, , , Ryan Tung
We present a new contact resonance force microscopy (CRFM) imaging technique, isomorphic contact resonance (iso-CR), that acquires data at a constant contact resonance (CR) frequency, and hence constant tip-sample contact stiffness across the scan area. Constant CR frequency is obtained by performing force versus distance curve measurements to vary the applied force at each pixel (i.e. force-volume mapping mode). In the ISO-CR mode, the cantilever maintains an invariant vibrational shape and a constant environmental damping, thus simplifying interpretation of amplitude and quality factor contrast compared to conventional CRFM. Iso-CR imaging is demonstrated for a piezoelectric AlN thin film grown epitaxially on Si, that contains nanoscale Al-face and N-face domains. Iso-CR piezoresponse force microscopy (iso-CR-PFM) images, obtained with electrical excitation of the tip-sample contact, and iso-CRFM images, obtained with mechanical (photothermal) excitation of the cantilever, are compared for the same sample area. The CR fit parameters, including drive amplitude, drive phase, and Q factor, are quantified by data analysis based on the damped harmonic oscillator model. The fit parameters show contrast between Al-face and N-face domains for iso-CR-PFM but not for iso-CRFM. In addition, the domain contrast of the PFM amplitude and Q factor decreases with increasing CR frequency, and the domain contrast of the PFM phase approaches 180 deg at high CR frequency. These frequency dependent domain contrast effects are ascribed to electrostatic contributions to the cantilever deflection, such that the ratio of the electrostatic to true piezoelectric (sample surface displacement) signal decrease strongly with increasing CR frequency. The ability to compare multiple excitation schemes at specific iso-CR frequencies helps elucidate the origin of the electromechanical and nanomechanical contrast in the images.
aluminum nitride, atomic force microscopy, contact resonance, electromechanical properties, nanomechanical properties, piezoelectric thin films, piezoresponse force microscopy