Characteristics and reduction of noise in scanning hot electron microscopy

Scanning hot electron microscopy (SHEM) is a method for observing the subsurface hot electron (HE) distribution in a solid. SHEM requires that both the HE and thermal-equilibrium electron (TE) cut-rents now through the tip current circuit. To obtain a large HE current, the top of the potential energy in the tip-sample gap region is lowered below the HE energy by the approach of the tip toward the sample and/or the increase in the voltage applied across the gap. However, even under this condition, the HE current is about one thousandth of the TE current. To improve the HE detection ability for SHEM, in this paper dependencies of the noise current caused by the gap space fluctuation on related parameters have been theoretically derived. Furthermore in the Au/Air/W structure, the power spectral density of the tunnel current is measured for SHEM. The noise current is proportional to the tunnel current as shown by theory. From the results, conditions for the observation of hot electrons are clarified.