V. R. Kolagunta

Self-Assembly of a Two-Dimensional Superlattice of Molecularly Linked Metal Clusters

Close-packed planar arrays of nanometer-diameter metal clusters that are covalently linked to each other by rigid, double-ended organic molecules have been self-assembled. Gold nanocrystals, each encapsulated by a monolayer of alkyl thiol molecules, were cast froma colloidal solution onto a flat substrate to form a close-packed cluster monolayer. Organic interconnects (aryl dithiols or aryl di-isonitriles) displaced the alkyl thiol molecules and covalently linked adjacent clusters in the monolayer to form a two-dimensional superlattice of metal quantum dots coupled by uniform tunnel junctions. Electrical conductance through such a superlattice of 3.7-nanometer-diameter gold clusters, deposited on a SiO2 substrate in the gap between two gold contacts and linked by an aryl di-isonitrile [1,4-di(4-isocyanophenylethynyl)-2-ethylbenzene], exhibited nonlinear Coulomb charging behavior.

Room temperature Coulomb blockade and Coulomb staircase from self‐assembled nanostructures

The self‐assembly of well‐characterized, nanometer‐size Au clusters into ordered monolayer arrays spanning several microns has been achieved. Techniques to insert molecular wires to link adjacent clusters in the self‐assembled array have also been developed. ‘‘Unit cell’’ nanostructures formed from individual Au clusters supported on a self‐assembled monolayer film of the double‐ended thiol molecule p‐xylene‐α,α′‐ dithiol show evidence for reproducible single electron effects at room temperature when studied by scanning tunneling microscopy. From these measurements, estimates for the electrical resistance of a single molecule can be obtained. The experimental values for this resistance are in reasonable agreement with theoretical calculations using the Landauer approach.

An ohmic nanocontact to GaAs

The formation and characterization of nanometer-size, ohmic contacts to n-type GaAs substrates are described. The nanocontacts are formed between a single-crystalline, nanometer-size Au cluster and a GaAs structure capped with layer of low-temperature-grown GaAs (LTG:GaAs). An organic monolayer of xylyl dithiol (p-xylene-α,α′- dithiol; C8H10S2) provides mechanical and electronic tethering of the Au cluster to the LTG:GaAs surface. The I(V) data of the Au cluster/xylyl dithiol/GaAs show ohmic contact behavior with good repeatability between various clusters distributed across the surface. The specific contact resistance is determined to be 1×10−6 Ω cm2. Current densities above 1×106 A/cm2 have been observed.

Nanoelectronic device applications of a chemically stable GaAs structure

We report on nanoelectronic device applications of a nonalloyed contact structure which utilizes a surface layer of low-temperature grown GaAs as a chemically stable surface. In contrast to typical ex situ ohmic contacts formed on n-type semiconductors such as GaAs, this approach can provide uniform contact interfaces which are essentially planar injectors, making them suitable as contacts to shallow devices with overall dimensions below 50 nm. Characterization of the native layers and surfaces coated with self-assembled monolayers of organic molecules provides a picture of the chemical and electronic stability of the layer structures. We have recently developed controlled nanostructures which incorporate metallic nanoclusters, a conjugated organic interface layer, and the chemically stable semiconductor surface layers. These studies indicate that stable nanocontacts (4 nm×4 nm) can be realized with specific contact resistances less than 1×10−6 Ω cm2 and maximum current densities (1×106 A/cm2) comparable ...

Annealing stability and device application of nonalloyed ohmic contacts using a low temperature grown GaAs cap on thin n+ GaAs layers

This letter summarizes a study of nonalloyed ohmic contact structures consisting of Au/Ti metallization deposited on a thin (3.5 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a thin (10 nm) layer of heavily doped n-type GaAs. We demonstrate that this Au/Ti:LTG:GaAs/n+GaAs contact structure can be used to make effective contacts to thin n+ layers, that the resulting contact survives annealing at temperatures between 300 °C and 400 °C, and that the contact resistivity, ρc, is reasonably stable for these anneals. This is contrasted with conventional Au/Ge/Ni alloyed contacts. The contact structure has also been applied to a resonant tunneling diode (RTD). The characteristic current-voltage curves of RTDs show that the performance of the intrinsic barrier/well/barrier region of the device is not degraded after anneal.

Self‐aligned sidewall gated resonant tunneling transistors

Three‐terminal vertical quantum structures using a self‐aligned sidewall gating technique have been developed. Resonant tunneling transistors with physical widths of about 0.7 μm demonstrate pinch‐off of the resonant peak at room temperature. Comparable gating characteristics for forward and reverse drain‐source biases indicate that the gating action is vertically uniform, making this topology suitable for the fabrication of low‐dimensional structures and study of multiple quantum well devices.

Sidewall gated double well quasi-one-dimensional resonant tunneling transistors

We present gating characteristics of submicron vertical resonant tunneling transistors in double quantum well heterostructures. Current–voltage characteristics at room temperature and 77 K for devices with minimum feature widths of 0.9 and 0.7 μm are presented and discussed. The evolution of the I–V characteristics with increasing negative gate biases is related to the change in the lateral confinement, with a transition from a large area 2D to a quasi-1D. Even gating of multiple wells and lateral confinement effects observable at 77 K make these devices ideally suited for applications in multi-valued logic systems and low-dimensional structures.

Self-assembly based approaches for metal/molecule/semiconductor nanoelectronic circuits

This paper describes a technological approach which combines the nanoscale elements available from molecular devices and self-assembled molecular/nanoparticle systems with semiconductor devices which can provide the gain or bistability required for computational functionality. The architectural motivation for these configurations and experimental demonstrations of several key technologies for this hybrid approach are described.

Quasi one-dimensional confinement in double-well sidewall gated resonant tunneling transistors

In this paper we present gating effects in double well resonant tunneling heterostructures with sub-micron minimum feature widths. Resonant tunneling through one dimensional states in the wells is observed as the device approaches pinch-off at temperatures as high as 77 K. This is the first clear demonstration of resonant tunneling through such laterally confined one-dimensional sub-bands at 77 K.