Electronic signal regulator for constant resolution inelastic electron tunneling spectroscopy

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Abstract
A relatively simple and inexpensive ac signal regulator is described which facilitates constant resolution inelastic electron tunnelingspectroscopy (IETS). Constant resolution is achieved by maintaining an approximately constant ac modulation voltage across IET junctions during spectral scans. The regulator circuit is based upon a field-effect transistor optoisolator with appropriate feedback control acting as a voltage comparator. It is modular in design and can easily be added in the signal path of existing IET spectrometers. A complete schematic diagram of the circuit is provided as well as a discussion on the theory of operation. IET spectra obtained from tunnel junctions with various degrees of nonlinear conductance-voltage behavior are presented with, and without, the circuit. Analysis of these spectra shows that the regulator increases the spectrometer’s signal-to-noise ratio, produces no distortion and, in the case of severely nonlinear junctions, reveals spectral features at mid to high bias, which are otherwise difficult or impossible to detect. Additionally, the regulator offers approximately an order of magnitude increase in data acquisition rate over software algorithms for maintaining constant resolution via IEEE-488 control of spectrometer instrumentation. Our results suggest that such a modular analog regulator would be a valuable addition to IET spectrometers, especially for workers wishing to investigate severely nonlinear IET junctions.