| Gallium nitride (GaN) is viewed as an excellent candidate for the fabrication of high-power and high-frequency transistors operating at elevated temperatures, due to its wide bandgap, high electron saturation velocity, high breakdown electric-field strength, and potential for lattice-matched heterojunctions with other group-III nitrides. This thesis focuses on the following three open areas of research important to the eventual commercialization of GaN-based field-effect transistors: (1) metal-organic chemical vapor deposition (MOCVD) of high-mobility, low-doped GaN, (2) n- and p-type conductivity control of GaN through ion implantation and in-situ doping during MOCVD growth, and (3) potential gate insulators deposited on GaN.; Parameters affecting the MOCVD growth of GaN, including source quality, buffer-layer thickness, growth temperature, and epitaxial-layer thickness, are explored, and optimum conditions for the MOCVD growth of unintentionally-doped GaN are identified. In addition, electron-density and electron-mobility values extracted from Hall-effect measurements are shown to be misleading indications of material quality for unintentionally-doped GaN, caused by structural non-homogeneity along the direction of growth in theses samples. N- and p-type doping of GaN is also investigated. Silane and Bis(cyclopentadienyl)magnesium are used for in-situ silicon and magnesium doping of GaN during MOCVD growth, respectively. Silicon is demonstrated to be a very effective donor in GaN, allowing for the growth of n-type GaN over a large range of electron densities (10 16--1019 cm-3) with high electron mobilities (maximum of 543 cm2/V·sec). Magnesium doping, while producing p-type conductivity in GaN (maximum p = 2.3 x 1017 cm-3 with a corresponding mu h = 17 cm2/V·sec), demonstrates an anomalous relationship between the Mg incorporation and the hole density. The cause of this behavior is identified as native-defect formation during GaN growth. Ion implantation of Si and Mg into GaN is also examined. Si implantation produces heavily doped n-type material when implanted under the correct conditions into both n- and p-type GaN. However, the results from Mg implantations are not as successful, as neither type conversion nor heavily p-type material was demonstrated convincingly. Si implantation into p-type GaN is used to fabricate p-n junctions that demonstrate, to our knowledge, the best-reported reverse leakage characteristics for ion-implanted GaN p-n junctions.; Finally, a comprehensive review of the performance of jet-vapor deposited silicon-oxide/silicon-nitride/silicon-oxide (ONO) stack insulators on GaN is reported. ONO/GaN MIS capacitors are characterized from room temperature to 450°C and demonstrate low interface-state densities (measured <5 x 1010 cm-2eV-1 using the AC-conductance technique at room temperature and 450°C, low oxide-charge densities, low leakage current densities, and high breakdown electric-field strengths. In addition, the interface states' capture cross-section and lifetimes are measured. The results, taken as a whole, demonstrate that jet-vapor deposited ONO stack insulators on GaN offer superior performance to other reported insulators on GaN and should perform well as the gate on GaN-based MISFETs. |
