Surfaces and nanofilms

Many physical phenomena are modified in the presence of a surface. This can be due to the lower coordination number at the surface, surface structural modifications such as reconstructions and surface layer expansions, or band bending at the surface. Furthermore, the study of thin films opens up new scientific and technological horizons. Basic physical phenomena such as magnetism, hydrogen interactions with materials, phase transitions, critical phenomena, and epitaxial growth can be modified in the thin film limit by controlling the influences of finite size, pseudomorphic crystal structures, quantum confinement, lattice clamping, and epitaxially induced anisotropic strain. This is especially so when films are only a few nanometers thick, i.e. nanofilms.

It is vital, however, that such samples be fabricated and carefully maintained in an ultra-high vacuum (UHV) chamber since even a sub-monolayer of surface contamination may disrupt surface effects and the composition of ultra-thin films is easily altered by exposure to air, through oxidation and absorption of Hydrogen, etc.

The NCNR Molecular Beam Epitaxy (MBE) Chamber for In-Situ Neutron Scattering can be used to fabricate single crystal thin films and multilayers, as a protective UHV environment for both bulk and thin film samples, and as an ultra-clean environment for loading interstitials, all while allowing neutron scattering investigations in-situ. MBE was chosen as the thin film growth technique because it produces the purest thin films with the best control of layer thickness and interface widths, and because it produces the highest quality epitaxial single crystal thin films, heterostructures, and superlattices. This is the only combined in-situ MBE / neutron scattering chamber in the world.

The study of SiO2 on Si provided a useful vehicle to develop techniques and test the limits of neutron reflectometry for nanofilm metrology while returning useful information on this technologically important system.

Additional uses of the MBE environment include the ability to control purity and very low pressure levels of gasses such as H used in adsorption and absorption studies. One example is the Reversible Tuning of the Magnetic Exchange Coupling in Fe/V (001) Superlattices Using Hydrogen .

Last modified 20-February-2003