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Molecular-beam epitaxy of III-N on novel ZrB2 substrates

Electrically conductive zirconium diboride (ZrB2) is a promising lattice-matched substrate for GaN-based nitride semiconductors. In this paper, important properties of ZrB2 as a substrate for nitrides, such as, thermal expansion coefficient, thermal conductivity, optical reflectivity and cleavage, are reviewed. Then, heteroepitaxial growth of GaN and AlN on the substrate by molecular-beam epitaxy (MBE) are discussed. Direct growth and two-step growth using low-temperature GaN nucleation layers as well as characterization of the surface condition of ZrB2 substrates by X-ray photoelectron spectroscopy (XPS) and the effect of surface treatment on grown layers are presented.