Quantum devices fabricated by hydrogen resist lithography

Nikola Pascher, Sigrun Köster and Andreas Fuhrer

IBM Research Zurich-Rüschlikon, Switzerland

On hydrogenated silicon (001) surfaces the tip of a scanning tunneling microscope (STM) can be used as a lithography tool to locally desorb a hydrogen passivation layer. Exposing STM-defined patterns to gas-phase dopants like phosphine (n-type) and diborane (p-type), atomically sharp and degenerately doped nanostructures can be fabricated in the silicon crystal. The high doping density makes gate-tunability of these devices difficult. Transport over the barrier between two doped contact areas is thus fully determined by the pattern geometry, specifically the length of the gap in the STM pattern and the width of the contacts. Thus a thorough characterization of barriers with varying geometry is crucial for device fabrication. We use cryogenic transport experiments on a 6 nm x 6 nm n-type quantum dot with four different contacts to characterize the functionality of barriers with varying length (d = 10, 12, 16 and 27 nm). We probe different voltage configurations while measuring the current into every contact and use Coulomb blockade spectroscopy to estimate the tunnel and capacitive coupling of each of the contacts to the dot. We find that the extracted mutual capacitances closely match the values calculated from the geometry using an electrostatic field solver. We also present initial results on p-type devices fabricated by STM-lithography and show that similar dopant devices can be realized with diborane. This will enable us to explore new atomic-scale bipolar dopant devices in the future.