The electrical conductivity of solid-state matter is a fundamental physical property and can be precisely derived from the resistance measured via the four-point probe technique excluding contributions from parasitic contact resistances. Over time, this method has become an interdisciplinary characterization tool in materials science, semiconductor industries, geology, physics, etc, and is employed for both fundamental and application-driven research. However, the correct derivation of the conductivity is a demanding task which faces several difficulties, e.g. the homogeneity of the sample or the isotropy of the phases. In addition, these sample-specific characteristics are intimately related to technical constraints such as the probe geometry and size of the sample. In particular, the latter is of importance for nanostructures which can now be probed technically on very small length scales. On the occasion of the 100th anniversary of the four-point probe technique, introduced by Frank Wenner, in this review we revisit and discuss various correction factors which are mandatory for an accurate derivation of the resistivity from the measured resistance. Among others, sample thickness, dimensionality, anisotropy, and the relative size and geometry of the sample with respect to the contact assembly are considered. We are also able to derive the correction factors for 2D anisotropic systems on circular finite areas with variable probe spacings. All these aspects are illustrated by state-of-the-art experiments carried out using a four-tip STM/SEM system. We are aware that this review article can only cover some of the most important topics. Regarding further aspects, e.g. technical realizations, the influence of inhomogeneities or different transport regimes, etc, we refer to other review articles in this field.

https://iopscience.iop.org/article/10.1088/0953-8984/27/22/223201/pdf

The 100th anniversary of the four-point probe technique: the role of probe geometries in isotropic and anisotropic systems

A tunneling microscope that is a hybrid between IBM Zurich designs and squeezable tunnel junctions has been operated in air, oil, and liquid nitrogen. Key design goals were (1) maximum rigidity and (2) minimum thermal drift. Images of individual atoms in a close packed layer have been obtained under liquid nitrogen.

A tunneling microscope for operation in air or fluids

An STM – SEM setup for characterizing photon and electron induced effects in single photovoltaic nanowires

We observe a composition modulated axial heterostructure in zincblende (ZB) InAs 0.90 Sb 0.10 nanowires initiated by pseudo-periodic twin boundaries using scanning tunneling microscopy. The twin boundaries exhibit four planes with reduced Sb concentration due to a lower Sb incorporation during lateral overgrowth of a 4H wurtzite as compared to a ZB stacking sequence. We anticipate that this leads to compositional band offsets in addition to known structural band offsets present between 4H and ZB polytypes, changing the band alignment from type II to type I.

Composition modulation by twinning in InAsSb nanowires

Laterally grown InxGa1-xAs nanowires (NWs) are promising candidates for radio frequency and quantum computing applications, which, however, can require atomic scale surface and interface control. This is challenging to obtain, not least due to ambient air exposure between fabrication steps, which induces surface oxidation. The geometric and electronic surface structures of InxGa1-xAs NWs and contacts, which were grown directly in a planar configuration, exposed to air, and then subsequently cleaned using atomic hydrogen, are studied using low-temperature scanning tunneling microscopy and spectroscopy (STM/S). Atomically flat facets witha root mean square roughness of 0.12 nm and the InGaAs (001) 4 × 2 surface reconstruction areobserved on the top facet of the NWs and the contacts. STS shows a surface bandgap variation of 30 meV from the middle to the end of the NWs, which is attributed to a compositional variation of the In/Ga element concentration. The well-defined facets and small bandgap variations found after area selective growth and atomic hydrogen cleaning are a good starting point for achieving high-quality interfaces during further processing. (Less)

Low temperature scanning tunneling microscopy and spectroscopy on laterally grown InxGa1-xAs nanowire devices

In order to understand the structural and electronic properties of semiconductor nanowires, scanning tunneling microscopy is an appealing technique that can supplement transmission electron microscopies and conventional electrical characterization techniques. It is able to probe the surface of semiconductor materials at the atomic scale and can be successfully applied to study the nanofaceting morphology, the atomic structure and the surface composition of oxide-free nanowire sidewalls. Based on the advantages provided by the unique geometry of semiconductor nanowires for a low-cost and efficient integration into nanoscale devices, additional characterization schemes performed with multiple probe scanning tunneling microscopy are also presented to get a deeper understanding of their transport properties.

Ph. Caroff

Papers

Electrical characterization of semiconductor nanowires by scanning tunneling microscopy