Suppression of charges within an Al ₂ O ₃ high- <i>κ</i> dielectric and reduction of interface traps at the cryogenic temperature of a Ge n-MOS capacitor fabricated by plasma postoxidation and forming gas annealing

Abstract Germanium (Ge)-based quantum devices are considered as promising candidates for scalable quantum information processing due to strong intrinsic spin–orbit coupling and the capability for ultrafast all-electrical spin manipulation. To fabricate fault-tolerant quantum encoding devices, it is important to improve the cryogenic properties of a Ge-based metal oxide semiconductor (Ge MOS) structure with thin gate dielectric layers as much as possible. Based on a thin Al 2 O 3 /GeO x dielectric stack, we investigate the effects of oxygen plasma postoxidation (PPO) power and hydrogen forming gas annealing (H 2 FGA) temperature on the cryogenic properties of Ge MOS capacitors in depth. By systematically depicting the distribution of the interface trap density ( D it ) within the Ge bandgap, we found that the formation of GeO x reduces the energy state density of characteristic defects across the Ge bandgap, thereby leading to the reduction of the D it . The contribution of FGA to the reduction of the fixed charge within the Al 2 O 3 dielectric is clarified, and the C–V hysteresis effect improvement is revealed by suppressing the trap charge area density within the Al 2 O 3 dielectric by integrating the PPO and FGA processes. Finally, the D it of the Ge n-MOS capacitor with the Al 2 O 3 /GeO x dielectric stack is reduced to 5 × 10 11 cm −2 eV −1 (@ E v + 0.13 eV) at 15 K. The area density of the fixed charge and the trap charge within Al 2 O 3 are reduced from 1.4 × 10 12 –7.2 × 10 11 cm −2 and from 1.28 × 10 12 –5.13 × 10 10 cm −2 , respectively. This provides a technical pathway for the development of Ge-based devices for quantum computation.