I. Introduction
III–V compound semiconductors have firmly established their importance in various electronic and optoelectronic applications. In particular, III-As-based materials have superior electron mobility and mature growth and process technologies. On the other hand, III-N-based materials are promising for power electronics applications due to their high breakdown voltage characteristics, which are deficient in III-As-based materials. Hence, proper hybridization of III-N and III-As materials may enable fabrication of novel devices targeting both the high-frequency and high-power applications with performance not achievable by each component material alone. Various GaAs/GaN heteroepitaxy experiments such as [1] and [2] have been attempted in the past. However, the quality of resulting materials was never satisfactory, mainly due to the huge lattice mismatch (43.5%) and the difference in the stable phases of GaAs (zinc-blende) and GaN (wurtzite).