The Microscopic Mechanism of 1/f Fluctuations in the Tunneling Current of Scanning Tunneling Microscopes

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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 and of W tips and graphite samples in ultra high vaccum. For 1/f noise, the noise intensity at a fixed current increases with increasing bias voltage. 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 phi 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. Tips capable of imaging graphite surface in atomic resolution sometimes yield random telegraphic noise with a Lorentzing spectrum, which is well understood by considering that the shar … More p tips select a single relaxation process that happens to be located in the narrow current path.The temperature dependence of 1/f fluctuations in the tunneling current of scanning tunneling microscopes was measured for graphite smaples in ultra high vacuum. The noise spectra in the temperature range between 65K and room temperature show the frequency dependence of 1/f^a type with alpha ranging around unity. The noise intensity at 100 Hz varies with temperature exhibiting two discernible peaks. The latter fact contradicts with the theoretical expectation from the phonon number fluctuation model which predicts monotonic increase of noise intensity with temperature. The experimental results are best explained by the model assuming thermally activated multiple relaxation processes. The frequency factor of the order of 1014 Hz which yields a good fit with the experimental data suggests that the origin of the 1/f noise is some mobile "defects" dispersed over the sample surfce at positions remote from the probe tip. Less