Scanning Tunneling Spectroscopy of Insulating Films

Paper for Advanced Solid State Physics class, Dmitri Y. Petrovykh.

Physics Department, University of Wisconsin, Madison, WI, USA (1999)

1 STM and STS of Bulk Insulators
2 STS of Thin Film Insulators
1 Tunneling Current Theory
2 Tunneling Barrier Parameters
3 The Simplest Model for T(E,V)
4 Thermal Broadening Included
5 More Accurate Tunneling Probabilities
Possible Additional Effects
Acknowledgments & References

I.2 STS of Thin Film Insulators

The normalization problem described in the previous section can be averted if we utilize thin films, where the tunneling current I remains finite inside the band gap. The states of the silicon substrate decay exponentially through the CaF₂ film, leaving a finite density of states at the CaF₂ surface available for tunneling.

Fig. 2 Tunneling spectra of CaF₁/Si(111) and CaF₂/CaF₁/Si(111).
Two sharp onsets in the (dI/dV) spectra characterize the respective conduction band minima and provide chemical selectivity. The normalized (dI/dV)/(I/V) spectra exhibit resonances at the CBM. Different line types represent different tip-sample separations.

The experimental⁵ (dI/dV)/(I/V) tunneling spectra obtained from CaF₂/Si(111) and CaF₁/Si(111) interfaces are presented in Fig. 2. The conduction band edges correspond to onsets in the (dI/dV) curves. In the normalized spectra well-defined peaks are observed at the CBM. The peak is three times as high as the continuum above it for CaF₂ and five times as high for CaF₁. In search of an explanation for this phenomenon several options can be considered⁵. Here a model for tunneling through a thin insulator film is developed, based on established approaches for planar tunneling⁶. The outcome produces the observed resonances and even their absolute height, suggesting that the most important physical effects are indeed captured in the model with a simple barrier structure. Some possible extensions of this approach for more realistic barrier potentials will be discussed as well.

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