Integrated Near-field Optoelectronic (INFO) Scanning Probe
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 with the RF Nanoelectronics 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). The microwave reflection coefficient S11 for scans over Au microcapacitors on oxidized Si is shown in Fig. 2. The overall contrast between the Au pads and the SiOx surrounds is 2x higher for the metal-coated nanowire tip than for state-of-the-art Pt tips. The results are described in a recent Applied Physics Letters article [J. C. Weber et al., APL 104, 023113 (2014)].
The next step in this project is to fabricate 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 NSMM function, which is sensitive to change in sample 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.