| With the rapid improvement of nitride semiconductor epitaxial growth technology, the precise and accurate electrical characterization of MBE grown nitride semiconductors and its alloys is essential for nitride device analysis and design.; In the first part of the dissertation, a variant of the conventional capacitance-voltage profiling technique is developed and applied to deduce interface charge densities and band-offset values at InxGa 1-xN/GaN heterojunction interfaces grown by molecular beam epitaxy (MBE). Conduction-band offsets of 0.09 +/- 0.07 eV and 0.22 +/- 0.05 eV, and polarization charge densities of (1.80 +/- 0.32) x 10 12 e/cm2 and (4.38 +/- 0.36) x 1012 e/cm2 are obtained for x=0.054 and x=0.09, respectively. The measured polarization charge densities are lower than those predicted theoretically, but are in good agreement with values inferred from optical data reported for InxGa1-xN/GaN quantum-well structures.; The second part of the dissertation focuses on reverse-bias leakage current analysis of Schottky diode fabricated on MBE-grown GaN and Al0.25Ga 0.75N/GaN heterojunction field effect transistor (HFET) devices. By combining temperature-dependent current-voltage measurements with conductive atomic force microscopy and analytical modeling, we conclude that below 150K, the leakage current is dominated by tunneling transport; while at higher temperatures, it is dominated by dislocation-assisted Frenkel Poole emission mechanism.; In the third part of the dissertation, we develop heterostructure design strategy, implemented using a GaN cap layer atop a conventional MBE Al xGa1-xN/GaN HFET structure, based on engineering the electric field at the metal-semiconductor interface of a Schottky contact. The Frenkel Poole emission of electrons into conductive screw dislocations is thus suppressed and the measured reverse-bias leakage currents are reduced by one to three orders of magnitude. Scanned probe measurements of local, nanoscale current distributions confirm directly that current flow via conductive dislocations is suppressed in structures incorporating the GaN cap layer.; In the appendix, possible applications of negative index material (NIM) in resonator structures are analyzed and followed by a discussion of a new NIM structural design and preliminary fabrication studies. |
