Feasibility of Ge double quantum dots with high symmetry and tunability in size and inter-dot spacing

We report the tunability of the sizes and inter-dot spacings of Ge coupled quantum dots (QDs) using nano-spacer technology in combination with selective oxidation of Si_0.85Ge_0.15. Spherical-shaped Ge QDs were formed at each sidewall corner of the nano-patterned Si₃N₄ ridges by thermal oxidation of poly-SiGe spacer layers encapsulating the Si₃N₄ nano-ridges. The diameters of the Ge spherical QDs are essentially determined by geometrical height, width, and length of the nano-spacer islands of poly-SiGe, which are tunable by adjusting the process times of their deposition and etch back. The inter-dot spacing between the Ge DQDs are controllable by adjusting the widths of the lithographically-patterned 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 sizes as well as the coupling barriers between the QDs and external electrodes in close proximity.