M. Wakui

A Freestanding GaN/HfO

Combination of GaN light source and Si-microelectromechanical systems (MEMSs) is a promising hybrid structure for optical MEMS. As one of GaN-Si hybrid structures, a freestanding GaN/HfO2 membrane was fabricated on Si substrate. Unlike conventional GaN membrane on Si substrate, the fabricated membrane had a tensile stress by using the HfO2 layer. Therefore, the GaN/HfO2 membrane was flat enough to be useful for several MEMS. The GaN crystal was grown by molecular beam epitaxy on the HfO2 layer deposited on Si substrate. The surface of the HfO2 layer was nitrified before GaN crystal growth, and thus, a part of HfO2 surface was changed to HfN, the lattice of which matched well to that of GaN. The characteristics of the GaN crystal grown on the nitrified HfO2 layer were also investigated.

_{\bf 2}

We theoretically and experimentally demonstrate a freestanding gallium nitride (GaN) resonant grating at telecommunication range. The optical responses of the freestanding GaN resonant gratings are analyzed by the rigorous coupled-wave analysis method. The freestanding GaN resonant gratings are validated on 850-nm freestanding membrane by a combination of electron beam lithography, fast atom beam, etching, and deep reactive ion etching. The polarization properties of such freestanding GaN resonant gratings are demonstrated in reflectance measurements, and the experimental results correspond well to the theoretical model. The strong resonant peaks show a clear dependence on the duty ratio under transverse magnetic polarization, and a promising resonant peak of the fabricated freestanding GaN resonant grating, in which the grating period P is 1500 nm, the grating height h is 230 nm, and the grating width is 280 nm, is observed at 1516.4 nm with a full-width at half-maximum of 4 nm.

Membrane Grown by Molecular Beam Epitaxy for GaN–Si Hybrid MEMS

Monolithic integration of GaN-based semiconductor with Si MEMS is demonstrated from a GaN/Si wafer, in which GaN crystal is grown by molecular beam epitaxy (MBE). A Light emitting diode is fabricated on the grown GaN crystal and the blue electro-luminescence is obtained. The GaN crystal property of is improved by using a template grown by metal organic chemical vapor deposition (MOCVD). A light distribution variable device with Si comb actuator is monolithically fabricated.

Freestanding GaN Resonant Gratings at Telecommunication Range

We study the growth of GaN crystal on Si substrate by molecular beam epitaxy (MBE), in order to integrate GaN light source and MEMS monolithically Since the lattice constant of HfN is close to that of GaN (only 0.35% mismatch), the crystal growth of GaN on HfN film is superior. On the other hand, HfO2 film is a good candidate for waveguide, dielectric and sacrificial layer. In this study, HfO2 film is surface-nitrified by a rf nitrogen plasma source of MBE to generate HfN layer. The morphology of the grown GaN crystal was better on the nitrified HfO2 layer. The photoluminescence (PL) efficiency of GaN quantum well grown on the nitrified HfO2 layer was better than that on Si substrate. As a simple hybrid lighting device structure, GaN grating on Si substrate was fabricated and the PL intensity from GaN diffraction grating was measured.