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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. [ABSTRACT FROM AUTHOR]

In this work, we explored the growth regime of (100) β-Ga2O3 homoepitaxial films on substrates with different miscut angles (1°, 2°, and 4°) in the MOVPE system. Under a low O2/Ga ratio growth condition, step-flow growth of (100) β-Ga2O3 homoepitaxial films can be maintained up to 3 μm on substrates with different miscut angles. Moreover, the results reveal that the growth rate decreases slightly with decreasing miscut angles, which matches estimation of the Burton–Cabrera–Frank theory and can be explained by the model of adsorption–desorption. By comparing the miscut-dependent growth rates, we give experimental evidence on the fundamental difference between the growth of (100) β-Ga2O3 films by MOVPE and MBE. In addition, a transport model is proposed to explain the desorption process in terms of the boundary layer and the kinetic resistance. [ABSTRACT FROM AUTHOR]

[Display omitted] • In-situ monitoring of the MOVPE process. • Reflectance spectroscopy application on the heteroepitaxial β-Ga 2 O 3 thin film on sapphire for solar-blind detector application. • Thin-film growth rate and surface roughness were determined during the MOVPE growth. • A potential method for monitoring (Al x Ga 1-x) 2 O 3 growth process. Metalorganic vapor-phase epitaxy of β-Ga 2 O 3 /c-plane Al 2 O 3 heterostructures was monitored in-situ by spectral reflectance in different wavelengths. The reflectance spectrum was analysed as a function of the growth time and the incident wavelength to estimate the growth rate and the refractive index at the growth temperatures. The obtained values are validated by ex-situ methods such as secondary ion mass spectrum measurement and spectroscopic ellipsometry. A theoretical simulation of the reflectance spectrum was carried out by combining a transfer matrix method with a multilayer model, and a good agreement with the experimental results is presented. [ABSTRACT FROM AUTHOR]

• In-situ monitoring the MOVPE process. • Reflectance spectroscopy application on the homoepitaxial (1 0 0) β-Ga 2 O 3 thin films. • Thin-film growth rate and surface growth mode determined during the MOVPE growth. We investigate Si-doped β-Ga 2 O 3 layers grown by metalorganic vapor phase epitaxy (MOVPE) on (1 0 0) β-Ga 2 O 3 semi-insulating substrates with the reflection spectrum incident normally onto the substrate surface. The reflectance spectrum transformed into the autocorrelation domain shows a more pronounced Fabry-Pérot oscillation than the raw spectrum, from which one can easily estimate the growth rate and the growth mode based on the period, and the damping behavior of the spectrum, respectively. The observed oscillation contributes to the refractive index difference between the bulk substrate and the grown film caused by the incorporated impurities and different preparation techniques. The high sensitivity of the reflectance spectroscopy is demonstrated to be an advantage as a powerful growth process monitoring tool. [ABSTRACT FROM AUTHOR]

• The parametric analysis was performed for MOVPE-grown β-Ga 2 O 3 thin film with the help of random forest model. • The contribution of the growth parameter to the thin-film growth rate was studied. • The outcome of model is applicable for both homo- and heteroepitaxy process, and for (1 0 0), (0 1 0) and (−2 0 1) orientations. In this work, we demonstrate a machine learning approach, Random Forest, for the β-Ga 2 O 3 growth rate prediction in the metal–organic vapor phase epitaxy (MOVPE) by analyzing the growth process of β-Ga 2 O 3 on sapphire optically. The proposed model can assess the complex non-linear dependencies among the growth parameters and optimize them for the optimal growth rate. The model based on the process parameters (e.g., precursor concentration, chamber pressure, and push gas) provides high predictive power, reaching the coefficient of determination (R2) of 0.95 and 0.92 for the training and testing sets. The outcome of the model is applicable to both homoepitaxial and heteroepitaxial processes and on different substrate orientations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Films show morphological instabilities at a certain thickness when grown under a high O 2 /Ga ratio. • Growth under low O 2 /Ga ratios (O 2 /Ga = 350) effectively suppresses meandering and bunching instabilities. • Observations are rationalized through the Burton-Cabrera-Frank (BCF) theory, linking theory and experiment. The morphology evolution of (1 0 0) β-Ga 2 O 3 films grown by metalorganic vapor phase epitaxy (MOVPE) is studied by atomic force microscopy (AFM). In particular, when grown under a high O 2 /Ga ratio (O 2 /Ga = 1250) and above a thickness of 350 nm, these films exhibit striking morphological instabilities, including step meandering and bunching, which contribute significantly to surface roughening. Transmission electronic microscopy (TEM) measurements reveal a morphological transition on the growing surface with a coexistence of step-flow and step-bunching growth modes resulting from these instabilities. In contrast, growth conditions with low O 2 /Ga ratios (O 2 /Ga = 350) effectively suppress meandering and bunching instabilities, resulting in films with excellent electrical properties. These observations are rationalized through the Burton-Cabrera-Frank (BCF) theory and Bales-Zangwill (BZ) instability (Gibbs-Thompson effect and Mullins-Sekerka instability) , thus bridging theoretical models and the experimental results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 < 200 pm) of the epi-ready wafers. The free electron concentration and Hall mobility at room temperature were in the range of 1.6–9 × 1018 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. [ABSTRACT FROM AUTHOR]

Epitaxially strained NaNbO3 films 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 NaNbO3 films on NdGaO3 exhibit the orthorhombic c phase. With increasing in-plane strain a first structural phase transition to the monoclinic r phase and, further on, for films grown under tensile strain on rare earth scandates, a second phase transition to the aa phase, 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 NaNbO3 films. [ABSTRACT FROM AUTHOR]

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-Ga 2 O 3 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-Ga 2 O 3 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-Ga 2 O 3 MOSFET devices. [ABSTRACT FROM AUTHOR]