P-type gallium nitride semiconductor development and characterization for LEDs and other devices
Togtema, Gregorey L.
Master of Science
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Migration Enhanced Afterglow (AIEAglow) is proposed as a fabrication technology to produce high indium content indium gallium nitride (IriGaN) LEDs operating in the green wavelengths. This research presents results for the first confirmed p-type gallium nitride films grown in a AIEAglow system for these applications. Films were fabricated at a variety of low temperatures, illustrating the technology’s capability to grow p-GaN at indium preserving conditions for high indium content InGaN LEDs. Films were resistive and exhibited a degree of codoping that stemmed from background oxygen concentration present during growth, rendering Hall effect measurements unsuitable. Electroluminescent spectra exposed three different emission sources: that from the bandgap, that from magnesium to nitrogen vacancy transitions, and the well known yellow defect. Current- Voltage characterization of films grown on top of an ii-GaN template determined the Fermi level inside the p-layer, yielding a net hole concentration of 9.9 x 10[superscript 16] cm [superscript -3] in one film. p-GaN semiconductors were investigated using Scanning Electron Microscopy (SEM), Atomic Force Alicroscopy (AFM), transmission spectroscopy and X-Ray Diffraction (XRD). It was found that growth rate decreased as the growth temperature increased from 545°C to 635°C indicating the desorption of gallium and nitrogen from the lattice at low growth pressures. Conditions used in the fabrication of films were also conducive to producing nanowires and nanostructures, which were shown to correlate with a decrease in luminosity; a worsening of the crystal structure, and is indicative of metal rich material. Attempts were made at optimizing the molecular flow rates of gallium, nitrogen and magnesium to yield better materials. A single heterojuriction GaN-IiiGaN LED was fabricated using the material studied in this thesis. It was shown that a small degree of codoping reduced the resistivity of commercially acquired p-GaN by two orders of magnitude, down to 0.038 [symbol]. cm. This p-GaN was placed below an IriGaN active layer, removing the need for magnesium doping at low temperatures. The low series resistance along the p-GaN material allowed large currents to be applied without much overheating. The high iridium content IiiGaN layer produced yellow light with high electrical conversion efficiency not usually characteristic of nitride or phosphide semiconductor LEDs.