Lithography-free, site-controlled germanium quantum dots in silicon nanowires for single-hole transistors operating up to 50 K

Precise control over quantum dot (QD) position and size is critical for quantum electronics but typically requires complex nanofabrication. Here, we report a step-confined heteroprecursor supply (sc-HPS) strategy for growth of crystalline germanium (c-Ge) QDs at predefined step edges within planar ultrathin silicon nanowire (SiNW) channels, eliminating the need for high-resolution lithography. Unlike conventional in-plane solid-liquid-solid growth using a uniform amorphous silicon precursor, sc-HPS uses a spatially confined amorphous germanium (a-Ge) layer defined by oblique patterning of a SiO₂/a-Ge stack. This restricts Ge supply to a narrow strip along the step edge, enabling nucleation and tunable QD diameters of ~25 to 150 nm, sandwiched between two epitaxially connected SiNWs. The SiNW/Ge-QD/SiNW heterostructure forms a Si/Ge interface that provides three-dimensional hole confinement. Single-hole transistors fabricated from these structures exhibit clear Coulomb oscillations and well-defined Coulomb diamonds up to 50 K, demonstrating single-hole charging behavior. This scalable and lithography-free approach enables previously unexplored opportunities in hole-based quantum devices and nanoelectronic architectures.