Meng Zhu

Molecular Self-Assembly at Bare Semiconductor Surfaces: Cooperative Substrate−Molecule Effects in Octadecanethiolate Monolayer Assemblies on GaAs(111), (110), and (100)

The structures of self-assembled monolayers formed by chemisorption of octadecanethiol onto the surfaces of GaAs(001), (110), (111-A)-Ga, and (111-B)-As have been characterized in detail by a combination of X-ray photoelectron, near-edge X-ray absorption fine structure, and infrared spectroscopies and grazing incidence X-ray diffraction. In all cases, the molecular lattices are ordered with hexagonal symmetry, even for the square and rectangular intrinsic substrate (001) and (110) lattices, and the adsorbate lattice spacings are all incommensurate with their respective intrinsic substrate lattices. These results definitively show that the monolayer organization is driven by intermolecular packing forces to assemble in a hexagonal motif, such as would occur in the approach to a limit for an energetically featureless surface. The accompanying introduction of strain into the soft substrate surface lattice via strong S substrate bonds forces the soft substrate lattice to compliantly respond, introducing quasi-2D strain. A notably poorer organization for the (111-A)-Ga case compared to the (111-B)-As and other faces indicates that that the Ga-terminated surface lattice is more resistant to adsorbate packing-induced stress. Overall, the results show that surface molecular self-assembly must be considered as a strongly cooperative process between the substrate surface and the adsorbate and that inorganic substrate surfaces should not be considered as necessarily rigid when strong intermolecular adsorbate packing forces are operative.

Observation of magnetoresistance polarity reversal in 3D to 2D tunneling in an asymmetric GaMnAs resonant tunneling diode

We investigate the magnetoresistance (MR) characteristics of a GaMnAs-based asymmetric resonant tunneling structure with a 3D GaMnAs top layer and a 2D GaMnAs quantum well (QW). The incorporation of a 2D layer distinguishes our device from the many conventional devices reported in the literature in that the MR characteristics of our device result from 3D to 2D tunneling, whereas MR in a conventional device results from 3D to 3D tunneling. By observing a shift of negative differential resistance (NDR) features as an in-plane magnetic field is varied, we infer that the 3D to 2D tunneling magnetoresistance (TMR) in our device has the opposite sign from conventional TMR in GaMnAs. We relate this difference to the effect of quantum confinement on the 2D density of states in the GaMnAs QW.

Tunneling Magnetoresistance in Exchange Biased Ferromagnetic Semiconductor Tunnel Junctions

Summary form only given.Here, we demonstrate the observation of TMR in exchange-biased MTJs derived from the ferromagnetic semiconductor (Ga,Mn)As. Although still limited to operation at low temperatures, these devices provide an important step forward in exploring proof-of-concept semiconductor spintronic tunneling devices.