Nanometer scale ohmic contact:
UHV STM images and current-voltage spectroscopic data

Takhee Lee, et al, Appl. Phys. Lett. vol. 74, 2869 (1999).
Takhee Lee, et al, Appl. Phys. Lett.vol. 76, 212 (2000).


A requirement of many of nano-devices is the presence of nearby contacts having dimensions of order 1 microns or greater. Thus, even though the device has shrunk well into the nanometer-scale, the contacts to the device still require areas of 102-104 greater than the active device.
Therefore, nano-contacts having dimensions comparable to a nano-device will be required to alleviate this difficulty in the future.

We have developed a nanometer-size, ohmic contacts to n-type GaAs substrates. The nano-contacts are formed between a single crystalline, nanometer-size Au cluster ( 4nm) and a GaAs structure capped with layer of low-temperature-grown GaAs (LTG:GaAs). A self-assembled monolayer of xylyl dithiol 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 x 10-6 Ohm cm2. Current densities above 1 x 106 A/cm2 have been observed.

Schematic of nanoscale ohmic contact structure

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25 x 25 nm UHV STM image of cluster on undoped LTG:GaAs surface.

The stability of the nano-contacts were checked by performing 100 consecutive images over an 80 minute period of time. From these measurements, the Au clusters were observed to remain stable and did not exhibit any damage due to the scanning conditions employed. These observations indicate that the clusters are well tethered to the LTG:GaAs cap-layer.
The electronic properties of the Au nano-contacts were investigated by measuring a series of I(V)s when the STM tip was positioned over the Au cluster and over the XYL-coated LTG:GaAs substrate. When compared to the XYL-coated substrate, the nano-contact is found to exhibit an ohmic behavior, with a significant enhancement in the conduction for low bias voltages.

I-V over cluster (ohmic)     I-V over substrate (Non ohmic)
(Different curves represent different tip position over the sample.)

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20 x 20 nm UHV STM image of cluster on p-doped LTG:GaAs surface.

Nano-ohmic contact behavior was also observed over Au nanocluster when p-doped LTG:GaAs cap layer was used.

I-V over cluster and substrate

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Last modified: Sun Oct 10 12:45:57 EST 1999