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.

Mapping emission heterogeneity in layered halide perovskites using cathodoluminescence.

Recent advancements in the fabrication of layered halide perovskites and their subsequent modification for optoelectronic applications have ushered in a need for innovative characterisation techniques. In particular, heterostructures containing multiple phases and consequently featuring spatially defined optoelectronic properties are very challenging to study. Here, we adopt an approach centered on cathodoluminescence, complemented by scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy analysis. Cathodoluminescence enables assessment of local emission variations by injecting charges with a nanometer-scale electron probe, which we use to investigate emission changes in three different systems: PEA2PbBr4, PEA2PbI4and lateral heterostructures of the two, fabricated via halide substitution. We identify and map different emission bands that can be correlated with local chemical composition and geometry. One emission band is characteristic of bromine-based halide perovskite, while the other originates from iodine-based perovskite. The coexistence of these emissions bands in the halide-substituted sample confirms the formation of lateral heterostructures. To improve the signal quality of the acquired data, we employed multivariate analysis, specifically the non-negative matrix factorization algorithm, on both cathodoluminescence and compositional datasets. The resulting understanding of the halide replacement process and identification of potential synergies in the optical properties will lead to optimised architectures for optoelectronic applications.

Additive GaN Solid Immersion Lenses for Enhanced Photon Extraction Efficiency from Diamond Color Centers.

Effective light extraction from optically active solid-state spin centers inside high-index semiconductor host crystals is an important factor in integrating these pseudo-atomic centers in wider quantum systems. Here, we report increased fluorescent light collection efficiency from laser-written nitrogen-vacancy (NV) centers in bulk diamond facilitated by micro-transfer printed GaN solid immersion lenses. Both laser-writing of NV centers and transfer printing of micro-lens structures are compatible with high spatial resolution, enabling deterministic fabrication routes toward future scalable systems development. The micro-lenses are integrated in a noninvasive manner, as they are added on top of the unstructured diamond surface and bonded by van der Waals forces. For emitters at 5 µm depth, we find approximately 2× improvement of fluorescent light collection using an air objective with a numerical aperture of NA = 0.95 in good agreement with simulations. Similarly, the solid immersion lenses strongly enhance light collection when using an objective with NA = 0.5, significantly improving the signal-to-noise ratio of the NV center emission while maintaining the NV's quantum properties after integration.

Scanning capacitance microscopy of GaN-based high electron mobility transistor structures: A practical guide.

The scanning capacitance microscope (SCM) is a powerful tool to characterise local electrical properties in GaN-based high electron mobility transistor (HEMT) structures with nanoscale resolution. We investigated the experimental setup and the imaging conditions to optimise the SCM contrast. As to the experimental setup, we show that the desired tip should be sharp (e.g., with the tip radius of ≤25nm) and its coating should be made of conductive doped diamond. Most importantly, its spring constant should be large to achieve stable tip-sample contact. The selected tip should be positioned close to both the edge and Ohmic contact of the sample. Regarding the imaging conditions, we also show that a dc bias should be applied in addition to an ac bias because the latter alone is not sufficient to deplete the two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure. The approximate range of the effective dc bias values was found by measuring the local dC/dV-V curves, yielding, after further optimisation, two optimised dc bias values which provide strong, but opposite, SCM contrast. In comparison, the selected ac bias value has no significant impact on the SCM contrast. The described methodology could potentially also be applied to other types of HEMT structures, and highly-doped samples.

Monitoring of the Initial Stages of Diamond Growth on Aluminum Nitride Using In Situ Spectroscopic Ellipsometry.

The high thermal conductivity of polycrystalline diamond makes it ideally suited for thermal management solutions for gallium nitride (GaN) devices, with a diamond layer grown on an aluminum nitride (AlN) interlayer atop the GaN stack. However, this application is limited by the thermal barrier at the interface between diamond and substrate, which has been associated with the transition region formed in the initial phases of growth. In this work, in situ spectroscopic ellipsometry (SE) is employed to monitor early-stage microwave plasma-enhanced chemical vapor deposition diamond growth on AlN. An optical model was developed from ex situ spectra and applied to spectra taken in situ during growth. Coalescence of separate islands into a single film was marked by a reduction in bulk void fraction prior to a spike in sp2 fraction due to grain boundary formation. Parameters determined by the SE model were corroborated using Raman spectroscopy and atomic force microscopy.

Complications in silane-assisted GaN nanowire growth.

Understanding the growth mechanisms of III-nitride nanowires is of great importance to realise their full potential. We present a systematic study of silane-assisted GaN nanowire growth on c-sapphire substrates by investigating the surface evolution of the sapphire substrates during the high temperature annealing, nitridation and nucleation steps, and the growth of GaN nanowires. The nucleation step - which transforms the AlN layer formed during the nitridation step to AlGaN - is critical for subsequent silane-assisted GaN nanowire growth. Both Ga-polar and N-polar GaN nanowires were grown with N-polar nanowires growing much faster than the Ga-polar nanowires. On the top surface of the N-polar GaN nanowires protuberance structures were found, which relates to the presence of Ga-polar domains within the nanowires. Detailed morphology studies revealed ring-like features concentric with the protuberance structures, indicating energetically favourable nucleation sites at inversion domain boundaries. Cathodoluminescence studies showed quenching of emission intensity at the protuberance structures, but the impact is limited to the protuberance structure area only and does not extend to the surrounding areas. Hence it should minimally affect the performance of devices whose functions are based on radial heterostructures, suggesting that radial heterostructures remain a promising device structure.

Three-Photon Excitation of InGaN Quantum Dots.

We demonstrate that semiconductor quantum dots can be excited efficiently in a resonant three-photon process, while resonant two-photon excitation is highly suppressed. Time-dependent Floquet theory is used to quantify the strength of the multiphoton processes and model the experimental results. The efficiency of these transitions can be drawn directly from parity considerations in the electron and hole wave functions in semiconductor quantum dots. Finally, we exploit this technique to probe intrinsic properties of InGaN quantum dots. In contrast to nonresonant excitation, slow relaxation of charge carriers is avoided, which allows us to measure directly the radiative lifetime of the lowest energy exciton states. Since the emission energy is detuned far from the resonant driving laser field, polarization filtering is not required and emission with a greater degree of linear polarization is observed compared to nonresonant excitation.

Core-Shell Nanorods as Ultraviolet Light-Emitting Diodes.

Existing barriers to efficient deep ultraviolet (UV) light-emitting diodes (LEDs) may be reduced or overcome by moving away from conventional planar growth and toward three-dimensional nanostructuring. Nanorods have the potential for enhanced doping, reduced dislocation densities, improved light extraction efficiency, and quantum wells free from the quantum-confined Stark effect. Here, we demonstrate a hybrid top-down/bottom-up approach to creating highly uniform AlGaN core-shell nanorods on sapphire repeatable on wafer scales. Our GaN-free design avoids self-absorption of the quantum well emission while preserving electrical functionality. The effective junctions formed by doping of both the n-type cores and p-type caps were studied using nanoprobing experiments, where we find low turn-on voltages, strongly rectifying behaviors and significant electron-beam-induced currents. Time-resolved cathodoluminescence measurements find short carrier liftetimes consistent with reduced polarization fields. Our results show nanostructuring to be a promising route to deep-UV-emitting LEDs, achievable using commercially compatible methods.

Dielectric behaviour of plasma hydrogenated TiO2/cyanoethylated cellulose nanocomposites.

The interface between the polymer and nanoparticle has a vital role in determining the overall dielectric properties of a dielectric polymer nanocomposite. In this study, a novel dielectric nanocomposite containing a high permittivity polymer, cyanoethylated cellulose (CRS) and TiO2 nanoparticles surface modified by hydrogen plasma treatments was successfully prepared with different weight percentages (10%, 20% and 30%) of hydrogenated TiO2. Internal structure of H plasma treated TiO2 nanoparticles (H-TiO2) and the intermolecular interactions and morphology within the polymer nanocomposites were analysed. H-TiO2/CRS thin films on SiO2/Si wafers were used to form metal-insulator-metal (MIM) type capacitors. Capacitances and loss factors in the frequency range of 1 kHz to 1 MHz were measured. At 1 kHz H-TiO2/CRS nanocomposites exhibited ultra-high dielectric constants of 80, 118 and 131 for nanocomposites with 10%, 20% and 30% weight of hydrogenated TiO2 respectively, significantly higher than values of pure CRS (21) and TiO2 (41). Furthermore, all three H-TiO2 /CRS nanocomposites show a loss factor <0.3 at 1 kHz and low leakage current densities (10-6 A cm-2-10-7 A cm-2). Leakage was studied using conductive atomic force microscopy (C-AFM) and it was observed that the leakage is associated with H-TiO2 nanoparticles embedded in the CRS polymer matrix. Although, modified interface slightly reduces energy densities compared to pristine TiO2/CRS system, the capacitance values for H-TiO2/CRS-in the voltage range of -2 V to 2 V are very stable. Whilst H-TiO2/CRS possesses ultra-high dielectric constants (>100), this study reveals that the polymer nanoparticle interface has a potential influence on dielectric behaviour of the composite.

Efficient and Bright Deep-Red Light-Emitting Diodes based on a Lateral 0D/3D Perovskite Heterostructure.

Bright and efficient deep-red light-emitting diodes (LEDs) are important for applications in medical therapy and biological imaging due to the high penetration of deep-red photons into human tissues. Metal-halide perovskites have potential to achieve bright and efficient electroluminescence due to their favorable optoelectronic properties. However, efficient and bright perovskite-based deep-red LEDs have not been achieved yet, due to either Auger recombination in low-dimensional perovskites or trap-assisted nonradiative recombination in 3D perovskites. Here, a lateral Cs4 PbI6 /FAx Cs1- x PbI3 (0D/3D) heterostructure that can enable efficient deep-red perovskite LEDs at very high brightness is demonstrated. The Cs4 PbI6 can facilitate the growth of low-defect FAx Cs1- x PbI3 , and act as low-refractive-index grids, which can simultaneously reduce nonradiative recombination and enhance light extraction. This device reaches a peak external quantum efficiency of 21.0% at a photon flux of 1.75 × 1021 m-2 s-1 , which is almost two orders of magnitude higher than that of reported high-efficiency deep-red perovskite LEDs. Theses LEDs are suitable for pulse oximeters, showing an error <2% of blood oxygen saturation compared with commercial oximeters.

Optical emission from focused ion beam milled halide perovskite device cross-sections.

Cross-sectional transmission electron microscopy has been widely used to investigate organic-inorganic hybrid halide perovskite-based optoelectronic devices. Electron-transparent specimens (lamellae) used in such studies are often prepared using focused ion beam (FIB) milling. However, the gallium ions used in FIB milling may severely degrade the structure and composition of halide perovskites in the lamellae, potentially invalidating studies performed on them. In this work, the close relationship between perovskite structure and luminescence is exploited to examine the structural quality of perovskite solar cell lamellae prepared by FIB milling. Through hyperspectral cathodoluminescence (CL) mapping, the perovskite layer was found to remain optically active with a slightly blue-shifted luminescence. This finding indicates that the perovskite structure is largely preserved upon the lamella fabrication process although some surface amorphisation occurred. Further changes in CL due to electron beam irradiation were also recorded, confirming that electron dose management is essential in electron microscopy studies of carefully prepared halide perovskite-based device lamellae. RESEARCH HIGHLIGHTS: Cathodoluminescence is used to study the emission of focused ion beam milled perovskite solar cell lamellae. The perovskite remained optically active with a slightly blue-shifted luminescence, indicating that the perovskite structure is mostly preserved.

Carrier dynamics at trench defects in InGaN/GaN quantum wells revealed by time-resolved cathodoluminescence.

Time-resolved cathodoluminescence offers new possibilities for the study of semiconductor nanostructures - including defects. The versatile combination of time, spatial, and spectral resolution of the technique can provide new insights into the physics of carrier recombination at the nanoscale. Here, we used power-dependent cathodoluminescence and temperature-dependent time-resolved cathodoluminescence to study the carrier dynamics at trench defects in InGaN quantum wells - a defect commonly found in III-nitride structures. The measurements show that the emission properties of trench defects closely relate to the depth of the related basal plane stacking fault within the quantum well stack. The study of the variation of carrier decay time with detection energy across the emission spectrum provides strong evidence supporting the hypothesis that strain relaxation of the quantum wells enclosed within the trench promotes efficient radiative recombination even in the presence of an increased indium content. This result shines light on previously reported peculiar emission properties of the defect, and illustrates the use of cathodoluminescence as a powerful adaptable tool for the study of defects in semiconductors.

Origin(s) of Anomalous Substrate Conduction in MOVPE-Grown GaN HEMTs on Highly Resistive Silicon.

The performance of transistors designed specifically for high-frequency applications is critically reliant upon the semi-insulating electrical properties of the substrate. The suspected formation of a conductive path for radio frequency (RF) signals in the highly resistive (HR) silicon substrate itself has been long held responsible for the suboptimal efficiency of as-grown GaN high electron mobility transistors (HEMTs) at higher operating frequencies. Here, we reveal that not one but two discrete channels distinguishable by their carrier type, spatial extent, and origin within the metal-organic vapor phase epitaxy (MOVPE) growth process participate in such parasitic substrate conduction. An n-type layer that forms first is uniformly distributed in the substrate, and it has a purely thermal origin. Alongside this, a p-type layer is localized on the substrate side of the AlN/Si interface and is induced by diffusion of group-III element of the metal-organic precursor. Fortunately, maintaining the sheet resistance of this p-type layer to high values (∼2000 Ω/□) seems feasible with particular durations of either organometallic precursor or ammonia gas predose of the Si surface, i.e., the intentional introduction of one chemical precursor just before nucleation. It is proposed that the mechanism behind the control actually relies on the formation of disordered AlSiN between the crystalline AlN nucleation layer and the crystalline silicon substrate.

Crystalline Interlayers for Reducing the Effective Thermal Boundary Resistance in GaN-on-Diamond.

Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBReff) between the GaN HEMT and the diamond, reducing the benefit of the diamond heat spreader. Replacing the amorphous interlayer with a higher thermal conductivity crystalline material would reduce TBReff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al0.32Ga0.68N interlayer has been integrated into a GaN/AlGaN HEMT device epitaxy. Two samples were studied, one with diamond grown directly on the AlGaN interlayer and another incorporating a thin crystalline SiC layer between AlGaN and diamond. The TBReff, measured using transient thermoreflectance, was improved for the sample with SiC (30 ± 5 m2 K GW-1) compared to the sample without (107 ± 44 m2 K GW-1). The reduced TBReff is thought to arise from improved adhesion between SiC and the diamond compared to the diamond directly on AlGaN because of an increased propensity for carbide bond formation between SiC and the diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.

Halide Homogenization for High-Performance Blue Perovskite Electroluminescence.

Metal halide perovskite light-emitting diodes (LEDs) have achieved great progress in recent years. However, bright and spectrally stable blue perovskite LED remains a significant challenge. Three-dimensional mixed-halide perovskites have potential to achieve high brightness electroluminescence, but their emission spectra are unstable as a result of halide phase separation. Here, we reveal that there is already heterogeneous distribution of halides in the as-deposited perovskite films, which can trace back to the nonuniform mixture of halides in the precursors. By simply introducing cationic surfactants to improve the homogeneity of the halides in the precursor solution, we can overcome the phase segregation issue and obtain spectrally stable single-phase blue-emitting perovskites. We demonstrate efficient blue perovskite LEDs with high brightness, e.g., luminous efficacy of 4.7, 2.9, and 0.4 lm W-1 and luminance of over 37,000, 9,300, and 1,300 cd m-2 for sky blue, blue, and deep blue with Commission Internationale de l'Eclairage (CIE) coordinates of (0.068, 0.268), (0.091, 0.165), and (0.129, 0.061), respectively, suggesting real promise of perovskites for LED applications.

InGaN as a Substrate for AC Photoelectrochemical Imaging.

AC photoelectrochemical imaging at electrolyte-semiconductor interfaces provides spatially resolved information such as surface potentials, ion concentrations and electrical impedance. In this work, thin films of InGaN/GaN were used successfully for AC photoelectrochemical imaging, and experimentally shown to generate a considerable photocurrent under illumination with a 405 nm modulated diode laser at comparatively high frequencies and low applied DC potentials, making this a promising substrate for bioimaging applications. Linear sweep voltammetry showed negligible dark currents. The imaging capabilities of the sensor substrate were demonstrated with a model system and showed a lateral resolution of 7 microns.

Characterisation of InGaN by Photoconductive Atomic Force Microscopy.

Nanoscale structure has a large effect on the optoelectronic properties of InGaN, a material vital for energy saving technologies such as light emitting diodes. Photoconductive atomic force microscopy (PC-AFM) provides a new way to investigate this effect. In this study, PC-AFM was used to characterise four thick (∼130 nm) In x Ga 1 - x N films with x = 5%, 9%, 12%, and 15%. Lower photocurrent was observed on elevated ridges around defects (such as V-pits) in the films with x ≤ 12 %. Current-voltage curve analysis using the PC-AFM setup showed that this was due to a higher turn-on voltage on these ridges compared to surrounding material. To further understand this phenomenon, V-pit cross sections from the 9% and 15% films were characterised using transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. This identified a subsurface indium-deficient region surrounding the V-pit in the lower indium content film, which was not present in the 15% sample. Although this cannot directly explain the impact of ridges on turn-on voltage, it is likely to be related. Overall, the data presented here demonstrate the potential of PC-AFM in the field of III-nitride semiconductors.

Ultra-low-threshold InGaN/GaN quantum dot micro-ring lasers.

In this work, we demonstrate ultra-low-threshold, optically pumped, room-temperature lasing in GaN microdisk and micro-ring cavities containing InGaN quantum dots and fragmented quantum wells, with the lowest measured threshold at a record low of 6.2 µJ/cm2. When pump volume decreases, we observe a systematic decrease in the lasing threshold of micro-rings. The photon loss rate, γ, increases with increasing inner ring diameter, leading to a systematic decrease in the post-threshold slope efficiency, while the quality factor of the lasing mode remains largely unchanged. A careful analysis using finite-difference time-domain simulations attributes the increased γ to the loss of photons from lower-quality higher-order modes during amplified spontaneous emission.

Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures.

We report the successful realisation of intrinsic optical polarisation control by growth, in solid-state quantum dots in the thermoelectrically cooled temperature regime (≥200 K), using a non-polar InGaN system. With statistically significant experimental data from cryogenic to high temperatures, we show that the average polarisation degree of such a system remains constant at around 0.90, below 100 K, and decreases very slowly at higher temperatures until reaching 0.77 at 200 K, with an unchanged polarisation axis determined by the material crystallography. A combination of Fermi-Dirac statistics and k·p theory with consideration of quantum dot anisotropy allows us to elucidate the origin of the robust, almost temperature-insensitive polarisation properties of this system from a fundamental perspective, producing results in very good agreement with the experimental findings. This work demonstrates that optical polarisation control can be achieved in solid-state quantum dots at thermoelectrically cooled temperatures, thereby opening the possibility of polarisation-based quantum dot applications in on-chip conditions.