Principle of ferroelectric domain imaging using atomic force microscope

The contrast mechanisms of domain imaging experiments assisted by atomic fo rce microscope (AFM) have been investigated by model experiments on nonpiez oelectric (silicon oxide) and piezoelectric [Pb(Zr,Ti)O-3] thin films. The first step was to identify the electrostatic charge effects between the tip , the cantilever, and the sample surface. The second step was to explore th e tip-sample piezoelectric force interaction. The static deflection of the cantilever was measured as a function of dc bias voltage (V-dc) applied to the bottom electrode (n-type Si wafers) for noncontact and contact modes. I n addition, a small ac voltage (V-ac sin omegat) was applied to the tip to measure the amplitude (A(omega)) and phase (Phi (omega)) of the first harmo nic (omega) signal as a function of V-dc. By changing from the noncontact t o the contact mode, a repulsive contribution to the static deflection was f ound in addition to the attractive one and a 180 degrees phase shift in Phi (omega) was observed. These results imply that in the contact mode the can tilever buckling is induced by the capacitive force between the cantilever and the sample surface. This interaction adds to the tip-sample piezoelectr ic interaction thereby overlapping the obtained tip vibration signal. There fore, the antiparallel ferroelectric domain images obtained at zero dc bias voltage will show a variation in A(omega) but a negligible one in Phi (ome ga). The capacitive force contribution to the tip vibration signal was furt her verified in piezoelectric hysteresis loop measurement assisted by the A FM. The observed vertical offset of the loops was explained by the contact potential difference between the cantilever and the bottom electrode. The s hape of the curve could be explained by the capacitive force interaction co mbined with the tip-sample piezoelectric interaction. The experimental resu lts obtained in this study support the interpretation of the cantilever-sam ple capacitive force contribution to the tip vibration signal in ferroelect ric domain imaging experiments using AFM as a probing tool. The use of a la rge area top electrode between the tip and the sample resulted in the elimi nation of the electrostatic cantilever-sample interaction with negligible d egradation of the domain contrast. This method proved to be successful beca use the cantilever-sample interaction was hardly detected and only the tip- sample interaction was observed. (C) 2001 American Institute of Physics.