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In this work, we train a hybrid deep-learning model (fDNN, Forest Deep Neural Network) to predict the doping level measured from the Hall Effect measurement at room temperature and to investigate the doping behavior of Si dopant in both (100) and (010) β-Ga2O3 thin film grown by the metalorganic vapor phase epitaxy (MOVPE). The model reveals that a hidden parameter, the Si supplied per nm (mol/nm), has a dominant influence on the doping process compared with other process parameters. An empirical relation is concluded from this model to estimate the doping level of the grown film with the Si supplied per nm (mol/nm) as the primary variable for both (100) and (010) β-Ga2O3 thin film. The outcome of the work indicates the similarity between the doping behavior of (100) and (010) β-Ga2O3 thin film via MOVPE and the generality of the results to different deposition systems.

In this work, we report on the realization of vertical (100) β-Ga2O3 FinFET devices for the use in power electronics applications. The experiments are carried out on structures consisting of highly conducting (100) β-Ga2O3 substrates with a doping concentration N D of 3 × 1018 cm−3, and epitaxially grown layers with N D of 5 × 1016 cm−3 for the drift and channel region. The fabricated FinFET devices feature enhancement-mode properties with a threshold voltage of +4.2 V and on/off-current ratio of 105. Moreover, breakdown measurements of these devices reveal an average breakdown strength of 2.7 MV cm−1. Additional device simulation indicates the presence of electric field peaks near the gate edge outside the active device as high as 7 and 5 MV cm−1 in the Al2O3 gate oxide and β-Ga2O3 semiconductor, respectively.

Absrtract In this work, we comprehensively investigate the development of unwanted parasitic particles in the MOVPE chamber while growing μm level films. The density of the parasitic particles is found to be pronounced at film thicknesses starting from >1.5 to 2 μm. These particles seem to induce structural defects such as twin lamellae, thereby harming the electrical properties of the grown film. The origin of the parasitic particle is attributed to the parasitic reactions within the chamber triggered by the promoted gas-phase reactions during the growth process, which can be largely reduced by increasing the total gas flow and decreasing the showerhead distance to the susceptor. A film thickness of up to 4 μm has been achieved after minimizing the density of parasitic particles. Thereby, RT Hall measurements reveal carrier mobilities of 160 cm2V−1s−1 at carrier concentrations of 5.7 × 1016 cm−3.

This work investigated the metalorganic vapor-phase epitaxy (MOVPE) of (100) β-Ga2O3 films with the aim of meeting the requirements to act as drift layers for high-power electronic devices. A height-adjustable showerhead achieving a close distance to the susceptor (1.5 cm) was demonstrated to be a critical factor in increasing the stability of the Ga wetting layer (or Ga adlayer) on the surface and reducing parasitic particles. A film thickness of up to 3 μm has been achieved while keeping the root mean square below 0.7 nm. Record carrier mobilities of 155 cm2 V−1 s−1 (2.2 μm) and 163 cm2 V−1 s−1 (3 μm) at room temperature were measured for (100) β-Ga2O3 films with carrier concentrations of 5.7 × 1016 and 7.1 × 1016 cm−3, respectively. Analysis of temperature-dependent Hall mobility and carrier concentration data revealed a low background compensating acceptor concentration of 4 × 1015 cm−3.

We have systematically studied the growth, by the Czochralski method, and basic physical properties of a 2 cm and 2 in. diameter bulk β-(AlxGa1−x)2O3 single crystal with [Al] = 0–35 mol. % in the melt in 5 mol. % steps. The segregation coefficient of Al in the Ga2O3 melt of 1.1–1.2 results in a higher Al content in the crystals than in the melt. The crystals were also co-doped with Si or Mg. [Al] = 30 mol. % in the melt (33–36 mol. % in the crystals) seems to be a limit for obtaining bulk single crystals of high structural quality suitable for homoepitaxy. The crystals were either semiconducting (no intentional co-dopants with [Al] = 0–30 mol. % and Si-doped with [Al] = 15–20 mol. %), degenerately semiconducting (Si-doped with [Al] ≤ 15 mol. %), or semi-insulating ([Al] ≥ 25 mol. % and/or Mg-doped). The full width at half maximum of the rocking curve was 30–50 arcsec. The crystals showed a linear but anisotropic decrease in all lattice constants and a linear increase in the optical bandgap (5.6 eV for [Al] = 30 mol. %). The room temperature electron mobility at similar free electron concentrations gradually decreases with [Al], presumably due to enhanced scattering at phonons as the result of a larger lattice distortion. In Si co-doped crystals, the scattering is enhanced by ionized impurities. Measured electron mobilities and bandgaps enabled to estimate the Baliga figure of merit for electronic devices.

Beta gallium oxide (β-Ga2O3) is a promising ultra-wide bandgap semiconductor with attractive physical properties for next-generation high-power devices, radio frequency electronics, and solar-blind ultraviolet radiation detectors. Here, we present an overview and perspective on the development of MOVPE-grown (100) β-Ga2O3 thin films and its role in supplementing high-power electronics. We review the development path of the growth process on (100) β-Ga2O3 thin films with a discussion regarding the solved and remaining challenges. The structural defect formation mechanism, substrate treatment strategies, and different growth windows are analyzed to optimize the grown film to fulfill the requirements for device fabrication. Toward industrial applications, MOVPE-grown β-Ga2O3 thin films are evaluated in two aspects: thick layers with smooth surface roughness and the electrical properties in terms of high carrier mobility and low doping concentration. Based on the reviewed results, we propose strategies in substrate preparation treatments and supportive tools such as the machine learning approaches for future growth process optimization and envision the rising interest of the β-Ga2O3-related alloy, β-(AlxGa1−x)2O3.

Lateral $\beta $ -Ga2O3 MOSFET for power switching applications with a 1.8 kV breakdown voltage and a record power figure of merit of 155 MW/cm2 are demonstrated. Sub- $\mu \text{m}$ gate length combined with gate recess was used to achieve low ON-state resistances with reasonable threshold voltages above −24 V. The combination of compensation-doped high-quality crystals, implantation-based inter-device isolation, and SiN x -passivation yielded in consistently high average breakdown field strengths of 1.8–2.2 MV/cm for gate–drain spacings between 2 and 10 $\mu \text{m}$ . These values outperform the results of more established wide-bandgap device technologies, such as SiC or GaN, and the major Ga2O3 material promise—a higher breakdown strength—is well demonstrated.

In this study, in situ synchrotron x-ray diffraction was used to investigate a high-temperature phase transition in tensile-strained epitaxial NaNbO3/(110)DyScO3 films. A ferro-to-ferroelectric phase transition was found in the temperature range between 230 and 350 °C, with a very pronounced thermal hysteresis. While at room temperature the well known monoclinic a1a2 in-plane ferroelectric phase is present, here, a slightly disturbed orthorhombic ferroelectric a1/a2 phase with exclusive in-plane polarization was found at high temperatures. This phase appeared in four variants, indicating the occurrence of four types of superdomains. The orthorhombic symmetry and the corresponding domain morphology observed at high temperatures showed a striking similarity to K0.9Na0.1NbO3 thin films grown on a (110) NdScO3 substrate.

Beta-gallium oxide (β-Ga2O3) is an emerging ultra-wide bandgap (4.8 eV) [1] semiconductor material. The high band gap leads to a theoretical breakdown field strength of 8 MV/cm [2], which enables the potential for β-Ga2O3 to outperform SiC and GaN and to become the next generation of high-performance material with respect to power electronic applications. Since transistors based on β-Ga2O3 benefit from a low on-resistance at a given breakdown voltage less power losses within a transistor switching operation can be achieved [3]. Thereby the electrically active part of the device is made up of a homoepitaxial thin layer. To fulfil the requirements from the device side, the epitaxial β-Ga2O3 layer has to have high crystalline perfections. To achieve such high quality layers was the aim of this work. β-Ga2O3 thin layer were grown homoepitaxially by MOVPE [4, 5] on Mg-doped β-Ga2O3 (100) substrates [6] with 2°, 4° and 6° miscut [7]. A development of the growth process with respect to growth modes and growth rate has been done. The Investigations were focusing on the interplay of the technical parameters i.e.: chamber pressure, Ar- push gas, precursor flux and their ratios. A transition from 2D island to step flow to step bunching growth modes has been achieved by varying the chamber pressure and/or the Ga/O2 ratio. This work shows that an adjusting of the effective diffusion length to the substrate step width is crucial to obtain step flow growth and with that high quality β-Ga2O3 (100) layers with electron mobilities above 150 cm²/Vs.

A Langmuir adsorption model of the Si incorporation mechanism into metalorganic vapor-phase epitaxy grown (100) β-Ga2O3 thin films is proposed in terms of the competitive surface adsorption process between Si and Ga atoms. The outcome of the model can describe the major feature of the doping process and indicate a growth rate-dependent doping behavior, which is validated experimentally and further generalized to different growth conditions and different substrate orientations.

We provide a combined theoretical and experimental study of the electronic structure and the optical absorption edge of the orthorhombic perovskite LaInO$_{3}$. Employing density-functional theory and many-body perturbation theory, we predict a direct electronic quasiparticle band gap of about 5 eV and an effective electron (hole) mass of 0.31 (0.48) m$_{0}$. We find the lowest-energy excitation at 0.2 eV below the fundamental gap, reflecting a sizeable electron-hole attraction. Since the transition from the valence band maximum (VBM, $\Gamma$ point) is, however, dipole forbidden the onset is characterized by weak excitations from transitions around it. The first intense excitation appears about 0.32 eV above. Interestingly, this value coincides with an experimental value obtained by ellipsometry (4.80 eV) which is higher than the onset from optical absorption spectroscopy (4.35 eV). The latter discrepancy is attributed to the fact that the weak transitions that define the optical gap are not resolved by the ellipsometry measurement. The absorption edge shows a strong dependency on the light polarization, reflecting the character of the involved valence states. Temperature-dependent measurements show a redshift of the optical gap by about 120 meV by increasing the temperature from 5 to 300 K. Renormalization due to zero-point vibrations is extrapolated from the latter measurement to amount to 150 meV. By adding the excitonic binding energy of 0.2 eV obtained theoretically to the experimental optical absorption onset, we determine the fundamental band gap at room temperature to be 4.55 eV.

The authors propose a statistical model for Kinetic Monte Carlo computations of the homoepitaxial growth of Ga2O3 and compare it with experimental results.

Two inch diameter, highly conducting (Si-doped) bulk β-Ga2O3 single crystals with the cylinder length up to one inch were grown by the Czochralski method. The obtained crystals revealed high structural quality characterized by narrow x-ray rocking curves (FWHM ≤ 25 arc sec) and high surface smoothness (RMS 18 cm−3 and 118 – 52 cm2 V−1 s−1, respectively, which are not affected by a heat treatment at temperatures up to 1000 °C in an oxidizing atmosphere. Temperature-dependent electrical properties of the crystals revealed a degenerated semiconducting state. Both high structural quality and electrical properties make the crystals well suited as substrates for homoepitaxy and electronic device fabrication in the vertical configuration.

In this work, we report on the realization of SnO/β-Ga2O3 heterojunction vertical diodes and lateral field-effect transistors for power electronic applications. The p-type semiconductor SnO is grown by plasma-assisted molecular beam epitaxy on n-type (100) β-Ga2O3 with donor concentrations of 3 × 1017 cm−3 for the diode devices and 8.1 × 1017 cm−3 for the field-effect transistors. The deposited films show a predominant SnO (001) phase featuring a hole concentration and a mobility of 7.2 × 1018 cm−3 and 1.5 cm2/V s, respectively. The subsequent electrical characterization of the heterojunction diodes and field-effect transistors show stable switching properties with on/off current ratios >106 and specific on-resistances below 4 mΩ cm2. Furthermore, breakdown measurements in air of the non-field-plated heterojunction transistor with a gate-to-drain distance of 4 μm yield a breakdown voltage of 750 V, which equals an average breakdown strength of nearly 1.9 MV/cm. The resulting power figure of merit is calculated to 178 MW/cm2 demonstrating state-of-the-art properties. This emphasizes the high potential of this heterojunction approach.

A high growth rate process above 1 µm/h was achieved for Si-doped (100) β-Ga2O3 homoepitaxial films grown via metalorganic vapor phase epitaxy (MOVPE) while maintaining high crystalline perfection up to a film thickness of 3 µm. The main growth parameters were investigated to increase the growth rate and maintain the step-flow growth mode, wherein the enhanced diffusion channel due to the formation of a Ga adlayer was proposed to be the possible growth mechanism. Si doping allowed precise control of the n-type conductivity of the films with electron concentrations ranging from 1.5 × 1017 to 1.5 × 1019 cm−3 and corresponding mobilities from 144 to 21 cm2 V−1 s−1, as revealed by Hall effect measurements at room temperature. Secondary ion mass spectrometry confirmed homogeneous Si doping through the film and a one-to-one correlation between the Si concentration and the electron concentration. Low defect density in the films was determined by x-ray diffraction measurements. The demonstration of a high growth rate process of β-Ga2O3 films with μm level thickness and smooth surface morphology via MOVPE is critical for high power electronics with vertical device architecture.

The influence of substrate miscut on the surface morphology and Hall mobility at optimized growth conditions of $\beta$ -Ga 2 O 3 layers was investigated. The results demonstrate that the surface morphology and the Hall mobility are impacted by the miscut angle of $\beta$ -Ga 2 O 3 substrates. For substrates with miscut angle of 6° and a growth rate of about 4 nm/min, a morphology with step bunches was observed and a Hall mobility of the layer of $115\pm 10$ cm2/(Vs) was obtained. Using substrates with 4° miscut, step-flow growth mode and an enhancement of the Hall mobility about $135\pm 10\ \mathrm{cm}^{2}/(\mathrm{Vs})$ was achieved.

In the course of development of transparent semiconducting oxides (TSOs) we compare the growth and basic physical properties bulk single crystals of ultra-wide bandgap (UWBG) TSOs, namely β-Ga2O3 and Ga-based spinels MgGa2O4, ZnGa2O4, and Zn1-xMgxGa2O4. High melting points of the materials of about 1800 -1930 °C and their thermal instability, including incongruent decomposition of Ga-based spinels, require additional tools to obtain large crystal volume of high structural quality that can be used for electronic and optoelectronic devices. Bulk β-Ga2O3 single crystals were grown by the Czochralski method with a diameter up to 2 inch, while the Ga-based spinel single crystals either by the Czochralski, Kyropoulos-like, or vertical gradient freeze / Bridgman methods with a volume of several to over a dozen cm3. The UWBG TSOs discussed here have optical bandgaps of about 4.6 - 5 eV and great transparency in the UV / visible spectrum. The materials can be obtained as electrical insulators, n-type semiconductors, or n-type degenerate semiconductors. The free electron concentration (ne) of bulk β-Ga2O3 crystals can be tuned within three orders of magnitude 1016 - 1019 cm−3 with a maximum Hall electron mobility (μ) of 160 cm2V−1s−1, that gradually decreases with ne. In the case of the bulk Ga-based spinel crystals with no intentional doping, the maximum of ne and μ increase with decreasing the Mg content in the compound and reach values of about 1020 cm−3 and about 100 cm2V−1s−1 (at ne > 1019 cm−3), respectively, for pure ZnGa2O4.

In this work, the fabrication of high-performance metal-oxide-semiconductor field-effect transistors (MOSFETs) on Si-doped homoepitaxial layers on (100) Mg-doped semi-insulating β-Ga2O3 substrates is demonstrated. Furthermore, the influence of additional gate and source field-plates is analyzed in regards to the breakdown properties of the MOSFET devices. The comparative study reveals comparable transistor performances of devices with and without field-plates showing maximum drain currents, on/off-current ratios and ON-resistances of ~75 mA/mm, 109 and 110 Ω∙mm, respectively. Moreover, both MOSFET structures show comparable breakdown voltages at around 1200 V which equals an average breakdown field strength of 2 MV/cm emphasizing no influence of the field-plates on the breakdown properties. Analysis of the thin film composition by secondary ion mass spectrometry on the processed β-Ga2O3 wafers indicates an increased concentration of silicon dopants at the interface between epitaxial layer and substrate. Our findings suggest that this might be the current main limitation factor in reaching higher breakdown voltages using field-plated β-Ga2O3 MOSFET devices.

NbO2 is a promising candidate for resistive switching devices due to an insulator-metal transition above room temperature, which is related to a phase transition from a distorted rutile structure to an undistorted one. However, the electrical resistivity of the NbO2 thin films produced so far has been too low to achieve high on-off switching ratios. Here, we report on the structural, electrical, and optical characterization of single-crystalline NbO2 (001) thin films grown by pulsed laser deposition on MgF2 (001) substrates. An annealing step reduced the full width at half maximum of the NbO2 (004) x-ray Bragg reflection by one order of magnitude, while the electrical resistivity of the films increased by two orders of magnitude to about 1 kΩcm at room temperature. Temperature-dependent resistivity measurements of an annealed sample revealed that below 650 K, two deep-level defects with activation energies of 0.25 eV and 0.37 eV dominate the conduction, while above 650 K, intrinsic conduction prevails. Optical characterization by spectroscopic ellipsometry and by absorption measurements with the electric field vector of the incident light perpendicular to the c-axis of the distorted rutile structure indicates the onset of fundamental absorption at about 0.76 eV at room temperature, while at 4 K, the onset shifts to 0.85 eV. These optical transitions are interpreted to take place across the theoretically predicted indirect bandgap of distorted rutile NbO2.

We measure the low-frequency noise in epitaxial β-Ga2O3 grown by MOVPE. Both 1/f and generation-recombination noise components are well resolved. The Hooge parameters characterizing the 1/f noise are 3 × 10–4 at room temperature and 2 × 10–5 at temperatures near 200 K. Mid bandgap trap states result in generation-recombination noise that is analyzed using temperature dependent low-frequency deep-level noise spectroscopy. Trap levels with energies of 165, 127, and 37 meV below the conduction band minimum are characterized in terms of density and activation energy.

Epitaxially strained NaNbO3films were grown by liquid-delivery spin metal–organic chemical vapour deposition on several oxide substrates, inducing tensile and compressive lattice strain. High-resolution X-ray diffraction measurements reveal that coherently grown compressively strained NaNbO3films on NdGaO3exhibit the orthorhombiccphase. With increasing in-plane strain a first structural phase transition to the monoclinicrphase and, further on, for films grown under tensile strain on rare earth scandates, a second phase transition to theaaphase, are observed. Our results are in good agreement with the pathway of phase transitions predicted by Diéguez, Rabe & Vanderbilt [Phys. Rev. B, (2005),72, 144101] for NaNbO3films.

The quench sensitivity and the effect of lithium content on the precipitation hardening of Al-x at% Li (x = 0.5, 1, 2, 3, 6, and 8) alloys have been investigated by microhardness tests, scanning electron microscopy (SEM),and x-ray diffraction. In addition, the grain-size dependence on lithium concentration has been studied. The results showed that: (i) alloys with less than 6 at% Li are insensitive to quench rate, whereas alloys containing 6 at% Li or more are, to some extent, quench sensitive; (ii) the microhardness increases with increasing lithium content; (iii) SEM results show that the grain size diminishes with increasing lithium concentration both in naturally and in artificially aged specimens, the grain size in artificially aged specimens being greater than that in naturally aged specimens; (iv) the coexistence of δ' and δ phases suggests that small amounts of the δ phase could be precipitated at the expense of the δ' phase via partial dissolution.

The kinetics of the reactions (dissolution and precipitation) in Al-8 at% Li alloy were investigated by differential scanning calorimetry (DSC). Analysis of nonisothermal DSC scans at different heating rates was carried out to evaluate the overall activation energies associated with the transformation processes during continuous heating of the quenched alloys. An average activation energy associated with dissolution of Guinier-Preston zones was found to be 73.42′5.4I kJmol - 1 , which implies that the dissolution process is controlled by the migration of lithium atoms through the aluminium matrix. The average activation energy of 6' phase precipitation was 62.65′4.43 kJ mol - 1 . The activation energies of dissolution of the δ' phase and dissolution of the δ phase or lithium in the matrix were 132. 47 ′4. 03 and 398. 40 ′ 3.98 kJ mol - 1 , respectively. The reaction order of all processes indicates that they occur three-dimensionally throughout the aluminium matrix. δ and δ' phases have been detected in naturally (1800 min) aged specimens by x-ray diffraction.