Tunable diameter and spacing of double Ge quantum dots using highly-controllable spacers and selective oxidation of...

We report the novel tunability of the diameters and spacings of paired Ge double quantum dots (DQDs) using nano-spacer technology in combination with selective oxidation of Si_0.85Ge_0.15 at high temperature. Pairs of spherical-shaped Ge QDs were formed by the selective oxidation of poly-SiGe spacer islands at each sidewall corner of the nano-patterned Si₃N₄/poly-Si ridges. The diameters of the Ge spherical QDs are essentially determined by geometrical conditions (height, width, and length) of the nano-patterned spacer islands of poly-SiGe, which are tunable by adjusting the process times of deposition and etch back for poly-SiGe spacer layers in combination with the exposure dose of electron-beam lithography. Most importantly, the separations between the Ge DQDs are controllable by adjusting the widths of the poly-Si/Si₃N₄ ridges and the thermal oxidation times. Our self-organization and self-alignment approach achieved high symmetry within the Ge DQDs in terms of the individual QD diameters as well as the coupling barriers between the QDs and external electrodes in close proximity.