Ohmic nanocontacts to GaAs using undoped and p-doped layers of low-temperature-grown GaAs

The development and characterization of high-performance nanocontacts to n-GaAs are reported. The nanocontacts can be made to both undoped and p-doped low-temperature-grown GaAs (LTG:GaAs) cap layers. The geometry of the nanocontact is well characterized and requires the deposition of a 4 nm single-crystalline Au cluster onto an ohmic contact structure which features a chemically stable LTG:GaAs surface layer prepared using an ex situ chemical self-assembly technique. A self-assembled monolayer of xylyl dithiol (HS–CH2–C6H4–CH2–SH) is required to provide mechanical and electronic tethering of the Au cluster to the LTG:GaAs surface. For the case of an undoped LTG:GaAs cap layer, a specific contact resistance of 1×10−6 Ω cm2 and a current density of 1×106 A/cm2 have been measured from scanning tunneling microscopy. When a p-doped LTG:GaAs cap layer is used, the corresponding values are 1×10−7 Ω cm2 and 1×107 A/cm2, respectively. Improved surface stability as evidenced by a lower oxidation rate for p-doped LTG:GaAs provides a natural explanation for the higher-quality ohmic contact properties of the nanocontact to the p-doped LTG:GaAs cap layer.