Phonon-Mediated Electron Transport through CaO Thin Films

Scanning tunneling microscopy has developed into a powerful tool for the characterization of conductive surfaces, for which the overlap of tip and sample wave functions determines the image contrast. On insulating layers, as the CaO thin film grown on Mo(001) investigated here, direct overlap between initial and final states is not enabled anymore and electrons are transported via hopping through the conduction-band states of the oxide. Carrier transport is accompanied by strong phonon excitations in this case, imprinting an oscillatory signature on the differential conductance spectra of the system. The phonons show a characteristic spatial dependence and become softer around lattice irregularities in the oxide film, such as dislocation lines.

Figure 1

(a) STM topographic image of a CaO(001) film of 25 ML thickness ($V_S=4.5\mathrm{V}$, $I=25\mathrm{pA}$, $100\times 100{\mathrm{nm}}^2$). (b) Current, first and second bias derivative of the current, taken across the CB onset on a wide oxide terrace (bias setpoint: 3.5 V).

Figure 2

(a) Differential conductance spectra and (b) their numerical differentiation taken on 5 and 25 ML thick CaO films. The $d^{2}I/d{V}^2$ traces have been fitted with a sine to determine the bias intervals of the oscillations. (c) Energy associated to the oscillatory behavior of the $dI/dV$ signal, measured for differently thick oxide films and different setpoint conditions. After correction for tip-induced band-bending effects, the energies fall into a narrow range of $\mathrm{\Delta }E=(82\pm 10\mathrm{meV})$. The inset in the bottom visualizes the gradual increase of the CB onset with film thickness. (d) Scheme of ballistic versus hopping transport of electrons through a thick CaO film, including the possible role of phonons.

Figure 3

(a) Conductance curves measured on top of a subsurface Mo impurity and a defect-free region of a 25 ML thick CaO film. The Mo donor in the inset shows up with a characteristic ring structure, as it temporarily changes the charge state in the tip electric field ($5\times 5{\mathrm{nm}}^2$) [25]. (b) Corresponding second-derivative traces, as used for sine fitting. The derived peak intervals are given in the panel.

Figure 4

(a) Spacing of $dI/dV$ oscillations as determined from a spectral series taken across an oxide dislocation line. The height profile of the defect is depicted as a red line, the corresponding STM topography is shown on the right ($V_S=4.0\mathrm{V}$, $16\times 16{\mathrm{nm}}^2$). (b) Characteristic spacing of $dI/dV$ maxima measured either on differently sized ad islands (No. 0–4) or the flat surface (No. 5–7). The right panel shows the associated STM image taken on a 25 ML thick CaO film. ($V_S=4.0\mathrm{V}$, $40\times 40{\mathrm{nm}}^2$). All islands are grown around screw dislocations in their center.