Nov 2, 2012
Measuring the Hall effect in nanowires
Researchers at Lund University in Sweden have succeeded in measuring the Hall effect in nanowires for the first time – a feat that was thought impossible until now. The new measurements will help fine tune the electronic properties of these technologically important materials and might perhaps even help in the development of a new generation of nanowire-based devices.
The classic Hall effect occurs when an electric current flows through a conductor in a magnetic field. If the current and magnetic field are at right angles to each other, the Lorentz force deflects the electrons to one side and a Hall voltage builds up in the direction that is at right angles to both the current and the magnetic field. This Hall voltage can provide invaluable information about a material’s fundamental electronic properties, such as doping and carrier mobility.
Researchers have been studying semiconducting nanowires for the last decade or so because these materials could be ideal for making efficient light-emitting diodes and photovoltaic energy-harvesting devices. However, before such applications become possible, the electronic properties of these materials needs to be analysed in detail – something that has not proved easy because of the shape and the 2D dimensional nature of nanowires.
A team led by Lars Samuelson has now developed a new way to embed a nanowire in a thin polymer film. The simple set-up allows the researchers to deposit multiple electrical contacts along the nanowire for subsequent Hall effect measurements. “Nanowires are difficult to characterize electrically, but our new technique enables us to determine both the electron density and electron mobility in a single measurement and also with high spatial resolution (on the 100 nm scale),” explained team member Kristian Storm. “The experiments will hopefully allow us to fine tune the electronic properties of nanowires and so take research on these materials into its next stage.”
A total of eight metal electrodes for sourcing currents and reading voltages were deposited onto the nanowire shell by a technique called metal evaporation. By sourcing a current through terminals AH (see figure) while simultaneously applying a magnetic field perpendicular to the sample surface, the Lund team was able to measure the Hall voltage across the nanowire shell between terminals BC, DE and FG.
The researchers made their measurements on hexagonal indium phosphide p-type core, n-type shell zinc-blende nanowires that had been grown by metal–organic vapour phase epitaxy on InP substrates. These materials are thought to be ideal for photovoltaic and infrared LED applications. But the good news is that, because the measurement technique is a general one, it is not limited to a specific material or nanowire system and might be used to study a wide range of semiconducting wires, says Samuelson.
“Such measurements – combined with nano-optical characterization techniques, like micro-photoluminescence and cathodoluminescence, as well as structural studies using scanning probe microscopy – mean that researchers now have a complete toolbox (or nanowire-facet characterization platform, as we like to call it) at their disposal for analysing nanowires,” he told nanotechweb.org.
The research is described in Nature Nanotechnology.
About the author
Belle Dumé is contributing editor at nanotechweb.org.