Dresden 2020 – scientific programme
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O: Fachverband Oberflächenphysik
O 28: Poster Session - Scanning Probe techniques: Method Development
O 28.3: Poster
Monday, March 16, 2020, 18:15–20:00, P1A
Fast low-noise transimpedance amplifier for scanning tunneling microscopy — Martin Štubian1,2, Juraj Bobek1,2, Martin Setvin1, and •Michael Schmid1 — 1Institute of Applied Physics, TU Wien, Austria — 2Brno University of Technology, Brno, CZ
Scanning tunneling microscopy is one of the most versatile techniques in surface physics. One of the factors limiting its performance is the bandwidth and noise of the preamplifier. Higher bandwidth enables faster scanning, and also implies low phase shifts, which reduces the susceptibility to feedback loop oscillations. STM preamplifiers are current-voltage converters (transimpedance amplifiers, TIAs), usually with a high feedback resistor. Increasing its resistance leads to lower current noise (Johnson noise of the resistor), but at the same time usually results in lower bandwidth. Using a multi-stage amplifier design, we could achieve an input noise of ≈ 5 fA/√Hz at room temperature and low frequencies, but nevertheless a large bandwidth of 50–200 kHz and large dynamic range (sub-pA to 50 nA). We also demonstrate that minimizing the input capacitance is of paramount importance for low noise. This means that connecting the STM tip to the preamplifier via a long coaxial cable is unfavorable, and the performance can be substantially improved by placing the first amplifier stage into vacuum. Additionally, for low-temperature STMs, the Johnson noise is reduced by placing the feedback resistor in thermal contact with the cryostat. We also discuss a source of noise in operational amplifiers usually not considered, but important for TIAs.