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The impact of polarization-induced electric fields on the coherent electron tunneling probability in AlN/GaN coupled double quantum wells (CDQWs) is examined by solving Schrodinger and Poisson equations. The coherent electron tunneling probability has decreased with the increased in the polarization-induced electric fields, which is attributed to the resonance between the [E.sub.2] and [E.sub.3] subbands weakening with the polarization-induced electric fields increasing in AlN/GaN CDQWs.

The transmission electron microscopy technique is employed to explain the microstructures of the various [In.sub.x][Al.sub.1-x]N epilayers that are grown on the GaN templates by the metalorganic chemical vapor deposition technique. The various effects of different strains on the structures and properties of these compounds are also analyzed.

Photoluminescence and monoenergetic positron annihilation spectroscopy are used for examining the yellow luminescence (YL) in as-grown high-resistance (HR) and unintentional-doped (UID) GaN films grown by metal organic chemical vapor deposition. The studies have shown that the increased wave function overlap of electrons and holes induced by the strong space localization effect of holes deduced from the short positron diffusion length is the vital factor to increase the YL efficiency in HR-GaN.

The microstructure and electrical properties of [Al.sub.0.25][Ga.sub.0.75]N/GaN heterostructures with various AlN thickness are examined. Transmission electron microscopy images have shown that the AlN interlayer with an optimum thickness has made the threading dislocations bend and be annihilated with each other in the vicinity of the heterointerface due to the larger mismatch strain between AlN interlayer and GaN.

The temperature dependence of the specific contact resistance in annealed Ti/Al/Ni/Au multilayers on [(N[H.sub.4]).sub.2][S.sub.x] treated n-type GaN is analyzed in a wide temperature range by the transmission line technique. A field emission model with a temperature-dependent effective SBH is responsible for the '25 min treated' sample in which metal/semiconductor interface potential pinch-off has occurred.

A time-of-flight secondary ion mass spectrometry is utilized to examine the diffusion mechanisms of fluorine ions in GaN. Strong local confinement and stabilization of fluorine ions in GaN crystal achieved by significant reduction in the number of continuous vacancy chains revealed excellent thermal stability of fluorine ions for device applications.

A combination of spectroscopic and microscopic studies is conducted to examine the morphology and microstructure of dislocation etch pits in GaN epilayers etched by molten KOH. The results revealed that the chemical stabilization of Ga-polar surface plays a significant role in the formation of various types of dislocation etch pits.

The morphology and microstructure evolution of [Al.sub.0.3][Ga.sub.0.7]N epilayers grown on GaN/sapphire templates with low-temperature (LT) AlN interlayers (IL) by using metal organic chemical vapor deposition are examined by transmission electron microscopy and atomic force microscopy. The strain in [Al.sub.x][Ga.sub.1-x]N epilayers is modified by inserting the AlN IL because of the lattice mismatch between AlN, GaN, and [Al.sub.x][Ga.sub.1-x]N, which has further changed the formation of the edge-type threading dislocations.