Spectroscopic ellipsometry is applied to determine the dielectric functions (DFs) epsilon and absorption coefficients alpha of orthorhombic multi-domain kappa-(InxGa1-x)(2)O-3 from 0.5 to 6.5 eV. The investigated (001)-oriented thin films with 0 <= x <= 0.175 were grown by molecular beam epitaxy on c-plane sapphire substrates. By an anisotropic analysis, we demonstrate that the in-plane DF epsilon(perpendicular to) can be decomposed into their components of the orthorhombic dielectric tensor epsilon(xx) and epsilon(yy). The resulting three characteristic transition energies (E-x, E-y,E-1, E-y,E-2) were found to shift to lower energies with increasing In-content. The results are compared to previous studies, and the origin of apparent agreements is critically examined.

Abstract Both homogeneous and inhomogeneous alloying significantly extend the spectrum of available materials and properties. In this study, (Mo,W)S2 monolayer alloys in the full composition range were grown by a modified atomic layer deposition process. The mixture of Mo and W in the crystal lattice can be controlled by the pulsed precursor schema and either a homogeneous distribution or nanostructures such as line structures and elongated islands were realized. The distribution of metal atoms is visualized by transmission electron microscopy. Raman spectroscopy is used to determine the composition x in Mo x W1−x S2 by the shift of the $${{\rm{A}}}_{1}^{{\prime} }$$ A 1 ′ mode. The optical properties are analyzed by photoluminescence spectroscopy, resulting in the determination of bowing parameters for the A and B excitonic emission. Deviations from the expected bowing are explained by a change of the energy landscape due to the presence of nanostructures. Besides the composition control, this sub-atomic layer deposition approach could pave the way to the intended formation of quantum dot and wire structures in 2D transition metal dichalcogenide heterostructures.

The pursuit of accurate, reproducible and non-invasive methods for measuring temperature in a wide range and in harsh environments for, e.g. space applications, is an enduring challenge. Luminescent thermometers have the potential of meeting these requirements, but care must be taken in choosing the appropriate materials and designs, in making a reliable and accurate calibration, and in testing their performance under different conditions.

The use of Ru(0001) films as substrates on α-Al2O3(0001) substrates for radio-frequency magnetron sputter deposition of gallium oxide (Ga2O3) thin films is investigated regarding its dependence on the Ru film roughness; gallium oxide deposition temperature, which ranges from room temperature (RT) to 600 °C; and post-annealing up to 900 °C. The films have been characterized using various techniques. Raman spectroscopy confirms the deposition of β-Ga2O3 at deposition temperatures above 200 °C. The surface morphology was studied by atomic force microscopy (AFM), revealing that the roughness of the Ga2O3 surface strongly depends on deposition temperature, with a maximum at intermediate temperatures and a significant decrease between 400 and 600 °C. This can be explained by enhanced surface diffusion at higher temperatures. X-ray diffraction reveals poor crystallinity of all as-deposited films. Upon post-annealing, only those RT-deposited Ga2O3 films substantially gained crystallinity, which were deposited on the rougher Ru films. The cross-sectional scanning transmission electron microscopy provides insights into the grain structure, clearly disclosing the poly-crystallinity of the post-annealed Ga2O3 films with β-Ga2O3 being the predominant crystal structure. The β-Ga2O3 grain sizes are ∼10 nm for RT-deposited films and ∼40 nm for Ga2O3 deposited at 600 °C. However, γ-Ga2O3 grains of 10–20 nm in size are also found. The initial deposition of Ga2O3 on the Ru(0001) surface was studied in detail using AFM and low-energy electron diffraction (LEED). AFM shows that Ga2O3 islands nucleate predominantly at Ru step edges. LEED analysis confirms the nucleation of compressively strained epitaxial β-Ga2O3 and reveals the appearance of a (3 × 3)-reconstruction.

Annealing can improve the structural quality of transition metal dichalcogenides grown by chemical vapor deposition, atomic layer deposition, and molecular beam epitaxy. However, decomposition and desorption of chalcogens from the layers limit process parameters, such as temperature and duration. In this study, it will be shown that using confined-space annealing, realized by close contact face-to-face sample arrangement, it is possible to extend this parameter range to higher temperatures and longer annealing times. The extended temperature range leads to significantly improved optical and structural quality of MoS2 and WS2 grown by atomic layer deposition. At temperatures above 1000 °C, a rearrangement of material leads to the formation of hexagonal structures, which are not present in as-grown samples. Confined-space annealing can also be used for conversion of MoS2 layers into ternary Mo(S,Se)2 and even binary MoSe2 using a proper Se containing reservoir. The confined space allows escape of organic precursor residuals but strongly reduces outdiffusion of S, resulting in an improved confinement of S in the interspace between the samples. Hence, no additional sulfur precursor is needed for annealing, leading to a simple, low-level technology process, which is non-toxic, environment friendly, and resource-efficient.

In this publication, we study strain in kappa-(In Ga-x(1-x))(2)O-3/kappa-Ga2O3/Al2O3 heterostructures grown by plasma-assisted molecular beam epitaxy using a combination of density functional theory (DFT), x-ray diffraction (XRD), nanobeam electron diffraction (NBED), and energy-dispersive x-ray spectroscopy (EDX) for layers with low (2%) and high (18%) In concentration. Lattice parameters and elastic moduli were computed using the density functional theory formalism. The strain between kappa-(In Ga-x(1-x))(2)O-3 and kappa-Ga2O3 as well as Al2O3 and kappa-Ga2O3 was measured in two different zone axes by NBED. Due to the quasi-hexagonality of the kappa-phase, this yields an approximation for the full strain tensor. The obtained NBED results are supported by reciprocal space maps from XRD measurements. The relaxation of the layers was analyzed by comparing the measured strain data with those computed from the DFT lattice parameters and elastic moduli using the element concentrations measured by EDX, thus allowing for direct validation of the theoretical calculations by experimental data. This analysis results in satisfactory agreement and reveals that the layer with a low In concentration is fully strained (measured epsilon(zz) = 0.0037, computed epsilon(zz) = 0.0039), whereas the layer with high In concentration is fully relaxed (measured epsilon(zz) = 0.0222, computed epsilon(zz) = 0.0219). NBED, XRD, and theoretical predictions agree within their respective error margins for the strain between kappa-(InxGa1-x)(2)O-3 and kappa-Ga2O3. Overall, the absolute value of the strain between kappa-Ga2O3 and Al2O3 was measured to be smaller by NBED (measured epsilon(zz) = -0.0628) and XRD than it was computed from literature lattice parameters (epsilon(zz) = -0.0716). (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) license (https://creativecommons.org/licenses/by-nc/4.0/)

Gadolinium (Gd) is a promising optically active lanthanide for UV emission. In this work, the optical emission properties of Gd-implanted monoclinic gallium oxide (beta-Ga2O3) thin films are investigated. Second phase formation (gamma-Ga2O3) is observed due to implantation-induced damage of the beta-Ga2O3 lattice. Annealing the implanted films results in various beta-Ga2O3 grain orientations. The relationship between the crystalline nature and the optical properties of the beta-Ga2O3:Gd3+ films is studied. Optical activation occurs after annealing 700 degrees C, revealing a photoluminescence (PL) band at 3.92 eV. This emission is attributed to the 6P7/2 , 8S7/2 transition of Gd3+ in beta-Ga2O3. Its four constituent emission components at 3.9118 eV, 3.9153 eV, 3.9221 eV and 3.9348 eV, due to the ion's 6P7/2 Stark splitting in the beta-Ga2O3 crystal field, are investigated. The transition energies are independent of annealing temperature and film growth method, highlighting the insensitivity the 4f7orbital to minor changes in the monoclinic crystal environment.

The interest in ultrawide bandgap semiconductor gallium oxide, especially its monoclinic phase (beta-Ga2O3), has been increasing significantly since the first transistors were created with this oxide, which demonstrated a critical field strength greater than that of SiC (> 2.5 MV/cm). Furthermore, its ultrawide band gap has led to extensive exploration of its photonic and optoelectronic properties over more than six decades, with numerous studies devoted to applications such as UV solar-blind detectors and luminescent devices. Nanostructured semiconductors are key to developing architectures that expand the possibilities of electronic and photonic devices. Nanostructured beta-Ga2O3 has not yet been extensively studied compared to other semiconductors, but the work conducted so far shows great potential. In this study, we present our recent investigation of two approaches that leverage several physical properties - such as temperature-dependent luminescence, refractive index or crystal anisotropy - of beta-Ga2O3 nanomembranes (NMs) and microwires (mu Ws) for photonic applications. We have studied luminescence-based microthermometers fabricated by defining optical microcavities with distributed Bragg reflectors (DBR) created using atomic-layer-deposited (ALD) multilayers that encapsulate beta-Ga2O3:Cr mu Ws. This approach improves the robustness and thermal stability of the thermometric devices that we had previously developed in a wide temperature range. An optical assessment of these sensors, comparing different ALD designs, is shown. Additionally, we have studied mechanically exfoliated beta-Ga2O3 NMs with thicknesses of a few hundred nanometers, evaluated using optical interferometry and synchrotron-based X-ray excited optical luminescence (XEOL). A strong ultraviolet-blue luminescence emission band was observed.

Luminescence Thermometry Atomic layer deposition (ALD) is a powerful technique for achieving smooth and robust distributed Bragg reflector (DBR) coatings on microwire ends. In article number 2400881, Manuel Alonso-Orts and co-workers create optical microcavities with ALD-coated, chromium-doped gallium oxide (Ga2O3:Cr) microwires and demonstrate their use for wide-range temperature sensors with high stability, precision and accuracy, monitoring the temperature-induced spectral shifts of the resonant peaks.

We investigate the composition of α-phase intermediate layers at epitaxial Ga2O3/Al2O3 interfaces using high angle annular dark field scanning transmission electron microscopy. Their presence is considered a general phenomenon as they are observed independent of the growth technique [Schewski et al., Appl. Phys. Exp. 8, 011101]. Samples were grown by plasma assisted molecular beam epitaxy using different growth conditions. Almost independent of these, the quantitative evaluation of the measured intensities gave Ga concentrations of ∼25%. We show that the previously published model, based on a pure α-Ga2O3 interlayer, fails if it is adapted to the measured composition. Density functional theory (DFT) computations were used to overcome the approximations made in this model and suggest that a stabilization of the layer is possible due to the low Ga concentration (≤35%) at which the α-phase is the most stable. Our surface model computations suggest an exchange of Ga atoms at the surface with Al atoms from the underlying substrate as a possible formation mechanism.

The growth of α-Ga2O3 and α-(InxGa1−x)2O3 on m-plane α-Al2O3(101̄0) by molecular beam epitaxy (MBE) and metal-oxide-catalyzed epitaxy (MOCATAXY) is investigated. By systematically exploring the parameter space accessed by MBE and MOCATAXY, phase-pure α-Ga2O3(101̄0) and α-(InxGa1−x)2O3(101̄0) thin films are realized. The presence of In on the α-Ga2O3 growth surface remarkably expands its growth window far into the metal-rich flux regime and to higher growth temperatures. With increasing O-to-Ga flux ratio (RO), In incorporates into α-(InxGa1−x)2O3 up to x ≤ 0.08. Upon a critical thickness, β-(InxGa1−x)2O3 nucleates and, subsequently, heteroepitaxially grows on top of α-(InxGa1−x)2O3 facets. Metal-rich MOCATAXY growth conditions, where α-Ga2O3 would not conventionally stabilize, lead to single-crystalline α-Ga2O3 with negligible In incorporation and improved surface morphology. Higher TTC further results in single-crystalline α-Ga2O3 with well-defined terraces and step edges at their surfaces. For RO ≤ 0.53, In acts as a surfactant on the α-Ga2O3 growth surface by favoring step edges, while for RO ≥ 0.8, In incorporates and leads to a-plane α-(InxGa1−x)2O3 faceting and the subsequent (2̄01) β-(InxGa1−x)2O3 growth on top. Thin film analysis by scanning transmission electron microscopy reveals highly crystalline α-Ga2O3 layers and interfaces. We provide a phase diagram to guide the MBE and MOCATAXY growth of single-crystalline α-Ga2O3 on α-Al2O3(101̄0).

To meet temperature goals that limit warming to well below 2 °C requires the removal of hundreds of billions of tonnes of CO2 from the atmosphere over the course of this century. Effective Carbon Dioxide Removal (CDR) methodologies will be required to reduce net emissions in the near term, counterbalance residual CO2 emissions to achieve net-zero in the medium term, and contribute to net-negative emissions in the longer term – all of this in a sustainable and safe manner. This paper summarizes the research objectives and selected initial results of a collaborative project to assess CO2 storage in the upper ocean crust south of Iceland.The AIMS3 project (www.aims3.cdrmare.de) will deliver new insights, monitoring tools and feasibility assessments for CO2 storage in young, reactive basalts with little sedimentary cover. Along the flank of the Mid-Atlantic Ridge, we have done geophysical surveys and drilled a transect of boreholes in order to identify fluid migration in the upper ocean crust. Both in situ heat flow and geochemical signatures provide irrefutable evidence for such transport, which will help distributing injected CO2 in future experiments.In parallel, our project also has mineralization experiments to assess optimal conditions for injection dissolved, liquid, or supercritical CO2), numerical modelling for upscaling our results from seagoing work, and development of cost-effective sensors and smart robotic landers for long-term monitoring of the vicinity of the boreholes. We outline the rationale of AIMS3, provide an overview of the activities, and highlight some of the expedition results, with the goal to stimulate communication and collaboration.

MoS 2 and WS 2 mono- and multilayers were grown on SiO 2 /Si substrates. Growth by atomic layer deposition (ALD) at fast growth rates is compared to sub-ALD, which is a slow growth rate process with only partial precursor surface coverage per cycle. A Raman spectroscopic analysis of the intensity and frequency difference of the modes reveals different stages of growth from partial to full surface layer coverage followed by layer-by-layer formation. The initial layer thickness and structural quality strongly depend on the growth rate and monolayers only form using sub-ALD. Optical activity is demonstrated by photoluminescence (PL) characterization which shows typical excitonic emission from MoS 2 and WS 2 monolayers. A chemical analysis confirming the stoichiometry of MoS 2 is performed by x-ray photoelectron spectroscopy. The surface morphology of layers grown with different growth rates is studied by atomic force microscopy. Plan-view transmission electron microscopy analysis of MoS 2 directly grown on freestanding graphene reveals the local crystalline quality of the layers, in agreement with Raman and PL results.

The anion-selective transport through subnanoporous liquid-crystalline (LC) water treatment membranes was quantitatively detected by the deposition and electrochemical analysis of the LC membrane on the GaN electrode. The time course of the capacitance and Warburg resistance of the LC membrane suggest that the interaction of the LC membrane with monovalent Cl- ions is distinctly different from that with SO42- ions. A continuous decay in capacitance suggests the condensation of Cl- ions in subnanopores, whereas the interaction between SO42- ions and the inner wall of subnanopores is much weaker. The chronoamperometry data further suggest that SO42- ions are transported through subnanoporous channels 10 times faster than Cl- ions. These results, together with the previous X-ray emission spectroscopy, suggest that SO42- ions, which possess similar hydrogen-bonded structures to the hydrogen-bonded networks inside the subnanopores, can exchange the associated water molecules and hop along the network of water molecules, but Cl- ions bind and accumulate inside subnanopores. The well-controlled supramolecular self-assembly of LC building blocks opens a large potential toward the fine adjustment of hydrogen-bonding networks in nanospace providing materials new functions, which cannot be realized by bulk water.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Sushma Raghuvansy, Jon P. McCandless, Marco Schowalter, Alexander Karg, Manuel Alonso-Orts, Martin S. Williams, Christian Tessarek, Stephan Figge, Kazuki Nomoto, Huili Grace Xing, Darrell G. Schlom, Andreas Rosenauer, Debdeep Jena, Martin Eickhoff, Patrick Vogt; Erratum: “Growth of β-Ga2O3 and ε/κ-Ga2O3 on AlN(0001) by molecular-beam epitaxy” [APL Mater. 11, 111113 (2023)]. APL Mater. 1 January 2024; 12 (1): 019902. https://doi.org/10.1063/5.0192370 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAPL Materials Search Advanced Search |Citation Search

The high spatial resolution and contactless optical readout capabilities of luminescence thermometry offer significant advantages in numerous fields, including biomedicine, space exploration and optoelectronics. In addition, robust, reproducible, and accurate temperature measurements are essential in these areas. The ultra-wide band gap semiconductor material Ga2O3 is a suitable host for optical sensing in harsh environments due to its high stability. In this work, the thermometric operation of Ga2O3:Cr-based microcavities are evaluated. They are designed as follows: Ga2O3:Cr microwires are encapsulated in multilayers fabricated by atomic layer deposition (ALD), which act as both Bragg reflectors and protective layers for the thermometric sensor. Prior to the ALD encapsulation step, focused ion beam carved trenches at the microwire ends are necessary to accommodate the multilayer coating. The structural and optical properties of the devices are assessed experimentally, analytically and by simulations. The developed microthermometers can be easily calibrated using a cubic polynomial for the temperature-dependent resonant peak position shift. A better than 0.5 degrees C temperature resolution and accuracy for temperatures above -80 degrees C is demonstrated. Additionally, the devices show robustness against excitation laser densities of at least 34 W mm(-2), can operate at temperatures up to 600 degrees C and remain functional in liquids.