Research

Nanopatterning of Si/SiGe Two-dimensional Hole Gases by PFOTS-aided AFM Lithography of Carrier Supply Layer

Abstract:
Our goal is to achieve Si-based quantum dots for single-electron devices with zero surface states from Si/SiO2 or other imperfect passivation, since one surface state can greatly alter a single electron device. We have previously shown the approach of defining SiGe quantum dots for holes by AFM lithography and wet etching (to avoid radiation damage defects from e-beam and RIE) and epitaxial regrowth of Si to passivate the dot surface with an epitaxial heterojunction to confine holes with ideally zero interface states. In Coulomb blockade single-hole FET¡¯s epitaxial regrowth showed a vast improvement in repeatability and sharpness of the linewidth of conductance oscillations compared to unpassivated devices [1]. This work was limited, however, by fact that (i) lithography and pattern transfer from AFM oxidation by wet etching were very non-reproducible, and (ii) the inevitability of interface contamination on the dot surface during regrowth, resulting in defect states. In this work, we report two significant advances to overcome these limitations: (i) the use of a self-assembled monolayer (PFOTS) as an etch resist to improve the pattern transfer uniformity and repeatability of AFM lithography, and (ii) the patterning of the carrier supply layer of a 2D hole gas by this method, as opposed to the doped conducting layer itself. This avoids the potential creation of defects of the surfaces of the resulting quantum dots, since the dot surface itself is never exposed.