Hidehisa Sameshima

Fabrication and characterization of nanoscale resonant gratings on thin silicon membrane

We report the design and fabrication of nanoscale resonant gratings which is of interest for narrow bandwidth filtering application. The linear/circular grating structures, of which the grating width is 200nm and the grating height is 260nm, are generated on silicon-on-insulator wafer. Nanoscale gratings are fabricated on the silicon device layer by a combination of electron beam lithography and fast atom beam etching. The silicon handle layer under grating region is removed by deep reactive ion etching, and the buried oxide layer is kept. The reflectance measurements are performed to characterize the optical response of fabricated freestanding nanoscale gratings. The resonant behavior of linear gratings agrees with the theoretical predication, and the polarization-independent responses of circular gratings are also experimentally demonstrated.

Fabrication and characterization of freestanding circular GaN gratings

Its of significant interest to combine freestanding nanostructure with active gallium nitride (GaN) material for surface-emitting optoelectronic application. By utilizing bulk micromachining of silicon, we demonstrate here a promising way to fabricate freestanding GaN nanostructures using a GaN-on-silicon system. The well-defined nanoscale circular GaN gratings are realized by fast-atom beam (FAB) etching, and the freestanding GaN gratings are obtained by removing silicon substrate using deep reactive ion etching (DRIE). The freestanding GaN slab is thinned from the backside by FAB etching to reduce the confined modes inside the GaN slab. The measured microphotoluminescence (micro-PL) spectra experimentally demonstrate significant enhancements in peak intensity and integrated intensity by introducing freestanding circular grating. This work represents an important step in combining GaN-based active material with freestanding nanostructures for further increasing light-extraction efficiency.

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.

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We develop a novel way to fabricate subwavelength nanostructures on the freestanding GaN slab using a GaN-on-silicon system by combining self-assemble technique and backside thinning method. Silicon substrate beneath the GaN slab is removed by bulk silicon micromachining, generating the freestanding GaN slab and eliminating silicon absorption of the emitted light. Fast atom beam (FAB) etching is conducted to thin the freestanding GaN slab from the backside, reducing the number of confined modes inside the GaN slab. With self-assembled silica nanospheres acting as an etching mask, subwavelength nanostructures are realized on the GaN surface by FAB etching. The reflection losses at the GaN interfaces are thus suppressed. When the InGaN/GaN multiple quantum wells (MQWs) active layers are excited, the light extraction efficiency is significantly improved for the freestanding nanostructured GaN slab. This work provides a very practical approach to fabricate freestanding nanostructures on the GaN-on-silicon system for further improving the light extraction efficiency.

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

An electromechanical tunable grating was fabricated by micromachining a GaN crystal grown on a Si substrate. The tunable grating consisted of an expandable freestanding grating and an electrostatic comb-drive actuator. In order to compensate the residual stress of the GaN crystal grown on the Si substrate, crystallization stress of an HfO2 layer deposited on the GaN crystal was utilized. The freestanding GaN structure of the grating was fabricated by etching the Si substrate with XeF2 gas. The freestanding grating consisted of twenty-four 12-μm-long and 85-nm-wide grating lines with a 720-nm period. By applying a voltage of 140 V, the grating was expanded by 0.6 μm to change the period by 3.5%. The proposed tunable grating can be used for monolithic integration of a GaN light source and a micro-spectrometer.

Fabrication and characterization of subwavelength nanostructures on freestanding GaN slab.

Ultra-small electromechanical comb-drive actuators made of GaN crystal were studied in order to apply them to optical micro-electromechanical systems. Using GaN crystals grown on Si substrates by metal-organic chemical vapor deposition, two kinds of electrostatic comb-drive actuators were designed and fabricated. In the fabrication, due to a residual stress of the grown crystal, the movable part of the actuator suffered considerable deformation depending on the growth conditions. The strain-stress issue of the grown crystal layer is discussed on the basis of lattice misfit and thermal expansion. To compensate for a convex deformation, crystallization tension of a thin HfO2 film deposited on a GaN layer was investigated. The displacement of the actuator having dimensions of 52.2 μm in width and 105.4 μm in length was 1.3 μm at 70 V. Several variable systems will be feasible by combining the actuators monolithically with GaN opt-electronic devices. Future applications are also briefly discussed.Ultra-small electromechanical comb-drive actuators made of GaN crystal were studied in order to apply them to optical micro-electromechanical systems. Using GaN crystals grown on Si substrates by metal-organic chemical vapor deposition, two kinds of electrostatic comb-drive actuators were designed and fabricated. In the fabrication, due to a residual stress of the grown crystal, the movable part of the actuator suffered considerable deformation depending on the growth conditions. The strain-stress issue of the grown crystal layer is discussed on the basis of lattice misfit and thermal expansion. To compensate for a convex deformation, crystallization tension of a thin HfO2 film deposited on a GaN layer was investigated. The displacement of the actuator having dimensions of 52.2 μm in width and 105.4 μm in length was 1.3 μm at 70 V. Several variable systems will be feasible by combining the actuators monolithically with GaN opt-electronic devices. Future applications are also briefly discussed.

A GaN Electromechanical Tunable Grating on Si Substrate

A tunable grating is fabricated by micromachining a GaN crystal layer grown on Si substrate. The tunable grating consists of grating lines, electro-static comb-drive actuator and connection springs. The grating consists of 85nm wide, 12µm long 24 grating lines with 674nm period. The crystallization stress of an HfO2 layer deposited on the GaN crystal is used to compensate the residual stress of the GaN crystal grown on Si substrate. The freestanding GaN structure consisting of the grating and the actuator is fabricated by etching the Si substrate with XeF2 gas. Applying the voltage of 140V, the grating is expanded by 600nm corresponding to the period change of 3.7%.

Design and fabrication of GaN crystal ultra-small lateral comb-drive actuators

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.

GaN pitch-variable grating fabricated on Si substrate

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.