This paper presents a detailed investigation of the structural, optical, and electronic properties of (AlxGa1-x)(2)O-3 thin films used as active layers in solar-blind ultraviolet C (UVC) photodetectors onboard the INSPIRE-Sat 7 nanosatellite. Engineered via alloying Ga2O3 with Al2O3, these deep UV detectors exhibit a Gaussian spectral response centered at 215 nm with a full width at half maximum of 40 nm, enabling selective monitoring of solar irradiance in the Herzberg continuum (200-242 nm). The (AlxGa1-x)(2)O-3 films are grown on c-plane sapphire substrates using pulsed laser deposition. Structural and optical analysis reveals that the engineered film exhibits the monoclinic beta phase with a dominant (-201) orientation and the Al fraction of 0.58, inducing a 0.93 eV bandgap widening from 4.89 eV (Ga2O3) to 5.82 eV. Complementary photoemission and X-ray absorption spectroscopy studies reveal a 0.17 eV valence band downshift and a 0.74 eV conduction band uplift due to the alloying at x = 0.58. Cathodoluminescence spectroscopy identifies broad emission peaks at 3.55, 3.34, and 3.10 eV, attributed to self-trapped hole states localized at O sites adjacent to Al and Ga, respectively, and donor-acceptor pair recombination, consistent with the band structure modifications induced by Al2O3 alloying. These findings provide critical insights into beta-(Al0.58Ga0.42)(2)O-3 photodetector performance for Herzberg continuum detection in space applications.

p-NiO/n-Ga2O3 heterostructures were formed on single crystal (-201) beta (monoclinic) Ga2O3 and r-sapphire substrates by Pulsed Laser Deposition. Ring mesa layer stacks were created using a shadow mask during growth. X-Ray diffraction studies were consistent with the formation of (111) oriented fcc NiO on the bulk Ga2O3 and randomly oriented fcc NiO on (102) oriented beta-Ga2O3 /r- sapphire. RT optical transmission studies revealed bandgap energy values of similar to 3.65 eV and similar to 5.28 eV for the NiO and Ga2O3 on r-sapphire. p-n junction devices were formed by depositing gold contacts on the layer stacks using shadow masks in a thermal evaporator. Both heterojunctions showed rectifying I/V characteristics. On bulk Ga2O, the junction showed a current density over 16mA/cm(2) at +20V forward bias and a reverse bias leakage current over 3 orders of magnitude lower at -20V (1 pA). On Ga2O3/r-sapphire the forward bias current density at +15V was about an order of magnitude lower than for the p-NiO/bulk n-Ga2O3 heterojunction while the reverse bias leakage current at -15V (similar to 20 pA) was an order of magnitude higher. Hence the NiO/bulk Ga2O3 junction was more rectifying. Upon illumination with a Xenon lamp a distinct increase in current was observed for the IV curves in both devices (four orders of magnitude for -15V reverse bias in the case of the p- NiO/bulk n-Ga2O3 heterojunction). The p-NiO/n-Ga2O3/r-sapphire junction gave a spectral responsivity with a FWHM value of 80nm and two distinct response peaks (with maxima at 230 and 270nm) which were attributed to carriers being photogenerated in the Ga2O3 underlayer. For both devices time response studies showed a 10%/90% rise and fall of the photo generated current upon shutter open and closing which was relatively abrupt (millisecond range), and there was no evidence of significant persistent photoconductivity.

ZnO layers were grown on (100) and (111) oriented YSZ substrates by pulsed laser deposition (PLD). X-ray diffraction studies revealed growth of wurtzite ZnO with strong preferential (0002) orientation. The ZnO layer on YSZ (111) showed distinct Pendellosung fringes and a more pronounced c-axis orientation (rocking curve of 0.08 degrees). Atomic force microscopy revealed RMS roughnesses of 0.7 and 2.2nm for the ZnO on the YSZ (111) and YSZ (100), respectively. YSZ was then grown on the ZnO buffered YSZ (111) substrate by PLD. XRD revealed that the YSZ overlayer grew with a strong preferential (111) orientation. The YSZ/ZnO/YSZ (111) top surface was temporary bonded to an Apiezon wax carrier and the sample was immersed in 0.1M HCl so as to preferentially etch/dissolve away the ZnO underlayer and release the YSZ from the substrate. XRD revealed only the characteristic (111) peak of YSZ after lift-off and thus confirmed both the dissolution of the ZnO and the preservation of the crystallographic integrity of the YSZ on the wax carrier. Optical and Atomic Force Microscopy revealed some buckling, roughening and cracking of the lifted YSZ, however. XRD suggested that this may have been due to compressive epitaxial strain release.

We present measurement protocols of performances, test and calibrations of new compact solid-state photodetectors based on β-Ga2O3 oxides, and optimized for the UVC. They present reduced dark currents, permitting room temperature operation suppressing need for a cooling system (mass and power savings) and avoiding cold surfaces that traps environmental contamination. Detectors' response peak around 215-220 nm with a bandpass of 30 nm, allowing to observe the UVC wavelength band responsible of ozone creation in the stratosphere (Herzberg continuum, 200-242 nm) and to achieve solar-blindness for wavelengths above 250 nm. Other key assets of β-Ga2O3 detectors are their radiation hard properties (longer lifetime), and possible sensitivity (several hundreds mA/W at -5 V) that allows operation at lower voltages (reduced power), a key asset for Space applications. These detectors, evaluated, tested and calibrated, will be integrated on the INSPIRE-7 nanosatellite to be launched in 2023.

Irradiation with high-energy electrons (HEE) at cryogenic temperatures is a subtle tool for shaping matter. Unlike irradiation with heavy particles, e.g. protons, neutrons, or ions, HEE irradiation produces very low local damage generating exclusively point lattice defects. In the interaction process, the primary high-energy electron transfers a minute quantity of energy to a lattice ion, just enough for displacing it from its lattice site. The concentration of induced vacancies depends on the irradiation dose and in this way can be carefully adjusted. Since the lattice defects can act as donor or acceptor states in semiconductors, electron irradiation enables accurately-controlled compensation of electrically-active impurities introduced in a semiconductor crystal during growth. In this article, we present a study of the evolution of electronic properties of β-gallium oxide with step-by-step compensation of initial n-type doping through controlled introduction of point defects (gallium vacancies) produced by a 2.5-MeV electron beam. Our analysis relies on a set of electron paramagnetic resonance, luminescence, and transport data obtained at different temperatures.

With the advent of "New Space" and the explosion of nanosatellite missions, an extended latitude is offered for the emergence of innovative technological devices such as novel compact solid state UVC sensors. In this context, β-Ga2O3-based photodetectors are emerging as very promising candidates to overcome current technological limits for UVC detection in Space. Indeed, monitoring UVC solar radiation, and more specifically the Herzberg continuum (200-242nm), is fundamental to understand its' impact on the earth's climate and build better chemistry-climate models [1]. It is also, however, extremely challenging to achieve due to the harsh operating environment including large thermal variations, high energy particles, ionizing radiation and filter contamination due to satellite outgassing. The Ultra Wide Band Gap semiconductor, β-Ga2O3 (Eg ~ 4.9eV at 253nm), is intrinsically solar blind, radiation-hard and thermally-robust. Furthermore, the authors have recently shown that the bandgap can be engineered upwards through Al alloying so as to obtain optical transitions from 253 down to 200nm [2,3]. This allows the realization of β-Ga2O3-based photodetectors with peak operating wavelengths which capture the Herzberg continuum selectively and thus, dispenses with the need for short pass filters. Therefore, these β-Ga2O3-based photodetectors are excellent candidates to monitor the Herzberg continuum from Space. Hence, they have been selected to be integrated on the INSPIRE-Sat 7 (International Satellite Program in Research and Education) nanosatellite ("2U" CubeSat) which will monitor the Herzberg continuum on a low Earth orbit, following a prototype mission UVSQ-Sat (INSPIRE-Sat 5) successfully launched in January 2021 [4]. This work presents the realization of β-Ga2O3-based photodetectors going from the wafer to the final packaged sensors including device architecture development, photolithography, contacting, probing, singulation, packaging, stringent robustness testing (in a simulated environment) and performance binning, so as to obtain the final flight model photodetectors.

Ga2O3 layers were grown on c-sapphire substrates by pulsed laser deposition. Optical transmission spectra were coherent with a bandgap engineering from 4.9 to 6.2 eV controlled via the growth conditions. X-ray diffraction revealed that the films were mainly β-Ga2O3 (monoclinic) with strong (-201) orientation. Metal-Semiconductor-Metal photodetectors based on gold/nickel Inter- Digitated-Transducer structures were fabricated by single-step negative photolithography. 240 nm peak response sensors gave over 2 orders-of-magnitude of separation between dark and light signal with state-of-the-art solar and visible rejection ratios ((I240 : I290) of > 3 x 105 and (I240 : I400) of > 2 x 106) and dark signals of <50 pA (at a bias of -5V). Spectral responsivities showed an exceptionally narrow linewidth (16.5 nm) and peak values exhibited a slightly superlinear increase with applied bias up to a value of 6.5 A/W (i.e. a quantum efficiency of > 3000%) at 20V bias.

275 nm-thick Yttria-stabilised zirconia (YSZ) layers were grown on 240 nm-thick epitaxial (0002)-oriented ZnO buffer layers on c-sapphire substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) studies revealed high quality epitaxial growth with the YSZ having a preferential (111) orientation and a root mean square surface roughness of 1.4 nm over an area of 10 um x 10 um. The YSZ top surface was then temporary bonded to an Apiezon W wax carrier and the sample was immersed in 0.1M HCl so as to preferentially etch/dissolve away the ZnO underlayer and release of the YSZ from the sapphire substrate. XRD revealed only the characteristic (111) peak of YSZ after lift-off and thus confirmed both the dissolution of the ZnO and the preservation of the crystallographic integrity of the YSZ on the wax carrier. Optical and Atomic Force Microscopy revealed some buckling, roughening and cracking of the lifted YSZ, however, which was probably due to tensile epitaxial strain release.

B-Ga2O3 is a novel Ultra Wide Bandgap semiconductor, with many emerging applications (1), including that of a solar blind UVC photodetector (2). A key limitation, as for many WBG oxides, is the difficulty in reliably creating high quality Schottky contacts (typical barrier heights are in the range of 1.00-1.50 eV3). However, some studies indicate that graphene- or graphite-based electrodes can give an increase of the barrier height (4). The challenge of our study was to develop a facile and cost-effective method in order to obtain a good quality Schottky contact with graphite. Many studies employ Au in order to make Schottky contacts to B-Ga2O3. This study involves comparison of Au and graphite contacting schemes for inter-digitated-transducer (IDT) Schottky Metal-Semiconductor-Metal (MSM) contacts on B-Ga2O3 thin films grown on c-Al2O3 substrates by Pulsed Laser Deposition (PLD).

Nitrogen can be incorporated into MgZnO using low-temperature deposition. Donor–acceptor pair emission from N-doped MgZnO is attributed to molecular N2.

Staggered back-gated Field Effect Transistor (FET) structures were made by growing Li-doped NiO on Si3N4/SiO2/Si (111) using room temperature pulsed laser deposition. Optical studies showed over 80% transmission for the NiO: Li channel at wavelengths > 500nm. The MISFET revealed rectifying transfer characteristics, with a V-ON close to zero, a channel mobility of similar to 1 cm(2)/Vs, a gate leakage current (at +5V) of 0.8 mA and an I-ON/I-OFF ratio (at a Vgs of -15V) of similar to 10(3). The transistors showed enhancement-mode output characteristics indicative of a p-type channel with sharp pinch-off, hard saturation, a comparatively high (milliampere range) Id and a relatively low on-resistance of similar to 11 k Omega. Hence the adoption of Li doping in NiO channels would appear to be a promising approach to obtain p-type TFTs with superior transparency, speed and energy efficiency.

The use of ultra-wide bandgap transparent conducting beta gallium oxide (β-Ga2O3) thin films as electrodes in ferroelectric solar cells is reported. In a new material structure for energy applications, we report a solar cell structure (a light absorber sandwiched in between two electrodes - one of them - transparent) which is not constrained by the Shockley–Queisser limit for open-circuit voltage (Voc) under typical indoor light. The solar blindness of the electrode enables a record-breaking bulk photovoltaic effect (BPE) with white light illumination (general use indoor light). This work opens up the perspective of ferroelectric photovoltaics which are not subject to the Shockley-Queisser limit by bringing into scene solar-blind conducting oxides.

Here, we report the analogy of an extremely stable topological-like ultra-wide bandgap insulator, a solid that is a pure insulator in its bulk but has a metallic conductive surface, presenting a two-dimensional conductive channel at its surface that challenges our current thinking about semiconductor conductivity engineering. Nominally undoped epitaxial beta-Ga2O3 thin films without any detectable defect (after a range of state-of-the-art techniques) showed the unexpectedly low resistivity of 3 x 10(-2) Omega cm which was found to be also resistant to high dose proton irradiation (2 MeV, 5 x 10(15) cm(-2) dose) and was largely invariant (metallic) over the phenomenal temperature range of 2 K up to 850 K. The unique resilience and stability of the electrical properties under thermal and highly ionizing radiation stressing, combined with the extended transparency range (thanks to the ultra-wide bandgap) and the already known toughness under high electrical field could open up new perspectives for use as expanded spectral range transparent electrodes (e.g., for UV harvesting solar cells or UV LEDs/lasers) and robust Ohmic contacts for use in extreme environments/applications and for novel optoelectronic and power device concepts. (C) 2018 Elsevier Ltd. All rights reserved.

The bandgap of wurzite ZnO layers grown on 2 inch diameter c-Al2O3 substrates by pulsed laser deposition was engineered from 3.7 to 4.8 eV by alloying with Mg. Above this Mg content the layers transformed from single phase hcp to mixed hcp/fcc phase before becoming single phase fcc above a bandgap of about 5.5 eV. Metal-Semiconductor-Metal (MSM) photodetectors based on gold Inter-Digitated-Transducer structures were fabricated from the single phase hcp layers by single step negative photolithography and then packaged in TO5 cans. The devices gave over 6 orders of magnitude of separation between dark and light signal with solar rejection ratios (I270 : I350) of over 3 × 105 and dark signals of 300 pA (at a bias of -5V). Spectral responsivities were engineered to fit the “Deutscher Verein des Gas- und Wasserfaches” industry standard form and gave over two decade higher responsivities (14 A/W, peaked at 270 nm) than commercial SiC based devices. Homogeneous Ga2O3 layers were also grown on 2 inch diameter c-Al2O3 substrates by PLD. Optical transmission spectra were coherent with a bandgap that increased from 4.9 to 5.4 eV when film thickness was decreased from 825 to 145 nm. X-ray diffraction revealed that the films were of the β-Ga2O3 (monoclinic) polytype with strong (-201) orientation. β-Ga2O3 MSM photodetectors gave over 4 orders of magnitude of separation between dark and light signal (at -5V bias) with dark currents of 250 pA and spectral responsivities of up to 40 A/W (at -0.75V bias). It was found that the spectral responsivity peak position could be decreased from 250 to 230 nm by reducing film thickness from 825 to 145 nm. This shift in peak responsivity wavelength with film thickness (a) was coherent with the apparent bandgap shift that was observed in transmission spectroscopy for the same layers and (b) conveniently provides a coverage of the spectral region in which MgZnO layers show fcc/hcp phase mixing.

Surface-Enhanced Raman spectroscopy (SERS) is a widely used technique adopted in both academia and industry for the detection of trace quantities of Raman active molecules. This is usually accomplished by functionalizing distributions of plasmonic metal nanoparticles with the analyte molecules. Recently metal-coated nanostructures have been investigated as alternatives to dispersions of metal nanoparticles in order to avoid clustering and homogeneity/reproducibility issues. In this paper, several samples of Au-coated ZnO nanoarrays are adopted as SERS substrates in order to investigate the molecular sensing capacity for methylene blue (MB) molecules. Self-forming ZnO nanoarrays were grown on both c-sapphire and silicon substrates by pulsed laser deposition. The nanoarrays were then coated with 30 nm of gold using thermal evaporation and the SERS signals of MB functionalized samples were obtained with a Raman microspectrometer. The ratio of SERS intensity to that of an MB functionalized glass substrate (I-SERS/I-Raman) was calculated based on the averaged SERS signals. A relatively good within-wafer homogeneity of the enhancement effect was found with I-SERS/I-Raman values as high as 64.2 for Au-coated nano ZnO grown on silicon substrates. The experimental results show that the Au-coated ZnO nanoarrays can be excellent SERS substrates for molecular/chemical analyte sensing.

Nominally-undoped Ga2O3 layers were deposited on a-, c-and r-plane sapphire substrates using pulsed laser deposition. Conventional x-ray diffraction analysis for films grown on a-and c-plane sapphire showed the layers to be in the beta-Ga2O3 phase with preferential orientation of the (-201) axis along the growth direction. Pole figures revealed the film grown on r-plane sapphire to also be in the beta-Ga2O3 phase but with epitaxial offsets of 29.5 degrees, 38.5 degrees and 64 degrees from the growth direction for the (-201) axis. Optical transmission spectroscopy indicated that the bandgap was similar to 5.2eV, for all the layers and that the transparency was > 80% in the visible wavelength range. Four point collinear resistivity and Van der Pauw based Hall measurements revealed the beta-Ga2O3 layer on r-plane sapphire to be 4 orders of magnitude more conducting than layers grown on a-and c-plane sapphire under similar conditions. The absolute values of conductivity, carrier mobility and carrier concentration for the beta-Ga2O3 layer on rsapphire (at 20 Omega(-1). cm(-1), 6 cm(2)/Vs and 1.7 x 10(19) cm(-3), respectively) all exceeded values found in the literature for nominally-undoped beta-Ga2O3 thin films by at least an order of magnitude. Glow discharge optical emission spectroscopy compositional depth profiling for common shallow donor impurities (Cl, Si and Sn) did not indicate any discernable increase in their concentrations compared to background levels in the sapphire substrate. It is proposed that the fundamentally anisotropic conductivity in beta-Ga2O3 combined with the epitaxial offset of the (-201) axis observed for the layer grown on r-plane sapphire may explain the much larger carrier concentration, electrical conductivity and mobility compared with layers having the (-201) axis aligned along the growth direction.

The SERS yield of Au/ZnO nanoarrays used as chemical sensors is monitored through growth deposition parameters on silicon and sapphire.

ZnO is a remarkable multifunctional material with intrinsically high transparency over the whole visible range and a resistivity that can readily be tuned from semi-insulating right through to semi-metallic. As such it is forming a cornerstone for a transparent electronics revolution which promises both novel applications and remarkable performance advantages for existing products. This article gives an overview of recent advances in processing, alloying and doping of ZnO and of the main applications that are currently emerging, including replacement of ITO as a transparent electrode and substitution for amorphous silicon as the channel in thin film transistors for display backplanes.