Scanning probe technology is an essential tool for measuring nanostructure properties. A major goal in our project is to develop unique probes that combine several functions simultaneously-optical excitation, electrical response from DC to GHz, and morphology.
In a collaboration between the Nanoelectromagnetics Project and the Quantitative Imaging Project, we have fabricated and tested a GaN nanowire mounted on an AFM tip as a near-field scanning microwave microscopy (NSMM) tip (see Fig. 1). A W atomic layer deposition (ALD) coating provides a microwave pathway to the tip/sample interface. The microwave reflection coefficient S11 for scans over Au microcapacitors on SiO2 steps is shown in Fig. 2. The overall S11 contrast on the calibration sample is an order of magnitude higher for the ALD-coated nanowire tip than for state-of-the-art commercial Pt tips. Owing to its flexible structure, NW probes have also proven to be insensitive to surface contamination during contact-mode scanning across 2D films. The results are described in two recent publications: Applied Physics Letters article [J. C. Weber et al., APL 104, 023113 (2014)] and Nanotechnology article [J. C. Weber et al., Nanotechnology 25, 415502 (2014)].
The next step in this project is to develop waferscale fabrication of nanowire tips that are also single-nanowire LEDs, enabling near-field optical excitation at wavelengths determined by InGaN quantum disks grown into the nanowire structure. The high index of refraction for the nanowire leads to a large divergence once light exits the crystal, generating a small volume about 40 nm in diameter that experiences intense optical excitation, with fields falling off rapidly at larger distances. Unlike flood illumination from remote lasers or optically pumped antennae, the sample under test is stimulated only directly under the tip. The electrical response is then measured by the microwave circuit integrated with the NSMM, which is sensitive to changes in the local electromagnetic properties of the sample such as sheet resistance and capacitance.
Some examples of applications for the simultaneous operation of the multiple functions include measurement of response of individual grains and grain boundaries in photovoltaic materials, identification and mapping of defects and charge states in nanoscale Si circuits, and measuring electrical and mechanical response of neurons and other cellular structures to light. The multiprobe concept is covered by U S Patent 8,484,756.
(Upper) Electron microscope image of GaN nanowire scanning probe tip, and (lower) microwave reflection coefficient scans of gold microcapacitors.
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