Origin of low frequency noise and 1/f fluctuations of tunneling current in scanning tunneling microscopes

Abstract

The generation mechanism of conspicuous low frequency noise (including 1/f fluctuations) observed in the tunneling current of scanning tunneling microscopes was studied experimentally by investigating combinations of Pt–Ir tips and Au films or crystalline graphite samples. For 1/f noise, the noise intensity at a fixed current increases with increasing bias voltage or increasing tip–sample distance z. The noise power varies from place to place in a sample surface of Au film. The spatial variation of the noise has a positive correlation with the magnitude of the tunneling barrier height φ measured by the z modulation method. The demodulated signal in the z modulation experiments also exhibits fluctuations with a 1/f spectrum of the relative magnitude nearly equal to that in the current noise. The above features are observed also in high vacuum (10−7 Pa), ruling out the possibility that the current noise is brought about by absorption/desorption of gas molecules in the ambient. Tips capable of imaging graphite surface in atomic resolution sometimes yield random telegraphic noise with a Lorentzian spectrum, which is well understood by considering that the sharp tips select a single relaxation process that happens to be located in the narrow current path. Nevertheless, the 1/f spectra were in many cases obtained even in the atomic-resolvable tips. The experimental results are discussed based on two models, the φ fluctuation model and the phonon density fluctuation model.