distribution in the c-plane QWs. This work not only provides insights into the structure of non-polar a-plane QWs but also shows that APT is capable of detecting as-grown nanoscale clustering in InGaN and thus validates the reliability of earlier APT analyses of the In distribution in c-plane InGaN QWs which show no such clustering.
Non-polar (11-20) InGaN quantum dots (QDs) were grown by metal organic vapour phase epitaxy. An I... more Non-polar (11-20) InGaN quantum dots (QDs) were grown by metal organic vapour phase epitaxy. An InGaN epilayer was grown and subjected to a temperature ramp in a nitrogen and ammonia environment before the growth of the GaN capping layer. Uncapped structures with and without the temperature ramp were grown for reference and imaged by atomic force microscopy. Micro-photoluminescence studies reveal the presence of resolution limited peaks with a linewidth of less than ∼500 μeV at 4.2 K. This linewidth is significantly narrower than that of non-polar InGaN quantum dots grown by alternate methods and may be indicative of reduced spectral diffusion. Time resolved photoluminescence studies reveal a mono-exponential exciton decay with a lifetime of 533 ps at 2.70 eV. The excitonic lifetime is more than an order of magnitude shorter than that for previously studied polar quantum dots and suggests the suppression of the internal electric field. Cathodoluminescence studies show the spatial distribution of the quantum dots and resolution limited spectral peaks at 18 K.
In this paper we present a detailed analysis of the structural, electronic, and optical propertie... more In this paper we present a detailed analysis of the structural, electronic, and optical properties of an m-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.
Non-polar (11–20) InGaN quantum dots (QDs) have been grown using a modified droplet epitaxy metho... more Non-polar (11–20) InGaN quantum dots (QDs) have been grown using a modified droplet epitaxy method by metal-organic vapour phase epitaxy on top of a 15-period AlN/GaN distributed Bragg reflector (DBR) on a-plane GaN pseudo-substrate prepared by epitaxial lateral overgrowth (ELOG), in which the QDs are located at the centre of a ca. 180 nm GaN layer. The AlN/GaN DBR has shown a peak reflectivity of ∼80% at a wavelength of ∼454 nm with a 49 nm wide, flat stop-band. Variations in layer thicknesses observed by cross-sectional scanning transmission electron microscopy have been identified as the main source of degradation of the DBR reflectivity. The presence of trenches due to incomplete coalescence of the ELOG template and the formation of cracks due to relaxation of tensile strain during the DBR growth may distort the DBR and further reduce the reflectivity. The DBR top surface is very smooth and does not have a detrimental effect on the subsequent growth of QDs. Enhanced single QD emission at 20 K was observed in cathodoluminescence.
A significant fraction of global electricity demand is for lighting.
Enabled by the realization a... more A significant fraction of global electricity demand is for lighting. Enabled by the realization and development of efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent and fluorescent lighting, which are being broadly implemented. However, a key challenge for this industry is to achieve the right photometric ranges and application-specific emission spectra via cost effective means. Here, we synthesize organic−inorganic lead halide-based perovskite crystals with broad spectral tuneability. By tailoring the composition of methyl and octlyammonium cations in the colloidal synthesis, meso- to nanoscale 3D crystals (5−50 nm) can be formed with enhanced photoluminescence efficiency. By increasing the octlyammonium cations content, we observe platelet formation of 2D layered perovskite sheets; however, these platelets appear to be less emissive than the 3D crystals. We further manipulate the halide composition of the perovskite crystals to achieve emission covering the entire visible spectrum. By blending perovskite crystals with different emission wavelengths in a polymer host, we demonstrate the potential to replace conventional phosphors and provide the means to replicate natural white light when excited by a blue GaN LED.
InGaN structures epitaxially grown on a-plane or m-plane GaN exhibit in-plane optical polarizatio... more InGaN structures epitaxially grown on a-plane or m-plane GaN exhibit in-plane optical polarization. Linear elasticity theory treats the two planes equivalently and is hence unable to explain the experimentally observed higher degree of linear polarization for m-plane than a-plane InGaN. Using density functional theory, we study the response of InGaN random alloys to finite biaxial strains on both nonpolar planes. The calculated m-plane InGaN valence band splitting is larger than that of the a-plane, due to a greater degree of structural relaxation in a-plane InGaN. We provide a parametrization of the valence band splitting of InGaN strained to a-plane and m-plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between a-plane and m-plane polarization.
Droplets grown by modified droplet epitaxy on non-polar (11-20) surfaces of InGaN epilayers on GaN... more Droplets grown by modified droplet epitaxy on non-polar (11-20) surfaces of InGaN epilayers on GaN have been seen to be associated with underlying ring-like structures.This work discusses droplet etching as a possible mechanism for ring formation, and droplet creeping as a possible explanation for the droplets sitting askew of the ring centre. Transmission electron microscopy (TEM) analysis shows the droplets to move along the (0001) c-axis, and indicates that they have a very high indium content.
Nano-cathodoluminescence reveals the spectral properties of individual InGaN quantum wells in hig... more Nano-cathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nano-cathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.
In this paper we describe the implementation and the characterisation of five different in-situ d... more In this paper we describe the implementation and the characterisation of five different in-situ defect reduction techniques for non-polar (a -plane) GaN growth on r -plane sapphire. Sample 1 (3D/2D) employs a methodology frequently applied on the c -plane, involving a low temperature nucleation layer (LTNL) followed by 3D GaN island formation and lateral coalescence. For Sample 2 (d3D) GaN islands are grown directly onto the sapphire with no LTNL, followed by lateral growth. Sample 3 (d3D Si) follows a similar procedure, but with high silicon doping in order to adjust the 3D GaN island shape. Sample 4 (SiNx) utilises a silicon nitride interlayer between a LTNL and subsequent growth of a GaN layer. Sample 5 is grown by epitaxial lateral overgrowth (ELOG) coupled with a SiNx interlayer. X-ray diffraction, scanning electron microscopy and cathodoluminescence are used to identify defects, and determined the threading defect density to vary from 1 x 10^10–1 x 10^9 cm^–2 and basal-plane stacking fault (BSF) density to vary from 5 x 10^5 – 5 x 10^3 cm-1. The improvement in crystal quality is reflected in the photoluminescence spectra by a comparison of the ratio of the GaN near band edge (NBE) emission to the BSF associated emission. It was determined that the ELOG method was most successful in blocking BSFs, with a density reduction of 2 orders of magnitude resulting in a fifteen-fold increase in the NBE:BSF emission ratio increase.
We report on the optical properties of non-polar m-plane InGaN/GaN multi-quantum wells (MQWs) gro... more We report on the optical properties of non-polar m-plane InGaN/GaN multi-quantum wells (MQWs) grown on ammonothermal bulk GaN substrates. The low temperature continuous wave (CW) photoluminescence spectra are broad with a characteristic low energy tail. The majority of the emission bands decay with a time constant ~300 ps, but detailed photoluminescence time decay and time resolved spectroscopy measurements revealed the existence of a distinct slowly decaying emission band. This slowly decaying component is responsible for the low energy tails observed in the CW spectra. Scanning electron microscopy–cathodoluminescence (SEM-CL) studies show that the low energy emission band originates from regions across step-bunches, which are associated to the GaN substrate miscut. Subsequent scanning transmission electron microscopy imaging demonstrates that semi-polar QWs had formed continuous layers on the step bunches between the m-plane QWs and were responsible for the slower decaying, low energy emission band. Thus we assign the asymmetric low energy emission tails observed in photoluminescence studies to the formation of semi-polar facet QWs across the step bunches associated with the GaN miscut.
Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non... more Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non-polar a-plane InGaN quantum wells (QWs) grown on a GaN pseudo-substrate produced using epitaxial lateral overgrowth. Application of the focused ion beam microscope enabled APT needles to be prepared from the low defect density regions of the grown sample. A complementary analysis was also undertaken on QWs having comparable In contents grown on polar c-plane sample pseudo-substrates. Both frequency distribution and modified nearest neighbor analyses indicate a statistically non-randomized In distribution in the a-plane QWs, but a random distribution in the c-plane QWs. This work not only provides insights into the structure of non-polar a-plane QWs but also shows that APT is capable of detecting as-grown nanoscale clustering in InGaN and thus validates the reliability of earlier APT analyses of the In distribution in c-plane InGaN QWs which show no such clustering.
Non-polar (11-20) InGaN quantum dots (QDs) were grown by metal organic vapour phase epitaxy. An I... more Non-polar (11-20) InGaN quantum dots (QDs) were grown by metal organic vapour phase epitaxy. An InGaN epilayer was grown and subjected to a temperature ramp in a nitrogen and ammonia environment before the growth of the GaN capping layer. Uncapped structures with and without the temperature ramp were grown for reference and imaged by atomic force microscopy. Micro-photoluminescence studies reveal the presence of resolution limited peaks with a linewidth of less than ∼500 μeV at 4.2 K. This linewidth is significantly narrower than that of non-polar InGaN quantum dots grown by alternate methods and may be indicative of reduced spectral diffusion. Time resolved photoluminescence studies reveal a mono-exponential exciton decay with a lifetime of 533 ps at 2.70 eV. The excitonic lifetime is more than an order of magnitude shorter than that for previously studied polar quantum dots and suggests the suppression of the internal electric field. Cathodoluminescence studies show the spatial distribution of the quantum dots and resolution limited spectral peaks at 18 K.
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Papers by James Griffiths
line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the
experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.
Enabled by the realization and development of efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent and fluorescent lighting, which are being broadly implemented. However, a key challenge for this industry is to achieve the right photometric ranges and application-specific emission spectra via cost effective means. Here, we synthesize organic−inorganic lead halide-based perovskite crystals with broad spectral tuneability. By tailoring the composition of methyl and octlyammonium cations in the colloidal synthesis, meso- to nanoscale 3D crystals (5−50 nm) can be formed with enhanced photoluminescence efficiency. By increasing the octlyammonium cations content, we observe platelet formation of 2D layered perovskite sheets; however, these platelets appear to be less emissive than the 3D crystals. We further manipulate the halide composition of the perovskite crystals to achieve emission covering the entire visible spectrum. By blending perovskite crystals with different emission wavelengths in a polymer host, we demonstrate the potential to replace conventional phosphors and provide the means to replicate natural white light when excited by a blue GaN LED.
calculated m-plane InGaN valence band splitting is larger than that of the a-plane, due to a greater degree of structural relaxation in a-plane InGaN. We provide a parametrization of the valence band splitting of InGaN strained to a-plane and m-plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between a-plane and m-plane polarization.
line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the
experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.
Enabled by the realization and development of efficient GaN blue light-emitting diodes (LEDs), phosphor-based solid-state white LEDs provide a much higher efficiency alternative to incandescent and fluorescent lighting, which are being broadly implemented. However, a key challenge for this industry is to achieve the right photometric ranges and application-specific emission spectra via cost effective means. Here, we synthesize organic−inorganic lead halide-based perovskite crystals with broad spectral tuneability. By tailoring the composition of methyl and octlyammonium cations in the colloidal synthesis, meso- to nanoscale 3D crystals (5−50 nm) can be formed with enhanced photoluminescence efficiency. By increasing the octlyammonium cations content, we observe platelet formation of 2D layered perovskite sheets; however, these platelets appear to be less emissive than the 3D crystals. We further manipulate the halide composition of the perovskite crystals to achieve emission covering the entire visible spectrum. By blending perovskite crystals with different emission wavelengths in a polymer host, we demonstrate the potential to replace conventional phosphors and provide the means to replicate natural white light when excited by a blue GaN LED.
calculated m-plane InGaN valence band splitting is larger than that of the a-plane, due to a greater degree of structural relaxation in a-plane InGaN. We provide a parametrization of the valence band splitting of InGaN strained to a-plane and m-plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between a-plane and m-plane polarization.