The objective of this book is to characterize the optoelectronic properties of quaternary n-Al0.08In0.08Ga0.84N thin films grown via plasma assistance molecular beam epitaxy on sapphire (Al2O3) and silicon (Si) substrates for different optoelectronic applications, including Al0.08In0.08Ga0.84N (MSM) photodetectors (PDs), solar cells and multi-quantum well (MQW) laser diodes (LDs). Defect-free films with high structural, optical and electrical qualities were obtained. X-ray diffraction analysis was used to characterize small full width at half maximum intensity of diffraction peaks, low compressive strain, relatively large grain size and low dislocation density which produced smooth surfaces without any phases separation or cracks. Scanning electron microscopy, energy-dispersive X-ray microscopy and atomic force microscopy images confirmed these characterizations. Furthermore, high optical quality, as well as high absorption and absorption coefficients were observed using PL and UV-VIS spectroscopy. Finally, the simulation of Al0.08In0.08Ga0.84N MQW LD using ISE TCAD software was designed and optimized. The best performance of the LDs was achieved at a quantum well number of 4.