III-nitride semiconductors are of great interest owing to their commercial and military applications due to their optoelectronic and mechanical properties. They have been synthesized successfully by many growth methods. Among them, molecular beam epitaxy (MBE) is a promising epitaxial growth method owing to precise control of growth parameters, which significantly affect the film properties, composition, and thickness. However, the understanding of growth mechanism of III-nitride materials grown in this growth regime is far from being complete.In this dissertation, GaN and AIGaN growth mechanism under metal-rich conditions were investigated. The Ga surface desorption behavior during the growth was investigated systematically using reflection high-energy electron diffraction (RHEED). It was found that desorption of Ga atoms from the (0001) GaN surfaces under different III-V ratios deviates from the zeroth-order kinetics in that the desorption rate is independent of the coverage of adsorbed atoms. The desorption energies of Ga are determined to be 2.76 eV with the Ga coverage closing to 100%, 1.89 eV for a ~45% coverage, and 0.82 eV for a 10% coverage, as monitored by the change of the RHEED specular beam intensity during growth. In addition, the GaN surface morphology under different III-V ratios on porous templates matches the dependence of Ga desorption energy on the metal coverage, and III/V ratio dominates the growth mode. In a related AIGaN growth mechanism study, a competition between A1 and Ga atoms to incorporate into the film was found under metal-rich conditions. Employing this mechanism, A1xGa1-xN layers with precisely controlled A1 mole fraction, x in the range 0xxGa1-xN films was determined to be about 1 eV. The A1xGa1-xN layers grown under metal-rich conditions, as compared to that under N-rich conditions, have a better structural and optical quality. Employing A1xGa1-xN layers grown under metal-rich conditions, a lateral geometry GaN/A1GaN MQW-based photodetector was fabricated. It exhibited a flat and narrow spectral response in the range of 297~352 nm in the backillumination configuration.