Stable and epitaxial metal/III-V semiconductor heterostructures

Abstract

Long before the advent of nanofabrication and quantum-effect devices, the technological limitations imposed by polycrystalline, multiphase and thermally unstable contacts to III-V semiconductors were of concern to forward-looking materials scientists. In the early 1980s, efforts to elucidate the complex behaviour of reactive metal/III-V systems were initiated. These early efforts evolved slowly and culminated in the recent achievement of stable and epitaxial metallizations to III-V semiconductors. In this review, we first describe the criteria that must be met for the fabrication of metal/III-V heterostructures. Bulk phase equilibria are useful guides for selecting metal/semiconductor combinations which will not react during growth at moderate temperatures or during subsequent processing steps. We show, however, that phase stability is not sufficient for the fabrication of ultrathin metal overlayers or buried metal heterostructures. Growth conditions must be carefully optimized and combined with the appropriate selection of metallic phases with high melting points in order to suppress the strong tendency for island formation during growth and film agglomeration during overgrowth or processing. In our discussion of metal/semiconductor hetero-structures we highlight the relationship between symmetry differences and defects (domain boundaries) with particular emphasis on semiconductor overlayers grown on high-symmetry metals. Our work and that of others has shown that stable and epitaxial metallizations to III-V semiconductors as well as more complex metal/III-V heterostructures can be achieved with two classes of metallic materials; the transition-metal gallides and aluminides with the CsCl structure (TM-III) and the rare-earth monopnictides with the NaCl structure (RE-V). We discuss and compare the growth of these III-V/TM-III/III-V and III-V/RE-V/III-V heterostructures by molecular beam epitaxy, focusing special attention to the initial stages of growth of metallic films on III-V substrates and III-V overlayers on metallic films. Going beyond the strictly materials issues, we describe the electrical properties of such heterostructures, including stable enhanced-barrier Schottky contacts and semiconductor-clad metallic quantum wells, structures which may be the basis for exciting and novel electronic, photonic and magnetic devices.