Che-Hao Liao

Gallium oxide (Ga₂O₃) has gained considerable attention due to its wide bandgap, the availability of native substrates, and its excellent properties for solar-blind photodetectors, transparent electronics, and next-generation power devices. However, the expensive Ga₂O₃ native substrates have restricted its widespread adoption. To reduce costs and further the development of β-Ga₂O₃-based devices, there is a need for bandgap-tunable oxide films with high crystal quality for deep-ultraviolet (DUV) photodetectors and high-breakdown-field power devices. This study introduces a Thermal Interdiffusion Alloying method to address these requirements. It focuses on developing deep ultraviolet (DUV) photodetectors using β-Ga₂O₃ thin films on sapphire substrates by promoting the diffusion of aluminum (Al) atoms from the substrate into the film, resulting in the formation of aluminum gallium oxide (β-(Al_xGa_1−x)₂O₃). The aluminum content is controlled by adjusting the process temperature, allowing for tunable detection wavelengths and enhanced DUV sensing capabilities. Radio frequency (RF) sputtering optimizes the film’s quality by adjusting the sputtering power and the argon/oxygen (Ar/O₂) flow ratio. Material analysis indicates that this method expands the optical bandgap and shifts the response wavelength to 210 nm, significantly boosting the performance of the fabricated photodetectors. This research presents considerable potential for advancing DUV photodetectors across various disinfection applications.

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).

High‐temperature in situ growth of Ga₂O₃ thin film on flexible substrates for flexible/portable electronic devices is never realized because the conventional polymer substrates cannot meet the thermal‐stability requirements. In the research, for the first time, Ga₂O₃ thin films are directly grown on SiO_x/Al₂O_x buffered Hastelloy substrates. With pulsed laser deposition under high temperature, as‐grown Ga₂O₃ thin films have a fabric texture with a preferred out‐of‐plane orientation along (−201) and a good field‐effect mobility of 18.07 cm² V⁻¹ s⁻¹. The Ga₂O₃ coated tape is patterned with transistor arrays and exhibited good performance homogeneity. The representative transistor devices have a threshold voltage of ≈−2.75 V and a breakdown voltage of 116 V measured under a −20 V gate voltage. Moreover, the tape transistors also have good mechanical robustness. The transistors’ good electrical performance, high uniformity, and mechanical robustness suggest that the in situ deposition technique using Hastelloy tape is promising for fabricating various flexible Ga₂O₃ channeled electronic circuits. Furthermore, it is believed that this technique can be extended to other flexible semiconductor devices that require high‐temperature processing.

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.

BGaN is an emerging ultrawide bandgap semiconductor with important applications ranging from power electronics to ultraviolet light emitters. To date, BGaN boron composition has been limited to µm/h. Triethylborane and triethylgallium were used as the source gases for boron and gallium, respectively. Pure nitrogen gas was used as the carrier for all reactants. A root mean square roughness value of 2.56 nm was determined using an atomic force microscopy scan on an area of 5 × 5 µm². X-ray diffraction (XRD) 2θ–ω scans show a nearly lattice-matched BGaN/AlN film corresponding to a boron composition of ∼10%. A mixed wurtzite and zincblende phase was confirmed via an XRD pole figure and transmission electron microscopy. Additionally, the high crystalline quality of the mixed (002)_wz/(111)_ZB planes was shown using an XRD rocking curve with 810 arcsec full width at half maximum. The boron composition was precisely measured as 15% using Rutherford backscattering spectrometry combined with nuclear reaction analysis.

GaN electronics have hinged on invasive isolation such as mesa etching and ion implantation to define device geometry, which, however, suffer from damages, hence potential leakage paths. In this study, we propose a new paradigm of polarization isolation utilizing intrinsic electronic properties, realizing in situ isolation during device epitaxy without the need of post-growth processing. Specifically, adjacent III- and N-polar AlGaN/GaN heterojunctions were grown simultaneously on the patterned AlN nucleation layer on c-plane sapphire substrates. The two-dimensional electron gas (2DEG) was formed at III-polar regions but completely depleted in N-polar regions, thereby isolating the 2DEG channels with a large 3.5 eV barrier. Structures of polarization-isolated high electron mobility transistors (PI-HEMTs) exhibit significantly reduced isolation leakage currents by up to nearly two orders of magnitude at 50 V voltage bias compared to the state-of-the-art results. Aside from that, a high isolation breakdown voltage of 2628 V is demonstrated for the PI-HEMT structure with 3 μm isolation spacing, which is two-times higher than a conventional mesa-isolation HEMT. Moreover, the PI-HEMT device shows a low off-state leakage current of 2 × 10⁻⁸mA/mm with a high I_on/I_off ratio of 10⁹ and a nearly ideal subthreshold slope of 61 mV/dec. This work demonstrates that polarization isolation is a promising alternative toward the plasma-damage-free isolation for GaN electronics.

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 work, the local atomic level composition of BAlN films with ∼20% B was investigated using atom probe tomography. Dislocations and elemental clustering were confirmed along which Al atoms tend to segregate. The presence of local compositional heterogeneities (dislocations and small clusters) and impurities is related to the variation of local alloy stoichiometry of the BAlN films. The roughness and interface abruptness of BAlN/AlN were investigated, and a few nm of B and Al composition gradient in BAlN adjacent to the interface was observed. The nanoscale compositional analysis reported here will be crucial for developing BAlN films with a high B content and larger thickness for future high power electronics and optical applications.

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.