Transfer printing is employed to demonstrate the integration of gallium nitride (GaN)-based distributed Bragg reflectors (DBR) with 100 μm lateral dimensions and reflectance of 90% in various formats. Mesoporous GaN DBRs are utilized as basic building blocks to fabricate more complex photonic devices directly on Silicon (Si) and glass receiving substrates. Multi-mode optical resonant cavities centered at 450 nm on Si are thus formed by direct stacking of two mesoporous DBR membranes. Furthermore, active devices are also demonstrated by combining mesoporous DBR with GaN-based light-emitting diodes membranes of similar dimensions, resulting in a Fabry-Perot-mediated emission with its main peak shifted by 14 nm compared to a reference device without DBR. Measured optical bandwidth of 136 MHz (-6 dB) in a small signal modulation scheme is also demonstrated from these devices.

Light emitting diodes based on c-plane (In,Ga)N/GaN quantum wells (QWs) can have > 90% emission efficiency at modest current densities but this drops significantly at higher excitation, an effect known as efficiency droop that limits device efficacy at high brightness. Several explanations for this have been proposed including the saturation of carrier localisation sites at high excitation densities, resulting in a greater exposure of carriers to defects and hence a significant increase in the associated non-radiative recombination processes. Here, power- and temperature-dependent photoluminescence spectroscopy of c-plane (In,Ga)N/GaN QWs is used to investigate the relationship between the saturation of localised states and emission efficiency. For the samples studied, we find that the saturation of localised sites broadly coincides with the onset of efficiency droop. However, it is also found that as the localised states saturate with increasing excitation, the relative contribution of defect-associated non-radiative processes to overall recombination decreases rather than increases. Based on these observations and on modelling of recombination processes in the QW, it is concluded that the saturation of localised states does not significantly contribute to the reduction in emission efficiency at high excitation. Our studies rather suggest that defect-related non-radiative recombination is out-competed by radiative and Auger-Meitner recombination at the carrier densities required for saturation.

The luminescence characteristics and the relation between the distribution of impurities and stacking faults (SFs) in Mg-doped zincblende gallium nitride (zb-GaN:Mg) have been investigated by cathodoluminescence (CL) and atom probe tomography (APT). Four peaks have been identified in the CL emission spectrum, and the possible related recombination mechanisms have been proposed. The main peak at 3.23 eV is associated with excitonic transitions, while the other three, having lower energies at about 3.15, 3.02, and 2.92 eV, respectively, are related to donor-to-acceptor (DAP) transitions involving different acceptor energy levels. These DAP peaks were significantly more intense on or close to SFs compared to the surrounding defect-free material, indicating an enrichment of point defects near SFs. This finding was supported by APT measurements, where Mg showed a tendency to segregate toward SFs in zb-GaN.

We demonstrate the first AlGaN/GaN-based photoelectric tunable-step (PETS) terahertz (THz) detector working on the in-plane photoelectric (IPPE) effect. A bow-tie antenna on top of the device is utilized to focus the incident THz radiation. This device is capable of operating at a higher frequency, 1.9 THz, compared with most reported AlGaN/GaN-based FETs. Higher responsivity is also achieved by the AlGaN/GaN PETS THz detector in comparison to the previously demonstrated AlGaAs/GaAs-based devices. The demonstration of the in-plane photoelectric effect in the AlGaN/GaN system suggests its universality in 2DEG-based systems.

Microwave plasma chemical vapour deposition (MP-CVD) of thick polycrystalline diamond (PCD) (t> 100 mu m) is demonstrated on flipped III-nitrides (III-N)/gallium nitride (GaN) on Si using a sample holder designed using iterative microwave plasma modelling. The damage of flipped III-N/GaN in H2 plasma is due to superheating, caused by expansion of voids in the bonding layer from the flipping process and etching of the III-N/GaN film at an onset of above similar to 720 degrees C. This study demonstrates that holders with a tapered base allow rapid sample cooling (T similar to 669 degrees C) to mitigate damage in a reactive hydrogen plasma at high-power and pressure. This holder enables, high quality thick PCD deposition and demonstrates the importance of microwave plasma modelling for cost-effective iteration of sample holder/susceptor design for temperature regulation.

Conventional c-plane wurtzite InGaN/GaN quantum wells are subject to a large internal field that acts to separate electrons and holes and thereby lowers the rate of radiative recombination. This effect is exacerbated for higher indium contents and so may contribute to the lower efficiency of c-plane wurtzite InGaN/GaN QWs when emitting at green and amber wavelengths. In comparison, InGaN/GaN QWs grown in the cubic zincblende phase along the (001) direction are free of such fields and so exhibit recombination lifetimes that are shorter by two orders of magnitude and independent of indium content. Here, we report on zincblende QWs grown by metal-organic chemical vapor deposition at different temperatures. This results in different indium contents and thereby allows tuning of the emission band from blue to yellow. For each indium content, the spectrally integrated emission quenches as the temperature rises. However, the ratio of room temperature to low temperature emission improves for higher indium contents, increasing from 18 % to 34 % as the emission peak is tuned from 2.8 eV to 2.1 eV. This behavior is attributed to the thermal escape of carriers from the QWs playing an important role in the temperature dependent quenching of emission.

In this paper, we investigate the optical properties of a zincblende InGaN single quantum well (SQW) structure containing stacking faults (SFs). Cathodoluminescence studies revealed the presence of sharp emission features adjacent to SFs, identified as quantum wires (Qwire) via their spatial anisotropy. Scanning transmission electron microscopy provided evidence of indium rich regions adjacent to SFs which intersect the QW along the [110] and [1-10] directions, whilst atom probe tomography revealed that the indium rich regions have an elongated structure, creating a Qwire. This work sheds light on the intricate relationship between SFs and Qwires in zincblende InGaN SQW structures, offering insights into the underlying mechanisms governing their optical behavior.

Ohmic contacts to wide bandgap nitrides have been realised, but little is known about their behaviour at low temperatures. To address this, an established Ti/Al/Ti/Au contact stack on AlGaN/GaN heterostructures has been characterised from 320 to 80 K. Two structures were investigated, with very similar ambient 2D electron gas transport characteristics despite their difference in AlGaN barrier thickness and composition. This allowed for direct comparison of contact behaviour across different heterostructures. Upon annealing at <800 degrees C for samples with 29 nm AlGaN barriers, contacts which had Ohmic characteristics at room temperature exhibited a gradual onset of Schottky behaviour as the measurement temperature was lowered. When non-Ohmic behaviour was observed, a combination of direct tunnelling, Fowler-Nordheim tunnelling and a thermally assisted Fowler-Nordheim mechanism is suggested to describe the carrier transport. In this case, annealing at 800 degrees C for 30 s proved sufficient to ensure Ohmic behaviour when tested from 320 to 80 K. For a heterostructure with 8 nm AlGaN, the required annealing temperature to maintain consistent Ohmic behaviour across the temperature range was reduced to 750 degrees C. From these observations, the determining factor for Ohmic behaviour is suggested to be the thickness of the AlGaN barrier-either as-grown, or the effective thickness following the formation of TiN protrusions into the AlGaN barrier during annealing. The understanding provided here allows tailoring of either the processing conditions or the heterostructure, and may aid with design of novel devices for low temperature operation.

In this study, we present an atom probe tomography investigation of zincblende InGaN-based multi-quantum well light-emitting diode (LED) structures with a specific focus on the influence of stacking faults within the system. We demonstrate that the visualisation of stacking faults in atom probe reconstructions is possible due to previously documented sensitivities of measured composition in III-V materials to local variations in electric field during the experiment. Meanwhile, we quantify the composition of indium (In) in the InGaN quantum wells and establish that elongated regions exist, parallel to ridges on the sample surface, in which the indium content is increased. We discuss this observation in the context of previous scanning transmission electron microscopy (STEM) data which suggested that such In rich regions are associated with stacking faults. Our experiments not only showcase the feasibility of stacking fault characterization in InGaN-based multi-quantum well LEDs through atom probe tomography (APT) but also offer a practical pathway towards three-dimensional imaging and compositional analysis of stacking faults at the atomic scale.

Cubic zincblende InGaN/GaN quantum wells are free of the electric fields that reduce recombination efficiency in hexagonal wurtzite wells.

III-nitride materials, such as GaN and its alloys, are essential for modern microelectronics and optoelectronics due to their unique properties. Focused ion beam (FIB) techniques play a crucial role in their prototyping and characterization at the micro- and nanoscale. However, conventional FIB milling with Ga ions presents challenges, including surface amorphization and point defect introduction, prompting the exploration of alternative ion sources. Xenon-based inductively coupled plasma or plasma FIB has emerged as a promising alternative, offering reduced damage and better sample property preservation. Despite extensive research on FIB-induced damage in GaN, systematic comparisons between Ga and Xe ion milling on the luminescence characteristics of GaN remain limited. This study aims to fill this gap by evaluating and comparing the extent of FIB-induced damage caused by Ga and Xe ions in wurtzite and zincblende GaN through cathodoluminescence measurements. Our findings indicate that Xe ion milling yields higher integrated intensities compared to Ga ion milling, attributed to shallower implantation depths and reduced lattice disorder. We also observe a decrease in integrated intensity with increasing ion beam acceleration voltage for both wurtzite and zincblende GaN layers. This study provides valuable insights into optimizing FIB-based sample preparation techniques for III-nitride materials, with implications for enhancing device performance and reliability. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0International (CC BY-NC) license.

Zincblende GaN has the potential to improve the efficiency of green- and amber-emitting nitride light emitting diodes due to the absence of internal polarisation fields. However, high densities of stacking faults are found in current zincblende GaN structures. This study presents a cathodoluminescence spectroscopy investigation into the low-temperature optical behaviour of a zincblende GaN/InGaN single quantum well structure. In panchromatic cathodoluminescence maps, stacking faults are observed as dark stripes, and are associated with non-radiative recombination centres. Furthermore, power dependent studies were performed to address whether the zincblende single quantum well exhibited a reduction in emission efficiency at higher carrier densities-the phenomenon known as efficiency droop. The single quantum well structure was observed to exhibit droop, and regions with high densities of stacking faults were seen to exacerbate this phenomenon. Overall, this study suggests that achieving efficient emission from zinc-blende GaN/InGaN quantum wells will require reduction in the stacking fault density.

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.

Distributed Bragg reflectors (DBRs) based on alternating layers of porous and non-porous GaN have previously been fabricated at the wafer-scale in heteroepitaxial GaN layers grown on sapphire substrates. Porosification is achieved via the electrochemical etching of highly Si-doped layers, and the etchant accesses the n+-GaN layers through nanoscale channels arising at threading dislocations that are ubiquitous in the heteroepitaxial growth process. Here, we show that the same process applies to GaN multilayer structures grown on silicon substrates. The reflectance of the resulting DBRs depends on the voltage at which the porosification process is carried out. Etching at higher voltages yields higher porosities. However, while an increase in porosity is theoretically expected to lead to peak reflectance, in practice, the highest reflectance is achieved at a moderate etching voltage because etching at higher voltages leads to pore formation in the nominally non-porous layers, pore coarsening in the porous layers, and in the worst cases layer collapse. We also find that at the high threading dislocation densities present in these samples, not all dislocations participate in the etching process at low and moderate etching voltages. However, the number of dislocations involved in the process increases with etching voltage.

Laser-written nitrogen vacancy (NV-) centers are combined with transfer-printed GaN micro-lenses to increase fluorescent light collection by reducing total internal reflection at the planar diamond interface. We find a 2x improvement of fluorescent light collection using a 0.95 NA air objective at room temperature, in agreement with FDTD simulations. The nature of the transfer print micro-lenses leads to better performance with lower numerical aperture (NA) collection, as confirmed by results with a 0.5NA air objective which show improvement greater than 5x. The approach is attractive for scalable integrated quantum technologies.

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.

Room temperature quantum emitters have been reported in aluminum nitride grown on sapphire, but until now they have not been observed in epilayers grown on silicon. We report that epitaxial aluminum nitride grown on silicon by either plasma vapor deposition or metal-organic vapor phase epitaxy contains point-like emitters in the red to near-infrared part of the spectrum. We study the photon statistics and polarization of emission at a wavelength of 700–750 nm, showing signatures of quantized electronic states under pulsed and CW optical excitation. The discovery of quantum emitters in a material deposited directly on silicon can drive integration using industry standard 300 mm wafers, established complementary metal-oxide-semiconductor control electronics, and low marginal-cost mass-manufacturing.

•Cubic zincblende GaN films were grown by MOVPE on 3C-SiC/Si (001) templates.•The samples were characterized using DIC optical microscopy, AFM, XRD, and STEM.•The GaN epilayer growth temperature and gas-phase V/III-ratio were varied.•A narrow growth window for high phase purity zincblende GaN epilayer was identified.

Aberration-corrected scanning transmission electron microscopy techniques are used to study the bonding configuration between gallium cations and nitrogen anions at defects in metalorganic vapor-phase epitaxy-grown cubic zincblende GaN on vicinal (001) 3C-SiC/Si. By combining high-angle annular dark-field and annular bright-field imaging, the orientation and bond polarity of planar defects, such as stacking faults and wurtzite inclusions, were identified. It is found that the substrate miscut direction toward one of the 3C-SiC ⟨110⟩ in-plane directions is correlated with the crystallographic [1–10] in-plane direction and that the {111} planes with a zone axis parallel to the miscut have a Ga-polar character, whereas the {111} planes in the zone perpendicular to the miscut direction have N-polarity. The polarity of {111}-type stacking faults is maintained in the former case by rotating the coordination of Ga atoms by 180° around the ⟨111⟩ polar axes and in the latter case by a similar rotation of the coordination of the N atoms. The presence of small amounts of the hexagonal wurtzite phase on Ga-polar {111} planes and their total absence on N-polar {111} planes is tentatively explained by the preferential growth of wurtzite GaN in the [0001] Ga-polar direction under non-optimized growth conditions.