Exotic and mundane substrates for gallium nitride heteroepitaxy

It is often asserted that the principal obstacle to the epitaxial growth of high quality gallium nitride is the lack of a lattice matched substrate. Although this view is overly simplistic and underly factual, it conveys the correct impression that the most commonly used substrate, sapphire, has many shortcomings. Are there substrates to improve on sapphire? We have studied the growth of gallium nitride on a variety of materials, including ZnO, LiGaO/sub 2/, LiAlO/sub 2/, ScMgAlO/sub 4/, Si, garnets and spinels. By growing on a variety of materials, we can assess the relative importance of lattice match, thermal expansion match, chemical stability and compatibility, polarity, and even cost of a substrate on the ensuing growth.