Oxide-based Materials and Devices XVI

Ga2O3 is a highly promising material for power electronics, thanks to its large band gap and high breakdown voltage. Better control of doping is still an active research topic, both in Ga2O3 and in (AlxGa1-x)2O3 alloys. In alloys, controlled doping has proven difficult, and native-defect compensation and DX-center formation limit doping at higher Al concentrations. First-principles modeling, using advanced hybrid functional calculations within density functional theory, can greatly help in resolving experimental puzzles and guiding optimal doping conditions. I will discuss new results for promising donor dopants. I will also address acceptors, discussing their stability as well as the details of carrier capture processes at these deep centers; the latter allows better understanding of experimental characterization and provides guidelines for device design. Work performed in collaboration with Y. Frodason, S. Mu, S. Karbasizadeh, J. L. Lyons, H. Peelaers, M. E. Turiansky, J. B. Varley, and D. Wickramaratne, and supported by AFOSR.

Gallium oxide’s rapid development as an ultrawide band gap semiconductor platform for next-generation power electronics is largely a consequence of the availability of high-quality single-crystal substrates and epitaxial layers. As single crystals are typically obtained via melt-growth processes that involve crucibles, the incorporation of transition metals such as Ir, Cr, and Fe can strongly impact the resulting optical and electrical properties. Here we survey the current understanding of these and several other transition metal impurities and dopants that have been shown to exhibit diverse electronic and optical behavior. Using insights gained from crystal growth efforts and first-principles-based calculations employing hybrid functionals, we discuss how some transition metals can act as efficient shallow donors, while others can act as deep acceptors and/or deep donors that may be strongly influenced by the concentration of other dopants or defects and how these elements may be incorporated within the lattice.

Ga2O3, a wide-band gap semiconductor, is of interest for high-power devices and deep-UV photodetectors. Many of these applications require the formation of heterostructures to create a conduction-band offset to confine charge carriers. This is commonly achieved through alloying with Al2O3. However, Al2O3 has a significantly smaller lattice constant than Ga2O3, which can introduce strain on the heterostructure. We use hybrid density functional theory simulations to design a heterostructure which closely matches the lattice constant of Ga2O3, while maintaining a conduction-band offset. We found that alloys of In2O3 and Al2O3 form a lattice-matched monoclinic structure with a 1 eV conduction-band offset [1]. Moreover, we show that this alloy can readily be n-type doped using Si [2]. [1] S. Seacat, J.L. Lyons, and H. Peelaers, Phys. Rev. Materials 8, 014601 (2024) [2] S. Seacat and H. Peelaers, J. Appl. Phys. 135, 235705 (2024)

Gallium vacancies (VGa) play a pivotal role in carrier compensation in n-type Ga2O3. Previous theoretical investigations of VGa formation energies and charge-state transition levels were limited to zero temperature.Finite temperature causes band renormalization of the host lattice and induces lattice vibrations, which contribute to the free energy and alter the formation energies of defects. Here we investigate the effect of lattice vibrations on the defect formation energy of VGa and its charge-state transition levels in monoclinic Ga2O3 at finite temperatures. Using hybrid density functional calculations, the formation energy and charge-state transition levels of four different gallium vacancy configurations are investigated. We reveal the role of lattice vibrations in the relative stability of gallium vacancies. All studied charge-state transition levels exhibit a blue shift with increasing temperature. The implications of this shift on electron capture processes, such as those observed in deep-level transient spectroscopy, will be discussed. We will also comment on the role of gallium vacancies in diffusion, and in stabilizing the spinel phase.

Over the past few years, there is a general boom in the research of Ga2O3 as a multifunctional material with promising applications in power converters, wireless charging systems and solar cells, among others. There are five Ga2O3 polymorphs, with the β-Ga2O3 monoclinic structure being the most stable one. Most of the recent research reports a rather controllable electron concentration, mobility and conductivity by suitable doping with Si, Sn or Ge, and many unipolar devices have already been demonstrated as a proof of concept, where majority carriers dominate their performance. The origin of the n-type conductivity in β-Ga2O3 is usually assigned to oxygen vacancies that introduce deep levels in the band gap. On the modelling side, a main focus has been the study of the influence of vacancies and interstitial impurities on the Ga2O3 electronic band structure, especially on the energetic position of donor levels inside the bandgap as well as on the migration barriers for point defects in the bulk. In this work, we will use the density functional theory approximation (DFT) to address the influence of doping on the electronic and structural properties of Ga2O3.

Ga2O3 has triggered a paradigm shift in the ultra-wide bandgap semiconductor arena with its high critical field strength (8 MV/cm), a wide direct bandgap energy (~ 4.8eV) and a decent mobility (~100cm2/Vs), enabling critical technologies in the field of high power switching, solar-blind detection, etc. However, one of the major drawbacks of Ga2O3 has been the lack of a reliable method to realize p-type Ga2O3. This is a major hindrance to realizing practical solutions for high voltage switching in electrical grids, which require p-n junctions for operation. In this talk the authors explain how this major bottleneck to realizing the potential of Ga2O3 for various applications can be overcome by utilizing N2 atoms as a p-type dopant through growth under N2 carrier gas. It is demonstrated that N2 substitutes on O-sites in Ga2O3, and thus promotes a transformation to predominantly p-type conduction. Owing to the novelty of this discovery, the exact physical mechanism of this hole-conduction remains unknown.

AIXTRON is utilizing its well-established Close-Coupled Showerhead (CCS) reactor technology for the MOCVD growth of gallium oxide using dynamic process gap adjustment to reduce the working distance between the gas injection showerhead surface and substrate in real-time during the growth. This helps to reduce parasitic reactions during growth and offers much more flexibility for growth parameters optimization. Phase-pure beta-Ga2O3 layers grown on 4” sapphire substrates exhibited excellent thickness uniformity with only 1% standard deviation while a wafer-to-wafer thickness variation of 0.56% was achieved in a 3x2” configuration. SIMS analysis of layers grown under suitable conditions using TMGa indicated a C level of 3.7e16 cm-3 and H concentration of 1.3e16 cm-3, both close to the SIMS detection limits. Whereas background Si levels in the undoped layer were 4e15 cm-3, resulting in an ultra-low background carrier concentration of 2e15 cm-3 as evident from Hg-probe CV measurements.

β-Ga2O3 has become increasingly important in power electronics due to its wide bandgap (~4.9 eV) and high breakdown field (~8 MV/cm). Metalorganic chemical vapor deposition (MOCVD) is particularly effective for growing high-purity, low-dislocation β-Ga2O3 films with excellent low-temperature electron mobility (10,000–23,000 cm²/Vs) and minimal impurities [1-4]. However, traditional MOCVD showerhead designs limit growth rates, doping precision, and surface smoothness on large substrates. Agnitron Technology has addressed these challenges with an innovative showerhead design that significantly improves film quality and uniformity. This presentation will discuss the successful development of extremely smooth (<1 nm) and uniform β-Ga2O3 films on larger area substrates using this new showerhead technology. Films with thickness and doping uniformity below 2.5% were achieved on 50 mm substrates at approximately 20 µm/hr growth rates. The electrical mobility values were independent of the growth rate, remaining constant (~91 cm²/Vs with an electron concentration of n=5x10¹⁷ cm⁻³) across growth rates ranging from 1.7 µm/hr to 13 µm/hr. This demonstrates the potential of this technology to a

In this talk we highlight progress in plasma-assisted MBE growth, doping, processing, and electron devices for (001) beta-Ga2O3. We focus on the (001) orientation because it is the largest area (diameter) substrate that is commercially available. We demonstrate controllable continuous silicon doping using a valved cell. We will demonstrate wet etching on wagon wheel patterns to determined optimal stripe orientations for finFETS and trench MOSFETs. We will demonstrate controlled intentional nitrogen doping by introducing nitrogen into the oxygen flow in the We will demonstrate Schottky barrier height control via oxidation of Pt. We will demonstrate high perforamnce Schottky barrier diodes using mid-K and high K dielectrics including novel metal-insulator-semiductor (MIS) diodes using TiO2 as the nominal thin insulating interlayer.

In this talk, both n-type doping using a SiO solid source, impurity coming from the plasma cavity, and acceptor compensating doping through Ba will be discussed. Moreover, the relationship between the Seebeck coefficient and the carrier density (1 x 10^17 - 4 x 10^19 cm-3) of β-Ga2O3 substrates and homolayers will be discussed. This knowledge will then be used to characterize the carrier system in Si/Ba-doped β-Ga2O3 (010) thin films grown for the first time using molecular beam epitaxy (MBE). The structural characterization of the film and the growth challenges will also be presented. Ab-initio calculations showing the formation energy as well as the defect level of Ba will also be discussed.

Ga2O3 is an emerging yet already technologically important ultrawide bandgap semiconductor. Defects play a vital role in the development of semiconductor devices, often thought to hamper the full potential of the technology. It is therefore crucial to better understand the properties of defects to identify “killer defects” and implement targeted mitigation methods that will lead to step increase in device performance and reliability. Dislocations is one of the most prominent defects in epitaxial Ga2O3, yet their properties are to date unknown. We employ high-resolution transmission electron microscopy to conduct a statistically significant investigation of the structural properties of threading dislocations in α-Ga2O3, which reveal the defect character, Burgers vector, dissociation mechanisms, and atomic core configurations. Using hyperspectral cathodoluminescence mapping at threading dislocations in α-Ga2O3, we identify that free electron density decreases at dislocations, highlighting their non-radiative recombination role, and that point defects accumulate at dislocations, causing an increase in density of donor and acceptor states.

Attoseconds: When intense light interacts with a gas of atoms (or a transparent solid), electron wave packets are released. Attosecond pulse formation exploits the correlated electrons and holes, forcing the electron to return. Without the plasma connection, two of the most important strong-field process that accompany attosecond pulse formation—hot electron formation (inverse Bremsstrahlung) and non-sequential double ionization (collisional ionization)—seemed mysterious. These plasma-like processes lead to laser induced electron diffraction and orbital tomography. THz generation: Terahertz pulse formation by ionization has a similar linage. Using PIC codes to describe azimuthally polarized l=4 mm and 2 mm light interacting with a 150 µm thick jet of helium, we calculate THz pulses reaching 8.5 Tesla. But 10 Tesla is not a limit. 30 THz azimuthally polarized beams can be amplified in high-pressure CO2 reaching isolated magnetic fields of 1-gigagauss.

In recent years, topological phenomena in photonic systems have attracted much attention, with their striking features arising from robust states in the energy gaps of spatially periodic media. However, light waves are entities that extend in space as well as time, such that one may ask whether topological effects can also occur in the temporal domain, or even space-time. Intuitively, systems that are periodic in time may be gapped in momentum, leading to topological states localized at time interfaces. However, time - in contrast to space - exhibits a unique unidirectionality often referred to as the “arrow of time”. Inspired by these features, I will present our most recent experiments on topological states residing at temporal interfaces. Moreover, I will discuss the formation of spacetime-topological events and demonstrate unique features such as their limited collapse under disorder and causality-suppressed coupling.

Quantum technologies promise a change of paradigm for many fields of application, for example in communication systems, in high-performance computing and simulation of quantum systems, as well as in sensor technology. However, the experimental realization of suitable system still poses considerable challenges. Current efforts in photonic quantum science target the implementation of practical devices and scalable systems, where the realization of quantum devices and controlled quantum network structures is key for envisioned future technologies. Here we present our progress on the engineering of integrated photonic systems, which can overcome current limitations for the realization of scalable photonic systems. Specifically, our research currently focuses on three different but complementary topics: integrated devices based on lithium niobate circuits, engineering and harnessing the temporal-spectral structure of quantum states of light, and photonic quantum computation.

Currently, 70% of all electrical power consumed is routed through power electronics and this fraction is set to increase even faster with further addition of renewable sources to the grid and electrification of transportation. A significant reduction in size, weight, and power loss (SWaP) of power electronics will revolutionize the way the electric grid operates. The lion’s share of power electronics is based on silicon, with a bandgap of 1.1 eV. Significant reduction in SWaP is possible by migrating to semiconductors with wider bandgaps than silicon. Over the past 10 years, β-Ga2O3 with an ultra-wide bandgap of 4.8 eV has emerged as a promising material for next generation power electronics. In this talk, I will outline our work on metal-organic chemical vapor deposition (MOCVD) synthesis, processing, and characterization of β-Ga2O3 epitaxial thin films and heterostructures for next generation power devices.

Very recently, we learned that the phase stability of Ga2O3 polymorphs is readily controllable by the accumulation of radiation damage. Thus, using ion beams, different Ga2O3 phases may be stabilized at different spatial locations of ion beam penetration, which can be exploited for thermal energy management. Particularly, in this work we present heat conduction properties of the double polymorph γ/β-Ga2O3 structures, fabricated by a self-organized γ-polymorph transformation induced in β-Ga2O3 by irradiation. The thermal conductivity was measured by time-domain thermoreflectance method. The results show an order of magnitude difference in thermal conductivity across the γ/β interface. Moreover, we demonstrate that γ-Ga2O3 films exhibit remarkably high radiation tolerance in terms of thermal transport for consequent radiations with swift heavy ions. We also present the thermal transport properties of other Ga2O3 polymorphs and discuss phase-dependent thermal conductivity through phonon transport properties like phonon mean free path and phonon density of states. The molecular dynamics simulations with machine-learned potential are also presented to validate the experimental results.

One of the primary advantages of β-Ga2O3 over incumbent wide band gap semiconductors is the ability to be grown directly from the melt. Melt growth using Czochralski or similar methods results in impurities in the crystal originating from the crucible, including iridium and other transition metals, notably chromium which can also be present in the source material. These impurities exhibit optoelectronic signatures that are not only useful in their identification, but also sensitive to the Fermi energy of a given crystal (i.e., they vary with the electrical conductivity of the matrix). In this work, we describe how a laser Raman system can be used to map and correlate spatially-dependent Cr3+ photoluminescence, electronic-coupled Raman scattering due to Ir4+ internal transitions and hydrogenic shallow donors. The spectroscopic signals resulting from heterogeneities in impurity concentrations throughout the Czochralski boules with striated morphology were confirmed with Laser ablation inductively-coupled plasma mass spectrometry. Mapping of photoluminescence and Raman-related signatures is demonstrated as an effective and facile method for spatial measurement of heterogeneities.

Compensating deep level defect states that influence the electrical properties of β-phase gallium oxide (β-Ga2O3) can be introduced intentionally through doping, or unintentionally through unwanted impurities and intrinsic defects. Understanding the complex behavior of these compensating centers is essential for optimizing β-Ga2O3 for both high voltage and RF electronics. Applications in harsh environments, such as in space, add further complications due to defect creation resulting from high energy irradiation. Due to its inherent n-type conductivity, compensating deep acceptor states intentionally introduced by doping with Nitrogen, Iron or Magnesium to create semi-insulating regions of device structures are of great interest. However, inadvertent impurities such as Carbon are also predicted to create deep acceptors and can be problematic. The presence of intrinsic defects that form during growth or introduced by harsh environments makes the understanding of how defects impact material and device properties very complex. Here we use deep level transient (thermal) spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) to identify, compare and characterize individual deep acceptor states created by both extrinsic and intrinsic sources in β-Ga2O3, and compare with theoretical predictions. Compensation efficiencies between acceptor choices are compared, and some of the more unusual behaviors of very deep states present below midgap are described, where both conduction and valence band transitions are observed and explained. Radiation studies used to differentiate extrinsic from intrinsic sources will be discussed in the context of unraveling the comprehensive impacts of defects in β-Ga2O3.

Uncontrolled fires are estimated to contribute as much to carbon gas emissions as all of commercial transport. A key problem is that these fires are often not detected in time to limit the damage because most commercial fire/smoke detectors usually do not go off until it is too late to intervene and quell the conflagration before it takes hold. Remote optical sensing should be a big part of the solution. Although infrared (IR) sensors are the conventional solution for heat detection, they are not ideal for remote optical fire detection because they are subject to multiple confounding signals. For this reason, there is a need for ultraviolet C band (UVC) flame sensors, which are not subject to such false positives because their photoresponse is solar blind. This talk will explore the use of Ga2O3/NiO heterojunctions for use as self-powered remote fire/flame sensors.

Ga2O3 represents a promising ultrawide bandgap semiconductor for power electronics. This talk will present the progresses and challenges of MOCVD growth of Ga2O3. Specifically, MOCVD growth of Ga2O3 along both (010) and (001) will be discussed. The recent demonstration of MOCVD growth of p-type NiO and n-Ga2O3/p-NiO will be presented.

This presentation covers our electron microscopy study of the atomic structure and defects at the NiO/β-Ga₂O₃ interface, a potential candidate for gallium oxide-based PN junctions. We explored key questions about the crystal orientation of NiO, defects, and how growth temperature impacts the interface and its electrical properties. Our results reveal a cubic spinel NiGaO intermediate layer forms at the interface, with its thickness influenced by growth temperature. This interlayer, acting as a strain mitigator, affects the orientation of NiO and the leakage current in the PN junction. Our findings provide insights for optimizing the interface structure for better performance.

The unique material properties of Gallium Oxide make it promising for a range of future applications, but innovative materials and device engineering are needed to translate these ultimate material limits to real technology. This presentation will discuss our recent work on epitaxy, heterostructure design, and electrostatics to achieve high-performance β-Ga2O3 lateral and vertical electronic devices, and photodetectors.

This first decade of monoclinic Ga2O3 device research has been incredible (underpinned by the availability of large area bulk substrates) in breakdown voltages, power device figure of merit and high-speed performance. It has emerged as a promising ultra-widebandgap semiconductor for next generation power, GHz switching and RF applications. In this talk, we will provide the status and challenges that needs to be addressed before this technology can be used in the field.

Transparent Photovoltaic (TPV) is the technology of solar cells to convert light to electric energy beyond the darkness. Different from the typically dark or opaque solar cells, TPV is transparent by passing the visible range lights due to the wide-energy bandgap of metal-oxides. And thus, human beings may not recognize the existence of TPV entities but the electric energy is generated through the invisible power generator. This kind of invisible TPV may open a new era for on-demand energy supplying system, by being applied in windows of cell phones, displays, vehicles, and buildings. The pyroelectric-photovoltaic device outperforms traditional photovoltaic devices due to the long-range electric field that occurs under pulse illumination. Optimization of parameters such as pulse frequency, scan speed, and illumination wavelength enhances power harvesting, as demonstrated by a power conversion efficiency of 11.9% and an incident photon-to-current conversion efficiency of 200% under optimized conditions. This breakthrough enables reconfigurable electrostatic devices and presents an opportunity to accelerate technology that surpasses conventional limits in energ generation.

Quantized nanolaminates (QNLs) are sequences of alternating layer stacks of high and low refractive index material in the nanometer regime (quantum well layer and barrier layer) resulting in a metamaterial. It has been shown that QNLs are able to overcome the natural interdependency between refraction index and optical bandgap in the deposition material, allowing both properties to be tuned individually. Beyond that, QNLs are said to improve the laser-induced damage threshold (LIDT), while tuning the optical bandgap to higher values. In this study, we used ion beam sputtering (IBS) and embed QNLs into a Ta2O5 - SiO2 high-reflectance (HR) mirror at 532 nm and 45° angle of incidence. The thickness of the quantum well layers varied from 0,5 nm, 1 nm, 2 nm, 4 nm, 8 nm. LIDT was determined by S-on-1 testing. We also performed an evaluation of the electric field intensity throughout the whole extent of the mirrors to better understand the impact of QNL-dimensions on the endurance of HR-mirrors.

Structured optical components formed by laser fabrication or nanostructured coatings enable a wide range of applications, such as polarizers, waveplates, spatial filters, antireflective coatings, etc. However, such elements often require functionalization to enhance their performance and manipulate the behavior of light. In this work, we present the application of the atomic layer deposition (ALD) technique to produce conformal coatings on structured optical components. Our recent experiments demonstrate that ALD coatings can increase the surface quality and reduce the surface roughness of the structured surfaces. Moreover, we investigated the potential of ALD to produce protective layers on porous anisotropic coatings to stabilize their optical properties in changing environments.

Sol-gel coatings and nano-imprint lithography are employed to directly frame 3D nano-structures composed of hard ceramics (e.g. SiO2, TiO2) over large surfaces. Photonic metasurfaces can be produced with high throughput at low cost for applications in structural colour, anti-reflection coatings for high power lasers, AR/VR, and light emission.

Numerous high-value applications within the energy sector involve environmental conditions that are incompatible with traditional sensor technology, due to degradation of electrical interconnects, packaging, or the sensors themselves. In this work, we discuss the utilization of nickel incorporated oxides on evanescent field optical fiber sensors for gas sensing under harsh conditions relevant to multiple energy infrastructure applications. Two major application areas will be discussed (1) hydrogen sensing for high-temperature applications (e.g., for operation within a solid oxide fuel cell or electrolyzer) and (2) for low-level detection of acetylene in an operational power transformer.

This study aims to address the limitations of traditional gas sensors that operate at high temperatures and pose explosion risks by utilizing terahertz photons as the detection source, allowing the sensor to function at room temperature. To develop a novel optical gas sensor, we synthesized a composite material of ZIF-8 and MoSe2 using a hydrothermal method and combined it with metamaterials for detection of toxic gas (NO2). ZIF-8, a special metal-organic framework material, has a large specific surface area, high porosity, and good chemical and thermal stability, providing advantages in the field of gas sensors. MoSe2, a transition metal dichalcogenide (TMDCs), has unique electrical and optical properties, showing great potential in gas sensing recently.

Transparent ceramics offer remarkable advantages with respect to mechanical and optical properties for the usage as transmissive windows for IR sensor applications in harsh environments, compared to glasses or polymers. However, the processing makes the fabrication of the monolithic transparent components comparably challenging. In this work, these challenges are emphasized for the production of transparent MgAl2O4, Y2O3 and MgO, with a focus on the granulation of the ceramic powders.

Bulk-conductive microchannel plates (MCPs) made of semiconducting glasses have peculiar characteristics such as slant electric field in microchannels, electron gain always obtained under non-saturated condition in channels, as well as long service life and high signal-to-noise ratio without ion feed-back during operation. The glass-formation region and properties of semiconducting glasses for making bulk-conductive MCPs have been discussed. Night vision devices with bulk-conductive MCPs are considered as enhanced image intensifier due to avoiding the electron scrubbing pre-treatment which greatly reduces the electron gain of conventional MCPs. A thickened piece of bulk-conductive MCP, which can be operated at higher voltage to obtain higher electron gain under non-saturated condition , may be used as a substitution of V- or Z-pack products of conventional MCP for photon counting.

Improved remote optical detection is urgently needed in order to more rapidly localise and extinguish the wildfires that plague our planet. Since IR detectors are subject to confounding signals from background heat sources, optical fire sensors usually couple them with solar-blind UVC sensors which are not susceptible to such false positives. These are usually vacuum tube photomultipliers (PM) which are highly sensitive but bulky, fragile, expensive and require high voltage operation. Recently the authors developed groundbreaking solid state UVC sensors which went up on a cubesat scientific mission in April 2023. Based on a next generation semiconductor, Ga2O3, these sensors have intrinsic solar blindness and state-of-the-art gain. Hence they do not need solar filters, pre-amps or high operating voltages and thus offer greatly reduced mass, size and cost compared to PMs. In this talk we wll describe the development of UVC imaging arrays designed for airborne early warning/localisation of fire ignition. .

In recent decades, deep-ultraviolet (DUV) optoelectronics have gained attention due to their unique properties, including intrinsic solar-blindness and high scattering. These characteristics provide low environmental noise and excellent signal transmission, making DUV photodetectors (PDs) valuable for applications such as risk monitoring and wireless optical communication. This talk will discuss overcoming challenges in metal oxide-based DUV PDs, like achieving efficient DUV absorption and maintaining material stability. Strategies include converting wide bandgap materials to ultra-wide bandgap materials using quantum dot size modulation and synthesizing ternary UWBG perovskite oxides. Emerging DUV PD applications like partial discharge detection, flame sensing, and real-time blood component identification will also be highlighted.

In this work, we demonstrate growth of highly-oriented β-Ga₂O₃, thin films on Si(111) substrates, and systematically analyse the performance of both metal-semiconductor-metal (MSM) and p-n-junction photodetectors, fabricated with these films. Wide-angle, out-of-plane ω-2θ X-ray scans revealed only the (2 ̅01),(4 ̅02) and (6 ̅03) reflections of β-Ga₂O₃, for deposition at 750 ºC and an argon partial pressure of 5 × 10^(-3) mbar, thus establishing the formation of (2 ̅01)-oriented β-Ga₂O₃. In MSM photodetectors, fabricated with Ti/Au and Ni/Au asymmetric contacts to the β-Ga₂O₃ epilayers, the dark current was measured to be as low as 5 nano Ampere, while under UV illumination at 253 nm, the photocurrent was obtained to be 1.5 micro-ampere. Interestingly, the photoresponse of the device spanned from DUV to the visible region of the spectrum, possibly due to contribution from the underlying Si substrate. Fabrication and measurement of vertical photodetectors in the p-Si/n-Ga₂O₃ geometry is currently underway.

Traditional force sensors based on piezoresistive, capacitive, piezoelectric, or triboelectric effects face significant obstacles. Achieving sustainable, low cost, self-powered flexible force sensors capable of transducing force without additional active electronic circuitry for long term applications is challenging. This study introduces a self-powered, flexible, battery-free, multi-layered, and multi-nanomaterial 3D-printed Triboelectric Nanogenerator (TENG)-based force sensor for long-term force sensing applications. The TENG device employs SEBS and MXene/Polyaniline composite, chosen for their excellent triboelectric properties, flexibility, and 3D printing compatibility. To mitigate conductivity degradation due to MXene oxidation, PANI, a conductive polymer, was integrated to form a protective layer for MXene. The TENG device achieves a peak voltage of 600V, a sensitivity of 5.8 V/N, and a peak power density of 0.8 W/m² at a force of 50 N and an 8 Hz contact frequency. This innovation marks a substantial advancement in the development of self-powered force sensors for long-term force-sensing applications.

A top-surface encapsulation layer, consisting of a conventional polymer and oxygen scavenging mesoporous silica nanospheres, is developed to prolong the stability of yellow-emitting HDASnBr4 perovskite thin-films. The solution-processed composite barrier layer effectively prolonged the ambient stability of the oxygen-sensitive HDASnBr4 perovskite thin-films and maintained the light emission for significantly longer time compared to control films without encapsulation.

We report recent progress on optoelectronic devices and metasurfaces involving surface plasmon polaritons, enabled by metal-oxide-semiconductor (MOS) structures where the semiconductor is indium tin oxide operating in its epsilon-near-zero regime. We discuss electrically tuneable metasurfaces, high-speed electro-absorption modulators, and reflection modulators. Hot carriers created by the absorption of plasmons in metallic nanostructures on MOS structures are also discussed as they lead to novel device physics that open the door to new device concepts.

CdO, and the ternary alloy with Zn, present very high intrinsic carrier concentrations and carrier mobilities, yielding plasma frequencies in the 3-8 um region of the mid-IR with very low optical losses. This combination of properties yields outstanding plasmon polariton figures of merit, opening the door to applications where the polarization state, intensity, propagation direction, or wave front shape of light needs to be controlled. Among these applications, surface-enhanced infrared absorption (SEIRA) spectrocopy uses localized plasmon polaritons (LSPs) to amplify the near field and enhance the absorption signal from molecules with bond frequencies in the mid-IR. Indeed, the C-H, C=O or O-H bonds, present in many molecules, have frequencies in this spectral range. In order to excite these LSPs, nanostructures defined with nanolithography are typically used. In contrast, we show here that using a self assemble approach, CdO-based nanoparticles can be used to obtain record SEIRA figures or merit, with large detection areas (in the cm^2 range), and in an spectral window (2.5-8 um) unreachable with any other doped semiconductors.

For potential applications of Ti3C2 MXene as energy storage material using hydrogen it is crucial to understand hydrogen bonding and diffusion. Here, it is shown that the chemical bonding of hydrogen atoms and molecules extends far beyond the simple picture of covalent, ionic and multicenter bonds. On the surface and between two Ti3C2 sheets, this bonding is restricted to the formation of Ti−H bonds. However, at interstitial sites, both H and H2 form multicenter bonds including the nearest neighbor Ti and C atoms. Interestingly, C−H bonds involve the formation of s−p hybrid orbitals. For H2 molecules the formation of multicenter bonds results in an increase in bond length. The vibrational eigenmodes for all complexes were calculated and vibrational frequencies ranging from 890 to 1610 cm−1 were obtained. Moreover, H diffusion was investigated and the diffusion coefficient for various migration paths were calculated showing that H diffusion is governed by interstitial diffusion.

This presentation will report on the fabrication and properties of quasi-freestanding graphene on silicon carbide (SiC). It will start with a concise overview of the growth and properties of epitaxial graphene on SiC. Then different recipes for the preparation of quasi-freestanding graphene by intercalation will reviewed, followed by an overview over different intercalants, their diverse structural and electronic properties and their effect on the overlying graphene.

The synthesis of graphene integrated Cr2AlC MAX phase using the sol-gel method was explored to investigate its potential biomedical applications. The sol-gel process, known for its precision in controlling the chemical composition and morphology of materials, facilitated the formation of a homogeneous and highly stable graphene integrated Cr2AlC MAX phase composite. Morphological characterization using Scanning Electron Microscopy (SEM) and structural analysis via X-ray Diffraction (XRD) confirmed the successful integration of chromium, aluminum, and graphene oxide within the matrix. The composite exhibited unique physicochemical properties, including enhanced surface area, mechanical strength, and biocompatibility, making it suitable for various biomedical applications. Preliminary in vitro studies demonstrated the material's potential in drug delivery systems, where it showed controlled release kinetics and improved drug loading capacity. Additionally, the graphene integrated Cr2AlC MAX phase and composite exhibited significant antibacterial activity against common pathogens, indicating its potential use in antimicrobial coatings and wound healing applications. Furthermore, cytot

Functionalization of 2D-conductive layers but also the passivation of underlying interfaces, e.g. by molecular monolayers, are of high technological interest for many applications from catalysis, optoelectronics to optical and electro-chemical sensors. Optical spectroscopies are very well suited for sensitive contactless and non-destructive analysis of the prepared interfaces. In particular spectroscopic Raman and infrared (IR) polarimetric analysis can access detailed informations on the 2D-layer properties as well as the molecular functionalization. Thereby bands due to molecular vibrations and phonons as well as absorptions of free charge carriers are related to structure and material properties. Complementary information of interface properties is revealed from PL and XPS measurements as well as DFT calculations. We acknowledge financial support by the European Union through EFRE 1.8/13.

Understanding the charge carrier dynamics in heterostructures of transition metal dichalcogenides (TMDs) is crucial for optimizing them for potential applications in electronic and optoelectronic devices. In this study, we investigate three different TMDs heterostructures using Raman spectroscopy and ultrafast pump-probe microscopy (UPPM). The carrier dynamics are discussed in relation to the stacking configuration of the MoS₂ and WS₂ layers as well as the quality of the layers.

The interest on ultrawide bandgap semiconductor gallium oxide, especially its monoclinic phase (β-Ga2O3) is strongly increasing since the first transistors were created and its very high critical electrical field was demonstrated. On the other hand, work on its photonic and optoelectronic properties has been also developed for more than six decades. Many works have been devoted to applications such as UV solar-blind detectors or as luminescent devices, harnessing its ultrawide band gap. In this study, we present our recent works on two approaches that harness the physical properties (luminescence, refractive index, crystal anisotropy,…) of β-Ga2O3 nanomembranes, micro- and nanowires for photonic applications. 1) Characterization of mechanically exfoliated nanomembranes with SEM, Raman, PL and synchrotron XANES and XEOL. 2) β-Ga2O3:Cr microwire-based luminescentthermometers where micro/nanowires are encapsulated in atomic layer deposited? multilayers that act as DBRs, resulting in robust microthermometers for a wide temperature range.

The Si nanorods and Si/Ge nanowires grown by molecular beam epitaxy (MBE) on silicon wafers were subject to oxidation using the vapor phase form of CP-4 etchant. These semiconductor quantum structures offer promising application possibilities ranging from photonics to advanced electronics device applications. Therefore, understanding of the oxidation mechanism and resulting effects would provide a valuable knowledge on physical and electrical properties of these commercially valuable materials. In this presentation, a comprehensive review of the properties is provided in order to clarify the origin of the observed oxidation based features including wafer recovery process. The results have been analyzed using state-of-the-art characterization techniques and compared with the current developments in the area. At the end of the paper, a lookout is provided for possible photonic applications.

The combination of a high electrical conductivity while maintaining a high optical transparency is rarely met in conventional materials as it requires to degenerately dope wide band gap semiconductors. The task to find a candidate material with high electrical conductivity and a good optical transparency in the ultraviolet spectrum is an even more daunting task as it requires to realize an ultra-wide band gap semiconductor with a very high carrier concentration. An elegant way out of this dilemma is the utilization of metals exhibiting strong electron correlation. Here, the free carrier absorption edge can be pushed into the infrared regime despite its high carrier concentration due to the carrier mass renormalization arising from a sizeable electron-electron interaction present in these materials, opening up the materials design space of transparent conductor materials towards correlated metals. In this talk I will demonstrate that among possible correlated metal are candidate materials with interband transition beyond 4.5 eV, rendering them suitable as transparent conducting electrodes for deep UV-LED applications.

In recent years, zinc germanate (Zn₂GeO₄) has garnered significant interest as a novel transparent conductive oxide, featuring a direct ultra-wide band gap of 4.5 eV at room temperature. This wide band gap facilitates higher breakdown electric fields and short-wavelength emission, making Zn₂GeO₄ particularly advantageous for high-power electronics and deep ultraviolet optoelectronics applications. In this work, we present an in-depth study of the optical properties of this material. High crystal quality Zn₂GeO₄ micro- and nanowires exhibit intense luminescence at room temperature, associated with native defects. This luminescence can be tuned from the visible to the ultraviolet range by doping with different ions. Furthermore, Zn₂GeO₄ microwires demonstrate waveguiding behavior and light confinement effects, attributable to their high refractive index, which has been estimated both experimentally and through first-principles calculations. These physical characteristics are crucial for tailoring the optical features of Zn₂GeO₄, enabling the exploitation of this oxide in optoelectronic devices with bespoke properties.

VO2, a transparent oxide with insulator-to-metal transition (IMT) around 65°C, is an intriguing candidate for the realization of nanoscale devices with dynamic control of the optical properties. Planar multi-layer structures have the potential to function as self-activating devices; numerical analysis of their opto-thermal properties maximized the transmittance variation by a planar structure made of a 52 nm VO2 layer placed atop a 7 nm Ag layer. The preparation conditions have a significant impact on both the hysteresis cycle and the quality of VO2 thin films. Room temperature sputtering deposition and annealing in inert environment ensured improved IMT properties. XRD, SEM, and Raman spectroscopy were used to examine the shape and phase of the polycrystalline thin film. The presence of the Ag layer and the orientation of the sapphire substrate (c-cut versus a-cut) affect the quality of the VO2 insulator to metal transition. IMT is lower in presence of Ag, as assessed by resistance and transmittance measurements. Transient spectroscopy was also exploited to characterize the VO2 in all the tested configurations.

Tungsten trioxide (WO3), an indirect bandgap semiconductor (Eg ≈ 2.5–2.8 eV) with excellent chemical stability, is highly suitable for optoelectronic devices. This study involved the fabrication of WO3 thin films on high-resistance silicon substrates using DC sputtering. The dielectric properties of the films were investigated under various conditions, including as-deposited and annealed states at 200℃, 350℃, and 500℃. The feasibility of laser annealing was also evaluated. The surface morphology and microstructures of the films were characterized using XRD, SEM, and AFM. Additionally, THz-TDS measurements and CST simulations were used to assess the potential of WO3 films for applications in terahertz devices.

This study primarily investigates the laser doping and activation process of polycrystalline silicon thin-film transistors using femtosecond lasers, optimizing the laser activation parameters after polycrystalline silicon ion implantation. Leveraging the advantages of short femtosecond laser pulse duration and low thermal budget, the goal of rapid thermal activation of polycrystalline silicon material is achieved. THz-TDS is used for non-destructive measurement to calculate the conductivity of polycrystalline silicon applied at the transistor's gate terminal, which affects the overall transistor leakage current, switching speed, and electron transmission efficiency. This type of research can build upon the structure of current advanced process transistors (such as fin field-effect transistors, surround gates, and metal-oxide-semiconductor field-effect transistors) to further optimize the production method.

An optical memristor based on the miniaturization of Ag/BiFeO3/ITO resistive random-access memory (ReRAM) placed immediately above a lithium niobate optical waveguide is developed. The device's working principle depends on the light-matter interaction of the evanescent wave with the formation/rupture of the filaments inside the ReRAM. By alternately applying positive and negative biases, the binary memory states of ReRAM can be toggled accordingly. Due to the filament formation attributed to the electrochemical metallization (ECM) and the bipolar switching characteristics of the device, applying a positive bias switches the ReRAM to a low resistance state, while applying a negative bias returns the device to the high resistance state as a result of filament breakage. To enhance the coupling between the optical waveguide and the ReRAM, a miniaturized ReRAM is deliberately placed within the wet etched groove, which facilitates the light-matter interaction. It is found that the memory device inserted into the etched groove has an extinction ratio of about 10.9%, which is noticeably higher than that of the control sample with the ReRAM situated directly above the waveguide.

Within electron plasma model, a complex crystal of ABСD as a result of phase separation into four subsystems is considered. During the phase transition in the ABCD crystal, the valence bond breaking associated with the crystal lattice destruction. It is caused by the balance energies of the A-B-C-D and A-A, B-B, C-C, D-D bonds. The basic principle of the phase transition is caused by bonds being broken and dumbbell configurations of atoms are formed. The pairing of electrons is guided by a plasma mechanism. Free electrons couple to lower system energy. When molecules are formed from individual atoms by dumbbell configuration, energy is released. The energy balance is caused by a change in the distances between atoms A-A, B-B, C-C, D-D, with decreasing temperature. It is necessary that in the phase transition region the total square plasma energy of the formed quasi-molecules is significantly greater than the square plasma energy of the original crystal. This is the condition for the superconducting phase transition in the crystal ABCD: 2 ABCD = A2 + B2 + C2 + D2. During phase separation of a polyatomic crystal, a multigap energy system is formed. Experimental limitations make it d

Development of new crystal products is a continuing effort at SYNOPTICS. Commercially successful products require harnessing our capabilities in high temperature bulk crystal growth and understanding structure-property relationships including crystallographic, optical, thermal, magnetic, mechanical, and thermodynamic properties of materials to provide viable products for our customers. This talk will review our most recent development work in bulk single crystal growth for several different Yb3Ga5O12, or YbGG.

A polycrystalline PbS thin film grown on quartz substrates using the chemical bath deposition method was employed to develop detectors for mid-infrared wavelengths. The typical peak detectivity of the PbS photoconductive detector in this experiment ranges from 1.0 to 3.0x10^11 cm-Hz0.5/W, with a corresponding noise equivalent power from 2.0 to 6.5x10-12 W/Hz0.5. With aging, some PbS detectors exhibited instability, characterized by high noise signals, elevated dark resistance, and significantly reduced or absent detection signals. Failure analysis revealed internal packaging issues, resulting in performance degradation due to gas molecule generation, adsorption, and electric field-induced migration phenomenon. However, the damage to detector performance was found to be reversible; annealing could restore original functionality after de-lidding the affected parts. The precise pathways of field failure in PbS detectors remain unclear, necessitating investigation into the underlying mechanisms and root causes. DOEs with TO39-packaged PbS detectors and evaluation using non-destructive impedance spectroscopy were conducted to elucidate potential root causes of detector failure mechanism

Within electron plasma model, a complex crystal of the A n B m as a result of phase separation into (n + m) subsystems is considered. During the phase transition in the A n B m crystal, the valence bond breaking associated with the crystal lattice destruction. It is caused by the balance energies of the A-A n▬B-B m and A▬A n, B▬B m bonds. The basic principle of the phase transition is caused by bonds being broken and dumbbell configurations of atoms are formed. The pairing of electrons is guided by a plasma mechanism. Free electrons couple to lower system energy. When molecules are formed from individual atoms by dumbbell configuration, energy is released. The energy balance is caused by a change in the distances between atoms A-A-A, B-B-B, with decreasing temperature. It is necessary that in the phase transition region the total square plasma energy of the formed quasi-molecules is significantly greater than the square plasma energy of the original crystal. This is the condition for the superconducting phase transition in the crystal A n B m. 2●A n B m = n●A2 + m●B2. q (A n B m) = 2β●Φ^2 (A n B m) / (n●Φ^2(A) + m●Φ^2(B), where parameters are square Φ^2 (M, ρ, s, q, β), square Φ

In this study, the optimization of a photovoltaic solar cell was investigated using SCAPS-1D simulation, focusing on both inorganic oxide semiconductor ETL (Electron Transfer Layer) and HTL (Hole Transfer Layer) in conjunction with a perovskite absorption layer. Inorganic oxide semiconductor materials, which are less sensitive to oxygen and H2O, simplify the overall structure of the photovoltaic solar cell. The ETL materials examined were selected based on their n-type oxide semiconductor characteristics, with ZnO and TiO2 being representative due to their wide bandgap energy properties. For HTL materials, p-type oxide semiconductors like NiOx and Cu2O were chosen, with NiOx known for its excellent electron blocking capability and chemical stability. Perovskite materials were used for the absorption layer. The initial focus was on optimizing the thickness of the ETL, HTL, and PAL layers, followed by minimizing defect density and interface defects in the PAL layer. By optimizing these parameters, a significant improvement in Power Conversion Efficiency (PCE) compared to the initial efficiency was achieved.

Spectrally selective materials filter the electromagnetic spectrum in terms of their optical properties, such as absorption, reflection, transmission, etc. Transition metal oxides like Sn-doped In2O3 (ITO) can be used for spectral selectivity due to their tunable plasmonic properties and chemical and thermal stability. Here, we fabricate a hybrid thin film stack of ITO and a semiconducting oxide CuCo2O4 (CCO) by co-sputtering deposition on a stainless steel substrate to produce spectrally selective reflectors. A reflectance of 90% was achieved in the visible range of the solar spectrum by tuning the ratio of the two oxides. This high reflectance in the visible range makes it a suitable candidate for optical solar reflectors, which play a crucial role in maintaining the optimum performance of satellites and space crafts by minimizing the solar energy input and emitting thermal radiation.

In the current work, TiO2 thin film is deposited by using RF sputtering technique on Si substrate, and W ultra-thin layer is deposited and subsequently annealed at high temperature to diffuse in TiO2. The change in structural, morphological, and superhydrophilic properties of TiO₂ film after W diffusion are investigated by using Grazing Incidence X-ray Diffraction (GI-XRD), Atomic Force Microscopy (AFM), Field Emission Gun Scanning Electron Microscopy (FEG-SEM), and static contact angle measurements. All the results suggest that W incorporation and subsequent annealing significantly enhance the surface roughness and surface wettability parameters of TiO2 film, which can be a potential candidate for microfluidic on-chip device applications.

In this study, we designed an optimal bandpass filter for aviation lighting systems, employing SiO2 and Nb2O5 materials, using the Essential Macleod program to ensure safe aircraft operations. The multilayer thin films were optimized for maximal transmittance, and the impact of varying argon flows during RF sputtering was investigated. Our results indicate that transmittance peaks at 50 sccm and decreases at higher flows, such as 100 sccm. Comprehensive characterization techniques including EDXRF, SEM, XRD, and XPS were utilized to assess the elemental composition, microstructure, and crystalline structure of the films, as well as to determine the oxidation states of niobium oxide. This study underscores the significance of argon flow control in the fabrication of efficient niobium oxide multilayer thin films for improved aviation lighting.

In this work, a numerical analysis of multilayered thermochromic surfaces composed of dielectrics and the transition metal oxide VO2 was carried out using the finite element method. The transmittance of the multilayered device has been analyzed by changing the incident angle and geometric parameters. It has been demonstrated that the optical properties of the analyzed structure changes with the temperature achieving a satisfactory performance and the solar light modulation occurs at the near infrared wavelength region of the solar spectrum. In this way they become suitable for smart windows applications

Since shallow p-type dopants are not available for Ga2O3, bipolar homojunction Ga2O3 devices are not possible, so the vast majority of the reported Ga2O3 devices have employed unipolar device architectures: i.e. Schottky barrier devices or lateral, isotype field-effect transistors (FETs). One potential answer to this limitation is to integrate n-type Ga2O3 with p-type NiO so as to design novel vertical p–n heterojunction devices (e.g. power diodes and/or photodetectors). With a bandgap of 3.7 eV, NiO is the complementary p-type oxide of choice through its’ robustness and flexible dopability. The minority carrier nature of these devices should allow lower on-resistances and, therefore, give better on-state performance and an improved range of functionality. The vertical architecture will also bring reductions in current crowding and a smaller footprint (which directly increases circuit integration density and reduces cost per device). In this work, we report the heteroepitaxy of p-Mg)NiO on n-(Al)Ga2O3 using pulsed laser deposition. This novel ternary approach gives better band matching between the p and n layers than the more conventional binary (p-NiO/n-Ga2O3) architecture.