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 nano contact is well characterized and requires the deposition of a 4 nm single- crystalline Au cluster onto an ohmic contact structure which features a che mically stable LTG:GaAs surface layer prepared using an ex situ chemical se lf-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 x 10(-6) Ohm cm(2) and a cur rent density of 1 x 10(6) A/cm(2) have been measured from scanning tunnelin g microscopy. When a p-doped LTG:GaAs cap layer is used, the corresponding values are 1 x 10(-7) Ohm cm(2) and 1 x 10(7) A/cm(2), respectively. Improv ed surface stability as evidenced by a lower oxidation rate for p-doped LTG :GaAs provides a natural explanation for the higher-quality ohmic contact p roperties of the nanocontact to the p-doped LTG:GaAs cap layer. (C) 2000 Am erican Institute of Physics. [S0003-6951(00)03002-3].