Importance of the tip shape in RF-STM measurements on an Induced Quantum Dot in Silicon

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 would allow material testing at the quantum dot level without damage to the substrate under study. To that end, our group has been measuring the 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). Preparation of the tungsten tip on an Au (111) crystal resulted in a broad metallic tip base with a 2 nm protrusion as the overall tip shape. Tip imaging was performed in-situ on pyramidal features on the Si (100) surface, which allow to resolve the last 20-25 nm of a tip. The broad tip delivered an unprecedented response in the capacitance signal both qualitatively and quantitatively. After a tip change the RF response changed. We were able to track tip shape and RF response through several deliberate tip changes induced by applying voltage pulses. Ultimately, we were unable to recover the broad tip shape. The dependence of C_TS(V) vs. tip shape will be discussed.

 

[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