Floating electrometer for scanning tunneling microscope applications in the femtoampere range

Abstract

In this work, a floating, high precision current–voltage converter, applicable to scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM) measurements, is described. The electrometer circuit presented shows a bias independent output offset, adds low noise, and has low thermal drift. The amplifier is useful for any floating applications where an ultrasmall current has to be measured with high resolution (±20 fA). The circuit is fast enough for typical sampling rates required for BEEM or STM image measurements and can also be used for fA measurements related to ground.