(page under construction)
A series of research papers [1-9] published by PI and other groups demonstrated that for atomically flat MBE grown films of certain high electron density metals (see Figs. 1ab) STM can be used for high resolution imaging of buried metal-substrate interfaces. In these films, electrons move in quasi-1D channels  in a perpendicular to film direction, typically  direction. As illustrated schematically in Fig. 1a, multiple electron reflections inside of such channels open the experimental possibility of deep subsurface imaging using STM with few-angstrom lateral resolution and sub-angstrom in depth resolution. The typical imaging results for differently prepared Pb/Si interfaces are shown in Figs. 2 and 3. Most of the published research was done using Pb(111) films on Si(111) substrates. The origin of quasi-1D channels, shown in Fig. 1a, is not fully established; these channels are possibly caused by charge fractionalization and confinement [1, 10].
Figure 1. (a) STM subsurface probe uses quantum electron echolocation. (b) A portion of periodic table where this imaging method has been demonstrated. All these elements have electronic configuration s2px characterized by partly filled p-orbitals. (c) -directed anisotropic p-orbitals are most likely responsible for transverse quasi-1D transport. The ABC-type layer stacking is taken into account. (d) Charges injected into surface p-orbitals (top of the figure) from the STM tip, at the next layer can separate on three fractional charges. Eventually these fractional charges can recombine (bottom of the figure). The mechanism resembles the confinement of quarks.
Figure 2. STM image of Pb(111) film on 7×7 reconstructed Si(111) substrate [3, 7]. The image consists of two components: the atomic lattice of metal and the additional electronic echo signal from the metal-substrate interface visible as bright triangular domains corresponding to Si(111) 7×7 structure. The interface echo part of the signal is best observed at certain resonant energies and sometimes reverses its contrast depending on tunneling bias. The maximum so far probed metal thickness was 30 monolayers .
Figure 3. STM images of interfacial strain patterns for three different Pb(111) islands on unreconstructed Si(111) . The upper and the lower sets of STM images show bias dependent contrast reversals and changes of pattern details.
Current plans involve using this established imaging technique for characterization of other interfaces including:
- Interfaces of different symmetry
- Interfaces of metals and wide band-gap semiconductors
- Interfaces of metals and van der Waals materials
- I. B. Altfeder, X. Liang, T. Yamada, D. M. Chen, V. Narayanamurti, “Anisotropic Metal-Insulator Transition in Epitaxial Thin Films”, Physical Review Letters 92, 226404 (2004)
- I. Altfeder, K. A. Matveev, A. A. Voevodin, “Imaging the Electron-Phonon Interaction at the Atomic Scale”, Physical Review Letters 109, 166402 (2012)
- I. B. Altfeder, V. Narayanamurti, D. M. Chen, “Imaging Subsurface Reflection Phase with Quantized Electrons”, Physical Review Letters 88, 206801 (2002)
- M. Hupalo and M. C. Tringides, “Correlation between height selection and electronic structure of the uniform height Pb/Si(111) islands”, Physical Review B 65, 115406 (2002)
- I. B. Altfeder, J. A. Golovchenko, and V. Narayanamurti, “Confinement-Enhanced Electron Transport across a Metal-Semiconductor Interface”, Physical Review Letters 87, 056801 (2001)
- L. Vitali, F. P. Leisenberger, M. G. Ramsey, and F. P. Netzer, “Thallium overlayers on Si(111): Structures of a “new” group III element”, Journal of Vacuum Science and Technology A 17, 1676 (1999)
- I. B. Altfeder, D. M. Chen, K. A. Matveev, “Imaging Buried Interfacial Lattices with Quantized Electrons”, Physical Review Letters 80, 4895 (1998)
- I. B. Altfeder, K. A. Matveev, D. M. Chen, “Electron Fringes on a Quantum Wedge”, Physical Review Letters 78, 2815 (1997)
- M. Schmid, W. Hebenstreit, P. Varga, and S. Crampin, “Quantum Wells and Electron Interference Phenomena in Al due to Subsurface Noble Gas Bubbles”, Physical Review Letters, 76, 2298 (1996)
- H. Steinberg et al., Charge fractionalization in quantum wires.