No abstract

We report the fabrication and electrical characterization of single-hole transistors (SHTs), in which a Ge spherical quantum dot (QD) weakly couples to self-aligned electrodes via self-organized tunnel barriers of Si3N4. A combination of lithographic patterning, sidewall spacers, and selfassembled growth was used for fabrication. The core experimental approach is based on the selective oxidation of poly-SiGe spacer islands located at the specially designed included-angle locations of Si3N4/Si-trenches. By adjusting processing times for conformal deposition, etch back and thermal oxidation, good tunability in the Ge QD size and its tunnel-barrier widths were controllably achieved. Each Ge QD is electrically addressable via self-aligned Si gate and reservoirs, thus offering an effective building block for implementing single-charge devices.

We reported few-photon detection and exciton binding-energy determination using tunneling-current spectroscopy of Ge-quantum dot (QD) single-hole transistors (SHTs) operating in the few-hole regime under 400 nm–1550 nm illumination. When the photon energy is smaller than the bandgap energy (1.46 eV) of the 20 nm Ge QD (for instance, under 1310 nm and 1550 nm illuminations), the peak voltage of tunneling current peaks remain intact even irradiation power is as high as mW. In contrast, 850 nm illumination (i.e., the photon energy is equal to the bandgap energy of the Ge QD) induces a considerable shift in the first hole-tunneling current peak towards positive VG (VG 0.08 V at 25.8 nW and 0.15 V at 112 nW) and even creates new additional photocurrent peaks at more positive VG (VG 0.2 V) at W irradiation. The abovementioned experimental observations were further strengthened for Ge-QD SHTs illuminated by 405 nm lasers with much lower power. The observed newly-photogenerated current peaks are ascribed to the interactions between few photoexcitons and single-hole tunneling within the Ge QD.