The Influence of Localized State Charging on 1/f Tunneling Current Noise Spectrum

We report the results of theoretical and UHV STM investigations of low frequency tunneling current noise spectra component (1/f) in vicinity of individual impurity atoms on the InAs (110) surface. It is found that power law exponent of tunneling current noise spectra strongly differs for measurements above flat surface and above impurity atom. Obtained results can be explained by switching ON and OFF of Coulomb interaction of conduction electrons with one or two charged localized states in tunneling junction which results in power law singularity of lowfrequency part of tunneling current noise spectrum 1/f. Power law exponent in different low frequency ranges depends on the relative values of Coulomb interaction of conduction electrons with different charged impurities. Introduction In the present work we discuss one of the possible reasons for the 1/f noise in the STM/STS junctions. Up to now the typical approach to 1/f noise problem consists of “by hand” introducing of the random relaxation time τ for two-state system with the probability distribution function A/τ. Therefore the averaged over τ noise spectrum of two-state system has power law singularity. But the physical nature and the microscopic origin of such probability distribution function in general is unknown. Although the current noise gives a basic limitation for the performance of scanning tunneling microscope, only the limited number of works was devoted to study of 1/f noise. The investigations of the noise spectra in two-level system were carried out in [2]. Authors studied current noise in a double-barrier resonant-tunneling structure due to dynamic defects that can be switched states because of their interaction with a thermal bath. The time fluctuations of the resonant level result in lowfrequency noise, the characteristics of which depend on the relative strengths of the electron escape rate and the defect's switching rate. If the number of defects is large, the noise is of the 1/f type. Experimental investigations of tunneling current noise spectra have been carried out in [3], [4], [5], [6]. The influence of fluctuations of the tunneling barrier height on tunneling current spectrum has been studied experimentally in [7]. The experiments have been performed under UHV conditions on graphite and gold samples using PtIr tips. From these measurements the authors have concluded that the intensity of barrier height fluctuations correspond to the intensity of tunneling current 1/f noise in the frequency range from 1 to 100 Hz. To the best of our knowledge tunneling current noise STM measurements were performed only for relatively simple surfaces like gold, graphite etc. Our main goal is to study one of the possible microscopic origins of 1/f noise in tunneling contact. We report the results of investigation of tunneling current noise spectrum modification by the Coulomb interaction of conduction electrons in the electrodes with non-equilibrium localized charges formed by different impurity atoms on the sample and on the STM tip apex. We suggest the theoretical model for tunneling current noise above the clean flat surface and above the impurity atoms on semiconductor or metallic surfaces. We have found that changing of the power law exponent above the flat surface and above impurity atom depends on the parameters of tunneling junction such as tip-sample separation [1]. * [email protected] 67 WDS'08 Proceedings of Contributed Papers, Part III, 67–73, 2008. ISBN 978-80-7378-067-8 © MATFYZPRESS