Isomorphic contact resonance force microscopy and piezoresponse force microscopy of an AlN thin film: demonstration of a new contact resonance technique

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 measurements to vary the applied force at each pixel (i.e. force-volume mapping mode). The CR frequency increases with increasing applied force; thus, a carefully selected target frequency will be reached for most pixels at some point in the force versus distance curve. 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 of a piezoelectric AlN thin film sample is demonstrated. Iso-CRFM images were obtained by mechanically driving the base of the cantilever, and iso-CR piezoresponse force microscopy (iso-CR-PFM) images were obtained by electrically biasing the tip. The PFM phase images reveal that the sample contains nanoscale Al-polar (or 'up') and N-polar (or 'down') domains, with ≈180° phase contrast between oppositely polarized domains. The PFM amplitude and Q-factor images also show 'up' vs. 'down' domain contrast, which decreases with increasing CR frequency. The frequency-dependent amplitude and Q contrast is ascribed to a frequency-dependent electrostatic contribution to the signal. Domain contrast is not observed in the CRFM (mechanically driven) images. To summarize, the iso-CR capability to control the resonance frequency across multiple excitation schemes helps elucidate the origin of the electromechanical and nanomechanical image contrast.