We investigated the hydrogen sensing characteristics of Pt Schottky diodes on and (010) β-Ga₂O₃ bulk crystals. The Pt Schottky diodes on β-Ga₂O₃ wafer exhibited the fast, reversible, and cyclic response upon hydrogen exposure. The maximum value of the relative current change of the Ga₂O₃ diode sensor was as high as 7.86 × 10⁷ (%) at 0.8 V, which is slightly higher than that of the (010) Ga₂O₃ diode. The hydrogen responses of both β-Ga₂O₃ diode sensors are believed to result from oxygen and gallium atomic configurations of Ga₂O₃ surfaces for hydrogen adsorption. The Pt Schottky diodes of Ga₂O₃ wafers did not show any clear response to other gases, such as N₂, CO, CO₂, O₂, CH₄, NO₂, and NH₃. Our finding suggests that the Pt Schottky diodes on β-Ga₂O₃ hold great potential for the applications of hydrogen gas sensors with high sensitivity and selectivity.

Enhanced hydrogen sensing performance of Pt Schottky diodes on ZnO single crystal wafers in humid ambient conditions is reported using a polymethylmethacrylate (PMMA) membrane layer. ZnO diode sensors showed little change in forward current when switching to wet ambient H₂ conditions with 100% relative humidity. This sensitivity drop in the presence of water vapor can be attributed to surface coverage of hydroxyl groups on the Pt surface in humid ambient conditions. The hydrogen sensitivity of PMMA-coated diode sensors recovered up to 805% in wet H₂ ambient conditions at room temperature. The PMMA layer can selectively filter water vapor and allow H₂ molecules to pass through the membrane layer. It is clear that the PMMA layer can effectively serve as a moisture barrier because of low water vapor permeability and its hydrophobicity. In both dry and wet conditions, ZnO diodes exhibited relatively fast and stable on/off switching in each cycle with good repeatability.

The extraction efficiency of nonpolar a-plane (11-20) GaN LEDs on sapphire substrates has been enhanced by selectively etching the mesa sidewall faces and the n-type GaN surfaces with photoenhanced chemical wet etching. Submicron-sized trigonal prisms having prismatic planes of {1-100} were clearly displayed on the n-type GaN surfaces as well as the sidewall face after 5 min etching at 60°C. The radiation patterns have shown that more light is extracted in all directions and the output powers of surface textured a-plane GaN LEDs have increased by 25% compared with control samples. PEC wet etching produced unique feature of etching morphology on the mesa sidewall faces and the n-type GaN surface.

Wet chemical etching using hot KOH and H₃PO₄ solutions was performed on semipolar (112) and nonpolar (110) GaN films grown on sapphire substrates. An alternating KOH/H₃PO₄/KOH etch process was developed to control the orientation of the facets on the thin-film surface. The initial etch step in KOH produced c- and m-plane facets on the surface of both semipolar (112) and nonpolar (110) GaN thin-films. A second etch step in H₃PO₄ solution additionally exposed a ( 2) plane, which is chemically stable in H₃PO₄ solution. By repeating the chemical etch with KOH solution, the m-plane facets as seen in the original KOH etch step were recovered. The etching methods developed in our work can be used to control the surface morphologies of nonpolar and semipolar GaN-based optoelectronic devices such as light-emitting diodes and solar cells.

There has been growing interest in developing nanoelectronic devices based on graphene because of its superior electrical properties. In particular, patterning graphene into a nanoribbon can open a bandgap that can be tuned by changing the ribbon width, imparting semiconducting properties. In this study, we report the effect of ribbon width on electrical transport properties of graphene nanoribbons (GNRs). Monolayer graphene sheets and Si nanowires (NWs) were prepared by chemical vapor deposition and a combination of nanosphere lithography and metal-assisted electroless etching from a Si wafer, respectively. Back-gated GNR field-effect transistors were fabricated on a heavily p-doped Si substrate coated with a 300 nm-thick SiO2 layer, by O2 reactive ion etching of graphene sheets using etch masks based on Si NWs aligned on the graphene between the two electrodes by a dielectrophoresis method. This resulted in GNRs with various widths in a highly controllable manner, where the on/off current ratio was inversely proportional to ribbon width. The field-effect mobility decreased with decreasing GNR widths due to carrier scattering at the GNR edges. These results demonstrate the formation of a bandgap in GNRs due to enhanced carrier confinement in the transverse direction and edge effects when the GNR width is reduced.

We studied the Ti/Au Ohmic contact on (001) plane Ga ₂ O ₃ single crystal wafer as a function of annealing temperature and crystallographic orientation. The lowest specific contact resistances of ∼5 × 10 ⁻⁴ Ω·cm ² were obtained at 400 °C annealing temperature under nitrogen ambience. The Ohmic metal contact became degraded when annealed above 500 °C. The formation of Ti oxide might be responsible for the degradation of Ohmic contact at high annealing temperatures. In comparison with (010) Ga ₂ O ₃ substrate, the Ohmic contact could be more easily formed on ( 2 ¯ 01 ) and (001) Ga ₂ O ₃ planes, possibly due to high surface energy and dangling bond density. It is notable that the contact properties did not show any particular dependence on in-plane azimuth angles on (001) Ga ₂ O ₃ substrate.