Andreas Fiedler

The transport properties and electrical bandgap of nominally undoped 75‐nm‐thick CrN layers simultaneously grown on AlN(0001) and AlN(112) templates using plasma‐assisted molecular beam epitaxy are investigated. The layers grown on AlN(0001) and AlN(112) exhibit (111) and (113) surface orientations, respectively. All layers exhibit antiferromagnetism with a Néel temperature of K, observed by temperature‐dependent magnetic and electrical measurements. Hall‐effect measurements demonstrate n‐type semiconducting behavior across a wide temperature range from 4 to 920 K. At low temperatures ( K), the data show parallel conduction channels from a metallic impurity band and the conduction band. The carrier mobility exhibits a temperature dependence consistent with a nondegenerate semiconductor, governed by ionized‐impurity scattering below 400 K and phonon scattering above 400 K. An analysis of the temperature‐dependent carrier density between 300 and 920 K yields two activation energies associated with intrinsic conduction: 0.15 eV (with an uncertainty of eV), which we attribute to the fundamental bandgap, and 0.50 eV (with an uncertainty of eV) representing a higher energy transition.

We successfully achieved the growth of β-(AlxGa1−x)2O3 (x = 0–0.55) on a (100) β-Al0.24Ga0.76O3 substrate using metalorganic vapor phase epitaxy (MOVPE). A stacked layer composed of β-(Al0.47Ga0.53)2O3/β-Ga2O3 on a β-Al0.24Ga0.76O3 substrate was demonstrated. High-resolution x-ray diffraction and reciprocal space mapping analysis verified the coherent epitaxial growth of phase-pure β-(AlxGa1−x)2O3 thin films up to an Al composition of x = 0.55. Films with higher Al content exhibited partial plastic strain relaxation. The multilayer structure of the β-(Al0.47Ga0.53)2O3/β-Ga2O3 thin film exhibited good quality and coherent growth. Atomic force microscopy measurements proved a surface roughness well below 0.42 nm for the fully strained films and about 1 nm for the relaxed ones. These results indicate that the MOVPE technique has significant potential for the fabrication of β-Ga2O3-based heterojunctions suitable for device applications.

Undoped and Sb‐doped rutile‐GeO ₂ single crystals of diameter up to 18 mm and length up to 25 mm were grown by the top‐seeded solution growth (TSSG) method using a K ₂ CO ₃ + Na ₂ CO ₃ solution and [001] growth direction. Epi‐ready wafers with (100), (001), (110), and (111) surface orientations, and sizes of 5 × 5 and 10 × 10 mm ² were prepared by chemical‐mechanical polishing. The full width at half maximum (FWHM) of the rocking curves for all orientations was confined between 13–26 arcsec, while the root mean square (RMS) roughness was between 100–150 pm. Wafer annealed in air at temperatures 600–900°C revealed that the RMS roughness could be further decreased to 70 pm for the (001) surface orientation. The prepared r‐GeO ₂ wafers are well suited for homo‐ and heteroepitaxial growth of r‐GeO ₂ , as well as r‐Ge _x Sn _1‐x O ₂ and r‐Ge _y Si _1‐y O ₂ films, respectively, and electronic power device fabrication.

The step-bunching instability in (100) β-Ga2O3 films grown via metalorganic vapor phase epitaxy was investigated using a machine learning approach based on Random Forest (RF). This study reveals the interplay of Ga supersaturation (O2/Ga) and impurity effects as coexisting mechanisms driving the morphological transition (from step-flow growth to step-bunching). The developed machine-learning framework accurately classifies growth morphology and offers valuable insights by correlating theoretical principles with experimental parameters. Critical growth parameters influencing the film morphology were identified. The corresponding strategy, high Ga supersaturation, is proposed to mitigate the step-bunching formation and maintain the desired step-flow growth mode. Despite the challenges posed by small datasets, the RF model demonstrates robust classification performance, establishing machine learning as a powerful tool for experimental science.

This study examines the feasibility of using in-situ reflectance measurement for Si-doped β-Ga2O3 films homoepitaxially grown by metalorganic vapor-phase epitaxy on substrates with varying conductivity [semi-insulating (Mg-doped) and conductive (Si-doped)]. Interference oscillation patterns were observed in the wavelength region well below the bandgap absorption, with the oscillation amplitudes showing a significant dependence on the doping concentrations of the grown films. Reflectance spectroscopy enables the estimation of growth rate and doping level based on the period and amplitude of the observed Fabry–Pérot oscillations, respectively. These oscillations occur due to the refractive index difference between the grown film and the substrate, which is influenced by doping in the homoepitaxy process and estimated by the Drude model.

In this Letter, we demonstrate the application of Deep Level Transient Spectroscopy (DLTS) and Laplace DLTS (L-DLTS) techniques to unintentionally doped β-Ga2O3 crystals grown by the Czochralski method. It is clearly shown that the capacitance signal associated with the electron emission from a trap level previously identified in the literature as E14 and characterized by an activation energy of 0.18 eV is found to be a superposition of electron emissions from two closely spaced energy levels. Furthermore, we noted that the corresponding L-DLTS signal splits into two well separated components with activation energies of 196 and 209 meV, and the splitting occurs as the electric field in the space charge region of a Schottky diode exceeds 2 × 107 V/m (0.2 MV/cm). Additionally, a strong dependency of DLTS and L-DLTS signals on the electric field strength and resulting enhancement of the electron emission from these two trap states agree well with the 1D Poole–Frenkel (PF) model, suggesting donor-like behavior of both states. Finally, we found that the barrier height for thermal emission of the electrons is significantly reduced in our samples by 121 meV due to the PF effect for experimental conditions corresponding to an electric field of 3.5 × 107 V/m (0.35 MV/cm).

This study focuses on the impact of high-doping impurities (>1018 cm−3) on the morphology of homoepitaxially grown (100) 4° off β-Ga2O3 film, as well as incorporating insights from the Cabrera–Vermilyea model (C–V model). Using atomic force microscopy imaging, we reveal that under low-supersaturation conditions, dopant-induced impurities lead to irregular step formation and growth stalling, inducing the step-bunching formation consistent with C–V model predictions. Conversely, higher supersaturation conditions restore desired step-flow morphology, resembling low-impurity growth states. It is also shown that the step-bunching formed under lower supersaturation conditions and high-impurity concentration might induce unwanted structural defects and compensate the free carriers. These findings underscore the delicate interplay between dopant concentrations, growth morphology, and supersaturation in metalorganic vapor phase epitaxy-grown (100) β-Ga2O3 films, providing a comprehensive understanding of optimizing their electrical properties with respect to power electronics applications.

Nickel-doped β-Ga2O3 crystals were investigated by optical absorption and photoconductivity, revealing Ni-related deep levels. The photoconductivity spectra were fitted using the phenomenological Kopylov and Pikhtin model to identify the energy of the zero-phonon transition (thermal ionization), Franck–Condon shift, and effective phonon energy. The resulting values are compared with the predicted ones by first-principle calculations based on the density functional theory (DFT). An acceptor level (0/−) of 1.9 eV and a donor level (+/0) of 1.1 eV above the valence band minimum are consistently determined for NiGa, which preferentially incorporates on the octahedrally coordinated Ga site. Temperature-dependent resistivity measurements yield a thermal activation energy of ∼2.0 eV that agrees well with the determined Ni acceptor level. Conclusively, Ni is an eminently suitable candidate for compensation doping for producing semi-insulating β-Ga2O3 substrates due to the position of the acceptor level (below and close to the mid-bandgap).

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.

In this work, we have investigated plasma-assisted deposition of Al2O3 on HVPE (001) β-Ga2O3 and evaluated the dielectric quality from electrical measurements on fabricated metal-oxide-semiconductor (MOS) capacitors. The interface structure and crystallinity of the films were investigated as a function of the growth temperature. The dielectric/semiconductor interfaces were found to have reverse breakdown electric fields up to 5.3 MV/cm in the β-Ga2O3, with relatively low hysteresis in capacitance–voltage and low leakage current. We determined a negative fixed interface charge density at the interface from analysis of thickness-dependent capacitance voltage data. This study shows the advantage of using plasma-assisted deposition to achieve high breakdown strength Al2O3/β-Ga2O3 MOS structures for device application purposes.