Michael G. Chapline

Carbon nanotube arrays on silicon substrates and their possible application

Abstract A method to grow regular arrays of oriented carbon nanotubes on silicon substrates is presented. It has been found that porous silicon is an ideal substrate for growing self-oriented carbon nanotubes on large surfaces. The growth mechanism of nanotube arrays has been discussed. The potential applications of carbon nanotube arrays in flat panel display and in synthesizing of other semiconducting nanorods on silicon substrates through the carbon nanotube-confined reaction have also been studied.

Observation of the Verwey transition in thin magnetite films

Magnetite (Fe3O4) has attracted much interest due to its unique properties such as the Verwey transition and 100% spin polarization at the Fermi surface. In this study thin films of epitaxial magnetite were grown on MgO and MgAl2O4 substrates using pulsed laser deposition. Previous studies suggested that the Verwey transition is typically not seen for film thicknesses less than 50nm. The existence of a sharp Verwey transition may indicate that the film has properties similar to that of bulk magnetite. We have investigated how stress, antiphase boundaries, imhomogenities, and deviations from stoichiometry affect the Verwey transition in thin films.

Spin filter based tunnel junctions

A theoretical estimate is given for the magnetoresistance ratio in ferromagnetic metal /nonmagnetic insulator/magnetic insulator/metallic junctions. Such a device has the potential to exhibit a room temperature magnetoresistive effect much larger than conventional magnetic tunnel devices. A half metallic electrode is desired but not required for achieving a sizable magnetoresistance in such devices. Some possible materials that could be used to fabricate such a device include ferrite based spin filters and CoFe∕MgO based electrodes. Such devices are predicted to give a magnetoresistance ratio >1000%.

Analytical formula for the tunneling current versus voltage for multilayer barrier structures

A formula is derived for the quantum mechanical tunneling current as a function of voltage bias for a multilayer barrier structure separated by similar electrodes. In the case of bilayer barriers, this formula has proven useful for confirming that the conduction is due to tunneling and can be used to determine the properties of the individual layers from fits to I-V curves. In contrast with Simmons’ formula [J. Appl. Phys. 34, 1793 (1963)] for the tunneling current in a single layer barrier, it is shown that the expected current versus voltage characteristics is polarity dependent. In the future this formula may prove useful for the analysis of resonant tunnel diodes and magnetic tunnel junctions utilizing multilayer barriers.