The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of β-Ga₂O₃. Epitaxial β-Ga₂O₃ layers at high growth rates (above 1 μm/h), at low reagent flows, and at reduced growth temperatures (740 °C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial β-Ga₂O₃ layers are demonstrated with a $\bar{2}$ 01 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown ($\bar{2}$ 01) β-Ga₂O₃ substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of β-Ga₂O₃.

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range ($2.45\times {10}^{18}$ cm⁻³ up to $1.10\times {10}^{20}$ cm⁻³) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance–voltage (C–V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art $p$ -GaN layers with a low resistivity and a high free-hole density (0.77 $\mathrm{\Omega }$ cm and $8.4\times {10}^{17}$ cm${}^{-3}$ , respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick Al_xGa_1−xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼ 9 $\times {10}^{12}$ cm⁻²) as the non-graded conventional structure, though the mobility drops from $\sim 2360$ cm²/V s in the conventional to $\sim 960$ cm²/V s in the graded structure. The transconductance g_m of the linearly graded channel HEMTs is shown to be flatter with smaller $g_{\mathrm{m}}^{\prime }$ and $g_{\mathrm{m}}^{″}$ as compared to the conventional non-graded channel HEMT implying improved device linearity.

Spray-coating, has recently fueled scientific interest as a versatile solution-processing technique for the realization of organic electronic devices, such as organic field-effect transistors (OFETs). In the present work, air-brush method was used for the deposition of semiconducting blends of triisopropylsilylethynyl-pentacene (TIPS-PEN) and common insulating polymers of polystyrene or polymethylmethacrylate. The use of such blend systems not only resulted in an improved wet film formation but also enabled efficient control over the crystallization process. A systematic study on the effect of different composition ratio on the morphology and crystallinity of the sprayed films as well as their macroscopic uniformity, was carried out. Both blend systems revealed well-ordered TIPS-PEN crystalline domains on the top surface, indicative of the pronounced phase separation phenomena. The optimized airbrushed OFETs exhibited excellent electrical characteristics with a maximum hole mobility value of 1.3 cm² V⁻¹ s⁻¹, negligible hysteresis, near-zero turn-on voltages and on/off current ratio greater than 10⁵. Additionally, the transistors revealed good long-term environmental stability, with no significant degradation after a period of 13 months. These results represent an important step for present and future applications of spaying techniques toward the controlled growth of high performance and environmentally stable OFETs.

Strain-stress relationships for physical properties are of interest for heteroepitaxial material systems, where strain and stress are inherent due to thermal expansion and lattice mismatch. We report linear perturbation theory strain and stress relationships for optical phonon modes in monoclinic crystals for strain and stress situations which maintain the monoclinic symmetry of the crystal. By using symmetry group analysis and phonon frequencies obtained under various deformation scenarios from density-functional perturbation theory calculations on $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, we obtain four strain and four stress potential parameters for each phonon mode. We demonstrate that these parameters are sufficient to describe the frequency shift of the modes regardless of the stress or strain pattern which maintain the monoclinic symmetry of the crystal. The deformation potentials can be used together with experimentally determined phonon frequency parameters from Raman or infrared spectroscopy to evaluate the state of strain or stress of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, for example, in epitaxial heterostructures.