Even though prototype electronic devices have shrunk into the nanometer
scale range, the contacts to nano-devices often have dimensions of
order ~ micrometer or greater. Thus, the contacts still require
~ 102-104 greater area than the active nano-devices. The
demands on ohmic contacts at the nanometer scale are quite stringent
and present significant problems for any
approach based on an alloying process.
As reported earlier, non-alloyed ohmic contacts may be
appropriate for nano-device applications since they are free from a
deep interface and they possess spatial uniformity.
These non-alloyed contacts employ low-temperature grown
GaAs (LTG:GaAs), i.e. GaAs grown
at a temperature of 250-300oC by molecular beam epitaxy.
We have previously shown that ohmic nano-contacts can be formed to n-GaAs device layers using this non-alloyed ohmic contact approach. In this presentation, we extend the previous work to the development and characterization of high performance nano-contacts to n-GaAs using both n-type and p-type LTG:GaAs caplayers. The controlled-geometry nano-contact was obtained by depositing a 4 nm single crystalline Au cluster on an ohmic contact structure with a chemically stable LTG:GaAs surface layer using ex-situ chemical self-assembly techniques. A self-assembled monolayer of xylyl dithiol (C8H10S2; denoted as XYL) was used to provide mechanical and electronic tethering of the Au cluster to the LTG:GaAs surface. The dimensions of the Au cluster determine the size of the nano-contact. The electrical characteristics of the Au cluster/XYL/GaAs were determined using UHV scanning tunneling microscopy (STM). It has been found that the cluster/XYL contact to samples with both n- and p-type LTG:GaAs provide an effective ohmic nano-contact with good repeatability between various clusters distributed on the surface. STM current-voltage spectroscopy for these nano-structures provide estimates for the specific contact resistance and current density of the nano-contacts. For the case of an n-type LTG:GaAs caplayer, a contact resistance of 1 x 10-6 ohm cm2 and a current density, 1 x 106A/cm2 have been measured. When a p-type LTG:GaAs caplayer is used, the corresponding values are 1 x 10-7 Ohm cm2 and 1 x 107A/cm2, respectively. These contact properties are comparable to those observed in high quality large area ohmic contacts to n-type GaAs. 
A model explaining the origin of the ohmic contact as well as the difference in the contact properties between the samples with the n-type and p-type caplayers will be presented. Possible applications for these nano-contacts will also be discussed.
. A. G. Baca, et al., Thin Solid Films 308-309, 599 (1997).
. Takhee Lee, et al., submitted to Appl. Phys. Lett.
. M. P. Patkar, et al., Appl. Phys. Lett. 66, 1412 (1995).