A team of researchers has demonstrated that atomic force microscope (AFM) probe tips made from its near-perfect gallium nitride nanowires are superior in many respects to standard silicon or platinum tips in measurements of critical importance to microchip fabrication, nanobiotechnology, and other endeavors. In addition, the scientists have invented a means of simultaneously using the nanowire tips as LEDs to illuminate a tiny sample region with optical radiation while it is scanning, adding an entirely new dimension to the characterization of nanoelectronics materials and devices.

A single GaN nanowire is removed from a forest of wires grown by molecular beam epitaxy. Image is false-colored for clarity. (Image: NIST)

By itself, an AFM provides topographical information at nanometer resolution as its probe tip — in the range of 100 nm wide and suspended from a cantilever arm — scans across a sample surface. When the tip is used at the same time to continuously transmit and receive a microwave signal, the system becomes capable of revealing charge-carrier concentrations or defect locations in specific regions of nanoscale materials and devices. That technique, called near-field scanning microwave microscopy (NSMM), had never before been attempted using a nanowire probe. But as the team showed recently, nanowire probe tips substantially outperformed commercial Pt tips in both resolution and durability.

According to the researchers, the problem with platinum probes is that, because they’re capacitively coupled, any deformation whatsoever will cause their shape to change, adversely affecting their calibration. That is not the case with the tips they made from near-perfect gallium nitride. In a series of 12 scans, the Pt tip radius changed from ~ 50 nm to ~150 nm. The nanowire, however, retained its original dimensions. Moreover, the GaN tips exhibited improved sensitivity and reduced uncertainty compared to a commercial Pt tip.

Illuminating an NSMM sample with a conventional laser brings light in at an angle and greatly increases the space occupied by apparatus. (Photo: NIST)

NSMM can produce very detailed imaging of the local density of positive and negative charge carriers inside a nanostructure — information of great practical significance to microde-vice fabricators — and scientists from PML’s Electromagnetics Division have made notable progress in the technique. They believe that the use of nanowire probes, in conjunction with the recent arrival of a brand-new, custom-built, four-probe NSMM instrument, will reveal new aspects of nanostructure composition and performance. In biological materials, it could locate the attachment of chemical agents or particles that are bound to a cell, and aid in the study of protein dynamics.

Deploying a nanowire as a probe tip sounds deceptively simple. The researchers obtain a conventional AFM cantilever and probe, remove the existing tip, and use a device called a focused ion beam to drill a hole about 5 micrometers deep in the tip mount. Then, using a minuscule manipulator, they break off a single nanowire from a “forest” of them grown by molecular beam epitaxy, insert the wire into the hole, and weld it in place. Finally, the wire is coated with thin layers of titanium (20 nm) and aluminum (200 nm) in order to conduct the microwave signal all the way to the end of the tip and back.

The new, four-probe NSMM instrument has four tips, permitting simultaneous comparisons of materials. The probes are enclosed in an ultra-high vacuum chamber to minimize interference and contamination. (Photo: NIST)

The researchers tested their tip against a silicon tip, a platinum tip, and an uncoated GaN nanowire, each of which was scanned across an array of microcapacitors of different sizes. The coated nanowire proved about twice as sensitive as the Pt probe, and four times as sensitive as the others, with superior mechanical performance. At present only a few GaN probes can be made at once, but the team is at work on developing ideas for producing them in wafer-scale quantities.

The scientists are also preparing to test a new technology for which they were awarded a patent in July, in 2013 — using the nanowire tip as a light source by doping it so that it functions as an LED. Optical radiation can serve to excite the sample in a different way from the microwave signal, and scientists are already using lasers to illuminate nanoscale samples during AFM scans. The problem with that approach is that the laser has to shine in from the side, which produces shadows and causes some degree of uncertainty regarding what area is being illuminated. Using the nanowire tip as a light source means the illumination will be applied directly over the probe tip at the same place on the sample that is being exposed to the microwave signal. That could be particularly beneficial in characterizing photovoltaic materials where you could apply a light and get the carrier concentration at the same time.

Reaching that goal, however, will require more research into how to dope the GaN nanowires so as to increase efficiency of light output, and how to coordinate and integrate measurements from topographic, microwave, and optical modalities.

For more information, visit http://www.nist.gov/pml/div686/manufacturing/gan-nanowires-as-afm-tips.cfm