Ballistic Electron Transport in Aluminum Oxide
Aluminum oxide (alumina) is becoming an increasingly important material in high performance electronics. It is the insulator in magnetic tunnel junctions used for MRAM and may allow for the fabrication of solid state qubits based on Josephson junctions. In this thesis I have used ballistic electron emission microscopy (BEEM) to study the physical and electronic structure of alumina. BEEM's high spatial resolution (~1 nm) was exploited to study the alumina surface where clusters of chemisorbed oxygen were observed. The overall behavior of these clusters helped piece together the electronic structure of the material, in addition to the oxidation process itself. While the data from the oxide surface studies can be interpreted under the standard set of assumptions people generally impose on BEEM (i.e. no scattering) the data obtained when the oxide is buried cannot. In the latter case the signal levels are too low to ignore scattering. Before the spectra of buried oxide films could be interpreted, scattering needed to be understood. Scattering in BEEM is nothing new. Kaiser and Bell (BEEM's inventors) did the experiment nearly a decade ago when they injected holes into a Au base and collected electrons with n-type Si. The essential physics behind the scattering process was properly described in their work. However, fits to data using their theory failed at electron energies above 1.3 eV. By properly accounting for the density of tunneling states, the Kaiser-Bell approach can be modified to correctly describe the data at higher electron energies. Once simple systems could be reliably fit, scattering BEEM could be used to study alumina. In the case of hole injection, a higher turn on voltage (relative to Au on Si) is observed which is described by an inelastic scattering process. In the case of electron injection, the same scattering is present and is observed because the alumina attenuates the otherwise dominant unscattered signal. Scattering also effects spin transport. As scattering increases, the polarization of a ballistic electron beam becomes diluted. This is observed as the fading of contrast in magnetic images as alumina forms between two ferromagnetic layers.
R.A. Buhrman D.C. Ralph P.W. Brouwer
DARPA, NSF, ONR
Ballistic electron emission microscopy; aluminum oxide; hot electron scattering; magnetic tunnel junction; oxidation; scanning tunneling microscopy
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