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WHITE PA P E R An Improved Method for Differential Conductance Measurements by Adam Daire Introduction As modern electronics continue to shrink, researchers are increasingly looking to nanotechnology as the basis for the next breakthrough in device size and power consumption. Indeed, as semiconductor structures are made smaller and smaller, the distinction between small silicon geometries and large molecules becomes blurred. Approached from either direction, the consequences are the same. Quantum behavior such as tunneling begins to play an important role in the electrical characteristics. In the macroscopic world, conductors may have obeyed Ohm's Law (Figure 1a), but in the nanoscale, Ohm's definition of resistance is no longer relevant (Figure 1b). Because the slope of the I-V curve is no longer a fundamental constant of the material, a detailed measurement of the slope of that I-V curve at every point is needed to study nanodevices. This plot of differential conductance (dG = dI/dV) is the most important measurement made on small scale devices, but presents a unique set of challenges. Figure 1a. Macroscopic scale Figure 1b. Nanoscale (Classical) (Quantum) Keithley Instruments, Inc. 28775 Aurora Road Cleveland, Ohio 44139 (440) 248-0400 Fax: (440) 248-6168 www.keithley.com A G r e a t e r M e a s u r e o f C o n f i d e n c e Who Uses Differential Conductance? Differential conductance measurements are performed in many areas of research, though sometimes under different names. When used to measure the electron energy structure of small devices such as quantum dots, nanoparticles, or artificial atoms, it is sometimes referred to as electron energy spectroscopy. When used to perform non-contact measurements on surfaces within a scanning tunneling microscope, it may be called tunneling spectroscopy. When studying ultra-small semiconductor structures or nanotubes with semiconducting properties, it might be called a density of states measurement. Still other researchers refer to it more mathematically, as the derivative of the I-V curve, or simply dI/dV or G. It can be used to understand conduction phenomenon in cryogenic environments or to observe and predict the conditions when tunneling is