Pin-Pointing Two Level Systems Originating from Defects in Si Using RF-STM

We present measurements in a milli-kelvin scanning tunneling microscope (mK-STM) that has an integrated LC resonator to enable sensitive reflectometry measurements at 300 MHz. This capability allows us to measure small changes in capacitance of local features in the vicinity of the sample surface. We’re specifically using the RF-reflectometry of the tunnel junction to target defects in phosphorus doped Si(100). In this geometry, applying a voltage bias to the Si sample induces band bending which forms a scannable quantum dot that is localized beneath the STM tip. In ‘lift-mode’ operation, we observe RF-imaged ring-like structures in the vicinity of sub-surface defects, which arise from strong tunneling coupling between the tip-induced quantum dot and states confined to the defects. This interaction leads to measurable shifts in quantum capacitance. Follow-up position dependent 1-D line spectroscopy measurements results in a detuning-like picture of the resonance conditions that gives rise to the strong contrast in capacitance. The spectroscopy measurements further reveal strong effects from so-called two level systems (TLS), which manifest as a telegraphing phase signal between two deterministic charge states.