Session K - Non-alloyed Ohmic Contact on GaAs at Nanometer Scale

Takhee Lee, Jia Liu, B.L. Walsh, R.P. Andres, E.H. Chen, D.B. Janes, M.R. Melloch, R. Reifenberger, (Purdue Univerisyt) and J. M. Woodall (Yale University),

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).