A versatile system for inelastic electron tunnelling spectroscopy

A spectrometer system for sensitive measurements of inelastic tunnelling spectra is described. Modulation and bridge techniques are used to determine the current I, and the derivatives dI/dV and d 2 I/dV 2 for a four-terminal tunnelling junction subject to an applied bias potential V. An operational amplifier circuit sweeps the DC bias voltage across the sample whilst holding the superimposed AC modulation voltage constant, independent of conductance changes. The current I and dI/dV are determined using DC and lock-in voltmeters connected across a current-sensing resistor, whilst a bridge configuration permits low-noise measurements of the second derivative. The bridge may be automatically balanced over the entire range of bias voltage using a voltage-controlled resistor and feedback, thus minimising source noise. The circuitry is designed to operate in association with a microcomputer which permits signal-to-noise enhancement by signal averaging. A dummy sample based on p n junction is used for test and calibration purposes.