Fabrication and characterization of subwavelength nanostructures on freestanding GaN slab.

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.

Fabrication and characterization of freestanding circular GaN gratings.

It's 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.

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.