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.[1]
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.[2]
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.[3]
We have previously shown that ohmic nano-contacts can be formed to n-GaAs
device layers using this non-alloyed ohmic contact approach.[2]
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. [3]
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.
[1]. A. G. Baca, et al., Thin Solid Films 308-309, 599 (1997).
[2]. Takhee Lee, et al., submitted to Appl. Phys. Lett.
[3]. M. P. Patkar, et al., Appl. Phys. Lett. 66, 1412 (1995).