High-aspect ratio β-Ga₂O₃ nanorods consisting of prism-like crystals were formed using gallium oxyhydroxide and ammonia hydroxide via a hydrothermal synthesis followed by the subsequent calcination process. The formation of high-aspect ratio β-Ga₂O₃ nanorods was attributed to the oriented attachment mechanism that was present during the hydrothermal synthesis. A field-effect transistor was fabricated using the high-aspect ratio β-Ga₂O₃ nanorod, and it exhibited the typical charge transfer properties of an n-type semiconductor. This facile approach to forming high-aspect ratio nanorods without any surfactants or additives can broaden the science of β-Ga₂O₃ and expedite the integration of one-dimensional β-Ga₂O₃ into future electronics, sensors, and optoelectronics.

This paper describes the heteroepitaxial growth of high-quality alpha-phase gallium oxide (α-Ga2O3) on a sapphire substrate without a buffer layer via the hydride vapor phase epitaxy method. Here, compressive strain arising from the difference in lattice constants between the substrate and α-Ga2O3 is relieved near the interface, resulting in a high crystal quality of 32.72 arcsec (full width at half maximum value) in the high-resolution x-ray diffraction 2θ scan spectrum. Subsequently, the fabricated hetero α-Ga2O3-based photodetector with a metal–semiconductor–metal structure operating under ultraviolet radiation in the C-band (UVC) demonstrates a high UVC responsivity of 5 × 102 A W−1 and a high visible blindness of 8.14 × 104 at 235 nm. The photodetector utilizes photogenerated holes trapped near the interface of the metal electrode, inducing amplified electron current flow. The developed hetero α-Ga2O3-based UVC photodetector can be used to detect early signs of fire, flames, or corona discharge in visible light environments for social and industrial safety applications.

In this study, the authors fabricate Sn‐doped 100‐nm thick polycrystalline β‐Ga₂O₃ synaptic field‐effect transistors (FETs) emulating optical and electrical spike stimulation. When stimulated by deep ultraviolet (UV) optical spikes or electric voltage spikes at the gate, the devices exhibit several essential synaptic functions of excitatory‐postsynaptic currents (EPSCs), inhibitory‐postsynaptic currents (IPSCs), paired‐pulse facilitation (PPF), spike‐number‐dependent plasticity (SNDP), and spike‐timing‐dependent plasticity (STDP). Following UV optical stimulation, the devices mimic synaptic plasticity with a photogate effect, and the gate voltage stimulation emulates the synaptic weights according to the state of the gate dielectric interface. The β‐Ga₂O₃ synaptic FET demonstrates synergistic functions in various optoelectronic stimulation modes and successfully mimics the visual memory formation in bees with UV photoreceptors. Moreover, to verify the translation of optoelectrical‐derived synaptic behaviors of β‐Ga₂O₃ synaptic FETs into artificial neuromorphic computing, handwritten digit image recognition of the Modified National Institute of Standards and Technology dataset is performed using a convolutional neural network, and a learning accuracy of 96.92% is achieved. The realization of these fundamental functions of biological synapses suggests the utility of Ga₂O₃‐based optoelectronic devices for next‐generation neuromorphic computing.

Monoclinic gallium oxide (β-Ga₂O₃) has attracted the interest of the scientific community due to its application in power electronics. Power electronics that need to handle a high voltage often uses a “normally off” device with a positive threshold voltage due to its fail-safe operation and its simple system architecture. In this work, 8-nm-thick Sn-doped polycrystalline β-Ga₂O₃ thin films were investigated as a channel material for power electronics, and their properties were characterized. The optical bandgap of the 8-nm-thick Sn-doped β-Ga₂O₃ was determined to be 5.77 eV, which is larger than that of 100-nm-thick Sn-doped β-Ga₂O₃ due to the quantum confinement effect. The developed back-gated device demonstrated normally off behavior and exhibited a voltage handling capacity as high as 224 V (2.88 MV/cm). This ultrathin β-Ga₂O₃ layer could also be applied to fields other than power electronics, including displays, optical sensors, photocatalytic sensors, and solar cells.

Deep ultraviolet (DUV) photodetectors (PDs) based on ultrawide bandgap β‐Ga₂O₃ have great potential for aerospace, military, and civilian applications, especially because of their inherent solar blindness. An 8 nm thick Sn‐doped polycrystalline β‐Ga₂O₃ semiconductor is used as the channel material for a solar‐blind PD in this study. The β‐Ga₂O₃ channel is fully depleted by upward bending of the energy band owing to the work function difference between the highly p‐type Si (gate electrode) and β‐Ga₂O₃, resulting in normally off behavior with a positive threshold voltage under dark conditions. The normally off behavior is beneficial for simplicity of the gate‐driver circuitry and low power consumption under standby conditions. The fully depleted 8 nm thick polycrystalline β‐Ga₂O₃ exhibits high‐performance DUV detection capability of light at 215 nm: a low dark current of 29.3 pA and high photo‐to‐dark‐current ratio of ≈10⁴ at zero gate voltage. This fully depleted n‐type β‐Ga₂O₃ semiconductor with a wafer‐scale substrate can expedite the realization of high‐performance and low‐power‐consumption solar‐blind PDs.

The optical and electrical characteristics of a graphene oxide solution (GS) with different graphene oxide (GO) concentrations in de-ionized water are investigated via the electrochemical impedance spectroscopy (EIS) method. The measurement results produced by the EIS for the GS are represented with both Bode and Nyquist plots in a frequency range from 1 kHz to 10 MHz. Using these results, we develop an equivalent circuit model as a function of the GO concentration, representing the GS as a mixed circuit of two-dimensional (2D) GO dispersed in parallel in de-ionized (DI) water. The underlying physics of the current-flowing behavior in the GS are explained and interpreted using empirical circuit models; the circuit model also shows that highly resistive GO becomes conductive in GS form in the DI water. The findings in this work should draw new attention toward GSes and related applications, including functional composite materials, catalysts, and filter membranes.