Comparison of RF response of a tip induced quantum dot and local density of states measurements of Silicon at mK temperatures by RF-STM

Qualifying the suitability of a specific material substrate to host spin qubits for quantum computing applications prior to device fabrication would be extremely advantageous. A scanning electrode has been proposed [1] to locally induce a quantum dot in a semiconducting material and interrogate its interaction with buried defects through dispersive readout. This method provides a non-invasive approach to testing material quality at the quantum dot level, preserving the substrate integrity. To that end, our group has been measuring the bias voltage dependent tip-sample capacitance C_TS(V) on Cl terminated Si samples using an RF tank circuit in a milli Kelvin scanning tunneling microscope (mK-STM). We recently observed apparent multistate structures in the bias dependent capacitance response. The structures manifest as bright sets of parabolas as a function of tip position. Single parabolas or line-like structures where also observed. Simultaneously acquired scanning tunneling spectroscopy (STS) data show similar features, albeit limited to voltage regions outside of the silicon bandgap. In rare examples, features from both measurements coincide, but more often seem exclusive. A state in STS has to be part of the tunneling process and therefore its origin has to be located close to the tip apex. In contrast, the RF based capacitance response has a longer detection range. This, however, only accounts for occasional coincidence and not the observed exclusivity. These findings highlight the complexity of subsurface defect interactions and their influence on quantum dot behavior, underscoring the need for further investigation into the detection limits and mechanisms at play.



[1] Yun-Pil Shim, Rusko Ruskov, Hilary M. Hurst, and Charles Tahan, Appl. Phys. Lett. 114 (15) 152105 (2019). https://doi.org/10.1063/1.5053756

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