This PDF file contains the front matter associated with SPIE Proceedings Volume 11687

Highly conductive Si-doped epitaxial β-Ga2O3 films with a wide bandgap and high critical field strength were fabricated by pulsed laser deposition. The carrier concentration and Hall mobility are 4.06 × 1020 cm-3 and 58.7 cm2/Vs, respectively. Thus, overall conductivity has been calculated to be 3800 S/cm, which is the highest value ever reported based on β-Ga2O3 material. This degenerately doped layer will enhance the overall transistor performance through an ohmic regrowth process. Additionally, this opens up potential applications as a transparent conductive oxide layer due to coexistence of transparency and low resistivity (2.63 x10-4 Ω-cm) similar to commercial ITO.

Monoclinic gallium oxide is a wide-gap (4.8 eV) semiconductor with a high breakdown field. To fully exploit the applications in high power electronics, high-quality epitaxial growth of gallium oxide is required. We use density functional theory calculations to explore the adsorption of Ga and In adatoms on the Ga2O3 (010) surface and the effect of In on the growth rate. We also study the co-adsorption of Al, Ga, and O adatoms on the Ga2O3 (010) surface to reveal the role of surface reconstructions and adatom diffusion in Al incorporation in (AlxGa1−x)2O3 alloys.

Aiming to enhance the dielectric constant and maintaining a relatively low dielectric loss, transparent acceptor-donor (Cu-Ga) co-doped ZnO films were fabricated on c-sapphire using pulsed laser deposition. With Ga=0.5 wt% and Cu=8 wt%, the dielectric constant was optimised while the dielectric loss is relatively low ( 204 and 0.27 respectively at the frequency of 1 kHz). The dielectric constant is stable over a wide range of frequency of ~10-10⁶ Hz. The film has good optical transmittance (>75 %) in the visible wavelength range (450-800 nm). Ac conductivity study reveals two relaxation processes in the sample, namely the correlated barrier hopping (CBH) and the small polaron tunneling (SPT). The enhancement of the dielectric constant was ascribed to the formation of new defect complexes induced by the acceptor-donor doping; and the CBH and SPT of electrons between these neighboring defect complexes.

We have applied positron annihilation spectroscopy to study a wide range of β-Ga₂O₃ bulk crystals and thin films with various doping levels. The Doppler broadening of the 511 keV positron-electron annihilation line exhibits colossal anisotropy compared to other three-dimensional crystalline semiconductors. State-of-the-art theoretical calculations of the positron characteristics in the β-Ga₂O₃ lattice reveal that the positron state is effectively 1-dimensional, giving rise to strong anisotropy. Strongly relaxed split Ga vacancies are found to exhibit even stronger anisotropy and to dominate the positron annihilation signals in almost all experiments. The evidence leads to the conclusion that split Ga vacancies are abundant, with concentration of 10¹⁸ cm^-3 or more, in β-Ga₂O₃ samples irrespective of conductivity.

We report the characteristics of luminescence bands in beta-Ga2O3 thin films and single crystals. The dominant UV emission at 3.4 eV exhibits strong thermal quenching but its peak shape remains unchanged. The blue and green bands, attributed to defects, are found to be strongly dependent on growth conditions. Additionally, we observe a distinct red luminescence at 1.9 eV upon hydrogen doping. The emergence of this emission is accompanied by substantially increased electrical conductivity. The red emission is shown to be consistent with shallow donor–deep acceptor pair recombination and will be discussed in the context of defect models.

In this talk, we will present fabrication and characterization of submicron Ga2O3 field-effect transistors (FETs) for high-frequency wireless communications. Superior small-signal characteristics of a current-gain cutoff frequency of 9 GHz and a maximum oscillation frequency of 27 GHz were achieved at a gate length of 200 nm. Simple delay-time analysis on the FETs was also performed to investigate an effective electron velocity and a proportion of each delay component to the total delay time.

In this paper, we analyze the threshold voltage stability of beta-Ga₂O₃ FinFETs for power applications using Al₂O₃ as gate insulator. In dynamic characterization measurements, when the filling bias condition is moved from off-state to on-state a positive threshold voltage shift is induced, caused by the trapping of electrons in the insulator or at the insulator interface with the semiconductor. The threshold voltage variation was found to be stable in rest condition, but illumination by 280 nm UV light was able to slowly recover the threshold voltage even below its value before the filling condition was applied, suggesting the presence of natively trapped charge into the oxide even in the as-grown device. In order to obtain more information on the role of the external illumination, monochromatic excitation in the range from 1.5 eV to 5 eV was applied to the device before a transfer characteristic measurement. Results show that photon energies lower than 2.2 eV cause a positive threshold voltage shift, caused by charge trapping during the measurement phase and not related to illumination. Photon energies between 2.2 eV and 3.5 eV promote electron detrapping, leading to a partial recovery in the threshold voltage. Finally, energies above 3.5 eV cause an additional charge trapping process. The physical origin of the photon energy difference was investigated by monochromatic light-induced current transients, and a suitable model considering the conduction band discontinuities between the gate metal and the oxide and between the oxide and the semiconductor was developed to explain the experimental data.

Vertical β-Ga2O3 Schottky diodes from metal-organic chemical vapor deposition (MOCVD) epitaxy are reported for high-power devices. The field plate Schottky barrier diode (SBD) showed a differential specific on-resistance (Ron,sp) of 0.67 mΩ-cm2 and an average breakdown electric field of 2.28 MV/cm. To the best of our knowledge, this Ron,sp is the lowest among the available vertical β-Ga2O3 SBD reports, and contributed from the high-mobility MOCVD β-Ga2O3 epitaxy. Moreover, the average electric field of 2.28 MV/cm is higher compared to most of the vertical β-Ga2O3 punch-through SBDs. These results suggest that the high-quality MOCVD β-Ga2O3 can be promising for high-power devices.

Tin monoxide (SnO) is one of the few p-type semiconducting oxides with reasonably high hole mobility (>1cm^2/Vs). It possesses a challenging metastability with respect to Sn and the n-type semiconducting oxide SnO2, which is adressed in this talk by a phase diagram, presenting a rapid in-situ approach to find the growth window for SnO during plasma-assisted molecular beam epitaxy, and by time and temperature stability investigations of the grown SnO layers. The nondegenerate hole transport properties of the obtained, (001)-oriented single crystalline layers are shown and discussed. As an application example SnO/beta-Ga2O3 vertical pn heterojunction diodes were prepared. Their high rectification (2x10^8 at +/-1V) allows for pn junction isolation of p-type SnO devices on top of n-type Ga2O3 substrates. The diodes exhibited an ideality factor of ~1.2, a built-in voltage of 0.96V, and an estimated peak breakdown electric field of 2.2 MV/cm in the Ga2O3.

Sn doping of β-Ga2O3 grown by conventional plasma-assisted molecular beam epitaxy (PAMBE) and via metal oxide catalyzed epitaxy (MOCATAXY) using a supplied indium flux during MBE growth was investigated. Sn doping of (010) β-Ga2O3 via MOCATAXY allowed for sharper doping profiles as well as a wider range of donor concentrations from 4 x 10^16 cm-3 to 2 x 10^19 cm-3 with a maximum Hall mobility of 136 cm2/Vs and a Sn donor level of 77 meV below the conduction band. Expansion of MOCATAXY to (001) β-Ga2O3 also showed improved Hall mobility, growth rates, and smoother films in this orientation.

Ga2O3 is a highly promising material for power electronics, thanks to its large band gap (4.8 eV) and high breakdown voltage. Better control of doping is still an active research topic, both in Ga2O3 and in (AlxGa1-x)2O3 alloys. First-principles modeling, using advanced hybrid functional calculations within density functional theory, can greatly help in resolving experimental puzzles and guiding optimal doping conditions. Work performed in collaboration with S. Mu, J. L. Lyons, H. Peelaers, J. B. Varley, and D. Wickramaratne.

This presentation will give an overview of our recent work on materials growth and device engineering of 𝛽-Ga2O3 electronic devices. We will discuss the design and properties of advanced modulation-doped (Al,Ga)2O3/Ga2O3 structures with high sheet charge density and excellent transport properties. We will then outline novel strategies for realizing high breakdown fields and low resistance within devices, and outline design, growth, and characteristics of state-of-art Gallium Oxide devices, including scaled transistors with cutoff frequency of 27 GHz, and transistors with a power switching figure of merit of 586 MW/cm2 and breakdown voltage of 660V.

The wide-bandgap semiconductor Ga2O3 is a promising candidate for high-power electronics. Alloying with Al for (AlxGa1-x)2O3 films enables heterostructures that are essential for device applications. However, the limited thickness of (AlxGa1-x)2O3 films grown on Ga2O3 substrates is a serious obstacle. Here we employ first-principles calculations to determine the brittle fracture toughness of such films for three growth orientations of the monoclinic structure: [100], [010] and [001]. Surface energies and elastic constants are computed for the end compounds—monoclinic Ga2O3 and Al2O3—and used to interpolate to (AlxGa1-x)2O3 alloys. The appropriate crack plane for each growth orientation is determined, and the corresponding critical thicknesses of (AlxGa1-x)2O3 films are calculated based on Griffith’s theory. Our in-depth analysis of surface energies for both relaxed and unrelaxed surfaces provides important insights into the factors that determine the relative stability of different surfaces. We conclude that the critical thickness is largest for (AlxGa1-x)2O3 films grown along [100].

Monoclinic gallium sesquioxide (b-Ga2O3) has emerged in recent years as a promising material for applications in power electronics and UV photo-detectors, but point defects in this interesting material is not well understood and may even limit device performance. Here, we will discuss the present status of understanding electrically active defects in β-Ga2O3, and present recent results related to intrinsic and impurity related defect centers. Indeed, deep level transient spectroscopy (DLTS) and steady state photocapacitance spectroscopy (SSPC) show that several electrically active defect levels are present in as grown material, or may arise after irradiation. Combining electrical characterization, secondary ion mass spectrometry (SIMS) and density functional theory (DFT), we have recently identified several of the defects and their electrical properties, and both intrinsic and extrinsic defect identification will be discussed.

Wide and ultrawide bandgap oxides, such as β-Ga2O3, ZnGa2O4 and Zn1-xGa2-2xGexO4, are of particular importance for a myriad of technological applications, including electronics, optoelectronics, and medical devices. In the case of the latter, the development of new, affordable and non-invasive methodologies for bioimaging and diagnosis is of crucial importance towards solutions that can improve health and wellbeing of the populations. For these purposes, red/near infrared emitters within the biological transparency window are required. Therefore, the here studied oxides were subjected to a controlled Cr-doping giving rise to intraionic emission in this region. In the case of the here studied oxide systems, we will investigate the intraionic luminescence properties of Cr3+, with particular emphasis on the persistent luminescence recorded in micro/nano particles of Cr-doped ZnGa2O4 and Zn1-xGa2-2xGexO4 synthesised by laser ablation in liquid media.

In the past year and a half, major progress has been realized in beta-Ga2O3 as a power electronic material. In comparison with other wide and ultrawide bandgap (WBG and UWBG) semiconductors, bulk beta-Ga2O3 can be grown directly from the melt. The material offers controlled donor doping with Si, Ge, or Sn, remarkably low background unintentional compensating acceptor concentration, intentional compensation doping with Mg, Fe, or N. The material offers controlled wet etching. Recent thermal calculations show that despite the modest thermal conductivity (~1/2 of GaAs), heat can be managed in canonical lateral and vertical devices designs. For vertical devices, taking into account dopant ionization energies, a revised figure of merit for minimum on-resistance shows the superior performance potential for beta-Ga2O3 compared to all WBG and UWBG semiconductors.

Atomic layer deposition (ALD) offers a low thermal budget method for producing α-Ga₂O₃ films on sapphire substrate. In this paper we review the recent progress on plasma-enhanced ALD growth of α-Ga₂O₃ and present the optical and photoconductive properties of the deposited films. We show that the deposited material exhibits an epitaxial relationship with the sapphire substrate, and with an atomically sharp film-substrate interface. The α-Ga₂O₃ films had an optical bandgap energy measured at 5.11 eV, and exhibited a broad luminescence spectrum dominated by ultraviolet, blue and green bands, in line with current literature. We finally demonstrate the suitability of the material for solar-blind photodetection.

Gallium oxide has emerged as a promising ultrawide-bandgap semiconductor for electronic applications. Part of the attraction of Ga2O3 is its ability to be alloyed with other materials (such as Al2O3) for band-gap engineering or doped with other elements (such as silicon) for modifying its electrical conductivity. But what is still unknown is how these alloying capabilities extend into the orthorhombic phases, or how well the ultrawide-bandgap AlGO alloys can be n-type doped. Here hybrid density functional theory calculations are used to determine the electronic structure of AlGO alloys. Conduction-band offsets of AlGO alloys in the orthorhombic phase are calculated, as are donor ionization energies as a function of Al content. In light of these results, we discuss band engineering and doping strategies in AlGO alloys for electronic device applications.

Ga2O3 is the only ultra-wide bandgap semiconductor with melt-growth substrate technology similar to that of Si, heterostructure device technology similar to that of the III-Nitride family, and high growth rate (GR) epitaxial technologies such as MOCVD and HVPE to support the development of ultra-high-breakdown voltage devices competitive with SiC technology. We have demonstrated for the first time a β-Ga2O3 MOSFET grown by high-GR MOCVD (Agnitron Technology’s Agilis 100 reactor) with record high mobility of 170 cm2/Vs, despite increased carrier scattering rate in the doped channel, facilitated by a significant improvement in epilayer quality. The high GR demonstrated via this method paves the road for demonstration of high breakdown voltage devices on a thick Ga2O3 buffer layer. [1] M.J. Tadjer et al., J. Phys. D: Appl. Phys. 54 (2021) 034005.

Ultra-wide bandgap (~ 4.8 eV) beta phase gallium oxide (β-Ga₂O₃) grown by metal organic chemical vapor deposition (MOCVD) has demonstrated promising electronic transport properties with room temperature electron mobilities reaching 194 cm²/V-s and background doping as low as 9×10¹⁴ cm^-3 [Zeng et al, Appl. Phys. Lett. 114, 250601 (2019)]. Commensurate with these values is a total trap concentration that is ~10x lower, with a different distribution of states throughout the bandgap than what has been observed for β-Ga₂O₃ grown by other methods [Zhang et al., Appl. Phys. Lett. 108, 052105 (2016), Farzana et al, Appl. Phys. Lett. 123, 161410 (2018)]. Given the promise of MOCVD-grown β-Ga₂O₃, a deeper understanding of the nature of defects in this material is of interest. This work provides a comprehensive picture of the current state of knowledge regarding deep levels in MOCVD-grown β-Ga₂O₃, including trapping properties, energy and concentration distributions in the bandgap, potential physical sources, and comparisons with other growth methods. By applying a suite of complementary defect spectroscopy methods-deep level optical spectroscopy, deep level transient spectroscopy, and admittance spectroscopy, quantitative characterization of defect states within the ~ 4.8 eV bandgap is possible. We find that, through systematically varying growth conditions, differing trends in concentrations for individual states are observed, implying that growth optimization is possible. Combined with observations made after high energy particle irradiation, we can differentiate between states of intrinsic and extrinsic origin.

Epitaxial growth of β-Ga2O3 was performed on (110) substrate by plasma-assisted molecular beam epitaxy (PAMBE). Investigation of (010) substrates has revealed that (110) facets are revealed the chevron consistent features in reflection high-energy electron diffraction (RHEED) studies, which indicates (110) is a natural plane in β-Ga2O3 and exhibits atomically flat surface after Ga polishing. The growth rate dependence on Ga flux study suggests that the growth rate is not reduced on the (110) plane compared to that of (010). Atomic force microscopy (AFM) shows smooth surface morphology was obtained by growing on (110) substrates.

This study has highlighted the formation of an amorphous-nano-oxide film on Si substrate dominating the preferred orientation of sputtered β-Ga2O3. After the rapid thermal annealing (RTA) process from 700 to 1000 °C for β-Ga2O3 (100)/SiO2/p-Si, all the amorphous transformed into a β-Ga2O3 structure. Meanwhile, the thermal-induced massive twin boundaries and stacking faults generation have been observed in the annealing process above 800 °C. Therefore, an optimum metal-semiconductor-metal photodetector performance is achieved for the 800°C-RTA-treated β-Ga2O3 samples with the photo/dark current ratio of 3.91×102 and responsivity of 0.702 A/W (at 5 V bias). Furthermore, the interface energies per area (Ei) by density functional theory between β-Ga2O3 films ((001), (010), (100), and (-201)) with various facets and amorphous SiO2 were determine to quantify the sequence of the preferred orientations.

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.

The formation energies, activation energies, and self-compensation effects of silicon (Si), germanium (Ge), carbon (C), beryllium (Be), and magnesium (Mg) in wurtzite (wz-) and zincblende (zb-) GaN are explored through a unified hybrid density-functional theory. The common donors (Si and Ge) are promising donors for both wz- and zb-GaN due to small activation energies (< 30 meV). The popular acceptor alternatives (C and Be) have smaller activation energies of 490 and 134 meV in zb-GaN relative to that of 590 and 205 meV wz-GaN, respectively. However, neither C nor Be is expected to outperform Mg as the former suffers from considerable activation energy, and a strong self-compensation effect limits the latter. Mg's activation energy in zb-GaN is 153 meV, which is lower than that of 226 meV in wz-GaN. For the selfcompensation effects, C, Si, and Ge favor the interstitial incorporation in wz-GaN than zb-GaN, while Be and Mg behave oppositely. This is attributed to the coherence between the orbital symmetry and the geometrical symmetry of the interstitial site.

ZnGa2O4 is a ultra-wide band bandgap transparent oxide with electron transport properties similar to those of the popular b-Ga2O3 but with a higher lattice symmetry. Recent experimental work has been producing high-quality ZnGa2O4 crystals. Here we present our ongoing first-principles modeling work on the structural, thermal, and electronic properties of ZnGa2O4. Elastic stiffness tensor modeling based on symmetry-allowed deformations provides an insight into the effect of symmetry on lattice dynamical properties. We obtain thermodynamical properties such as thermal expansion in the quasiharmonic approximation based on phonons from finite-displacement supercell approach. The phonon dispersions and density of states are compared to those of b-Ga2O3. We find a large number of optical phonons at low energies <15 meV but with higher symmetry by comparison to b-Ga2O3. The large number of optical modes has a signature both in quantum magnetoconductance measurements and in the breakdown field, the latter being a metric relevant for applications in power electronics.

First-Principles DFT calculations are carried out to investigate electronic and structural properties of (BxGa1-x)2O3 alloys in both monoclinic and orthorhombic phases. Generally, the alloying with boron results in the increasing of the bandgap energy and reduction of the lattice constants of (BxGa1-x)2O3 alloys. In addition, the formation enthalpy is calculated to predict its growth feasibility. The band alignment between Ga2O3 and B2O3 is also investigated, which shows the type-II offset in monoclinic phase and type-I offset in orthorhombic phase, respectively. Our studies provide important insight regarding the potential of (BxGa1-x)2O3 alloys for III-Oxide based electronic and optoelectronic device applications.

Recently, materials with vanishing refractive index, near-zero-index (NZI), have garnered considerable amount of attention, primarily for their ability to exhibit enhanced light matter interaction, due to slow light affects. Furthermore, effects such as static light, enhanced nonlinearities and emission tailoring have made such materials a heavily researched area. Amongst them, transparent conducting oxides (TCOs), a class of materials that have vanishing index at technologically relevant near-IR spectral range, are increasingly being investigated, for their potential use in photonic circuits. Wide natural abundance of ZnO together with well-studied properties make ZnO-based TCOs, such as aluminum- and gallium-doped ZnO particularly attractive for NZI applications.

Recently, ternary perovskite oxides have attracted great attention as alternative transparent conducting oxides (TCOs) because their structures are compatible with many other perovskite oxides that allow devices to be fabricated comprised entirely of perovskite oxides. Among these perovskite oxides, BaSnO₃ has attracted considerable attention as a promising TCO because of its high mobility at room temperature (~320 cm²V^-1s^-1 in bulk single crystals and ~100 cm²V^-1s^-1 in epitaxial thin films) and high temperature stability in oxygen atmospheres compared to other TCOs, such as In₂O₃, ZnO, and SnO₂. The electrical and optical properties of the BaSnO₃ can be improved by either inducing oxygen vacancies or cationic doping. We have grown epitaxial La-doped BaSnO₃ (LBSO) thin films on MgO (001) substrates by pulsed laser deposition using a La_0.04Ba_0.96SnO₃ target, and investigated their structural, electrical, and optical properties as a function of the oxygen pressure during deposition. The permittivity of the LBSO films can be modified as a function of the oxygen pressure during deposition allowing tuning of their epsilon-near-zero (ENZ) wavelength from 2.2 μm to 7 μm. We will present details of the deposition conditions on the properties of LBSO films and the ability to tune the permittivity in this infrared range.

Many systems within the energy sector necessitate high-temperature or chemically harsh conditions (e.g., solid oxide fuel cells, power plant boiler systems, post-combustion facilities). Significant economic and technological value can be added through the integration of in-situ sensor technology; unfortunately, harsh environments pose a major challenge to traditional sensor materials. Optical fiber-based sensors provide a robust solution to this problem and offer capability for spatially distributed sensing. Silica fiber, with cladding removed and coated with bare or metallic nanoparticle incorporated sensing layers, exhibits stability up to 800-900°C under a wide range of chemical environments. As sensing layers, complex perovskites oxides - studied extensively as anode and cathode materials within the solid oxide fuel cell (SOFC) community – provide ideal tunability, stability, and defect-dependent optical properties for high-temperature gas-sensing applications. Modeled defect chemistry kinetics are presented in the context of experimental high-temperature (600-800°C) optical gas sensor data at visible and NIR wavelengths, both on planar substrates and on optical fiber. Doped SrTiO₃ is highlighted as a model sensing material, due to strong Drude response under chemically reducing conditions, and due to its well-documented material / chemical properties. Equilibrium calculations are performed for ionic and electronic motion within thin films on fiber – using a ray-based approach for guided optical modes.

The main obstacle towards widespread industrial adoption of THz quantum cascade lasers is the requirement of cryogenic cooling. Room-temperature operation using the conventional GaAs/AlGaAs material system is inherently limited by the optical phonon energy (ELO=36 meV) being close to the laser transition energy (~10-20 meV). In contrast, the ZnO/ZnMgO material system has a significantly higher ELO of 72 meV, pushing the theoretical high-temperature limit far above room temperature. At the same time, ZnO comes with it's own challenges, such as significantly broadened energy levels and short upper laser state lifetime. In the present talk, these considerations will be discussed in the light of non equilibrium Green’s function modelling, which is necessary to correctly treat the strong electron-phonon scattering. In addition, design schemes suitable for m-plane (avoiding internal fields) ZnO QCLs will be presented and analysed, providing pathways towards room temperature THz QCLs.

Zincoxide is a rather new material system and promising candidate for mid-infrared (mir) and THz optoelectronic devices like quantum cascade lasers (QCLs) and detectors (QCDs) due to its twice as high LO-phonon energy as GaAs. The non-polar m-plane orientation allows designing and realizing such complex devices without internal electrical fields. We present the full fabrication scheme of such QCL/QCD devices including novel optimized etching techniques, surface leakage current suppression by multiple orders of magnitude and low resistance Ohmic contacts (~10^(-5) Ohm x cm^2). Optimized fabrication schemes resulted in fabrication yielding up to more than 80% of operational devices.

5G networks are currently being deployed around the world, introducing a new era in machine-to-machine communications and reinforcing the Internet of Things. The 5G radiofrequency bands range from sub-1 GHz to 70 GHz, while the 6th generation (6G) is expected to cover bands at hundreds of GHz. There is a need for devices with high frequency performance and scalable manufacturing using inexpensive techniques and materials. Herein we present ZnO-based Schottky diodes, processed from solution on wafer scale with high yield. Coplanar nanogap electrodes are fabricated using a high-throughput low-cost technique, named adhesion lithography. The diodes’ cutoff frequency exceeds 100 GHz.

Dynamic control over the permittivity of materials enables control over the amplitude, phase, and polarization of light. Thus, to realize practical tunable devices, it is important to perform a detailed dynamic characterization of technology-relevant materials with substantially tunable optical properties. In this work, we demonstrate extraordinarily large, unity-order permittivity modulation in zinc oxide through interband pumping. The large permittivity changes actively enable large reflectance modulation in both lithography-free mirrors (70% at 31.6 mJ/cm2 pump) and nanodisk resonators (55% at 7.6 mJ/cm2 pump fluence). The relaxation time for this response is 20 ps. We explore the physical origins of the permittivity modulation and determine the physical limits. The results of this study will advance the realization of ultrafast dynamic optical devices for optical switching, beam-steering, and spectroscopy.

We report the direct current (DC) and microwave performance of BeMgZnO/ZnO heterostructure field effect transistors (HFETs) on sapphire substrates. The devices fabricated using Al2O3 as the gate dielectric with a gate length of 1.5 um and a gate width of 75 um exhibited a pinch-off voltage of −4.0 V and a maximum peak transconductance of 63 mS/mm. A current gain cutoff frequency fT of 5.0 GHz was achieved, highest among ZnO-based FETs. The corresponding electron velocity of above 1E7 cm/s estimated based on the gate transit time inches closer to the theoretical peak velocity in ZnO (3.5E7 cm/s). This value is significantly higher than the previously reported values in ZnO-based HFETs, which is attributed to the two-dimensional electron gas (2DEG) concentration at or near the resonance of longitudinal optical (LO)-phonon and plasmon frequencies as well as the improved quality of the heterostructure owing to optimized ZnO buffer growth and BeO and MgO alloying in the barrier. To probe the high-frequency response of the HFETs, extrinsic and intrinsic parameters of the small-signal equivalent circuit for the BeMgZnO/ZnO HFETs were investigated using the hybrid extraction method.

Al-doped ZnO (AZO) has received significant attention due to its inherent properties like wide bandgap, high optical transparency, and electrical conductivity that has established its potential application in optoelectronic devices. The primary challenge in the efficient use of AZO thin films is the un-intentional formation of intrinsic defects, which deteriorate the device performance. The research community has made a significant effort to minimize these intrinsic defects and obtained high-quality films using low-cost growth techniques followed by a post-growth annealing treatment that has successfully suppressed defect states' formation. This presentation provides a comprehensive picture of the current state of knowledge on AZO's growth using spatial atomic layer deposition (SALD), comparing it with other growth methods such as ALD and physical vapor deposition. In this report, a series of AZO thin films were prepared by SALD on a glass substrate, followed by post-growth annealing from 400-550 °C, and the optical, structural, and electrical qualities of the formed thin films were evaluated using various techniques. X-ray (002) peak reveals the formation of stoichiometric films with increasing annealing temperature. UV-Vis results exhibited a marginal decrease in the bandgap until 500 °C that can be attributed to the suppression of oxygen vacancies. The refractive index monotonically increases with annealing temperatures, which correlates with the formation of higher film density due to the increase in grain size demonstrated in atomic force microscopy results. We find that the systematic variation of annealing temperature from 300–500 °C increases the film resistivity, implying a monotonic decrease in carrier concentration that matches X-ray and UV-Vis results.

The tremendous advances flexible electronics has experienced have led to the demonstration and - in some cases - commercialization of a plethora of devices, such as foldable displays, ubiquitously integrated sensor systems, and imperceptible implants. All these systems call for specialized analog circuits capable to transmit and receive data, condition sensors, drive actuators or control powering devices. Nevertheless, the current availability of materials and processes compatible with flexible foils imposes limitations to the realization of high-performance flexible analog systems. Among state-of-the-art technologies, amorphous metal oxides – and especially Indium-Gallium Zinc-Oxide (IGZO) – thin-film-transistors (TFTs), are extremely suited due to their electrical and mechanical performance. Here, we present TFTs based on IGZO semiconductor and Al2O3 insulating layers on polyimide substrates. First, we summarize different approaches to reduce the transistor channel length (down to 160 nm), together with their influence on the AC performance. Even though sub-500 nm lengths are demonstrated for TFTs fabricated using vertical structures, direct laser writing and focused ion beam milling, the highest transit and oscillation frequencies of respectively 135 MHz and 398 MHz, are achieved by 500 nm long self-aligned TFTs. We then show how flexible IGZO TFTs enable the realization of complex, flexible analog circuits operating at frequencies up to 20 MHz. However, even if remarkable performances are demonstrated for flexible NMOS circuits, their unipolar characteristics results in limited gain, high power consumption and complex design. To overcome this, we also show how the complementation of IGZO with ptype carbon nanotubes results in flexible common-source CMOS amplifiers with gain of 28.7 dB.

Thin-film transistors deliberately comprising rectifying source contacts have attractive properties for sensor and driver circuits: high performance uniformity and geometrical tolerance; superior saturation; and high intrinsic gain. The paper reviews the source-gated and multimodal thin-film transistor configurations, and presents their proposed applications to ultra-compact sensing and data processing circuits. Source-gated transistors with nanoscale tunneling contacts offer an alternative to the Schottky-contact fabrication route, which presents processing challenges. Emerging multimodal transistors overcome limitations of traditional contact-controlled devices and add to the list of useful properties: high gain or constant transconductance by design; immunity to drain voltage variations in floating gate configuration; and a significantly faster response time than source-gated transistors. These devices form the foundation for the design of compact, yet extremely versatile, thin-film circuits for sensing, signal conditioning and signal conversion. Finally, a vision is presented in which the properties of these circuits will be essential to convey seamless user interactivity to physical objects, transforming them into intuitive user interfaces beyond traditional displays screens.

We discuss the role of aluminum oxide (i.e. Al₂O₃ when stoichiometric) for transistors and sensors based on oxide semiconductors such as InGaZnO (IGZO) and two-dimensional (2D) semiconductors, such as monolayer MoS₂. Aluminum oxide is a well-known capping and dielectric layer in semiconductor technology typically deposited by atomic-layer deposition (ALD), which offers a dense and high-quality film with low gas permeability even when deposited on flexible substrates. However, when deposited at low temperature (< 200°C), aluminum oxide can include a significant amount of fixed charges and defects, which lead to unusual charge trapping and doping effects in semiconductor devices. For example, such charge trapping can cause (apparent) sub-60 mV/decade subthreshold swing at room temperature in IGZO transistors, but can also lead to potential applications in neuromorphic computing. We also discuss effective doping (~10¹³ cm^-2) of 2D semiconductors by thin ALD-grown non-stoichiometric AlO_x capping layers. This is achieved with an aluminum seed layer, which enables uniform growth of the subsequently deposited ALD film. This approach leads to a negative shift in threshold voltage, record on-state current (~700 μA/μm) in a monolayer semiconductor, and drastic reduction in contact resistance. Finally, we investigate the passivation effects of Al₂O₃ capping, which limits the interaction of the underlying semiconductors with ambient air and moisture. We demonstrate improved response in MoS₂ temperature sensors and long-term stability in flexible MoS₂ transistors (8 months). Further, we evaluate the effects of Al₂O₃ passivation on IGZO transistors after aging for 80 months.

This talk is on the growth of semiconducting oxides for transistors. The films are grown by oxide MBE and the materials focus is La-doped BaSnO3 for the channel of n-type transistors and Sn2+-based oxides for the channel of p-type transistors. In addition to producing the highest mobility BaSnO3 films, we have achieved a fully transparent thin-film transistor utilizing them that provides a drain current exceeding 0.45 mA/μm and an on-off ratio of 1.5 × 10^8. A growth variant of MBE--“suboxide MBE”--will also be described that makes it possible to deposit Sn2+-based compounds at BEOL-compatible growth temperatures.

This paper presents recent progress in resistive oxide memories and their integration into advanced embedded nonvolatile memory technology nodes. With the downscaling trends in emerging semiconductor manufacturing and novel user needs such as higher density, low power consumption, high speed and reliable memories are needed by manufacturers. Two terminal memory cells based on resistive devices as oxides, phase change materials or magnetism are discussed in terms of power consumption, read/write speeds, scalability and effective cost. The experimental results are focused on oxide RRAMs with an emphasis on resistive switching of Metal/HfO₂/Metal memory stacks and associated physical and electrical characteristics.

Single-crystalline ZnGa2O¬4 epilayers were successfully grown on sapphire substrates and fabricated into phototransistor. The operational modes of phototransistors were dependent of the thickness of epilayers. It was found that the enhancement mode n-channel transistor can be applied for DUV phototransistor as the thickness of ZnGa2O¬4 was less than 50 nm. The thermal treatment can improve the epilayer quality. The annealed ZnGa2O4-based phototransistor demonstrates a responsivity of 4.74×102 A/W at 240 nm, a DUV to visible light rejection ratio (240 nm to 470 nm) of 1.54×102, trise = 0.4 sec and tfall = 0.7 sec for response time.

The field of nanophotonics is based on the ability to confine light to sub-diffractional dimensions. In the infrared, this requires compression of the wavelength to length scales well below that of the free-space values, which requires the implementation of polaritons, such as the plasmon and phonon polaritons. Here we will discuss the opportunity to implement polaritonic strong coupling between different media in an effort to dictate the polaritonic dispersion relation, and thus, the propagation and resonant properties of these materials. Within the talk results highlighting ultra-strong coupling in both forms of polaritons will be presented in the context of infrared emitters, as a means to control planar propagation using hyperbolic polaritons and in an effort to dictate the IR dielectric function using superlattice designs, with n-doped CdO offering an exemplary material in this regard.

Chromogenic materials exhibit tunable properties as a consequence of an external stimulus such as light (photochromism), temperature (thermochromism) or potential (electrochromism..). Those smart compounds find applications in buildings and automobile industry by controlling light and heat transfer through windows for transmissive devices while colour changes in reflective devices offer great interest in the field of displays and printed electronics. Focusing on electrochromism, an appropriate answer to the various fields of applications requires an adjustment of both the material and the device architecture. Through various examples, we will illustrate how we can tune either the optical contrast, the memory effect, the processability, of oxides or polymer based hybrids ink , of which electrochromic performance are highlighted in 5 to 3 layers devices built in vertical or side to side configuration.

Quantum magnetoconductance, delta-sigma(B)=sigma(B)-sigma(0), offers a new, unique way to study phonons in heavily-doped, complex semiconductors, including beta-Ga2O3 and Sn-doped In2O3 (ITO). At low temperature, theory predicts delta-sigma(B)=2.908B^(1/2) S/cm, shown to be true for thick, but not thin, samples. We grew ten ITO films by PLD on fused silica, d=13–292 nm. The thickness dependence was explained by a new delta-sigma(d)-vs-d theory based on a second source of disorder, interface-generated defects that decrease exponentially with distance from the ITO/FS interface. A fit of delta-sigma(d)-vs-d gives three parameters, including d*, the thickness above which the surface is not affected by interface damage.

The field of ultraviolet (UV)-laser applications is currently experiencing rapid growth in the semiconductor processing, laser micromachining and biomedical markets. A key enabler for these technologies are optical coatings used to manipulate and direct laser beams in a targeted manner. As laser power, laser fluence and pulse frequencies increase, the demands on the physical properties of the coating materials become more stringent. We demonstrate how ion beam sputtering and post-deposition heat treatment are utilized to produce low loss optical coatings at 355 nm and 266 nm. The importance of precisely controlling the sputtering conditions for individual materials is highlighted and the influence of different process parameters on the resulting material properties is discussed. The effect of annealing on key performance parameters for optical coatings such as absorption, stress, roughness, and film structure is investigated. The low absorption achieved in this work results in high laser induced damage thresholds (LIDT) exceeding 2.5 J/cm² and 6.5 J/cm² for highly reflective (HR) mirrors and 7.6 J/cm² and 15.7 J/cm² for antireflective coatings at 266 nm and 355 nm, respectively.

Devices made with functional oxide materials are known for their versatility and high-performance. This presentation will cover the use of a variety of oxides including transparent indium tin oxide, zinc oxide, titanium dioxide, vanadium oxide, and strontium titanate thin films for stretchable and conformal applications. The presentation will also discuss results from the various oxide based stretchable platforms for applications in sensing, optics, and memories. The use of oxides for biosensors and electronic skin with integration of brain mimicking electronics will also be presented.

We are exploring the versatility of doped functional oxides integrated on silicon for the telecom wavelength range (1.3µm-1.55µm). One core concept is to exploit the giant nonlinear optical coefficients and rare-earth doping (e.g. Erbium) of crystalline oxides to provide novel solutions for Si photonic devices. Our focus is on yttria-stabilized zirconia (YSZ) as oxide matrix, having obtained first results in terms of light amplification and nonlinear effects with YSZ waveguides. Superlattice approach for Er-doped layers will be described, together with the first steps towards the implementation of functional oxides for nonlinear optical applications. The presented work was done in collaboration with A. Ruiz-Caridad, G. Marcaud, S. Serna, L. Largeau, S. Matzen, G. Agnus, P. Aubert, P. Lecoeur, K. Panaghiotis, M. Rerat, N. Dubreuil, X. Le Roux, C. Alonso-Ramos, and L. Vivien.

Nano and microstructures of ternary oxide compounds, such as nickel gallate, indium-zinc-oxide compounds, and lithium stagnates, have been successfully synthetized by a vapor-solid method. Following this synthesis process, a significant amount of material is produced in an economic and scalable way. NiGa₂O₄ nano- and microneedles are grown using mixtures of Ga₂O₃ powders and Ni and Ga metallic powders as main precursors. In the case of In₂Zn_kO_3+k compounds, the precursor blend contains ZnO and InN or In₂S₃ as a source for indium atoms, producing 1-dimensional or 2-dimentional preferential growth, respectively. The growth of complex branched structures of Li₂SnO₃ has been achieved with the use of SnO₂ and Li₂CO₃. The temperature and precursor selection allow us to engineer the size, grade of complexity and final morphology of the structures. The growth mechanism of the obtained nano and microstructures is discussed and the driven force behind it is identified as anisotropic growth, autocatalytic process and dislocation driven mechanisms, depending on the specific materials and experimental conditions. Ternary compounds will be presented together with their properties characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), transmission electron microscopy (TEM) and Raman spectroscopy techniques. The possibility of synthetizing nanocomposites will be also briefly discussed.

Ultra-wide bandgap (UWBG) semiconductors and ultra-thin two-dimensional materials (2D) are at the very frontier of the electronics for energy management or energy electronics. A new generation of UWBG semiconductors will open new territories for higher power rated power electronics and deeper ultraviolet optoelectronics. Gallium oxide - Ga₂O₃ (4.5-4.9 eV), has recently emerged as a suitable platform for extending the limits which are set by conventional (~3 eV) WBG e.g. SiC and GaN and transparent conductive oxides (TCO) e.g. In₂O₃, ZnO, SnO₂. Besides, Ga₂O₃, the first efficient oxide semiconductor for energy electronics, is opening the door to many more semiconductor oxides (indeed, the largest family of UWBGs) to be investigated. Among these new power electronic materials, ZnGa₂O₄ (~5 eV) enables bipolar energy electronics, based on a spinel chemistry, for the first time. In the lower power rating end, power consumption also is also a main issue for modern computers and supercomputers. With the predicted end of the Moore’s law, the memory wall and the heat wall, new electronics materials and new computing paradigms are required to balance the big data (information) and energy requirements, just as the human brain does. Atomically thin 2D-materials, and the rich associated material systems (e.g. graphene (metal), MoS₂ (semiconductor) and h-BN (insulator)), have also attracted a lot of attention recently for beyond-silicon neuromorphic computing with record ultra-low power consumption. Thus, energy nanoelectronics based on UWBG and 2D materials are simultaneously extending the current frontiers of electronics and addressing the issue of electricity consumption, a central theme in the actions against climate change.

“Sybilla” equipment has been developed for two decades to propose solutions to the challenges faced by the very promising and expanding field of oxide thin film deposition. The underlying technology named Chemical Beam Vapor Deposition (CBVD) inherits its basic concept from Chemical Beam Epitaxy (CBE), and consists in effusing (in high vacuum conditions) beams of organometallic compounds towards a substrate on which they decompose under energy activation to form the film. The technique enables deposition of multi-element oxides (up to 3 was tested, up to 5 possible), either homogenously or in combinatorial mode (i.e with controlled precursor flow gradient emitted onto the substrate, in good agreement with theoretical model predictions). High homogeneity films can be achieved, even on large substrates (scaling between 150 mm and 450 mm wafer was shown). Precursor decomposition can be initiated either thermally (substrate heating) or by irradiating with energetic beams (laser and electron activations were studied). Additive growth can be obtained by such localized irradiation, or alternatively depositing through shadow masks and benefiting from the line-of-sight nature of the technique (and exploiting the precursor decomposition kinetics not to damage masks). The multi-parameter nature of the deposition technology (precursor nature, different flows, temperature) allows to tune growth rate (from few nm/h to several μm/h) as well as thin film physico-chemical properties (chemical composition, film morphology, crystallinity, etc.) and functional properties. Combinatorial growth reveals a very efficient facility to optimize processes (in one shot, saving time and resources) and address new thin film architectures.

Despite being a bona-fide bulk insulator, diamond develops an intriguing two-dimensional (2D) p-type surface conductivity when its surface is terminated by hydrogen and exposed to appropriate surface adsorbate layer as a result of the surface transfer doping process. Consequently, the surface of diamond presents a versatile platform for exploiting some of the extraordinary physical and chemical properties of diamond, leading to applications such as chemical/biological sensing and the development of high-power and high-frequency field-effect transistors (FETs). In this talk, I will describe our recent work on the surface transfer doping of diamond by transition metal oxides (TMOs), which give rise to an underlying two-dimensional (2D) hole conducting layer on diamond that can be harnessed for building devices and the exploration of quantum transport properties.

We propose to explore tandem junctions associating single crystalline silicon bottom cell (Eg = 1.12 eV) and wide bandgap (1.7 eV) CIGS top cell, using GaP intermediate layer. Our purpose is to grow CIGS films under epitaxial conditions on GaP to improve the top cell efficiency, thanks to a reduction of the structural defects density detrimental for the cell performance, so that CIGS-Si tandem cells can emerge as cost competitive for the next generation of PV modules. Epitaxy of CIGS (CIGSe or CIGSu) on GaP/Si platform is demonstrated and preliminary results on AZO/ZnMgO/CdS/CIGS cells on Mo/Glass and GaP/Si are reported.

In this talk I will show how the Rashba spin-orbit coupling present in two-dimensional electron gases (2DEGs) based on SrTiO3 can be used to interconvert spin and charge currents with great efficiency. The conversion is highly tuneable by gate voltages, as the Fermi level is displaced across the complex multi-orbital band structure of the 2DEG. These results offer possibilities for device applications, for instance into Intel's MESO transistor. Finally, I will present a new ferromagnet-free approach to achieve a non-volatile control of spin-charge interconversion with ferroelectric Rashba 2DEGs, that may pave the way to an entirely new family of ultralow power spintronics device.

Electronic excitations and their real-time dynamics are critical for how we use optical materials in applications and how we probe carrier thermalization and energy transfer between electrons and ions. Recent experimental advances allow us to do so with unprecedented accuracy and time resolution, however, their interpretation relies on detailed theoretical understanding. This can be provided by first-principles theoretical-spectroscopy, based on many-body perturbation theory and time-dependent density functional theory. This talk will briefly discuss how we use quantum-mechanical first-principles simulations, based on the GW+BSE approach, to provide an accurate description of optical properties for oxide materials. I will then show how different dielectric screening contributions affect this description: More specifically, we compare screening due to free carriers for doped systems, electronic interband screening, and lattice polarizability. The first effect can be modeled using a Thomas-Fermi description of free electrons, and the latter can be described using the Froehlich model. Incorporating these into our simulations, allows us to quantify how exciton binding energies are reduced by additional screening and how optical and excitonic properties of various transparent conducting oxides are impacted. Finally, we explore carrier dynamics emerging in ZnO after an initial optical excitation by imposing occupation numbers on electron and hole Kohn-Sham states. The resulting temperature-dependent optical spectra allow us to draw conclusions about contributions from band-gap renormalization, Burstein-Moss shift, and inter-band transitions in excellent agreement with experiment.

It has been a strong interest in the laser systems with segmented gain/loss with PT-symmetry properties. This paper shows that a fiber laser pumped from one end with a depleting pump meets the conditions of PT-symmetry in the vicinity of the turning point where the gain becomes equal to the loss. The real component of the square of the complex refractive index is symmetric with respect to the turning point while the imaginary part is anti-symmetric. When the gain grows, the laser becomes segmented into two regions with dominating gain or loss. The turning point starts moving towards the other end of the laser. It leads to the appearance, in addition to the laser mode evenly distributed over the entire laser cavity, of a mode that is pushed in the lossy region. The frequency of that mode is in resonance with the length of the lossy region. The experimental data for a 15-m long Erbium doped fiber laser correlated with such hypothesis. We also built a nanocolloid capillary optical amplifier using synthesized phosphor NaY_0.83 F₄: Yb³⁺ _0.14, Er³⁺ _0.03. The effect of upconversion emission in such amplifier leads to an additional channel for the pump depletion. This potentially can cause even stronger PT-symmetry effects in a laser based on such optical amplifier.

Down-conversion phosphors based on ions of lanthanides can be used to convert UV/blue components of solar spectrum in near-infrared radiation more suitable for conventional silicon photovoltaic (PV) cells. Comparing to other down convertors, their advantages are long Stokes shift (>300 nm) excluding re-absorption, and environment safety. However, down-conversion efficiency remains an issue. The paper reports on synthesis and characterization of down-conversion phosphor based on lanthanide-doped fluoride NaYF₄:Yb³⁺, Er³⁺. The phosphor was synthesized using inexpensive wet method and further baked at ~500°C for 1 h to convert NaYF₄ matrix from cubic to hexagonal phase. The obtained micro-powder was ball-milled to nano-powder. The phosphor demonstrated down-conversion radiation from 830 to 1100 nm attributed to Yb³⁺. The Stokes shift was ~600 nm. The intensity of NIR peak radiation increased three times with increasing concentration of Yb³⁺ from 3 to 14%. A thin-sheet down-convertor of a Luminescent Solar Concentrator (LSC) was made in the form of a projector Mylar transparency coated with nanocolloid of the phosphor in a solution of polymer PMMA in chlorobenzene. Such thin sheet improved by 15% the PV power produced by the LSC being illuminated with a solar simulator as compared to an incandescent light bulb. The obtained results can be used in building more efficient PV green power.

Material characterizations were carried out to investigate the optical properties and surface morphology of Ga2O3:Se wafer obtained through ion implantation. Se concentration in Ga2O3 varies by adjusting the implant dosage up to 1⨉10^16 ions/cm2. The scanning electron microscopy (SEM) and atomic force microscope (AFM) were conducted to reveal the surface morphology, which shows the surface quality of the samples is likely to be improved with Se-implantation. Optical absorption measurement was also carried out to determine the effect of Se on the properties of Ga2O3. The results indicate the potential use of Ga2O3:Se in ultraviolet photodetector or electronic device applications.

Gallium oxide (Ga₂O₃), an ultra-wide bandgap semiconductor with potential applications in power devices, may be doped with Mg to control the native n-type conductivity. The charge transitions associated with Mg in Mg-doped β-Ga₂O₃ crystals are studied using photoinduced electron paramagnetic resonance (photo-EPR) spectroscopy to understand the mechanisms that produce stable semi-insulating substrates. The steady state photo-EPR measurements are performed at 130 K by illuminating the samples with photon energy from 0.7 to 4.7 eV. Our results show that there are two transitions associated with Mg in the bandgap: onset of quenching of neutral Mg at 1.5 eV and excitation at 3.0 eV. The quenching threshold is consistent with several DFT predicted values for Mg^-/0 level. Therefore, we suggest the quenching is due to transition of an electron from the valence band to the neutral Mg. For photoexcitation, hole capture is the only viable process due to polaronic nature of neutral Mg in Ga₂O₃. The measurements demonstrate that electron excitation to impurities, such as Fe and Ir, does not contribute to creation of the holes. Further, gallium vacancies must not participate since their characteristic EPR spectrum is never seen. Thus, we speculate that the defects responsible for the hole formation and consequent excitation of the neutral Mg are oxygen vacancies.

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.

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.