State-of-the-art electronic and optoelectronic devices require electronic materials with specialized properties that cannot be characterized with standard methods, or that must be characterized with extra precision. As a result of this research, the following new materials characterization methods have been developed:

  1. Carrier density gradient analysis method: Semiconductor uniformity is essential for all semiconductor applications, including optoelectronic light emitters & sensors, IC's, and logic devices. This method extends the van der Pauw method of electrical characterization so that one can measure small variations in the doping density of semiconductor samples typical to semiconductor devices.
  2. Fourier-domain mobility spectral analysis: This method allows for electrons and holes of differing mobilities to be separated out from magnetotransport data. The experimental system of interest was lightly n-type quantum wells of Hgl-xCdxTe, which revealed ambipolar conduction of both electrons and holes.
  3. Heterodyne 4-point method for electrical characterization of time varying conductivities: A new method was developed to improve the sensitivity of Hall measurements by orders of magnitude. This heterodyne Hall effect technique uses ac signal multiplication to measure a pure Hall resistance Rxy (B) in arbitrarily shaped samples.
  4. Anisotropic conductor characterization: With five contacts, it was demonstrated in a black phosphorus nanolayer that three independent four-point resistance measurements can determine the three independent components of an inplane anisotropic resistivity tensor. Also, a detailed method for measuring in-plane and cross-plane conductivities of a superlattice has been formally described.
  5. Heavy-tail transient analysis: Disordered systems exhibit a range of time-scales and manifest slow switching transients, or “heavy-tail” relaxations, which limit performance. An equation was derived that fits all classes of heavy-tail functions and was experimentally applied in both the low- and high-disorder limits in 2D transistors of black phosphorus.
  6. Disorder scaling in non-ohmic conductivity of 2D materials: In 2D conductors, the gated conductivity is typically non-linear, with changes in the surface adsorbed gases leading to changes in both the doping level and the mobility. We demonstrate that all of the characterization curves at different adsorbate concentrations, dopings, and disorder can all be collapsed onto a single universal curve that is characteristic of that particular material.
  7. Percolation model for electrical and thermal conductivity in disordered media: Disordered porous media are described in a modified percolation model that is adapted for polymer composites under uniaxial pressure. Percolation is also shown to describe thermal conductivity in metal-organic frameworks fabricated by combustion synthesis.

This work was done by Matthew Grayson of Northwestern University for the Air Force Research Laboratory. AFRL-0270

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Novel Characterization Methods for Anisotropic and Mixed Conduction Materials

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This article first appeared in the February, 2019 issue of Aerospace & Defense Technology Magazine.

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