Scanning-tunneling-microscope investigations on Rb exposed TaS2 and light emission from silicon

Schmidt, Patrick

Two different aspects of surface science were analyzed in this thesis. First, the light emission properties of single crystal silicon with different orientations and doping were studied using the scanning tunneling microscope. Tunneling electrons excite several optical transitions in silicon bulk. The direct interband transitions are well known, but this is the first time these transitions were observed with scanning-tunneling-microscope induced luminescence. For the case of electron tunneling from the metallic tip of the scanning tunneling microscope into the silicon sample (V > 0), a presented model, based on Auger mediated electron-hole recombination, explains the observed light emission spectra. For a change of polarity (V < 0), light emission was found only from the reverse driven p-type samples while spectra from the n-type samples appear still dark. A new mechanism of light emission is proposed which explains the experimental observations. The interplay between band bending, Zener tunneling and Auger processes enables light emission in this case. The findings in this study confirm and fulfil the theoretical predictions of direct interband transitions between 3.3 eV and 5.4 eV photon energy under tunneling contacts which have never been observed before experimentally. Further investigation at substantially higher tunneling currents shows that the energies of a series of quantum well states in silicon, which were induced in the potential well of the band bending, can be resolved within the light emission spectra. Second, the influence of in vacuo Rb adsorption on 1T-TaS_2 was explored under different aspects. It is known that the Rb drives a Mott Hubbard transition; and, with photo emission spectroscopy, the energy of the lower Hubbard band with respect to the Fermi energy was measured. Concurrently, the adsorbate induces a transition from a p(13 x 13)R13,9° charge density wave into a c(2 sqrt{3} x 4) one, as observed recently by low energy electron diffraction experiments; and a nanostructured network establishes, as seen by scanning tunneling microscope studies. In this study the c(2 sqrt{3} x 4) charge density wave was mapped in real space, and the complete Mott Hubbard gap was measured using scanning tunneling microscopy and scanning tunneling spectroscopy, respectively. Experimental spectroscopy data atop a Rb driven nanostructures indicate that these are most likely not fabricated from Rb. Further, the experimental evidence for the intercalation of Rubidium by using this in vacuo sample preparation method was given.

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Schmidt, Patrick: Scanning-tunneling-microscope investigations on Rb exposed TaS2 and light emission from silicon.

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