AlN layers annealed at high temperatures offer low threading dislocation densities of mid 108 cm−2 and are therefore increasingly used as base layers in ultraviolet (UV) light emitting diode (LED) heterostructure growth. These LEDs, just like those grown on conventional metalorganic vapor phase epitaxy (MOVPE) AlN templates, often suffer from long-wavelength parasitic luminescence. In this work, luminescence properties of far-UVC LED heterostructures grown on MOVPE-AlN/sapphire templates and high-temperature annealed AlN/sapphire templates are compared. To investigate the origin of parasitic emission with high spatial resolution, cross section scanning transmission electron microscopy was combined with cathodoluminescence measurements. As a result, the main origin of the parasitic luminescence band centered at 3.5 eV (354 nm) for the heterostructure grown on annealed AlN is assigned to point defects related to oxygen in the AlN template layer. The defect band centered at 3.0 eV (413 nm) for the heterostructure grown on MOVPE-AlN was found to be related to self-compensating VAl-Si point defect complexes in the n-AlGaN layer and oxygen incorporation close to the AlN/sapphire interface. The results also suggest that the type of AlN template determines the kind of parasitic luminescence from the n-AlGaN layer.

Herein, integrating GaN quantum dots (QDs) within a resonant cavity is focused on. Utilizing metal-organic vapor phase epitaxy, controlled growth of GaN QDs on AlN is achieved. A deep-UV distributed Bragg reflector (DBR) with high reflectivity in the 250-300 nm range, using AlN and Al0.7Ga0.3N layers to maximize refractive index contrast, is developed. A 50-period DBR achieves 98% reflectivity at a wavelength of 272 nm. Scanning transmission electron microscopy and electron energy loss spectroscopy analyses reveal a trisection of DBR periods, attributed to a Ga composition pulling effect during growth. The real structure's reflectivity is simulated and matched well with measured data, though actual reflectivity is lower than the ideal. Cathodoluminescence studies at T = 17 K show emission peaks from both the DBR and the GaN QDs. Further, single-photon emission is demonstrated with a g(2)(t = 0) value of 0.41 at 272.7 nm, confirming the potential for deep-UV single-photon sources. Additionally, the creation of a planar resonant cavity with enhanced emission intensity and vertical nanopillar structures with an aspect ratio of 11 and a diameter of 400 nm confirm the successful integration of GaN QDs in advanced UV photonic structures.

Using nano-cathodoluminescence performed in scanning transmission electron microscope (STEM-CL), we have investigated a photonic-bandgap-crystal (PBC) laser structure at T = 17 K. In cross-sectional STEM images the full device structure is clearly resolved. The most dominant luminescence originates from the 3-fold MQW of the active region. The MQW shows a distinct peak wavelength change in growth direction indicating different structural and/or chemical properties of the individual quantum wells. In detail, a clear shift from 427 nm to 438 nm from the first to the top QW is observed, respectively.

Unitized regenerative fuel cells have emerged as promising energy conversion and storage systems for various applications. However, in order to optimize their efficiency, it is crucial to enhance the performance of the bifunctional catalyst. This study aims to provide deeper insights into the electrochemical behavior and performance of the bifunctional catalyst. Several electrocatalysts were prepared and evaluated using rotating disc electrode measurements. The primary focus was placed on investigating the interaction between Pt, Ir, and the support material, antimony doped tin oxide (ATO), and their impact on the oxygen evolution reaction and oxygen reduction reaction. Among the analyzed catalysts, Pt black mixed with synthesized IrO2 supported on developed ATO exhibited the highest performance, considering the results from both the fuel cell and electrolyzer systems.

AlN films, a-plane and m-plane oriented, were obtained by molecular beam epitaxy overgrowth on GaN isolated nanopillars following a three-step process involving: dry etching of a GaN buffer layer to obtain nanopillars; GaN overgrowth on nanopillars and finally AlN overgrowth until nanocrystals coalescence. The resulting a-plane AlN layer shows a roughness, with a RMS of 90 nm over a 5 x 5 mu m2, higher than the m-plane AlN one with a RMS of 35 nm over the same area, which shows much better morphological quality. Plan-view transmission electron microscopy images of the m-plane AlN layer reveal stacking faults, but threading dislocations are barely seen. Cross-sectional transmission electron microscopy image shows stacking faults in bunches running through the sample volume, separated by areas (50 to 100 nm wide) which seem free of them.

The 2022 edition of the 9th International Workshop on Nitride Semiconductors (IWN) took place in Berlin, Germany, from October 9 to 14, 2022. The conference was chaired by Michael Kneissl (TU Berlin) and Jürgen Christen (University of Magdeburg). This important biennial event encompasses all aspects of III-nitride semiconductor science, engineering, and industry. The 2022 workshop continued the tradition of successful gatherings in various locations: Kanazawa, Japan (2818), Orlando, USA (2016), Wrocław, Poland (2014), Sapporo, Japan (2012), Tampa, USA (2010), Montreux, Switzerland (2008), Kyoto, Japan (2006), Pittsburgh, USA (2004), Aachen, Germany (2002), and Nagoya, Japan (2000). The participation at IWN 2022 in Berlin surpassed expectations following a two-year break due to the pandemic, with over 800 scientists representing 32 countries in attendance. The topics of the workshop ranged from novel nitride materials and nanostructures, to growth and fabrication, to characterization and fundamental physics, to electrical and optical devices. Two tutorial speakers (Andreas Waag, TU Braunschweig) and Chris Van de Walle (UCSB) opened the workshop, by discussing GaN-based micro-LEDs and point defects, respectively. Excellent presentations were given by six plenary speakers, who covered a wide range of topics of great interest to the community: Hiroshi Amano (Nagoya University) discussed about deep UV laser diodes, as an example of overcoming the semiconducting limits; Stacia Keller (UCSB) talked about lattice constant engineering for long wavelength nitride emitters; Matteo Meneghini (University of Padova) gave a presentation on defect- and reliability-related aspects, covering both GaN electronics and optoelectronics; Herbert Pairitsch (Infineon) gave a perspective on the applicability of recent advances in material research performed within the research project UltimateGaN; Euijoon Yoon (Seoul National University) gave an overview of recent progress and prospects of micro-LEDs grown on sapphire nano-membranes; Huili Grace Xing (Cornell University) gave a talk entitled “Reveal the true self of GaN by tunneling and avalanche”. The program was then enriched by the presentations of 67 invited speakers, 251 oral presenters, and 416 poster presentations, who covered all topics of interest for optoelectronic and electronic applications of GaN. The workshop organizers express their gratitude to the plenary and invited speakers, rump session chairs, panelists, and short-course presenters for their exceptional contributions to an engaging workshop. They would also like to recognize the industrial sponsors for their generous financial support. Additionally, the program committee's relentless efforts in soliciting and reviewing abstracts are greatly appreciated. A total of 62 submissions were received for the conference publications in the special issues of physica status solidi (a) and (b). All submissions underwent the standard peer-review process of the journals. Out of these submissions, 39 were selected for publication in physica status solidi (a), and 16 were chosen for publication in physica status solidi (b), all featured in a dedicated issue of pss. Among these, two were Review papers: an article entitled “Recent Progress of E-mode GaN MIS-HEMTs with Hybrid Ferroelectric Charge Trap Gate (FEG-HEMT) for Power Switching Applications”, by Jui-Sheng Wu, Edward Yi Chang et al. (National Yang Ming Chiao Tung University) [Phys. Status Solidi A 2023, 220, 2300018], and a paper entitled “On the Origin of the Yellow Luminescence Band in GaN”, by Michael A. Reshchikov (Virginia Commonwealth University) [Phys. Status Solidi B 2023, 260, 2200488]. The published articles encompass a comprehensive array of topics that were addressed during the workshop. Our appreciation goes to Gaia Tomasello and Stefan Hildebrandt for their valuable editorial peer-review work and to Heike Höpcke and Matt Lock for the post-acceptance assembly of this publication. In conclusion, we extend our gratitude to all the workshop attendees who actively contributed to making IWN 2022 a lively and enjoyable event. We eagerly anticipate the opportunity to reunite with you in O'ahu (Hawaii) for the next International Workshop on Nitride Semiconductors in November 2024. Michael Kneissl, TU Berlin (Germany) Jürgen Christen, University of Magdeburg (Germany) Conference Chairs Axel Hoffmann, TU Berlin (Germany) Bo Monemar, Linköping University (Sweden) Honorary Chairs Tim Wernicke, TU Berlin (Germany) Conference Secretary Ulrich Schwarz, TU Chemnitz (Germany) Local Chair Åsa Haglund, Chalmers University of Technology (Sweden) Matteo Meneghini, University of Padova (Italy) Guest Editors and Publication Chairs

The lateral variation of the emission energy of GaAs masklessly grown on V-grooved Si is imaged with cathodoluminescence wavelength imaging. This newly developed unique experimental approach allows, for the first time, to directly visualize and quantify the extreme homogeneity of this novel growth mode and the lateral variation of Si impurity incorporation in such semiconductor microstructures. It represents an essential characterization tool for micropatterned optoelectronic monolithic integrated circuits. At the request of the authors and editor, this article is being retracted effective 22 June 2023.

We present a nanoscopic investigation of the carrier transport into individual single InP quantum dots (QDs) of a membrane external-cavity surface-emitting laser structure (MECSEL) by means of highly spatially resolved cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope (STEM-CL). The lateral STEM-CL spectrum linescans across a single InP QD exhibit a characteristic change of excitonic transitions during this linescan. This gives direct access to the QD population by the generated excess carriers and the renormalization of the QD ground state while the electron beam approaches and subsequently recedes the QD position.

Sputter epitaxy is a low-cost process suited for the deposition of group-III-nitride semiconductors and allowing the deposition on large substrate areas at lower growth temperatures than in metal-organic vapor phase epitaxy (MOVPE). High-quality AlN, AlGaN, and GaN epitaxially grown on Si(111) substrates by reactive magnetron sputtering are demonstrated and details on process parameters are given. With an ammonia-based reactive sputtering process in a high-purity environment, AlN can be grown with high crystalline quality comparable to the best MOVPE-grown samples regarding twist and tilt and with a very low surface roughness, free of the typical columnar structure of sputtered AlN and pits. Also AlGaN, typically required for strain engineering of GaN layers grown on Si, can be grown in the entire compositional range by co-sputtering of Al and Ga. Thin undoped buffer layer samples show high breakdown field strengths well above 2.5 x 10(6) V cm(-1) and demonstrate the possible use for field effect transistor (FET) buffer layers.

We report on the formation process of GaN/AlN quantum dots (QDs) which arises after the deposition of 1–2 monolayers of GaN on an AlN/sapphire template followed by a distinct growth interruption (GRI). The influence of the duration of a GRI on the structural and optical properties of the GaN layer has been systematically investigated. QDs develop from initially bulky GaN islands, which nucleate in close vicinity to bundles of threading dislocations (TDs). For prolonged GRIs, a decreasing island size is observed which is consistent with a systematic blue shift of the emission wavelength. In addition, fragmentation of the bulky GaN islands into several smaller islands occurs, strongly dependent on local strain fields caused by TDs as well as on the different facet orientation of the islands. This morphological transition during GRI eventually leads to GaN QD formation, which assemble as clusters with a density of 10 8 cm −2 . Desorption of GaN is identified as the major source for this morphological transition. The GRI time allows for tuning of the QD emission wavelength in the ultraviolet spectral range.

The characteristic energies of traps in InAlN/AlN/GaN high-electron mobility transistor structures on Si(111) substrates formed after irradiation with 75 MeV S-ions are studied by means of c-lattice parameter analysis, vertical IV-characteristics, micro-photoluminescence (µ-PL), photocurrent (PC) and thermally stimulated current (TSC) spectroscopy. From the lattice parameter analysis, point defect formation is concluded to be the dominant source of defects upon irradiation. A strong compensation effect manifests itself through enhanced resistivity of the devices as found in vertical IV-measurements. Defect formation is detected optically by an additional PL-band within the green spectral region, while defect states with threshold energies at 2.9 eV and 2.65 eV were observed by PC spectroscopy. The TSC spectra exhibit two defect-related emissions between 300 K and 400 K with thermal activation energies of 0.78–0.82 eV and 0.91–0.98 eV, respectively. The data further supports the formation of Ga vacancies (V Ga) and related complexes acting mainly as acceptors compensating the originally undoped n-type GaN buffer layers after irradiation.

Sputtering epitaxy is a low cost process allowing large area deposition at lower growth temperatures than metalorganic vapour phase epitaxy (MOVPE) and may ease e.g. integration with Si CMOS technology. We present high quality AlN and GaN epitaxial layer structures grown on Si(111) substrates by reactive magnetron sputtering. By optimizing nucleation and growth on Si(111) substrates AlN layers are obtained with twist and tilt values comparable to MOVPE grown samples and with very low surface roughness free of any columnar structure. Also the entire compositional range of AlGaN has been addressed by co-sputtering of Al and Ga. Thin undoped buffer layer samples show high breakdown field strengths well above 1×10 6 V/cm which is prerequisite for FET applications. We also report on reactive sputtering of material combinations that are largely inaccessible to MOVPE, e.g., transition metal nitrides and AlScN.

Selective area epitaxial growth is an important technique, both for monolithic device integration as well as for defect reduction in heteroepitaxy of crystalline materials on foreign substrates. While surface engineering with masking materials or by surface structuring is an effective means for controlling the location of material growth, as well as for improving crystalline properties of epitaxial layers, the commonly involved integral substrate heating presents a limitation, e.g., due to constraints ofr the thermal budget applicable to existing device structures. As a solution, an epitaxial growth approach using a laser source only locally heating the selected growth area, in combination with metal-organic precursors to feed a pyrolithic chemical reaction (also known as metal-organic vapor phase epitaxy, MOVPE), is presented. Without masking or surface structuring, local epitaxial growth of III-V compound semiconductor layers on a 50-1500 µm length-scale, with high structural and optical quality, is demonstrated. We discuss general design rules for reactor chamber, laser heating, temperature measurement, sample manipulation, gas mixing, and distinguish laser-assisted local MOVPE from conventional planar growth for the important compound semiconductor GaAs. Surface de-oxidation prior to growth is mandatory to realize smooth island surfaces. Linear growth rates in the range 0.5-9 µm/h are demonstrated. With increasing island diameter, the probability for plastic deformation within the island increases, depending on reactor pressure. A step-flow mode on the island surface can be achieved by establishing a sufficiently small temperature gradient across the island.

We report on nanoscopic exploration of the luminescence from individual InP quantum dots (QDs) by means of highly spatially resolved cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM). A 7-fold layer stack with high-density InP quantum dots is embedded as an active medium membrane in an external-cavity surface-emitting laser. We characterize the vertical transfer of carriers within the periodic separate confinement heterostructure and determine the capture efficiency of carriers from the cladding layer into the quantum dot layers. Benefiting from the nanoscale resolution of our STEM-CL, we perform single-dot spectroscopy on single isolated QDs in the STEM lamella resolving the details of the excitonic structure of individual quantum dots. Executing highly spatially resolved spectrum line scans within the QD layers, we directly visualize the lateral transport, i.e., the efficient lateral capture of carriers into an individual QD. We observe a characteristic change of the spectral fingerprint during this line scan, while the electron beam is approaching and subsequently receding from the quantum dot position. This directly correlates to the increase and decrease of the numbers of excess carriers reaching the dot, i.e., altering the quantum dot population. The characteristic shift of emission energies visualize the renormalization of the ground-state energy of the single dot, and the intensity ratio of the excitonic recombinations verifies this change of the occupation and the state-filling.

We report on the realization of highly conductive and highly reflective n-type AlInN/GaN distributed Bragg reflectors (DBR) for use in vertical cavity surface emitters in a metalorganic vapor phase epitaxy process. While Ge-doping enables low-resistive n-type GaN/AlInN/GaN heterostructures, very high Ge doping levels compromise maximum optical reflectivities of DBRs. Simulations of the Bragg mirror’s reflectivities together with structural analysis by X-ray diffraction reveal an increased absorption within the doped AlInN layers and interface roughening as major causes for the observed reduction of the optical reflectivity. By adjusting the Ge doping level in the AlInN layers, this structural degradation was minimized and highly conductive, 45-fold AlInN/GaN DBR structures with a maximum reflectivity of 99% and vertical specific resistance of 5 × 10 –4 Ω cm 2 were realized.

Blue and yellow emission bands in carbon-doped GaN grown by MBE were investigated in low-temperature cathodoluminescence measurements performed in a scanning transmission electron microscope (STEM-CL) with high spatial resolution. Blue luminescence at 2.85 eV and two contributions in the spectral range of the yellow emission band around 2.2 eV separated by 120 meV are observed in carbon-doped material, whereas only one distinctive yellow luminescence contribution was found in unintentionally-doped GaN.

Freshly introduced a-screw dislocations in gallium nitride are an effective source of ultraviolet radiation, characterized by intense emission of narrow luminescence doublet lines in the spectral range of 3.1-3.2 eV. Furthermore, an additional narrow spectral line with an energy of 3.3 eV has been found at the points of intersection of such dislocations, where extended dislocation nodes were formed. In this communication, we report on the spectral properties of the characteristic luminescence of such nodes, which were obtained for the (0001) gallium nitride samples with dislocations introduced by nanoindentation. The spectral position of the dislocation-related luminescence doublet experiences a redshift with increasing distance from the indentation site. It follows the spectral shift of the excitonic near-bandgap emission, associated with stress relaxation. The luminescence of the intersection points exhibits a similar tendency. At certain local positions, its doublet fine structure is observed, which has a spectral linewidth of the order of or even less than that of the exciton. In this case, the spectral splitting between components of the doublet varies irregularly depending on the position of the exciton (i.e., on the mechanical stress). We see a clear indication of quantum dot-like emission. The fine structure of the luminescence of the intersection points can be easily explained by the energy dependence of emission on their size, as well as on their density, in particular, by the formation of paired nodes, which were previously observed in experiments in a transmission electron microscope.

•Epitaxial growth of AlN on Si (111) by pulsed reactive sputtering.•Change of N-precursor change growth mode from three-dimensionel to two-dimensional.•Change in growth mode approved by scanning transmission electron microscopy images.•Increase of ad-atom mobililty by use of NH3.•Smooth surface morphology for growth with NH3 with low roughness as 0.14 nm.