Semiconductor materials have been studied intensively since the birth of silicon technology more than 50 years ago. The ability to physically and chemically t- lor their properties with precision is the key factor responsible for the electronic revolution in our society over the past few decades. Semiconductor material s- tems (like silicon and GaAs-related materials) have now matured and found well established applications in electronics, optoelectronics, and several other ?elds. Other materials such as III-Nitrides were developed later, in response to needs that the above mentioned semiconductors were unable to ful?ll. The properties of I- nitrides (AlN, GaN InN, and related alloy systems) make them an excellent choice for ef?cient light emitters in the visible as well as the UV region, UV detectors, and for a variety of electronic device such as high frequency unipolar power devices. There was a major upsurgein the research of the GaN material system around1970.