We introduce a two-fluid mobility model incorporating fundamental aspects of electron-hole (e-h) scattering such as momentum conservation for simulating laser-driven semiconductor switches (LDSSs). Compared to previous works that use Matthiessen's rule, the two-fluid mobility model predicts distinct ac responses of e-h plasmas in semiconductors. Based on the two-fluid mobility model, we develop a theory with very few adjustable parameters for simulating the switching performance of LDSSs based on high-purity indirect-gap semiconductors such as silicon (Si). As a prototypical application, we successfully reproduce experimentally measured reflectance at around 320 GHz in a laser-driven Si switch. By injecting e-h plasmas with densities up to 1020 cm-3, we reveal the importance of carrier-screening effects in e-h scattering and Auger recombination for carrier densities above the critical carrier density for excitonplasma Mott transition. Our results also suggest a way to characterize the intrinsic momentum-relaxation mechanism, e-h scattering, and the intrinsic e-h recombination mechanism in indirect-gap semiconductors, Auger recombination. We reassess the ambipolar Auger coefficient of high-purity Si with high injection levels of e-h plasmas up to 1020 cm-3 and find a minimal value of 1.8 x 10-41 cm6/ns. This value is more than one order of magnitude smaller than the ambipolar Auger coefficient widely used for simulating LDSSs, 3.8 x 10-40 cm6/ns, which was deduced from the minority-carrier lifetime in highly doped silicon more than four decades ago.

Investigation of the nucleation and growth of Al 2 O 3 and HfO 2 films grown directly on chemical vapour-synthesised monolayer MoS 2 via atomic layer deposition.

The integration of single-layer transition metal dichalcogenides (TMDCs) in nanoscale field-effect transistor devices requires the deposition of a high dielectric constant (high-κ) material to act as the gate dielectric. Traditional thermal atomic layer deposition (ALD) is commonly used to deposit dielectrics on three-dimensional substrates, but ALD of high-κ materials on monolayer TMDCs is more challenging. Thermal ALD with water (H2O) co-reactant often results in incomplete and nonuniform dielectric growth on atomically thin TMDCs, owing to a chemically inert basal plane. The development of alternative ALD processes for the realization of dielectric layers on monolayer TMDCs is therefore important. Here, we study oxygen (O2) plasma and ozone (O3) as co-reactants for the ALD of aluminum oxide (Al2O3) and hafnium dioxide (HfO2) on monolayer molybdenum disulfide (1L MoS2) films. By employing a robust characterization process that combines atomic force microscopy, Raman/photoluminescence spectroscopy, and X-ray photoelectron spectroscopy, we reveal growth of high-κ dielectrics by plasma-enhanced ALD with O2 plasma oxidant damages the underlying 1L MoS2 via oxidation to molybdenum trioxide (MoO3). No significant deleterious oxidation to MoO3 is observed following O3-based deposition on 1L MoS2, and we demonstrate the growth of HfO2 via thermal ALD with O3 co-reactant. This work reveals the impact of ALD processes on 1L MoS2 during the growth of high-κ dielectrics, highlighting O3-based thermal ALD as a potential route for the integration of dielectric layers on 1L MoS2 for nanoscale optoelectronic device fabrication.

Recombination at the metal-silicon interface is a major barrier to reaching the theoretical power conversion efficiency limits. We present a PL method for evaluating the metal contact recombination current ( J 0,c ) of rear TOPCon metallisation.

A two-fluid mobility model is derived for simulating laser-driven semiconductor switches. We reveal the importance of carrier-screening effects in electron-hole scattering and Auger recombination for carrier densities above the critical carrier density for Mott transition.

Surface passivating stacks are fabricated on n-type silicon using plasma-enhanced atomic layer deposition (ALD) with the structure comprising a positively charged silicon oxide interlayer capped with negatively charged hafnium oxide. Without the inclusion of an ALD-grown silicon oxide interlayer, high-resolution transmission electron microscopy reveals the existence of a similar to 2 nm thick silicon oxide layer between the hafnium oxide and the silicon before and after a 450 degrees C activation anneal. The thickness of the silicon oxide interlayer is increased with the intentional deposition of silicon oxide by plasma-enhanced ALD (up to 33 nm). By increasing the thickness of silicon oxide, we demonstrate control of the stack's passivating properties, finding an inverse relationship between interlayer thickness and passivation level. Corona charging experiments demonstrate a reduction in the net negative charge density (from -10(12) q cm(-2) to -10(11) q cm(-2)) with increasing interlayer thickness. For the interlayer thicknesses considered, Kelvin probe measurements confirm that the charge polarity was not flipped as has previously been observed for an analogous stack with an aluminium oxide capping layer. ALD-SiOx/HfOx stacks provide well-defined effective fixed charge densities which can be used to provide flexibility in field-effect passivation for silicon photovoltaic solar cells.

Silicon-based terahertz (THz) photomodulators suffer from a modulation speed limited by the lifetime of the charge carriers photoexcited in the silicon. We report a silicon-based THz photomodulator scheme offering real-time reconfiguration of the switching behavior by manipulation of effective charge carrier lifetime. Atomic layer deposition was used to coat silicon samples with dielectric layers to passivate the surfaces with a conductive polymer subsequently deposited to enable electrical gating over the whole surface. The resulting gated photomodulators are characterized using photoconductance decay and photoluminescence imaging. A gated photomodulator with HfO2 passivation is then implemented into a THz time domain spectroscopy setup to demonstrate the potential for live photomodulation optimization during a single-pixel imaging experiment. We use the device to achieve a real-time improvement of the signal-to-noise ratio of the images by a factor of 8.

Photovoltaics play a key role in the United Kingdom (UK) meeting its net-zero targets. In this analysis, we draw on governmental datasets and projections, technical data from across the photovoltaic (PV) manufacturing industry and technoeconomic research to determine the UK's waste processing needs in the event it meets its deployment targets. Our estimates suggest that the-18 GW of current UK PV capacity corresponds to approximately 70m installed modules. To meet the 2050 target of 90 GWpeak, there needs to be sustained average annual installation of-5-10m individual PV modules over the coming decades, the majority of which are likely to be silicon-based technologies. Many current PV modules are nearing the end of their lifespan or are on sites selected for "repowering", and, based on our estimates, cumulative end-of-life UK PV module numbers will exceed 100m (-2m tonnes) by 2050. The tonnage of UK solar waste is expected to exceed the current pan-European solar waste recycling capacity by 2035, demonstrating the need for further investment in recycling infrastructure and capacity. Domestic processing of this waste would enable retention of contained high-value or critical materials within UK supply chains, but this requires technical infrastructure to be in place.

AbstractMonolayer molybdenum disulfide (1L MoS2), a promising optoelectronic material, emits strong visible photoluminescence (PL). Systematic control of the intensity, energy, and spectral width of PL from 1L MoS2 on silicon dioxide/silicon (SiO2/Si) is demonstrated via simple external treatments. Treating MoS2 with solutions formed from the superacid bis‐(trifluoromethanesulfonyl)amide (TFSA) enhances, blueshifts, and sharpens the PL. Treatments with solutions from structurally analogous chemicals that lack sulfur, in the case of bis(trifluoroacetamide) (BTFA), or lack fluorine, in the case of methanesulfonamide (MSA), show the same trend, suggesting a two‐component mechanism for TFSA involving the presence of electronegative species and sulfur vacancy passivation. Up to ≈100× enhancement of the PL intensity is achieved, with the peak blueshifted by ≈30 meV and the spectral linewidth halved. Conversely, direct thermal atomic layer deposition (ALD) of aluminum oxide (Al2O3) or hafnium oxide (HfO2) is found to suppress the PL by up to a factor of ≈3, redshift by up to ≈70 meV, and broaden by ≈3×. Single‐spot and mapping Raman/PL techniques are combined in a robust characterization process to associate changes in the PL character to charge doping. This work demonstrates the convenient tunability of the optical behavior of 1L MoS2 by varying the electron density.

Solar-based power generation presents challenges for system and grid operators due to the intermittent nature of power supply. Predicting the performance of photovoltaic (PV) power plants and rooftop systems can often be challenging due to difficulties in data collection and incoherencies in interconnected systems. Following the hierarchical aggregation structure from geographical and temporal similarities between PV systems, we suggest a simplified approach to predicting the performance of individual installations and evaluating the impact of these hypothetical installations on the overall grid. We use the hierarchical nature of power generation and ascertain weather datasets to predict the performance of new or existing systems for locations with unmeasured input data. We demonstrate an approach that could improve grid stability by using a hierarchical model on publicly available datasets on utility and rooftop installations. Ensemble machine learning algorithms are trained with 16 weeks of known hourly input training features to form a baseline model for known locations. The prediction accuracy is then directly compared for locations with known and unknown input features, both on a granular and subregion level. We observe a reduction in prediction accuracy by 6-8% using the hierarchical approach. The accuracy of the hierarchical model can be further enhanced beyond our work by increasing the training dataset temporally, as well as by augmenting nested layers of the hierarchy.

Muons are part of natural cosmic radiation but can also be generated at spallation sources for material science and particle physics applications. Recently, pulsed muons have been used to characterize the density of free charge carriers in semiconductors and their recombination lifetime. Muon beam irradiation can also result in the formation of dilute levels of crystal defects in silicon. These crystal defects are only detected in high carrier lifetime silicon samples that are highly sensitive to defects due to their long recombination lifetimes. This work investigates the characteristics of these defects in terms of their formation, recombination activity, and deactivation. Charge carrier lifetime assessments and photoluminescence imaging have great sensitivity to measure the generated defects in high-quality silicon samples exposed to ∼4 MeV (anti)muons and their recombination activity despite the extremely low concentration. The defects reduce the effective charge carrier lifetime of both p- and n-type silicon and appear to be more detrimental to n-type silicon. Defects are created by transmission of muons through the wafer, and there are indications that slowed or implanted muons may create additional defects. In a post-exposure isochronal annealing study, we observe that annealing at temperatures of up to 450 °C does not by itself fully deactivate the defects. A recovery of charge carrier lifetime was observed when the annealing was combined with Al2O3 surface passivation, probably due to passivation of bulk defects from hydrogen from the dielectric film.

Understanding surface passivation arising from aluminium oxide (Al2O3) films is of significant relevance for silicon-based solar cells and devices that require negligible surface recombination. This study aims to understand the competing bulk and surface lifetime effects which occur during the activation of atomic layer deposited Al2O3. We demonstrate that maximum passivation is achieved on n- and p-type silicon with activation at similar to 450 degrees C, irrespective of annealing ambient. Upon stripping the Al2O3 films and re-passivating the surface using a superacid-based technique, we find the bulk lifetime of float-zone and Czochralski silicon wafers degrade at annealing temperatures > 450 degrees C. By accounting for this bulk lifetime degradation, we demonstrate that the chemical passivation component associated with Al2O3 remains stable at activation temperatures of 450 & horbar;500 degrees C, achieving an SRV of < 1 cm/s on n- and p-type silicon. In conjunction with the thermal stability, we show that films in the range of 3-30 nm maintain an SRV of < 1 cm/s when annealed at 450 degrees C. From atomic-level energy dispersive X-ray analysis, we demonstrate that, post deposition, the interface has a structure of Si/SiO2/Al2O3. After activation at > 300 degrees C, the interface becomes Si/SixAlyO2/Al2O3 due to diffusion of aluminium into the thin silicon oxide layer.

Hafnium oxide (HfOx) films grown by atomic layer deposition (ALD) have recently been demonstrated to provide high‐quality silicon surface passivation. Reports have suggested that changing the composition of the hafnium‐containing precursor can enable films of both charge polarities to be produced. Herein, the passivation quality of hafnium oxide grown with metal amide precursors and a tetrakis(ethylmethylamido)hafnium (TEMAHf) precursor is examined, considering film charge polarity, chemical‐ and field‐based passivation effects, and film crystallinity. Throughout, the properties of TEMAHf‐HfOx are benchmarked against that of hafnium oxide grown with a tetrakis(dimethylamido)hafnium precursor. It is found that precursor choice has no influence on the fixed negative charge polarity (of order −1012 q cm−2) of HfOx films grown via plasma‐enhanced ALD. TEMAHf‐HfOx passivation is influenced by post‐deposition annealing temperature and can passivate with a surface recombination velocity ≤3 cm s−1 on n‐type silicon, compared to surface recombination velocities ≤11 cm s−1 for TDMAHf‐HfOx of a similar thickness.

Spatial and temporal control of thermally emitted terahertz (THz) radiation could pave the way for a new family of devices in imaging, spectroscopy and communication systems. We demonstrate a computational THz imaging method enabled by a structured illumination provided by a spatio-temporal emissivity modulation. We apply a surface passivation stack composed of ZnO and Al2O3 layers to a high-resistivity silicon wafer to increase the effective carrier lifetime of the electron-hole pairs by a factor of 16. The emitted power is further boosted by increasing the temperature of the modulator to 390 K. Using a low power LED and a digital micromirror device we optically control the local THz emission from the modulator and use it to produce structured THz beams in a single-pixel imaging setup. We employ a ghost imaging procedure with a single-pixel detector and sequential illumination with patterns from a Hadamard basis set to allow for computational reconstruction of the object from the temporal signal. We evaluate the performance of the technique and its potential trade-offs with respect to resolution and acquisition time and apply a simple compressed sensing protocol to speed up the imaging process.

Hafnium oxide thin films have attracted considerable interest for passivation layers, protective barriers, and anti-reflection coatings. This study presents a systematic investigation into the role of film growth co-reactant on film properties.

Photovoltaics (PVs) are a critical technology for curbing growing levels of anthropogenic greenhouse gas emissions, and meeting increases in future demand for low-carbon electricity. In order to fulfill ambitions for net-zero carbon dioxide equivalent (CO _2 eq) emissions worldwide, the global cumulative capacity of solar PVs must increase by an order of magnitude from 0.9 TW _p in 2021 to 8.5 TW _p by 2050 according to the International Renewable Energy Agency, which is considered to be a highly conservative estimate. In 2020, the Henry Royce Institute brought together the UK PV community to discuss the critical technological and infrastructure challenges that need to be overcome to address the vast challenges in accelerating PV deployment. Herein, we examine the key developments in the global community, especially the progress made in the field since this earlier roadmap, bringing together experts primarily from the UK across the breadth of the PVs community. The focus is both on the challenges in improving the efficiency, stability and levelized cost of electricity of current technologies for utility-scale PVs, as well as the fundamental questions in novel technologies that can have a significant impact on emerging markets, such as indoor PVs, space PVs, and agrivoltaics. We discuss challenges in advanced metrology and computational tools, as well as the growing synergies between PVs and solar fuels, and offer a perspective on the environmental sustainability of the PV industry. Through this roadmap, we emphasize promising pathways forward in both the short- and long-term, and for communities working on technologies across a range of maturity levels to learn from each other.

The carrier lifetime stability of gallium-doped silicon wafers and performance stability of industrial PERC solar cells produced from sister wafers were investigated under four different illumination conditions and temperatures. The seven investigated materials feature a resistivity variation of 0.4–1.0 Ωcm and lifetime samples were processed to create high hydrogen content (with PECVD SiNx) or low hydrogen content (with ALD Al2O3 or HfO2). Our results confirm that the material itself is prone to light and elevated temperature induced degradation (LeTID), however experiments on PERC cells produced utilising the same silicon material indicate that the production process can successfully suppress LeTID. In contrast to earlier studies, we observe only small levels of degradation at the cell level, with some showing an improvement in cell parameters under LeTID testing conditions. Our results indicate that LeTID is not necessarily a major issue for the performance of modern passivated emitter and rear cells made from gallium-doped silicon substrates.

We investigate the potential of ultra-thin HfO2 films grown by atomic layer deposition for passivating contacts to silicon focusing on variations in film thickness and post-deposition annealing temperature. A peak in passivation quality - as assessed by carrier lifetime measurements - is reported for 2.2 nm thick films annealed at 475 degrees C, for which a surface recombination velocity <1 cm/s is determined. For films <2.2 nm thick, there is a marked decrease in passivation quality. X-ray diffraction highlights a change from crystallised monoclinic to amorphous HfO2 as film thickness decreases from 12 nm to 2.2 nm. Kelvin probe results indicate that as-deposited 2.2-12 nm films have similar effective work functions, although the work function of 1 nm films is considerably lower. Upon post-deposition annealing in vacuum, all films exhibit a reduction in effective work function at temperatures coincident with the onset of passivation in air-annealed samples. An initial investigation into the contact resistivity in a passivating contact structure utilizing HfO2 reveals a strong post-deposition annealing temperature dependence, with the lowest resistance achieved below 375 degrees C, followed by a decrease in performance as temperature increases towards the optimal temperature for passivation (475 degrees C). Limitations of the contact structure used are discussed.

Monolayer molybdenum disulfide (MoS _2 ) is a promising candidate for inclusion in optoelectronic technologies, owing to its two-dimensional (2D) nature and resultant novel photoluminescence (PL). Chemical vapour deposition (CVD) is an important method for the preparation of large-area films of monolayer MoS _2 . The PL character of as-prepared monolayer MoS _2 must be well understood to facilitate detailed evaluation of any process-induced effects during device fabrication. We comparatively explore the PL emission from four different commercially available CVD-grown MoS _2 monolayer films. We characterize the samples via Raman and PL spectroscopy, using both single-spot and mapping techniques, while atomic force microscopy (AFM) is applied to map the surface structure. Via multipeak fitting, we decompose the PL spectra into constituent exciton and trion contributions, enabling an assessment of the quality of the MoS _2 monolayers. We find that the PL character varies significantly from sample to sample. We also reveal substantial inhomogeneity of the PL signal across each individual MoS _2 film. We attribute the PL variation to non-uniform MoS _2 film morphologies that result from the nucleation and coalescence processes during the CVD film development. Understanding the large variability in starting PL behaviour is vital to optimize the optoelectronic properties for MoS _2 -based devices.

Thin film dielectrics are ubiquitous in the manufacture of electronic devices and are frequently deposited and etched away at various stages of device fabrication. We demonstrate that hafnium oxide (HfO2) thin films grown via atomic layer deposition on silicon and silicon pre-coated with aluminum oxide (Al2O3) have etch resistance properties, which can be tuned simply by changing the post-deposition annealing temperature. The etching rates of films in hydrofluoric acid (HF) solutions were found to be dependent on annealing temperature, with the etch rate decreasing with increasing temperature. A transition region in the etch rate was identified between 300 and 350 °C, corresponding to the crystallization of the HfO2 films, as identified via x-ray diffraction. HfO2 films deposited directly onto silicon annealed above 350 °C were resistant to 10% HF solutions over the course of several hours. In the case of Si/Al2O3/HfO2 stacks, closer inspection reveals the existence of channels, which reduces the etch resistance of HF acid, as evidenced by tetramethylammonium hydroxide etching of the silicon substrate. Crystallized HfO2 can be used to protect other dielectrics in device processing, and we demonstrate its use in single-sided fabrication of patterned structures of Al2O3, which can control the effective charge-carrier lifetime in silicon wafers for use in modulating THz and mm-wave radiation.