Gallium Nitride Nanowire Based Electronic and Optical Devices
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Gallium nitride nanowires have significant potential for developing nanoscale emitters, detectors, and biological/chemical sensors, as they possess unique material properties such as wide direct bandgap (3.4 eV), high critical breakdown field, radiation hardness, and mechanical/chemical stability. Although few results of individual GaN nanowire devices have been reported so far, most of them often utilize fabrication processes unsuitable for large-scale nanosystems development and do not involve fundamental transport property measurements. Understanding the transport mechanisms and correlating the device properties with the structural characteristics of the nanowires are of great importance for realizing high performance devices. Focused ion beam induced metal deposition was used to make individual GaN nanowire devices, and assessment of their electrical properties was performed. The nanowires were grown by direct reaction of Ga and NH3, with diameters ranging from 80 nm to 250 nm and lengths up to 200 µm. Dielectrophoretic alignment was used to assemble these nanowires from a suspension on to a large area pre-patterned substrate. A fabrication technique, utilizing only conventional microfabrication processes, has been developed for realizing robust nanowire devices including field effect transistors (FETs), light emitting diodes (LEDs), Schottky diodes, and four-terminal structures. Nanowire FETs with different gate geometries were studied, namely bottom gate, omega-backgate, and omega-plane gate structures. Utilizing omega-backgated FETs, transconductance as high as 0.34 103 µS mm-1 has been obtained. Room temperature field effect electron mobility in excess of 300 cm2 V-1 s-1 have been exhibited by a nanowire FET, with a 200 nm diameter nanowire and Si substrate as the backgate. The observed reduction of mobility in the GaN nanowire FETs with decreasing diameter of the nanowire is attributed to the surface scattering. Electron beam backscattered diffraction revealed that the grain boundary scattering is present in some of the nanowires. Temperature dependent mobility measurements indicated that the ionized impurity scattering is the dominant mechanism in the transport in these nanowires. GaN nanoLEDs have been realized by assembling the n-type nanowires on a p-GaN epitaxial layer using dielectrophoresis. The resulting p-n homojunctions exhibited 365 nm electroluminescence with a full width half maximum of 25 nm at 300 K.