Fangren Hu

Variable optical reflectance of a self-supported Si grating

Variable reflectance of a self-supported Si grating mirror is reported. A grating suspended in air consisted of an array of 300nm wide, 300nm thick, and 20μm long beams. The period of the grating was varied with an electrostatic microactuator from 600to720nm. A broadband reflection in the wavelength longer than the grating period was observed, which was caused by a resonant interference of the light wave propagating along the grating similar to the guided-mode resonant gratings. Increasing the period of the grating with a microactuator, the reflection spectrum changed. The results were explained by the theoretical calculations based on rigorous coupled-wave analysis.

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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Column-crystallized InGaN∕GaN quantum-well flower structure was deposited on pillared Si (111) substrate. Dotted GaN nuclei grew along the direction of the coming Ga and N atoms, forming arrays of InGaN∕GaN quantum-well flower structure. Raman spectra measurement demonstrated that these crystals were fully relaxed. Photoluminescence measurement showed a room temperature peak position of 556nm and two peak positions of 400 and 549nm at low temperature. Hg lamp excited photoluminescence demonstrated a clear fluorescence distribution from the low to the top part of the flower structure and much stronger emission compared with the quantum-well crystals on the flat Si substrate.

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

We report here a double-sided process for the fabrication of a comb-drive GaN micro-mirror on a GaN-on-silicon platform. A silicon substrate is first patterned from the backside and removed by deep reactive ion etching, resulting in totally suspended GaN slabs. GaN microstructures including the torsion bars, movable combs and mirror plate are then defined on a freestanding GaN slab by the backside alignment technique and generated by fast atom beam etching with Cl2 gas. Although the fabricated comb-drive GaN micro-mirrors are deflected by the residual stress in GaN thin films, they can operate on a high resistivity silicon substrate without introducing any additional isolation layer. The optical rotation angles are experimentally characterized in the rotation experiments. This work opens the possibility of producing GaN optical micro-electro-mechanical-system (MEMS) devices on a GaN-on-silicon platform.

High-efficiency light-emitting column-crystallized InGaN∕GaN quantum-well flower structure on micropillared Si substrate

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.

Comb-drive GaN micro-mirror on a GaN-on-silicon platform

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.

Fabrication and characterization of subwavelength nanostructures on freestanding GaN slab.

We propose, fabricate and characterize the freestanding GaN nanocolumn membrane with bottom subwavelength nanostructures. The GaN nanocolumns are epitaxially grown on freestanding nanostructured silicon substrate that is achieved by a combination of self-assemble technique and silicon-on-insulator (SOI) technology. Optical reflection is greatly suppressed in the visible range due to the graded refractive index effect of subwavelength nanostructures. The freestanding GaN nanocolumn membrane is realized by removing silicon substrate from the backside, eliminating the silicon absorption of the emitted light and leading to a strong blue emission from the bottom side. The obtained structures also demonstrate the potential application for anti-reflective (AR) coating and GaN-Si hybrid microelectromechanical system (MEMS).

Freestanding GaN Resonant Gratings at Telecommunication Range

We report here the epitaxial growth of InGaN/GaN quantum wells on freestanding GaN gratings by molecular beam epitaxy (MBE). Various GaN gratings are defined by electron beam lithography and realized on GaN-on-silicon substrate by fast atom beam etching. Silicon substrate beneath GaN grating region is removed from the backside to form freestanding GaN gratings, and the patterned growth is subsequently performed on the prepared GaN template by MBE. The selective growth takes place with the assistance of nanoscale GaN gratings and depends on the grating period P and the grating width W. Importantly, coalescences between two side facets are realized to generate epitaxial gratings with triangular section. Thin epitaxial gratings produce the promising photoluminescence performance. This work provides a feasible way for further GaN-based integrated optics devices by a combination of GaN micromachining and epitaxial growth on a GaN-on-silicon substrate.PACS81.05.Ea; 81.65.Cf; 81.15.Hi.

Large area, freestanding GaN nanocolumn membrane with bottom subwavelength nanostructure

We report here the lateral epitaxial overgrowth (LEO) of GaN on a patterned GaN-on-silicon substrate by molecular beam epitaxy (MBE) growth with radio frequency nitrogen plasma as a gas source. Two kinds of GaN nanostructures are defined by electron beam lithography and realized on a GaN substrate by fast atom beam etching. The epitaxial growth of GaN by MBE is performed on the prepared GaN template, and the selective growth of GaN takes place with the assistance of GaN nanostructures. The LEO of GaN produces novel GaN epitaxial structures which are dependent on the shape and the size of the processed GaN nanostructures. Periodic GaN hexagonal pyramids are generated inside the air holes, and GaN epitaxial strips with triangular section are formed in the grating region. This work provides a promising way for producing novel GaN-based devices by the LEO of GaN using the MBE technique.