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.

Gallium oxide (Ga2O3) microwires, characterized by a high surface-to-volume ratio and wide bandgap, enhance the responsivity (R-lambda) of UVC photodetectors beyond conventional planar Ga2O3 designs. This study reports the microfabrication and characterization of Schottky photodiodes (SPDs) based on single Sn-doped beta-Ga2O3 microwires. Platinum (Pt) and single-layer graphene (SLG) serve as Schottky contacts, combined with complementary metal-oxide-semiconductor-compatible ohmic contacts (Ti/Al98.5Si1.0Cu0.5/TiW). Nano X-ray fluorescence and electrical analysis reveals a Sn doping range of (4 x 10(18)-1.24 x 10(19) cm(-3)) and a significant density of surface states. Under UVC illumination, these surface states dynamically modulate the Schottky barrier height through the trapping and detrapping of photogenerated holes at the Schottky junction, leading to high internal gain and fast decay without persistent photoconductivity. The Pt/SLG-based SPDs achieves R-lambda up to approximate to 713 A W-1 at -3 V and self-powered operation with R-lambda approximate to 1.92 A W-1 at 0 V. Additionally, the device exhibits self-powered photoresponse under 30.5 keV X-ray irradiation. While Pt-based SPDs show long rise times due to delayed hole transit, SLG-based SPDs demonstrate faster response (<150 ms) enabled by SLG's high UVC transparency. These results highlight the potential of microwire-based Ga2O3 SPDs for fast, high-gain, self-powered photodetection.

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.

Integration of semiconductor nanowires is critical for developing scalable and versatile nanodevices, but challenges remain in tailoring optical emission, forming reliable p-n junctions, and ensuring consistent nanoscale interconnection. Here, we investigate Ga2O3/SnO2 multiwire architectures using synchrotron-based X-ray fluorescence (XRF), X-ray excited optical luminescence (XEOL), X-ray absorption near-edge spectroscopy (XANES), and first-principles simulations. We map dopant distribution, analyze nanoscale optical responses, and determine dopant atomic coordination. The central wire is predominantly Sn-doped Ga2O3, while crossed wires are Ga-doped SnO2. XEOL maps reveal a pronounced enhancement of the 3.5 eV ultraviolet emission in Ga2O3 at the junctions, enabling controlled optical modulation. XANES and ab initio calculations confirm that Sn and Ga dopants preferentially occupy octahedral sites, introducing donor levels in Ga2O3 and acceptor levels in SnO2. This research significantly advances our understanding of dopant effects in complex semiconductor nanowire systems, paving the way for controlled optical emissions in Ga2O3/SnO2 multiwire architectures.

Gallium oxide (Ga 2 O 3 ) microwires, characterized by a high surface‐to‐volume ratio and wide bandgap, enhance the responsivity (R λ ) of UVC photodetectors beyond conventional planar Ga 2 O 3 designs. This study reports the microfabrication and characterization of Schottky photodiodes (SPDs) based on single Sn‐doped β–Ga 2 O 3 microwires. Platinum (Pt) and single‐layer graphene (SLG) serve as Schottky contacts, combined with complementary metal–oxide–semiconductor‐compatible ohmic contacts (Ti/Al 98.5 Si 1.0 Cu 0.5 /TiW). Nano X‐ray fluorescence and electrical analysis reveals a Sn doping range of (4 × 10 18 –1.24 × 10 19 cm −3 ) and a significant density of surface states. Under UVC illumination, these surface states dynamically modulate the Schottky barrier height through the trapping and detrapping of photogenerated holes at the Schottky junction, leading to high internal gain and fast decay without persistent photoconductivity. The Pt/SLG‐based SPDs achieves R λ up to ≈713 A W −1 at −3 V and self‐powered operation with R λ ≈1.92 A W −1 at 0 V. Additionally, the device exhibits self‐powered photoresponse under 30.5 keV X‐ray irradiation. While Pt‐based SPDs show long rise times due to delayed hole transit, SLG‐based SPDs demonstrate faster response (<150 ms) enabled by SLG's high UVC transparency. These results highlight the potential of microwire‐based Ga 2 O 3 SPDs for fast, high‐gain, self‐powered photodetection.

Monoclinic beta-Ga2O3 is a key representative material of the ultrawide-bandgap semiconductor family. The distinct atomic arrangement in beta-Ga2O3 introduces two coordination environments for Ga ions, resulting in pronounced anisotropy in its optical, electronic, and thermal properties. In this study, a synchrotron nanoprobe to investigate the anisotropic optical properties of well-oriented (100) beta-Ga2O3 nanomembranes with a thickness of 200 nm, produced through mechanical exfoliation, is employed. Polarization-resolved X-ray excited optical luminescence (XEOL) measurements reveal a strong ultraviolet (UV) emission band at 3.4 eV, which is strongly polarized along the c-axis. Additionally, XEOL data show blue (2.9 eV) and deep-UV (3.8 eV) emissions. Notably, the deep-UV band, rarely reported in conventional photoluminescence studies, is attributed to the presence of Ga vacancies, as supported by first-principles calculations. Polarization-dependent X-ray absorption near-edge structure (XANES) spectroscopy allows one to probe the distinct symmetries of the b and c crystallographic planes. Furthermore, by combining XANES and XEOL, this study investigates the site-specific contributions of Ga ions to the luminescence process. These findings highlight the potential of beta-Ga2O3 nanomembranes as a robust material platform for developing polarization-sensitive devices. The pronounced anisotropy of beta-Ga2O3 causes orientation-dependent optoelectronic properties, making it a highly promising candidate for a wide range of advanced applications.

Monoclinic β ‐ is a key representative material of the ultrawide‐bandgap semiconductor family. The distinct atomic arrangement in β ‐ introduces two coordination environments for Ga ions, resulting in pronounced anisotropy in its optical, electronic, and thermal properties. In this study, a synchrotron nanoprobe to investigate the anisotropic optical properties of well‐oriented β ‐ nanomembranes with a thickness of nm, produced through mechanical exfoliation, is employed. Polarization‐resolved X‐ray excited optical luminescence (XEOL) measurements reveal a strong ultraviolet (UV) emission band at eV, which is strongly polarized along the c ‐axis. Additionally, XEOL data show blue ( eV) and deep‐UV ( eV) emissions. Notably, the deep‐UV band, rarely reported in conventional photoluminescence studies, is attributed to the presence of Ga vacancies, as supported by first‐principles calculations. Polarization‐dependent X‐ray absorption near‐edge structure (XANES) spectroscopy allows one to probe the distinct symmetries of the b and c crystallographic planes. Furthermore, by combining XANES and XEOL, this study investigates the site‐specific contributions of Ga ions to the luminescence process. These findings highlight the potential of β ‐ nanomembranes as a robust material platform for developing polarization‐sensitive devices. The pronounced anisotropy of β ‐ causes orientation‐dependent optoelectronic properties, making it a highly promising candidate for a wide range of advanced applications.

Gallium oxide is being widely studied, mainly for high-power electronics applications. It is a very promising material for photonic/optoelectronic applications, such as solar-blind UV detectors and light emitters. In this work, we study the temperature-dependent behavior of the optical properties of microcavities based on luminescent beta-Ga2O3:Cr nanowires that emit an intense red-infrared band. Two distributed Bragg reflectors (DBR), when milled with a focused ion beam (FIB) and separated some microns, result in an optical microcavity that confines the light longitudinally. Both chromium R lines and Fabry- Perot spectral resonances (FPSR) are observed to shift as temperature varies, making these DBRs a valuable thermometer in a wide temperature range, due to both luminescent and interferometric transducing mechanisms. The underlying origin of this shift, in the case of the FPSR, is mainly the variation of the refractive index with temperature and the thermal expansion of the cavity. Ellipsometry studies carried out at different temperatures in bulk beta-Ga2O3 yielded the dispersion relations for the three main crystal axes, i.e. its temperature-dependent anisotropic refractive index. These results were implemented in finite-difference time-domain (FDTD) simulations to compare the expected spectral shift of the FPSR in the modelled system with the experimental shift in the DBR cavities, as obtained experimentally by micro-photoluminescence. The results from these two approximations, and a third one based on solving the relevant analytical equations, are compared.

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.

An accurate knowledge of the optical properties of β-Ga2O3 is key to developing the full potential of this oxide for photonics applications. In particular, the dependence of these properties on temperature is still being studied. Optical micro- and nanocavities are promising for a wide range of applications. They can be created within microwires and nanowires via distributed Bragg reflectors (DBR), i.e., periodic patterns of the refractive index in dielectric materials, acting as tunable mirrors. In this work, the effect of temperature on the anisotropic refractive index of β-Ga2O3 n(λ,T) was analyzed with ellipsometry in a bulk crystal, and temperature-dependent dispersion relations were obtained, with them being fitted to Sellmeier formalism in the visible range. Micro-photoluminescence (μ-PL) spectroscopy of microcavities that developed within Cr-doped β-Ga2O3 nanowires shows the characteristic thermal shift of red–infrared Fabry–Perot optical resonances when excited with different laser powers. The origin of this shift is mainly related to the variation in the temperature of the refractive index. A comparison of these two experimental results was performed by finite-difference time-domain (FDTD) simulations, considering the exact morphology of the wires and the temperature-dependent, anisotropic refractive index. The shifts caused by temperature variations observed by μ-PL are similar, though slightly larger than those obtained with FDTD when implementing the n(λ,T) obtained with ellipsometry. The thermo-optic coefficient was calculated.

In this work, we present our recent results on the applicability of optical microcavities based on Cr doped Ga2O3 wires to operate as a nanothermometer in a wide temperature range (at least from 150 up to 550 K) and achieving a temperature precision of around 1 K. To this purpose, DBR (distributed Bragg reflectors) have been used to enhance the reflectivity at the lateral ends of the wires. The transduction mechanism encompasses both the luminescence features of the characteristic R-lines of Cr3+ ions in this host as well as the interferometric effects of the Fabry-Perot resonances within the cavity.

Remote temperature sensing at the micro- and nanoscale is key in fields such as photonics, electronics, energy, or biomedicine, with optical properties being one of the most used transducing mechanisms for such sensors. Ga2O3 presents very high chemical and thermal stability, as well as high radiation resistance, becoming of great interest to be used under extreme conditions, for example, electrical and/or optical high-power devices and harsh environments. In this work, a luminescent and interferometric thermometer is proposed based on Fabry-Perot (FP) optical microcavities built on Cr-doped Ga2O3 nanowires. It combines the optical features of the Cr3+-related luminescence, greatly sensitive to temperature, and spatial confinement of light, which results in strong FP resonances within the Cr3+ broad band. While the chromium-related R lines energy shifts are adequate for low-temperature sensing, FP resonances extend the sensing range to high temperatures with excellent sensitivity. This thermometry system achieves micron-range spatial resolution, temperature precision of around 1 K, and a wide operational range, demonstrating to work at least in the 150-550 K temperature range. Besides, the temperature-dependent anisotropic refractive index and thermo-optic coefficient of this oxide have been further characterized by comparison to experimental, analytical, and finite-difference time-domain simulation results.

Ga2O3 micro- and nanowires-based optical microcavities have been obtained by patterning pairs of distributed Bragg reflectors (DBRs) with a focused ion beam (FIB) microscope. DBRs result in widely tunable high reflectivity bands. The microcavities have been designed and optimized with the aid of simulations and optically characterized by micro-photoluminescence. Tunable strong modulations are confirmed in the NUV-blue as well as in the red-NIR ranges for unintentionally doped and chromium doped wires, respectively. Experimental, analytical and simulations results will be compared and some possible applications of these cavities will be assessed.