Bipolar device fabrication using a scanning tunnelling microscope

Hydrogen resist lithography with the tip of a scanning tunneling microscope (STM) can be used to fabricate atomic-scale dopant devices in silicon substrates and could potentially be used to build a dopant-based quantum computer. However, all devices fabricated so far have been based on the n-type dopant precursor phosphine. Here, we show that diborane can be used as p-type dopant precursor, allowing p-type and bipolar dopant devices to be created. Characterisation of diborane \(\delta\)-layers reveals that similar mobilities and densities can be achieved as for phosphine, with sheet resistivities as low as 300 \(\Omega\). STM imaging and transport measurements of a 5.5 nm-wide p-type dopant nanowire give an estimated upper bound of 2 nm for the lithographic resolution of the p-type dopant profiles. By combining our p-type doping approach with established phosphine-based n-type doping, we fabricate a 100 nm wide pn-junction and show that its electrical behaviour is similar to that of an Esaki diode.