We have demonstrated wide bandgap and composition range β-(Al_xGa_1−x)₂O₃ thin films by employing thermal annealing of β-Ga₂O₃/sapphire templates. With proper annealing conditions at 1000–1500 °C, the β-Ga₂O₃ thin films transformed to the β-(Al_xGa_1−x)₂O₃ thin films with different bandgaps and compositions due to the Al diffusion from sapphire. Meanwhile, the Ga atoms diffused into sapphire. The interdiffusion process caused an increased film thickness, which was enhanced in proportion to the annealing temperature. It was confirmed by secondary ion mass spectrometry (SIMS) and transmission electron microscopy. Thus, higher temperatures resulted in high Al contents in the β-(Al_xGa_1−x)₂O₃ films. Also, the SIMS measurements show highly homogeneous Al contents throughout the β-(Al_xGa_1−x)₂O₃ films annealed at 1200 °C and above. Evaluated by x-ray diffraction (XRD), the Al content range of the samples is 0–0.81 for the β-Ga₂O₃ templates without annealing and with annealing up to 1500 °C. Evaluated by UV-Vis spectroscopy, the optical bandgap range of the samples is 4.88–6.38 eV for the β-Ga₂O₃ templates without annealing and with annealing up to 1400 °C, translating to the Al content range of 0–0.72. Moreover, the crystal quality of β-(Al_xGa_1−x)₂O₃ improved as the Al composition became larger due to higher annealing temperatures. The proposed technique is promising for the preparation of β-(Al_xGa_1−x)₂O₃ thin films without employing “direct-growth” techniques.

In this research, β-Ga₂O₃/NiO heterostructures were grown directly on CeO₂ buffered Hastelloy flexible substrates. With pulsed laser deposition under high temperatures, as-grown β-Ga₂O₃ and NiO thin films have a preferred out-of-plane orientation along the ⟨−201⟩ and ➎111➉ directions. This is due to the ideal epitaxial ability of the CeO₂ buffer layer, which serves as a perfect template for the epitaxial growth of single-oriented NiO and β-Ga₂O₃ by creating a constant gradient from CeO₂ (2.7 Å along ➎001➉) to NiO (2.9 Å along ➎110➉), and eventually to β-Ga₂O₃ (3.04 Å along ➎010➉). The Hastelloy substrates endow photodetectors with good deformability and mechanical robustness. Moreover, owing to the type-II band alignment of β-Ga₂O₃/NiO heterostructures, the photodetectors have a good photocurrent at zero bias under 284 nm of light illumination. In addition, the photocurrent is significantly higher than when using an analogous heterostructure (as described in some previous reports), because the β-Ga₂O₃ and NiO thin films are crystalized along a single orientation with fewer defects.

Molecular beam epitaxy (MBE) was adopted to grow zinc oxide (ZnO) film on sapphire substrate and improve the quality of ZnO thin film epitaxy using a magnesium oxide (MgO) buffer layer and a two-segment temperature scheme for ZnO thin film growth. The influence of thermal annealing of different layers on the optical and crystalline features, stress expression, as well as surface morphology of ZnO thin film was examined. SEM images showed smooth surfaces were formed, and these surfaces allowed the low-temperature ZnO buffer layer to have better epitaxial environment at the very beginning. X-ray diffraction (XRD) analysis revealed that a lower thermal annealing temperature more effectively released the stress of materials. The thermally annealed MgO buffer layer had 26% less strain than the purely thermally annealed, high-temperature-grown ZnO (HT-ZnO), and 33% less strain than the unannealed samples. Atomic force microscopy results showed that the root-mean-square of surface roughness of thermally annealed MgO was 0.255 nm, which was 80% lower than that of thermally annealed HT-ZnO (1.241 nm). Photoluminescence measurement showed that the thermally annealed MgO buffer layer had the highest strength for near-band-edge emission because of improved crystalline quality. HRTEM results showed that the stress caused by the mismatch between the sapphire lattice was effectively released because the MgO buffer layer was annealed at a high temperature. The surface of the MgO buffer layer became smooth and the stress mismatching with the ZnO lattice did not obviously extend upwards. When MBE was used to grow ZnO thin film, a lower thermal annealing temperature for the MgO buffer layer more effectively controlled stress accumulation and produced high-quality ZnO thin film.

This study investigated the growth of AlN epitaxial films on 2-in. Si(111) via metal-organic chemical vapor deposition. By introducing triethylboron (TEB) during trimethylaluminum pretreatment, a nearly crack free AlN epilayer with a thickness of 500 nm was acquired. The x-ray diffraction rocking curves of AlN (002) and (102) exhibited full width at half maximum values of 0.22° and 0.36°, respectively. Atomic force microscopy image analysis showed that after the introduction of TEB, larger grains appeared on the surface of Si(111) substrate, promoting the 3D growth pattern of the subsequent AlN buffer layer. Laytec reflection curves depicted the morphological transition from 3D to 2D growth mode during AlN deposition. At the same time, the curvature value was significantly reduced by 20 km−1, and the Raman spectrum peak of E2(high) shifted from 648.7 to 652.5 cm−1, indicating that the surface tensile stress was greatly reduced, effectively suppressing the crack problem of AlN on Si(111).

Wurtzite BAlN alloys are emerging ultrawide bandgap III-nitride semiconductors promising for optical and electronic devices. Yet the boron compositions of the grown alloys have been limited. In this Letter, we report on the demonstration of a thick single-phase wurtzite BAlN film with a boron composition over 20%. The growth was conducted at 1010 °C and 150 Torr with continuous flows of group-III precursors and ammonia with a growth rate of 2.2 μm/h by metalorganic chemical vapor deposition. The boron composition was studied by x-ray diffraction (XRD), secondary neutral mass spectrometry (SNMS), and Rutherford backscattering spectrometry (RBS). The XRD 2θ scan exhibited the clear wurtzite BAlN peak 1.82° larger than the AlN peak, indicating the boron composition of 30.9% based on the lattice constants of wurtzite AlN and BN. The SNMS and RBS experiments, independent of strain and defects, revealed that the boron content was 22%. The microstructures of the wurtzite BAlN film were further studied by transmission electron microscopy, showing an initial 5 nm thick layer free of crystal twinning followed by widespread crystal twinning with lattice rotations of 60° clockwise and anti-clockwise. The optical transmission experiment manifested that the bandgap of the smaller-lattice BAlN film was 5.1 eV, smaller than that of larger-lattice AlN. This trend was the opposite of the conventional InGaAlN but consistent with theoretical predictions. This study would greatly facilitate the research of material, physics, and devices incorporating the wurtzite BAlN alloys.