arXiv subjects

Rachel A. Oliver

Publications and source records attributed to Rachel A. Oliver.

At least 19 recordsLinked to original sources

arXiv2026

Improving feature resolution and pore back effect in focused ion beam tomography of porous GaN thin films

Porous gallium nitride (GaN) is a mesoporous crystalline material, typically in the form of a thin film on an unlike substrate, prepared by electrochemically etching conductive GaN. The use of porous GaN in electronics and optoelectronics is rapidly expanding, but is held back significantly by a lack of structural control and understanding of the principles of pore formation from underlying electrochemical mechanisms, where high-quality characterisation of pore morphology is essential to understanding these principles. Focused ion beam (FIB) tomography is an invaluable tool for such characterisation, but is hindered greatly by the pore back effect, where unwanted contrast appears in an image due to electrons scattering out from the back wall of a pore. No major attempts to formally quantify or assess the extent of the effect for different tomography experiments has been demonstrated. In this work, we demonstrate an advanced methodology for performing FIB tomography on porous GaN thin films, where the experiment is rotated to image pores perpendicular to the surface of the sample, and introduce new voxel intensity-based formalisms for assessing the impact of the pore back effect based on voxel intensities for individual features and the whole dataset. The new approach, which requires more complex preliminary setup, is found to significantly mitigate the pore back effect in porous GaN thin films with a range of pore morphologies without increasing the total experimental duration. The pore back effect can thus be quantified and mitigated in FIB tomography of porous GaN and similar mesoporous thin films.

cond-mat.mtrl-sci↗

arXiv2026

What enables GaOx as hole transport layer for a 16 percent 1.0 eV CuInSe2 Bottom Cells with VOC above 550 mV?

Among the highly efficient photovoltaic technologies, that do not rely on epitaxy, only chalcopyrites have a bandgap tunable down to 1.00 eV, the ideal for tandem applications. This is obtained with a pure CuInSe2 absorber without Ga. GaOx has been shown to be an efficient hole transport layer that prevents recombination at the metallic back contact. On the other hand, GaOx has proven detrimental, when it forms on In containing transparent back contacts in bifacial solar cells. Here, we investigate the conditions that make the GaOx layer conductive. We employ a GaOx hole transport layer that is formed through ion exchange during co-evaporation of the low band gap absorber layer. We find that no additional Cu is needed, and that Na is not necessary for a conductive GaOx. Nor did we find a systematic influence of oxygen flow during the sputtering process of the oxide layer. The GaOx layer is partly crystalline. The optimized passivating hole transport layer enables a CuInSe2 bottom solar cell, without any addition of Ag or heavy alkalis, with an active area efficiency above 16% and a record-certified open-circuit voltage VOC of 552meV

cond-mat.mtrl-sci↗

arXiv2026

A stochastic simulation of the dislocation-mediated etching of porous GaN distributed Bragg reflectors

Distributed Bragg reflectors (DBRs) can be fabricated by electrochemically etching nitride epitaxial structures consisting of alternating layers of highly n-type doped and non-intentionally doped (NID) GaN. Threading dislocations (TDs) can be electrochemically etched into transport pipelines that can carry the etchant through the NID layers to access the doped material. Experimentally this has been shown to involve a mechanism where the etching pathway may follow one TD into a doped layer and then propagate sideways through the doped layer to continue via a different TD. Across multiple layers this process creates complex pore structures that have been described as 'cascades'. Here, we build a stochastic simulation for the DBR etching process that can reproduce some key features of the observed microstructures including the cascade morphology. By comparing the simulation output to samples etched at a range of voltages, we show that we can reproduce variations in experimental chronoamperometry data with applied bias by varying the probability of etching the doped layers within the simulation. The outputs of the resulting simulations replicate the experimentally observed cascade morphology. At higher voltages, experimental data reveal a lower proportion of cascade features, a trend that is also replicated by the simulations for relevant probability values. Outputs of the simulations also correlate well with experimental chronoamperometry data for samples where - unlike in a DBR - the thicknesses of the doped layers vary through the epitaxial multilayer, suggesting that the probabilistic simulation can be applied to a range of structures to help understand the dislocation-mediated electrochemical etching process.

cond-mat.mtrl-sci↗

arXiv2026

Controlled epitaxy of room-temperature quantum emitters in gallium nitride

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 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.

cond-mat.mtrl-sci↗

arXiv2025

Silver Alloyed Wide Bandgap (Ag,Cu)(In,Ga)S2 Thin Film Solar Cells With 15.5% Efficiency

Sulfide chalcopyrite Cu(In,Ga)S2 (CIGS) is a wide bandgap semiconductor suitable for the top cell of a tandem solar device. Here we demonstrate significant improvements in absorber quality by alloying with Ag to form (Ag,Cu)(In,Ga)S2 (ACIGS) absorbers. We report the Ag alloying effect on compositional, structural, and optoelectronic properties of absorbers. We demonstrate suppressed bulk recombination and improved carrier lifetime in ACIGS, as a result of improved grain size, porosity reduction and defect passivation. We also show that Ag alloying flattens the Ga gradient. Consideration of this impact of Ag will be necessary in future engineering of the Ga profile to maximize charge carrier collection and avoid interface recombination. Exploiting the beneficial effects of Ag alloying, we report a wide bandgap (1.58 eV) ACIGS solar cell with a high power conversion efficiency of 15.5% and a large open-circuit voltage (VOC) of 948 mV, improving on the reference pure CIGS solar cell, with an 11.2% efficiency and an 821 mV VOC. Ag alloying is a useful route to further increase the efficiency of CIGS solar cells and future tandem devices.

cond-mat.mtrl-sci↗

arXiv2025

A cascade model for the defect-driven etching of porous GaN distributed Bragg reflectors

Fabrication of porous GaN distributed Bragg reflectors (DBRs) via the selective electrochemical etching (ECE) 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 ECE 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 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 ECE voltages. We propose that higher ECE voltages alter the probability of dislocation etching relative to doped layer etching, thereby empowering morphological optimization through improved mechanistic understanding of ECE.

cond-mat.mtrl-sci↗

arXiv2025

Sodium induced beneficial effects in wide bandgap Cu(In,Ga)S2 solar cell with 15.7% efficiency

This study underscores the pivotal role of sodium (Na) supply in optimizing the optoelectronic properties of wide bandgap (~1.6 eV) Cu(In,Ga)S2 (CIGS) thin film absorbers for high efficiency solar cells. Our findings demonstrate that the synergistic use of Na from the glass substrate, in conjunction with in-situ sodium fluoride (NaF) co-evaporation, significantly enhances the structural and optoelectronic properties of the CIGS. CIGS grown under either Na-deficient or excess conditions exhibits inferior microstructural and optoelectronic properties, whereas an optimal Na supply leads to enhanced photovoltaic performance. Optimal Na incorporation minimizes vertical gallium fluctuations and improves the grain size and crystallinity. An absolute 1 sun calibrated photoluminescence (PL) measurement reveals a substantial suppression of bulk defects and a reduction in non-radiative losses, resulting in a high quasi-fermi level splitting (ΔEF) of 1.07 eV, 93 meV higher than in Na-deficient CIGS with the same bandgap. Optimal Na supply further increases excited carrier decay time, as revealed from time-resolved PL, and hole doping density. Cross-sectional hyperspectral cathodoluminescence mapping reveals that optimal Na supply significantly reduces defect density near the surface, thereby effectively translating ΔEF to open-circuit voltage (VOC). As a result, a champion wide bandgap CIGS solar cell with a cadmium-free ZnSnO buffer layer achieved an impressive VOC of 971 meV and an active area power conversion efficiency of 15.7%, highlighting its potential for advancing tandem photovoltaic technologies with stable inorganic top cell.

cond-mat.mtrl-sci↗

arXiv2025

The effect of a band gap gradient on the radiative losses in the open circuit voltage of solar cells

The radiative open circuit voltage loss in a solar cell occurs because the absorptance spectrum near the band gap shows gradual increase rather than sharp step function like transition. This broadening effect has been attributed to band gap fluctuations and or to Urbach tails. In this report, we use modelling based on Planck s generalized law to distinguish between these two effects. Our results demonstrate that Urbach tails have only a minimal effect on the absorptance edge broadening and clarify that even an ideal direct semiconductor with no band gap fluctuations shows broadening at the absorptance onset. Furthermore, state of the art inorganic thin film solar cells often incorporate a band gap gradient across their thickness, which can further contribute to absorptance broadening. Using Cu(In,Ga)Se2 (CIGSe) absorbers as a case study, we perform a comprehensive analysis of voltage losses through absolute photoluminescence and electroluminescence spectroscopy, combined with photospectrometry and high-spatial-resolution cathodoluminescence measurements. We find that the loss analysis based on the combination of radiative, generation and non-radiative losses is complete. Samples with a graded band gap profile show more pronounced broadening of the absorptance onset and up to 16 mV higher radiative losses compared to the samples with uniform band gap. There is indication, that band gap-graded samples also have larger lateral band gap inhomogeneity.

cond-mat.mtrl-sci↗

arXiv2024

Buffer-less Gallium Nitride High Electron Mobility Heterostructures on Silicon

Thick metamorphic buffers are perceived to be indispensable for the heteroepitaxial integration of III-V semiconductors on silicon substrates with large thermal expansion and lattice mismatches. However, III-nitride buffers in conventional GaN-on-Si high electron mobility transistor (HEMT) heterostructures impose a substantial thermal resistance, throttling heat extraction, which reduces device efficiency and lifetime. Herein, bypassing the buffer, we demonstrate the direct growth of GaN after the AlN nucleation layer on silicon by metal-organic vapor phase epitaxy (MOVPE). By varying reactor pressure, we modulate the growth stress in the submicron epilayers and realise threading dislocation densities similar to that in thick buffered structures. We achieve a GaN-to-substrate thermal resistance of (11(+/-)4) ((m^2)K(GW^-1)), an order of magnitude reduction over conventional designs on silicon and one of the lowest on any non-native substrate. AlGaN/AlN/GaN heterojunctions on this platform show a characteristic 2D electron gas (2DEG), the room-temperature Hall-effect mobility of which, at over 2000 (cm^2/(V-s)), rivals the best-reported values. 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 may establish a new paradigm for nitride HEMTs, potentially accelerating applications from energy-efficient transistors to fundamental investigations on electron dynamics in this 2D wide-bandgap system.

physics.app-ph↗

arXiv2024

Improved sequentially processed Cu(In,Ga)(S,Se)2 by Ag alloying

Alloying small quantities of silver into Cu(In,Ga)Se2 was shown to improve the efficiency for wide and low band gap solar cells. We study low band gap industrial Cu(In,Ga)(S,Se)2 absorbers, substituting less than 10% of the copper with silver, using absolute photoluminescence and cathodoluminescence spectroscopy. Silver improves the grain size and promotes the interdiffusion of Ga and In across the depth of the absorber, resulting in a smoother band gap gradient. However, a certain lateral inhomogeneity is observed near the front and back sides. The non-radiative losses in the bare absorbers are reduced by up to 30 meV.

cond-mat.mtrl-sci↗

arXiv2023

Composition variations in Cu(In,Ga)(S,Se)2 solar cells: not a gradient, but an interlaced network of two phases

Record efficiency in chalcopyrite-based solar cells Cu(In,Ga)(S,Se)2 is achieved using a gallium gradient to increase the band gap of the absorber towards the back side. Although this structure has successfully reduced recombination at the back contact, we demonstrate that in industrial absorbers grown in the pilot line of Avancis, the back part is a source of non-radiative recombination. Depth-resolved photoluminescence (PL) measurements reveal two main radiative recombination paths at 1.04 eV and 1.5-1.6 eV, attributed to two phases of low and high band gap material, respectively. Instead of a continuous change in the band gap throughout the thickness of the absorber, we propose a model where discrete band gap phases interlace, creating an apparent gradient. Cathodoluminescence and Raman scattering spectroscopy confirm this result. Additionally, deep defects associated to the high gap phase reduce the absorber performance. Etching away the back part of the absorber leads to an increase of one order of magnitude in the PL intensity, i.e., 60 meV in quasi Fermi level splitting. Non-radiative voltage losses correlate linearly with the relative contribution of the high energy PL peak, suggesting that reducing the high gap phase could increase the open circuit voltage by up to 180 mV.

cond-mat.mtrl-sci↗

arXiv2023

Additive GaN solid immersion lenses for enhanced photon extraction efficiency from diamond color centers

Effective light extraction from optically active solid-state spin centres inside high-index semiconductor host crystals is an important factor in integrating these pseudo-atomic centres in wider quantum systems. Here we report increased fluorescent light collection efficiency from laser-written nitrogen vacancy centers (NV) in bulk diamond facilitated by micro-transfer printed GaN solid immersion lenses. Both laser-writing of NV centres and transfer printing of micro-lens structures are compatible with high spatial resolution, enabling deterministic fabrication routes towards future scalable systems development. The micro-lenses are integrated in a non-invasive manner, as they are added on top of the unstructured diamond surface and bond by Van-der-Waals forces. For emitters at 5 micrometer depth, we find approximately 2x 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.

physics.optics↗

arXiv2023

Three-photon excitation of quantum two-level systems

We demonstrate that semiconductor quantum dots can be excited efficiently in a resonant three-photon process, whilst resonant two-photon excitation is highly suppressed. Time-dependent Floquet theory is used to quantify the strength of the multi-photon processes and model the experimental results. The efficiency of these transitions can be drawn directly from parity considerations in the electron and hole wavefunctions in semiconductor quantum dots. Finally, we exploit this technique to probe intrinsic properties of InGaN quantum dots. In contrast to non-resonant 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 non-resonant excitation.

quant-ph↗

arXiv2023

Dislocation-induced structural and luminescence degradation in InAs quantum dot emitters on silicon

We probe the extent to which dislocations reduce carrier lifetimes and alter luminescence and growth morphology in InAs quantum dots (QD) grown on silicon. These heterostructures are key ingredients to achieving a highly reliable monolithically integrated light source on silicon necessary for photonic integrated circuits. We find up to 20-30% shorter carrier lifetimes at spatially resolved individual dislocations from both the QD ground and excited states at room temperature using time-resolved cathodoluminescence spectroscopy. These lifetimes are consistent with differences in the intensity measured under steady-state excitation suggesting that trap-assisted recombination limits the minority carrier lifetime, even away from dislocations. Our techniques also reveal the dramatic growth of misfit dislocations in these structures under carrier injection fueled by recombination-enhanced dislocation glide and III-V/Si residual strain. Beyond these direct effects of increased nonradiative recombination, we find the long-range strain field of misfit dislocations deeper in the defect filter layers employed during III-V/Si growth alter the QD growth environment and introduce a crosshatch-like variation in the QD emission color and intensity when the filter layer is positioned close to the QD emitter layer. Sessile threading dislocations generate even more egregious hillock defects that also reduce emission intensities by altering layer thicknesses, as measured by transmission electron microscopy and atom probe tomography. Our work presents a more complete picture of the impacts of dislocations relevant for the development of light sources for scalable silicon photonic integrated circuits.

cond-mat.mes-hall↗

arXiv2022

Magnetic properties of nickel electrodeposited on porous GaN substrates with infiltrated and laminated connectivity

We studied the magnetic properties of ferromagnetic-semiconductor composites based on nickel and porous-GaN, motivated by the effort to couple magnetic and semiconductor functionality. Nickel-infiltrated and nickel-coated (laminated thin-film) porous GaN structures were fabricated by electrodeposition, and their magnetic properties were subsequently examined collectively, by vibrating sample magnetometry and on the nanoscale, by magnetic force microscopy. We successfully demonstrated the ability to realize nickel infiltrated porous GaN, where the magnetic properties were dominated by the infiltrated material without a measurable surface contribution. We found that the structure and magnetization of electrodeposited porous-GaN/Ni composites depended on GaN degree of porosity and the amount of deposited nickel. The magnetization evolves from a nearly isotropic response in the infiltrated structures, to a shape-anisotropy controlled magnetic thin-film behaviour. Furthermore, both infiltrated and thin-film nickel electrodeposited on porous GaN were found to have low (< 0.1%) strain and corresponding low coercivity: < 6.4 and < 2.4 kA/m for infiltrated and thin-film, correspondingly. The most likely cause for the lowered strain is increased compliance of the porous GaN compared to bulk. These results encourage deeper investigation of magnetic nanostructure property tuning and of magnetic property coupling to GaN and similar materials.

cond-mat.mtrl-sci↗

arXiv2022

Fabrication and transfer print based integration of free-standing GaN membrane micro-lenses onto semiconductor chips

We demonstrate the back-end integration of broadband, high-NA GaN micro-lenses by micro-assembly onto non-native semiconductor substrates. We developed a highly parallel micro-fabrication process flow to suspend micron scale plano-convex lens platelets from 6" Si growth wafers and show their subsequent transfer-printing integration. A growth process targeted at producing unbowed epitaxial wafers was combined with optimisation of the etching volume in order to produce flat devices for printing. Lens structures were fabricated with 6 to 11 $μ$m diameter, 2 $μ$m height and root-mean-squared surface roughness below 2 nm. The lenses were printed in a vertically coupled geometry on a single crystalline diamond substrate and with $μ$m-precise placement on a horizontally coupled photonic integrated circuit waveguide facet. Optical performance analysis shows that these lenses could be used to couple to diamond nitrogen vacancy centres at micron scale depths and demonstrates their potential for visible to infrared light-coupling applications.

physics.app-ph↗

arXiv2021

Thermal stress modelling of diamond on GaN/III-Nitride membranes

Diamond heat-spreaders for gallium nitride (GaN) devices currently depend upon a robust wafer bonding process. Bonding-free membrane methods demonstrate potential, however, chemical vapour deposition (CVD) of diamond directly onto a III-nitride (III-N) heterostructure membrane induces significant thermal stresses. In this work, these thermal stresses are investigated using an analytical approach, a numerical model and experimental validation. The thermal stresses are caused by the mismatch in the coefficient of thermal expansion (CTE) between the GaN/III-N stack, silicon (Si) and the diamond from room temperature to CVD growth temperatures. Simplified analytical wafer bow models underestimate the membrane bow for small sizes while numerical models replicate the stresses and bows with increased accuracy using temperature gradients. The largest tensile stress measured using Raman spectroscopy at room temperature was approximately 1.0 $\pm0.2$ GPa while surface profilometry shows membrane bows as large as \SI{58}{\micro\metre}. This large bow is caused by additional stresses from the Si frame in the initial heating phase which are held in place by the diamond and highlights challenges for any device fabrication using contact lithography. However, the bow can be reduced if the membrane is pre-stressed to become flat at CVD temperatures. In this way, a sufficient platform to grow diamond on GaN/III-N structures without wafer bonding can be realised.

physics.app-ph↗

arXiv2020

Gender issues in fundamental physics: Strumia's bibliometric analysis fails to account for key confounders and confuses correlation with causation

Alessandro Strumia recently published a survey of gender differences in publications and citations in high-energy physics (HEP). In addition to providing full access to the data, code, and methodology, Strumia (2020) systematically describes and accounts for gender differences in HEP citation networks. His analysis points both to ongoing difficulties in attracting women to high-energy physics and an encouraging-though slow-trend in improvement. Unfortunately, however, the time and effort Strumia (2020) devoted to collating and quantifying the data are not matched by a similar rigour in interpreting the results. To support his conclusions, he selectively cites available literature and fails to adequately adjust for a range of confounding factors. For example, his analyses do not consider how unobserved factors -- e.g., a tendency to overcite well-known authors -- drive a wedge between quality and citations and correlate with author gender. He also fails to take into account many structural and non-structural factors -- including, but not limited to, direct discrimination and the expectations women form (and actions they take) in response to it -- that undoubtedly lead to gender differences in productivity. We therefore believe that a number of Strumia's conclusions are not supported by his analysis. Indeed, we re-analyse a subsample of solo-authored papers from his data, adjusting for year and journal of publication, authors' research age and their lifetime "fame". Our re-analysis suggests that female-authored papers are actually cited more than male-authored papers. This finding is inconsistent with the "greater male variability" hypothesis Strumia (2020) proposes to explain many of his results.

physics.soc-ph↗