Electronic properties of gold nanoclusters/semiconductor structures with low resistance interfaces
Abstract
Self-assembled metal/molecule/semiconductor nanostructures are utilized to define nanoelectronic device contact structures and are characterized using ultra high vacuum (UHV) scanning tunneling microscopy (STM). As examples of the controlled nanostructures for nanoelectronic device applications, nonalloyed ohmic contact nanostructures have been utilized on a surface layer of LTG:GaAs, i.e., GaAs grown at a low temperature by molecular beam epitaxy. The controlled-geometry nanocontact is obtained by depositing a 4 nm diameter single crystal Au cluster (truncated octahedral shape) onto n-GaAs(100) having LTG:GaAs based ohmic contact layers using ex-situ chemical self-assembly techniques. A self-assembled monolayer (SAM) of xylyl dithiol (HS-CH2-C6H4-CH2-SH) is formed on LTG:GaAs and provides an effective organic metal/semiconductor interface having both a robust mechanical tethering and a strong electronic coupling between the Au nanoclusters and the LTG:GaAs surface. UHV STM is used to locate and probe the electronic properties of the nanocontacts. STM current versus voltage (I-V) data measured over Au nanoclusters exhibit an ohmic behavior with a significant enhancement in the conduction for low bias voltages compared to I-V data over the SAM-coated LTG:GaAs substrate. A specific contact resistance of 10 −6–10−7 Ω·cm 2 and a current density of 106–107 A/cm2 have been measured on the nanocontacts from STM. The ohmic nanocontact is mainly due to the sequential tunneling through the xylyl dithiol layer and the LTG:GaAs layer while a midgap band of defect states in the LTG:GaAs layer assists conduction as if it effectively reduces the barrier width. Another interesting approach is to combine the nanoscale elements (Au clusters) and ordering from self-assembly processes with a procedure which can impose an arbitrary larger-scale pattern to form the specific configurations and interconnections needed for computation. Toward this goal, high-quality hexagonal close-packed arrays of Au nanoclusters (5 nm in diameter) are formed within patterned regions on active GaAs substrates having LTG: GaAs cap layer and are characterized using STM. This approach utilizes a patterned template which guides Au nanoclusters into pre-selected regions with xylyl dithiol. The directed self-assembly techniques used to fabricate these structures have the potential to provide high-throughput fabrication of nanostructures for nanoelectronics and other nanoscale applications.
Degree
Ph.D.
Advisors
Reifenberger, Purdue University.
Subject Area
Condensation
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