Noise in Scanning Tunneling Microscopy

We give a brief overview of our work on noise in scanning tunneling microscopy. We first estimate the electronic noise generated by the control circuit, then we present results on noise measurements performed under ultra high vacuum conditions. Results will be discussed for in situ preparations of both polycrystalline metal films (Au, Ag, Cu) evaporated onto silicon wafers, and for cleaved GaAs(110). Metal films had a high dynamic range of noise, and we could distinguish noisy areas of size >10 nm within a ‘low noise’ matrix. In most situations the noise was of 1/f type. The noise in the ‘low noise’ area behaved according to Hooges formula for the power spectral density (PSD ) SI = αH . I t 2 /N . f β with N set to unity, and β ≈ 1.0 and αH ≈ 4 . 10−3 . We attribute this noise to thermally activated processes in the metal film. Conversely, in noisy areas SI =αH . It/fβ. The PSD on GaAs(ll0) varied on an atomic scale with enhanced noise at the positions of the Ga atoms. Our explanation employs 1/f fluctuations in the phonon density (number fluctuations ).