The Study Of Surface2D Antireflection Nanostructures Realized By Nanoimprint Lithography

The reflection loss is commonly exist at the material interfaces of optical and optoelectronic devices, which seriously influence the photoelectric (electro-optic) conversion efficiency of the corresponding devices. Recently, the surface2D antireflection nanostructures, especially including photonic crystal and subwavelength structures, has become a research hotspot due to its excellent antireflection performance at the material interfaces. However, the2D antireflection nanostructures strongly rely on the corresponding nano-fabrication techniques in real application fields. Due to the reduction of the features size, the conventional techniques, such as the extreme ultra-violet lithography (EUVL), the e-beam lithography (EBL) and the focused ion beam (FIB) lithography, will inevitably to face the high cost problem, especially when the fabrication is performed on wafer scale. As a major candidate of next generation lithography (NGL), the nanoimprint lithography (NIL), which has the advantages of low cost, suitable for large scale production, high efficiency as well as resolution, has become the best choice for the fabrication of2D antireflection nanostructures.To date, the NIL has developed for only20years, and there are some problems remain to be resolved regarding the fabrication of low dimentional nanostructures. In this thesis, a three-mask-layer (TML) soft UV NIL process is proposed to overcome the aspect ratio problem which commonly exists in the one-step NIL process. Benefit from the high etching selectivity of SiO2intermediate layer to the bottom resist layer, it demonstrates a success for the fabrication of the grating array with line width of40nm and aspect ratio as high as6. Due to the flattening effect of bottom resist layer in TML soft UV NIL, uniform photonic crystal structures are successfully fabricated on the p-GaN surface with large area (>114×85μm2).In the studies of photonic crystal LED devices, photonic crystal LEDs with different diameter/period (d/a) values are successfully fabricated via the as-proposed TML soft UV NIL process. Compared to the un-patterned LED, the photonic crystal patterned LEDs show the enhancements in PL intensities are1.54fold,2.31fold and1.62fold for d/a values of0.57,0.66and0.77, respectively. A dry lift-off (DLO) soft UV NIL process is proposed for the fabrication of photonic crystal structures on p-GaN surface with perfect structure morphology. The LED device is grown on the patterned sapphire substrate (PSS), and has a surface nonflatness of micrometer scale. The PL intensities of the LED fabricated by DLO soft UV NIL method to the one with conventional soft UV NIL and un-patterned LED are1.41fold and3.48fold, respectively.Furthermore, the initial template of NIL is usually fabricated by the costly EBL (or FIB) method, which improves the cost of fabrication process and thus becomes the bottleneck of NIL regarding the process of industrialization. In this thesis, an anodic aluminum oxide (AAO) technique is adopted to be used in the2D antireflection nanostructure fabrication filed. The AAO template is fabricated by the electrochemistry process, which thus can vastly reduce the fabrication cost of NIL template. First, it is infeasible to perform the NIL process by direct using the AAO template due to the poor surface flatness of conventional AAO (with period of100nm). Based on the above consideration, an anodization combined with dry etching method is proposed. A2inch porous Si template is successfully fabricated and used to the soft UV NIL field. The imprinted resist has a surface fluctuation of less than3nm and a nanopore dimension of36～97nm. By using the as-fabricated porous Si template into soft UV NIL process, the nanopore structure is successfully transferred to Si surface.In the wavelength range of400～800nm, the surface reflectivity of the as-fabricated porous Si decreases from near34%to be less than5%. Next, A2inch big pore (250～500nm) AAO template is fabricated via the improved anodization process. By using the soft UV NIL process, the AAO structure are successfully transferred to p-GaN surface. Compared to the un-patterned LED, the150-nm-deep AAO structure patterned LED shows the enhancement in PL intensity of45%and the electroluminescence (EL) enhancement of11.4%when20mA of current is injected.