Fabrication Of Plasmonic And Random Semiconductor Nanostructures On GaN And Their Applications

GaN-based light emitting diodes (LEDs) have attracted great attention because of their wide range of applications, such as solid-state lighting, back lighting in cell phones, and traffic signals. However, GaN LEDs have not fully fulfilled their original promise. One of the major issues is their limited internal quantum efficiency, which determined by the crystal quality and epitaxial layer structures. Another major issue is their limited light extraction efficiency. The principal reason lies in the large refractive index (n) difference between GaN (nGaN=2.5at460nm) and air (nair=1). Snell’s law predicts that only～4%of the luminescence from conventional GaN LEDs can be extracted, while most photons are trapped inside the chips in the form of guided modes until absorbed and converted to heat. Extensive efforts over the past decade have yielded many techniques that can increase the internal and light extraction efficiency of LEDs, including coupling between surface plasmon and semiconductor, chip shaping, resonant cavity, surface roughening, photonic crystals, and use of a patterned substrate. Among these, surface roughening and surface plasmon coupling are the most economical and feasible for integration into an LED production line.In order to couple the surface plasmon and LED, firstly, we should grow plasmonic nanostructures on GaN epitaxial film. The main topics of this dissertation are as follows:(1) In this dissertation, we developed a simple photochemical method to grow Ag nanostructures on GaN. It is confirmed that the types of doping and carrier concentration are determinant factors for the growth of Ag NPs on GaN. On n-type GaN, the Ag NPs are spheroidic and uniform, with a high density. The localized surface plasmon resonances (LSPRs) of the Ag NPs enable coupling with light suffering internal reflection at both GaN/air and GaN/sapphire interfaces, induce a fabry-perot type surface plasmon resonance (SPR). Reducing the concentration of AgNO3leads to fractal Ag dendrites, adding absolute ethyl alcohol was added to the AgNO3can increase the density of branched Ag nanostrutures. For p-type GaN, Ag polyhedral NCs with shapes ranging from cubes, truncated cubes, cuboctahedra, truncated octahedra to octahedra were sobtained. It is found that the shape of Ag NCs can be precisely tuned by adjusting the concentration of AgN03. To our best knowledge, this is the first report for the growth of polyhedral Ag NCs of these shapes without surfactant at room temperature. It is believed that surface states on p-type GaN enables separation of the nucleation and growth processes, resulting in the selective formation of anisotropic Ag NCs.(2) The N-polar n-GaN can be textured through photochemical wet etching in KOH solution. After etching, hexagonal pyramids formed on the wafer surface. As increasing the duration of wet etching, the filling factor of the pyramid increased and density of that decreased. However, some regions still remain smooth. In our experiments, we have solved this problem by adding K2S2O8into the etching solution. The N-polar wafer surface completely covered with pyramids after wet etching with KOH-K2S2O8solution. The pyramids cuddled up to each other, with no gaps visible. When the concentration of KOH decreases to0.1M, the shape of pyramids turns into hemisphere. In addition, the pyramids are still covered on entire surface. Variation of the concentration of KOH opens the possibility of tuning the pyramid geometry, which can be applied to mediate the output profile of LED.(3) We developed metal-assisted electroless fabrication of nanoporous p-GaN to improve the light extraction efficiency of GaN-based light emitting diodes (LEDs). Although it has long been believed that p-GaN cannot be etched at room temperature, in this study we find that Ag nanocrystals (NCs) on the p-GaN surface enable effective etching of p-GaN in a mixture of HF and K2S2O8under ultraviolet (UV) irradiation. It is further shown that the roughened GaN/air interface enables strong scattering of photons emitted from the multiple quantum wells (MQWs). The light output power measurements indicate that the nanoporous LEDs obtained after10min etching show a32.7%enhancement in light-output relative to the conventional LEDs at an injection current of20mA without significant increase of the operating voltage. In contrast, the samples etched for20min show performance degradation when compared with those etched for10min, this is attributed to the current crowding effect and increased surface recombination rate.(4) The selective wet etching behavior of Ga-polar n-GaN and p-GaN were studied. Three size grades etch pits (EPs) formed on the Ga-polar GaN epitaxial films after selective wet etching. The size of EPs can be controlled by tuning the duration of etching, and the EPs density dependent on the etching temperature. The differences in EPs size and dependence of EPs density on temperature were explained based on thermodynamics theory. Over etching of p-GaN layer in LED wafer produces terraced EPs.In conclusion, I developed a simple photochemical method to grow Ag nanostructures on GaN and metal-assisted electroless fabrication of nanoporous GaN to improve the quantum efficiency of GaN-based LEDs. On n-type GaN, the productions after photochemical reaction are spheroidic and uniform Ag NPs, with a high density. Under the same reaction condition, Ag polyhedral NCs were obtained. Ag nanocrystals (NCs) on the p-GaN surface enable effective etching of p-GaN in a mixture of HF and K2S2O8under ultraviolet (UV) irradiation. The roughened GaN/air interface enables strong scattering of photons emitted from the multiple quantum wells. After selective wet etching, hexagonal etch pits were produced in the GaN expitaxial film. The roughened surface can be applied to increase the extraction efficiecy of LED.