Point defects are known to degrade LED performance by lowering efficiencies, maximum output power and device lifetimes. Here we show that growth temperature is a key variable, affecting both point defect concentrations and distributions. Cathodoluminescence and electron beam induced current measurements elucidate the role these defects play in carrier recombination within the wells. Combining such measurements with atomic force microscopy allows us to identify the growth mechanisms at play and help explain the point defect distributions observed. We find that in all cases, the presence of threading dislocations with a screw component LED to the formation of spiral hillocks. Desorption of gallium along ridges and wide atomic terraces lead to blue-shifted quantum well emission energies but also impacted point defect populations. As growth temperatures were increased, dislocation mediated gettering counteracts a rising population of point defects. This restricts their impact below 1060 ∘C, above which, performance regresses and point defects dominate.

Thermal management is the main technological challenge for next generation electronic devices. Recently, several groups successfully demonstrated boron arsenide (BAs) microcrystals with an ultrahigh thermal conductivity approaching that of diamond. The development of scalable epitaxial BAs growth techniques is urgently required to enable a transition of BAs material to real applications. We have grown boron arsenide layers on 3C-SiC/Si and sapphire substrates over a wide temperature range using molecular beam epitaxy (MBE). We have confirmed the incorporation of arsenic by a wide range of characterization techniques. The best quality of the boron arsenide layers was achieved at high growth temperatures of around 750 °C. We have demonstrated that high temperatures nucleation of the boron arsenide layer started with deposition of boron-rich monolayers on the substrate surface. For the epitaxy on sapphire during the initial growth phase, the cubic boron arsenide layers align with the hexagonal structure of the sapphire substrate and grow in the ⟨111⟩ direction for a few crystalline monolayers; however, currently, we are not able to sustain that, and the boron arsenide layer becomes amorphous. For boron arsenide layers grown at high temperatures, we have observed an increase in the thermal conductivity and cathodoluminescence optical response with a reproducible peak centered at ∼1.67 eV. The experimental results are explained by increased chemical interaction between arsenic and boron at growth temperatures above ∼600 °C. Our experimental data show that MBE growth conditions need to be further optimized first to improve stoichiometry and after that to decrease point-defect densities in boron arsenide layers to achieve an increase in the thermal conductivity.

Within the field of perovskite photovoltaics, there has been heavy focus on either improving the conductivity/mobility of the charge transport layers [electron transport (ETL) or hole transport layers (HTL)], or tuning their energy alignment with the perovskite absorber for optimising the device efficiency, with little attention paid to the impact of the underlying charge transport layer on the structural and optoelectronic properties of the perovskite overlayer. For example, in the n-i-p device architecture, the ETL provides a key surface upon which the perovskite film grows. In this work, electron backscatter diffraction (EBSD) and cathodoluminescence (CL) spectroscopy are used to show a direct correlation between optical emission and structural properties of all-inorganic CsPbI2Br perovskite absorber thin films with a selection of inorganic underlying ETLs, giving insights into the vital role of the ETL. Comparisons are drawn between the effect of three commonly used electron transport layers (zinc oxide, titanium dioxide and tin oxide) on the optical emission and crystallographic properties of the CsPbI2Br perovskite thin films processed at two different annealing temperatures. Among the ETLs, zinc oxide is found to promote perovskite films with enhanced grain size and preferred growth along the [100] orientation, and relatively uniform light emission for the high temperature processed layer, showing its strong potential as a low-cost electron transport layer for the development of perovskite solar cells. Titanium dioxide is found to result in a high level of heterogeneity in the light emission when the perovskite is processed at low temperature, while tin oxide is found not to promote large grain growth. The observed variations are understood in terms of the differences in thermal expansion coefficient of the perovskite as compared to those of the ETLs as well as the leading strain in the lattice. The results from the study show the importance of considering perovskite growth effects when selecting an underlayer.

Dislocations in epitaxial lateral overgrown alpha-Ga2O3 are investigated using hyperspectral cathodoluminescence spectroscopy. The dislocations are associated with a reduction of self-trapped hole-related luminescence (ca. 3.6 eV line) which can be ascribed to their actions as non-radiative recombination sites for free electrons, to a reduction in free electron density due to Fermi level pinning or to electron trapping at donor states. An increase in the intensity of the ca. 2.8 eV and 3.2 eV lines are observed at the dislocations, suggesting an increase in donor-acceptor pair transitions and providing strong evidence that point defects segregate at dislocations.

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. Most of the studies report that the metal cation in the dopant plays a greater role in the improvement of the film properties than the chloride anions, which are thought to be removed during annealing in some studies. The majority of the research to date on this topic has focussed on investigating 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 valuable insight into the impact chloride dopants have on all-inorganic perovskite absorbers, helping to influence future dopant strategies. Cl- anions remain in the CsPbI2Br film after annealing directing the preferential crystal orientation of the films along [100].

Electron beam techniques have been used to analyze the impact of substrate choice and growth parameters on the compositional and optical properties of tin gallium oxide [(SnxGa1−x)2O3] thin films grown by plasma‐assisted molecular beam epitaxy. Sn incorporation and film quality are found to be highly dependent on growth temperature and substrate material (silicon, sapphire, and bulk Ga2O3) with alloy concentrations varying up to an x value of 0.11. Room temperature cathodoluminescence spectra show the Sn alloying suppressing UV (3.3–3.0 eV), enhancing blue (2.8–2.4 eV), and generating green (2.4–2.0 eV) emission, indicative of the introduction of a high density of gallium vacancies (VGa) and subsequent VGa–Sn complexes. This behavior was further analyzed by mapping composition and luminescence across a cross section. Compared to Ga2O3, the spectral bands show a clear redshift due to bandgap reduction, confirmed by optical transmission measurements. The results show promise that the bandgap of gallium oxide can successfully be reduced through Sn alloying and used for bandgap engineering within UV optoelectronic devices.

Gallium nitride co-doped with magnesium and europium shows great potential for active layers in red light emitting diode structures due to strong and sharp luminescence emission around 620 nm. In this work, sharp and intense Eu3+ luminescence lines from the excited states of the D-5(J) (J = 0, 1) multiplets to the ground states of the F-7(J) (J = 0, 1, 2) multiplets have been analyzed using a C-2v crystal-field equivalent operator Hamiltonian. A model of Eu centers with the C-2v symmetry has been proposed to be an Eu3+ complex accompanied by either a pair of nitrogen and gallium vacancies (V-N-V-Ga) or a pair consisting of a nitrogen vacancy and magnesium impurity (V-N-Mg-Ga) in the vicinity of the Eu ion based on the crystal-field analysis, the selection rules and the observed polarization of the Eu3+ luminescence lines. Energy transfer from the host to the Eu ions under band-to-band excitation occurs through electron-hole recombination between V with the electron-like state and V-Ga or Mg-Ga with the hole-like state; these may be associated with the shallow-trapped or deep-trapped states, respectively, proposed as the energy transfer mechanism in previous literature.

Electron backscatter diffraction and cathodoluminescence are complementary scanning electron microscopy modes widely used in the characterisation of semiconductor films, respectively revealing the strain state of a crystalline material and the effect of this strain on the light emission from the sample. Conflicting beam, sample and detector geometries have meant it is not generally possible to acquire the two signals together during the same scan. Here, we present a method of achieving this simultaneous acquisition, by collecting the light emission through a transparent sample substrate. We apply this combination of techniques to investigate the strain field and resultant emission wavelength variation in a deep-ultraviolet micro-LED. For such compatible samples, this approach has the benefits of avoiding image alignment issues and minimising beam damage effects.

Supraparticles comprising semiconductor colloidal quantum dots as building blocks are a new class of microscopic lasers with a wide host of applications, including photocatalysis, biological and environmental sensing, integrated photonics, and medicine. Despite the recent advances in their fabrication, there have been no reports of their quantum dot components being recovered for use in a circular economy. Herein, we demonstrate a novel method for the recycling of these whispering-gallery-mode supraparticle lasers with a quantum dot recovery yield of 85%. The photoluminescence quantum yield of the recycled quantum dots is retained at 83 +/- 16% from the initial batch of 86 +/- 9%. These recycled quantum dots are then used again to synthesize distinct supraparticles via an oil-in-water emulsion self-assembly technique, allowing for the recreation of lasing supraparticles with similar thresholds to their freshly made precursors at 32.8 +/- 8.2 mJ cm-2 and 34.8 +/- 8.6 mJ cm-2, respectively. This proof-of-concept for recyclability has the potential to complement and enhance the manufacturing of supraparticle lasers, as well as to contribute to the overall recycling efforts of a broad spectrum of colloidal nanoparticle species, aiming to improve the economic and environmental sustainability of the technology.

Luminescent supraparticles of colloidal semiconductor nanocrystals can act as microscopic lasers and are hugely attractive for biosensing, imaging, and drug delivery. However, biointerfacing these to increase functionality while retaining their main optical properties remains an unresolved challenge. Here, we propose and demonstrate red-emitting, silica-coated CdS x Se1-x /ZnS colloidal quantum dot supraparticles functionalized with a biotinylated photocleavable ligand. The success of each step of the synthesis is confirmed by scanning electron microscopy, energy dispersive X-ray and Fourier transform infrared spectroscopy, zeta-potential, and optical pumping measurements. The capture and release functionality of the supraparticle system is proven by binding to a neutravidin functionalized glass slide and subsequently cleaving off after UV-A irradiation. The biotinylated supraparticles still function as microlasers; e.g., a 9 mu m diameter supraparticle has oscillating modes around 625 nm at a threshold of 58 mJ/cm2. This work is a first step toward using supraparticle lasers as enhanced labels for bionano applications.

This study introduces and compares the lasing performance of micron-sized and sphere-shaped supraparticle (SP) lasers fabricated through bottom-up assembly of II-VI semiconductor colloidal quantum wells (CQWs) with their counterparts made of quantum dots (CQDs). CQWs consist of a 4-monolayers thick CdSe core and an 8-monolayers thick CdxZn1-xS shell with a nominal size of 14 x 15 x 4.2 nm, and CQDs of CdSxSe1-x/ZnS with 6 nm diameter. SPs are optically characterized with a 0.76 ns pulse laser (spot size: 2.88 x 10-7 cm2) at 532 nm, and emit in the 620-670 nm spectral range. Results show that CQW SPs have lasing thresholds twice as low (0.1-0.3 nJ) as CQD SPs (0.3-0.6 nJ), and stress tests using a constant 0.6 nJ optical pump energy demonstrate that CQW SPs withstand lasing emission for longer than CQD SPs. Lasing emission in CQW and CQD SPs under continuous operation yields half-lives of tau CQW SP approximate to 150 min and tau CQD SP approximate to 22 min, respectively. The half-life of CQW SPs is further extended to tau QW approximate to 385 min when optically pumped at 0.5 nJ. Such results compare favorably to those in the literature and highlight the performance of CdSe-based CQW SPs for laser applications. CdSe-based colloidal quantum well supraparticles display a lower laser threshold than their quantum dot counterparts. When benchmarked against other state-of-the-art CdSe-based microlasers, these devices also score amongst the ones with the longest half-lives (between 102 and 103 min), and their degradation fluences (108-109 mJ cm-2) surpass those of all the CdSe-based quantum dot microlasers. image

Chlorine incorporation in triple halide perovskites reduces trap density and enhances the efficiency of indoor light harvesting.

Here, we explore a catalyst-free single-step growth strategy that results in high-quality self-assembled single-crystal vertical GaN nanowires (NWs) grown on a wide range of common and novel substrates (including GaN, Ga2O3, and monolayer two-dimensional (2D) transition-metal dichalcogenide (TMD)) within the same chamber and thus under identical conditions by pulsed laser deposition. High-resolution transmission electron microscopy and scanning transmission electron microscopy (HR-STEM) and grazing incidence X-ray diffraction measurements confirm the single-crystalline nature of the obtained NWs, whereas advanced optical and cathodoluminescence measurements provide evidence of their high optical quality. Further analyses reveal that the growth is initiated by an in situ polycrystalline layer formed between the NWs and substrates during growth, while as its thickness increases, the growth mode transforms into single-crystalline NW nucleation. HR-STEM and corresponding energy-dispersive X-ray compositional analyses indicate possible growth mechanisms. All samples exhibit strong band edge UV emission (with a negligible defect band) dominated by radiative recombination with a high optical efficiency (∼65%). As all NWs have similar structural and optical qualities irrespective of the substrate used, this strategy will open new horizons for developing III-nitride-based devices.

Indoor photovoltaics are receiving tremendous attention due to the continuous development of the Internet of Things (IoT). Here we report a triple anion (TA) perovskite CH3NH3PbI2.6(BrCl)0.2 with a tailored bandgap suitable for maximizing indoor light harvesting compared to methyl ammonium lead iodide CH3NH3PbI3. The best-performing TA perovskite indoor-photovoltaic device achieved a steady-state power conversion efficiency (PCE) of 25.1 output power density of 75 microW/cm2 under 1000 lux indoor illumination (0.3 mW/cm2 irradiance). This PCE is almost 40 equivalent CH3NH3PbI3-based devices (PCE of 17.9 reduced density of trap states and improved crystalline quality were achieved by the triple anion alloying method. The decisive role of chlorine (Cl) in the better performance of TA-based indoor photovoltaic devices was further investigated by successively reducing the Cl content and correlating it with the corresponding photovoltaic device performance. Replacing the commonly used hole transporting layer of Spiro-MeOTAD with undoped P3HT was found to significantly reduce the current-voltage hysteresis under indoor lighting conditions. A graphene-coated textile fiber-based temperature sensor was successfully powered by the triple anion perovskite indoor photovoltaic devices. The results from the present study demonstrate a novel route to maximize the PCE of halide perovskite indoor photovoltaic devices and their potential for application in the IoT industry.

Tin-gallium oxide (TGO) epilayers have been characterized through the electron microscopy techniques of wavelength-dispersive X-ray spectroscopy (WDX) and cathodoluminescence. Tin incorporation was found to be highly dependent on growth conditions with (0001)-sapphire and (010)-Ga2O3 substrates enhancing tin incorporation. Cathodoluminescence measurements show that TGO luminescence consists of an enhanced blue emission and quenched UV when compared to Ga2O3.and the onset of new green emission originating from the TGO, further correlated through cross-sectional WDX and cathodoluminescence mapping. As well as luminescence intensity changes TGO films display redshifted luminescence bands associated with a bandgap reduction due to the alloying, confirmed through optical transmission measurements.

We demonstrate the use of a low-cost liquid-crystal-based wavelength-tunable filter and CMOS video camera to add hyperspectral imaging capabilities to a probe station equipped with a simple optical microscope. The resultant setup is used to rapidly resolve the spectral and spatial variations in electroluminescence typically observed for InxGa1−xN/GaN light-emitting diodes. Applying standard statistical analyses of variation within the multivariate datasets, such as moments and principal components, we observe inhomogeneities on a spectral scale significantly smaller than the bandwidth of the tunable filter. The resultant tool offers an alternative to scanning beam luminescence techniques for high-throughput hyperspectral analysis of optoelectronic devices.

Titanium nitride (TiN) has emerged as a highly promising alternative to traditional plasmonic materials. This study focuses on the inclusion of a Cr90Ru10 buffer layer between the substrate and thin TiN film, which enables the use of cost-effective, amorphous technical substrates while preserving high film quality. We report best-in-class TiN thin films fabricated on fused silica wafers, achieving a maximum plasmonic figure of merit, -& varepsilon;'/& varepsilon;'', of approximately 2.8, even at a modest wafer temperature of around 300 degrees C. Furthermore, we delve into the characterization of TiN thin film quality and fabricated TiN triangular nanostructures, employing attenuated total reflectance and cathodoluminescence techniques to highlight their potential applications in surface plasmonics.