We investigated the effect of forming gas annealing (FGA) on the electrical properties of SiO 2 /GaN MOS devices. We conducted systematic capacitance-voltage measurements on metal-oxide-semiconductor structures and observed a significant amount of anomalous positive fixed charge (3 × 10 12 cm −2 ) at the interface between SiO 2 and GaN for FGA temperatures over 250 °C. The fixed charges generated by FGA may be attributed to oxygen vacancies formed in the gallium oxide (GaO x ) interlayer by the reaction between GaO x and hydrogen.

We examine the far-field and near-field properties of complementary screens made of nanostructured gold thin films, a rectangular nanowire and a nanovoid, using an aperture-type scanning near-field optical microscope and electromagnetic field calculations, and discuss the applicability of Babinet's principle in the optical region. The far-field transmission spectra of the complementary screens are considerably different from each other. On the other hand, genuine near-field extinction spectra exhibit nearly complementary characteristics. The spatial features of the observed near-field images for the complementary screens show little correlation. We have found from the Fourier analysis of the simulated images that high spatial-frequency components of the electromagnetic fields show mutual spatial correlation. These results suggest that Babinet's principle is applicable to the high spatial-frequency components of electromagnetic fields for the complementary screens.

GaN-based metal–oxide–semiconductor (MOS) devices, such as n- and p-type capacitors and inversion- and accumulation-type p-channel field effect transistors (MOSFETs), were fabricated by Mg-ion implantation and ultra-high-pressure annealing (UHPA) under 1-GPa nitrogen pressure. Even though UHPA was conducted at 1400 °C without protective layers on GaN surfaces, n-type MOS capacitors with SiO2 gate dielectrics formed on non-ion-implanted regions exhibited well-behaved capacitance–voltage characteristics with negligible hysteresis and frequency dispersion, indicating distinct impact of UHPA in suppressing surface degradation during high-temperature annealing. Efficient activation of the implanted Mg dopants and reasonable hole accumulation at the SiO2/GaN interfaces were also achieved for p-type capacitors by UHPA, but the fabricated inversion- and accumulation-type p-channel GaN MOSFETs were hardly turned on. The findings reveal extremely low hole mobility at GaN MOS interfaces and suggest an intrinsic obstacle for the development of GaN-based MOS devices.

A recent study has shown that anomalous positive fixed charge is generated at SiO2/GaN interfaces by forming gas annealing (FGA). Here, we conducted systematic physical and electrical characterizations of GaN-based metal-oxide-semiconductor (MOS) structures to gain insight into the charge generation mechanism and to design optimal interface structures. A distinct correlation between the amount of FGA-induced fixed charge and interface oxide growth indicated the physical origins of the fixed charge to be defect formation driven by reduction of the Ga-oxide (GaOx) interlayer. This finding implies that, although post-deposition annealing in oxygen compensates for oxygen deficiencies and FGA passivates defect in GaN MOS structures, excessive interlayer GaOx growth leads to instability in the subsequent FGA treatment. On the basis of this knowledge, SiO2/GaOx/GaN MOS devices with improved electrical properties were fabricated by precisely controlling the interfacial oxide growth while taking advantage of defect passivation with FGA.

We developed an aperture-type near-field reflection spectral imaging method to obtain optical properties of materials with a nanometer spatial resolution. We adopted a phase-stepping technique to extract genuine near-field signals of a sample from the observed intensity spectra dominated by the large background, in a multiplex manner. We performed near-field reflection spectral imaging of a single silver nanoplate to evaluate the developed system and also to examine near-field optical properties of the nanoplate. The near-field reflection spectrum of the nanoplate shows multiple resonant features that are not observable by conventional methods. The near-field reflection image taken at the resonance shows unique spatial features attributable to the plasmon mode resonantly excited. The developed system is applicable to transparent samples as well as opaque ones and enables spectral as well as spatial characteristics of the samples to be revealed.