Electrostatic modification of functional materials by electrolytic gating has
demonstrated a remarkably wide range of density modulation, a condition crucial
for developing novel electronic phases in systems ranging from complex oxides
to layered chalcogenides. Yet little is known microscopically when carriers are
modulated in electrolyte-gated electric double-layer transistors (EDLTs) due to
the technical challenge of imaging the buried electrolyte-semiconductor
interface. Here, we demonstrate the real-space mapping of the channel
conductance in ZnO EDLTs using a cryogenic microwave impedance microscope. A
spin-coated ionic gel layer with typical thicknesses below 50 nm allows us to
perform high resolution (on the order of 100 nm) sub-surface imaging, while
maintaining the capability of inducing the metal-insulator transition under a
gate bias. The microwave images vividly show the spatial evolution of channel
conductance and its local fluctuations through the transition, as well as the
uneven conductance distribution established by a large source-drain bias. The
unique combination of ultra-thin ion-gel gating and microwave imaging offers a
new opportunity to study the local transport and mesoscopic electronic
properties in EDLTs.Comment: to be published on Nano Lette
