Paper of the Month
B. Guilhabert, M. Toon, S. Ghosh, D. Jevtics, Z. Xia, M. J. Kappers, M. D. Dawson, R. A. Oliver, and M. J. Strain
Opt. Lett. 51, 993-996 (2026)
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.
DOI: 10.1364/OL.584532
B. Thornley, M. Sarkar, S. Ghosh, M. Frentrup, M. J. Kappers, T. R. Harris-Lee, and R. A. Oliver
Acta Materiala 308, 121957 (2026)
Fabrication of porous GaN distributed Bragg reflectors (DBRs) via the selective electrochemical etching of conductive Si-doped layers, separated by non-intentionally doped (NID) layers, provides a straightforward methodology for producing highly reflective DBRs suitable for device overgrowth and integration, which has otherwise proven difficult in the III-nitride epitaxial system via conventional alloying. Such photonic materials can be fabricated by a lithography-free defect-driven etching process, where threading dislocations intrinsic to heteroepitaxy form nanoscale channels that facilitate etchant transport through NID layers. Here, we report the first three-dimensional characterisation of porous GaN-on-Si DBRs fabricated in this methodology with different etching voltages, using serial-section tomography in a focused ion beam scanning electron microscope (FIB-SEM). These datasets reconstruct the pore morphology as etching proliferates through the alternating Si-doped/NID layer stack. Volumetric reconstruction enabled us to enhance the established ‘kebab’ model for defect-driven etching by proposing a ‘cascade’ model where the etchant cascades through the material via vertical etching down nanopipes and horizontal etching across pores, forming complex networks directly related to the pathways taken. This accounts for premature nanopipe termination and discontinuities in nanopipe formation, where dislocations are observed to activate and deactivate individually. Statistical analysis of individual etching behaviour, across all dislocations for each tomograph, revealed a greater tendency to form continuous structures that follow conventional ‘kebab’ behaviour at higher etching voltages. We propose that higher etching voltages alter the probability of dislocation etching relative to doped layer etching, thereby empowering morphological optimisation through improved mechanistic understanding of electrochemical etching.
K. M. Eggleton, J. K. Cannon, S. G. Bishop, J. P. Hadden, C. Zhao, M. J. Kappers, R. A. Oliver, and A. J. Bennett
APL Photonics 11, 016103 (2026)
The ability to generate quantum light at room temperature on a mature semiconductor platform opens up new possibilities for quantum technologies. Heteroepitaxial growth of gallium nitride on silicon substrates offers the opportunity to leverage existing expertise and wafer-scale manufacturing to integrate bright quantum emitters (QEs) in this material within cavities, diodes, and photonic circuits. Until now, it has only been possible to grow GaN QEs at uncontrolled depths on sapphire substrates, which is disadvantageous for potential device architectures. Here, we report a method to produce GaN QEs by metal-organic vapor phase epitaxy at a controlled depth in the crystal through the application of silane treatment and subsequent growth of 3D islands. We demonstrate this process on highly technologically relevant silicon substrates, producing room-temperature QEs with a high Debye–Waller factor and strongly anti-bunched emission.
J. Wang, S. Hu, Z. Chen, Z. Yuan, P. Zhao, A. Dasgupta, F. Yang, J. Yao, M. Anh Truong, G. Kusch, E. Y-H. Hung, N. R. M. Schipper, L. Bellini, G. J. W. Aalbers, Z. Liu, R. A. Oliver, A. Wakamiya, R. A. J. Janssen, and H. J. Snaith
Energy & Environmental Science 18, 7680 (2025)
Improved understanding of heterojunction interfaces has enabled multijunction photovoltaic devices to achieve power conversion efficiencies that exceed the detailed-balance limit for single-junctions. For wide-bandgap perovskites, however, the pronounced energy loss across the heterojunctions of the active and charge transport layers impedes multijunction devices from reaching their full efficiency potential. Here we find that for polycrystalline perovskite films with mixed-halide compositions, the crystal termination—a factor influencing the reactivity and density of surface sites—plays a crucial role in interfacial passivation for wide-bandgap perovskites. We demonstrate that by templating the growth of polycrystalline perovskite films toward a preferred (100) facet, we can reduce the density of deep-level trap states and enhance the binding of modification ligands. This leads to a much-improved heterojunction interface, resulting in open-circuit voltages of 1.38 V for 1.77-eV single-junction perovskite solar cells. In addition, monolithic all-perovskite double-junction solar cells achieve open-circuit voltage values of up to 2.22 V, with maximum power point tracking efficiencies reaching 28.6% and 27.7% at 0.25 and 1.0 cm^2 cell areas, respectively, along with improved operational and thermal stability at 85°C. This work provides universally applicable insights into the crystalline facet-favourable surface modification of perovskite films, advancing their performance in optoelectronic applications.
X. Xu, D. Dyer, M. Frentrup, W. R. Fieldhouse-Allen, M. J. Kappers, G. Kusch, D. J. Wallis, R. A. Oliver, and D. J. Binks
Journal of Physics D: Applied Physics 58, 475101 (2025)
InGaN/GaN quantum wells grown in the zincblende phase along the [001] direction are free of the internal electric fields that reduce the radiative recombination rate in conventional quantum wells grown along the c-axis in the wurtzite phase. However, heteroepitaxial growth and reduced thermodynamic stability compared to the wurtzite phase typically results in a significant density of stacking faults (SFs) in zincblende GaN, which impacts emission efficiency when they intersect quantum wells. Here it is shown that increasing the buffer layer thickness that lies between the substrate and the active region significantly reduces the density of SFs reaching the quantum well, and thereby increases the emission efficiency.
T. R. Harris-Lee, B. Thornley, J. Zhang, M. J. Kappers, and R. A. Oliver
ACS Applied Materials & Interfaces 17, 64931-64941 (2025)
Porous GaN has emerged as a promising material for enhancing the performance of optoelectronic devices and broadening the range of possible GaN applications. However, the electrochemical etching (ECE) process used to create porosity remains poorly understood, particularly regarding the impact of the chemical environment on pore morphology. Here, the controlled ECE of n-type GaN is systematically investigated across a range of etchant chemicals and pH values. It is shown that the identity, speciation, and relative concentrations of anionic species play dominant roles in dictating porous morphology. Through deliberate manipulation of anion compositions within an etchant solution, for example, by adjusting initial polyprotic acid concentration and/or addition of a conjugate salt, porous morphology and surface structure can be controlled and tuned effectively. Further, ECE-generated current oscillations, previously interpreted as evidence for an oxidation–dissolution ECE mechanism, are shown to correlate with the presence of dynamic anion equilibria, providing an additional mechanistic interpretation of n-type GaN ECE. This furthered understanding enables more tailored and application-specific control over porous structure, offering opportunities for optimized, bespoke GaN-based porous architectures.
A. Griesi, Y. P. Ivanov, S. M. Fairclough, A. V. Oli, G.Kusch, R. A. Oliver, P. De Padova, C. Ottaviani, U. Wijesinghe, S. Siebentritt, A. Di Carlo, O. S. Hutter, G. Longo, and G. Divitini
Small Methods 9(11), e01334 (2025)
In thin film photovoltaic devices, the control of grain structure and local crystallography are fundamental for high power conversion efficiency and reliable long-term operation. Structural defects, grain boundaries, and unwanted phases can stem from compositional inhomogeneities or from specific synthesis parameters, and they need to be thoroughly understood and carefully engineered. However, comprehensive studies of the crystallographic properties of complex systems, including different phases and/or a large number of grains, are often prohibitively challenging. Here, the use of 4D Scanning Transmission Electron Microscopy (4D-STEM) is demonstrated on cross-sections to unravel the nanoscale properties of three different materials for photovoltaics: Cu(In,Ga)S2, halide perovskite, and Sb2Se3. These materials are chosen because of the variety of challenges they present: the presence of multiple phases and complex stoichiometry, electron beam sensitivity, and very high density of grains. 4D-STEM provides comprehensive insights into crystallinity and microstructure, but navigating its large datasets and extracting actionable, statistically sound information requires advanced algorithms. How unsupervised machine learning, including dimensionality reduction and hierarchical clustering, can extract key information from 4D-STEM datasets is demonstrated. The analytical framework follows FAIR principles, employing open-source software and enabling data sharing.
S. Rezaie, G. Kusch, L. Samuelson, J. B. Wagner, and S. Yazdi
physica status solidi (RRL)–Rapid Research Letters 19, 2500145 (2025)
Nanowires are promising structures for next-generation photonic devices due to their superior structural, optical, and electronic properties compared to thin films. In this study, unexpected electrostatic potential wells across the non-polar m-plane and at the core/shell interface in n-type GaN core/shell nanowires, grown via metal-organic vapor phase epitaxy, are reported. Using advanced electron microscopy, including off-axis electron holography, electrostatic potential distributions are mapped and shallow quantum wells are identified at the core/shell interface and core center. High-resolution transmission electron microscopy ruled out planar and line defects, implicating point defects as their source. Valence electron energy loss spectroscopy revealed localized bandgap narrowing due to strain from concentrated point defects. Hyperspectral cathodoluminescence linked lower potential in the core to CN defects, while the absence of related luminescence at the core/shell interface suggests VGaON defect complexes as plausible causes. These findings highlight the critical role of point defects in GaN nanowires, with significant implications for device performance.
X. Xu, M. Frentrup, G. Kusch, R. Shu, C. Hofer, P. A. J. Bagot, M. P. Moody, M. J. Kappers, D. J. Wallis, and R. A. Oliver
Journal of Applied Physics 137, 235301 (2025)
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.
J. A. Cuenca, A. Al-Moathin, M. J. Kappers, S. Mandal, M. Kuball, R. A. Oliver, C. Li, O. and A. Williams
Carbon 241, 120349 (2025)
Microwave plasma chemical vapour deposition (MP-CVD) of thick polycrystalline diamond (PCD) (t > 100 µm) 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 H 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 720 °C. This study demonstrates that holders with a tapered base allow rapid sample cooling (T ~ 669 °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.
Y. Chen, R. Wang, G. Kusch, B. Xu, C. Hao, C. Xue, L. Cheng, L. Zhu, J. Wang, H. Li, R. A. Oliver, N. Wang, W. Huang, and J. Wang
Nature Communications 16, 3254 (2025)
Perovskite light-emitting diodes have drawn great attention in the fields of displays and lighting, especially for applications requiring high efficiency and high brightness. While three-dimensional perovskite light-emitting diodes hold promise for achieving higher brightness compared to low-dimensional counterparts, efficient blue three-dimensional perovskite light-emitting diodes have remained a challenge due to defect formation during the disordered crystallization of multiple A-cation perovskite. Here we demonstrate an all-site alloy method that enables sequential A-site doping growth of formamidinium and cesium hybrid perovskite. This approach significantly reduces the trap density of the perovskite film by approximately one order of magnitude. Consequently, we achieve efficient and bright blue perovskite light-emitting diode with an external quantum efficiency of 23.3%, a luminous efficacy of 33.4 lm W−1, and a luminance of approximately 5700 cd m−2 for the emission with a peak at 487 nm. This work provides a strategy for growing high-quality multicomponent perovskite for optoelectronics.
R. Shu, R. A. Oliver, M. Frentrup, M. J. Kappers, H. Xiu, G. Kusch, D. J. Wallis, C. Hofer, P. A. J. Bagot, and M. P. Moody
Materialia 40, 102417 (2025)
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.
Y. Hu, G. Kusch, D. Adeleye, S. Siebentritt, and R. A. Oliver
Journal of Microscopy 298 (1), pp 106-117 (2025)
Multi-microscopy offers significant benefits to the understanding of complex materials behaviour by providing complementary information from different properties. However, some characterisations may strongly influence other measurements in the same workflow. To acquire reliable and valid datasets, optimising multi-microscopy procedure is necessary. In present work, we studied the influence of the measurement order on the quality of multi-microscopy datasets. Multi-microscopy incorporating tunnelling current AFM (TUNA), electron backscatter diffraction (EBSD), and cathodoluminescence (CL) on a polycrystalline solar cell absorber, Cu(In,Ga)S2 (CIGS), is used as an example. The investigation revealed potential characterisation-induced contaminations, such as surface oxidation and hydrocarbon layer coating, of the sample surface. Their subsequent influence on the measurement results of following correlation techniques was examined. To optimise the dataset quality, multi-microscopy should be carried out in TUNA-EBSD-CL order, from the most to the least surface sensitive techniques. With the optimised multi-microscopy measurement order on a CIGS absorber, we directly correlated the local changes in electrical and opto-electronic properties with the microstructure of grain boundaries (GBs). The described methodology may also provide insightful concepts for applying other AFM-SEM-based multi-microscopy on different semiconductor materials.
S. Ghosh, M. Frentrup, A. M. Hinz, J. W. Pomeroy, D. Field, D. J. Wallis, M. Kuball, and R. A. Oliver
Advanced Materials 37, 2413127 (2025)
Thick metamorphic buffers are considered indispensable for III-V semiconductor heteroepitaxy on large lattice and thermal-expansion mismatched silicon substrates. However, III-nitride buffers in conventional GaN-on-Si high electron mobility transistors (HEMT) impose a substantial thermal resistance, deteriorating device efficiency and lifetime by throttling heat extraction. To circumvent this, a systematic methodology for the direct growth of GaN after the AlN nucleation layer on six-inch silicon substrates is demonstrated using metal-organic vapor phase epitaxy (MOVPE). Crucial growth-stress modulation to prevent epilayer cracking is achieved even without buffers, and threading dislocation densities comparable to those in buffered structures are realized. The buffer-less design yields a GaN-to-substrate thermal resistance of (11 ± 4) m2 K GW−1, an order of magnitude reduction over conventional GaN-on-Si and one of the lowest on any non-native substrate. As-grown AlGaN/AlN/GaN heterojunctions on this template show a high-quality 2D electron gas (2DEG) whose room-temperature Hall-effect mobility exceeds 2000 cm2 V−1 s−1, rivaling the best-reported values. As further validation, the low-temperature magnetoresistance of this 2DEG shows clear Shubnikov-de-Haas oscillations, a quantum lifetime > 0.180 ps, and tell-tale signatures of spin-splitting. These results could establish a new platform for III-nitrides, potentially enhancing the energy efficiency of power transistors and enabling fundamental investigations into electron dynamics in quasi-2D wide-bandgap systems.
F. Adams, S. Ghosh, Z. Liang, C. Chen, N. Suphannarat, M. J Kappers, D. J. Wallis, and R.A. Oliver
J. Phys. D: Appl. Phys. 58, 135117 (2025)
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 K 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 °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 °C for 30 s proved sufficient to ensure Ohmic behaviour when tested from 320 K 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 °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.
K. Loeto, G. Kusch, O. Brandt, P.-M. Coulon, S. Hammersley, J. Lähnemann, I. Girgel, S. Fairclough, M. Sarkar, P. A Shields, and R.A. Oliver
Nanotechnology 36, 025703 (2024/2025)
This study examines the exciton dynamics in InGaN/GaN core–shell nanorods using time-resolved cathodoluminescence (TRCL), which provides nanometer-scale lateral spatial and tens of picoseconds temporal resolutions. The focus is on thick (>20 nm) InGaN layers on the non-polar, semi-polar and polar InGaN facets, which are accessible for study due to the unique nanorod geometry. Spectrally integrated TRCL decay transients reveal distinct recombination behaviours across these facets, indicating varied exciton lifetimes. By extracting fast and slow lifetime components and observing their temperature trends along with those of the integrated and peak intensity, the differences in behaviour were linked to variations in point defect density and the degree and density of localisation centres in the different regions. Further analysis shows that the non-polar and polar regions demonstrate increasing lifetimes with decreasing emission energy, attributed to an increase in the depth of localisation. This investigation provides insights into the intricate exciton dynamics in InGaN/GaN nanorods, offering valuable information for the design and development of optoelectronic devices.
X. Bai, S.M. Fairclough, L. Dai, M. Sarkar, P.H. Griffin, A. Gundimeda, Y. Sun, N.C. Greenham, M.I. Dar, R.A. Oliver, and R.H. Friend
Adv. Optical Mater. 12, 2400221 (2024)
Blue gallium nitride (GaN) light-emitting diodes (LEDs), combined with red/green fluorescent converters, have broad potential for display applications. Metal halide perovskites now show excellent luminescence properties and may be suitable as light converters. Here a simple solution-processed method is reported to prepare a methylammonium lead bromide (MAPbBr3) nanoporous GaN composite. Fast (within 2 ps) energy transfer is demonstrated from photoexcited nanoporous GaN to encapsulate MAPbBr3 nanocrystals, as observed by transient absorption spectroscopy. The spatial confinement of the perovskite within the nanoporous GaN is shown to increase the perovskite radiative recombination rate. These results offer guidelines for developing high-performance perovskite/nanoporous GaN optoelectronics.