Theoretical analysis of effect of tip atomic species on apparent barrier height on Al(1 0 0) surface

We have analyzed the effect of the tip atomic species on the tip-sample separation and the bias-voltage dependence of apparent barrier height (ABH) on an Al(1 0 0) surface using the boundary-matching scattering-state density functional method, which can be used to calculate electron states under applied bias voltages self-consistently within the density functional theory. We found that, from the dependence of the tip-sample separation, the difference between measurements with the two tip atomic species is larger in the ABH than in the maximum barrier height evaluated from the calculated potential profile. Furthermore, we found that the bias-polarity dependence of the ABH measured with the Na tip shows behavior opposite to that shown by the ABH measured with the Al tip. These results can be understood from the difference in the degree of lateral confinement of tunneling electrons in the tunneling barrier region between the two atomic species.