Principle of the experiment:
Ballistic Electron Emission Microscopy (BEEM) [1] not only represents an ideal technique to study the electronic transmission at the Schottky-interface between a metal and a semiconductor, but moreover it offers capabilities to analyze the ballistic transport through adsorbates on top of the metal [2]. By using a metal layer either on a n-doped or on a p-doped semiconducting substrate it is possible to analyse the ballistic transport of electrons or holes through the Schottky diode [3].
Experimental:
Bismuth (Bi) films with a thickness of 3-4nm were grown on n-doped and p-doped Si(100) and Si(111). Recently we could analyze the ballistic transport of electrons through two different molecular adlayers, PTCDA and C60, deposited on top of the Bi film. In addition the ballistic transport of holes through an adlayer of C60 was studied. All experiments were performed by a modified "Nanoprobe" system (Omicron) providing three STMs which may be operated independently on the same sample. One STM unit was only used to form an electric contact to the metal layer, the second unit is operated as a conventional STM at negative or positive tip bias, hence injecting electrons or holes into the sample surface.
Results:
For electron transport, the STM- as well as the simultaneously recorded BEEM-images exhibit intramolecular resolution for a monolayer of PTCDA and C60. The comparison to the bare Bi surface reveals that the total BEEM current is reduced by the molecular layer. Since the observation is almost identical for Bi on Si(111) and on Si(100) it may be excluded that this is due to an alteration of the k-vectors associated to the tunneling electrons. These results will be compared to very recent studies of ballistic hole transport through C60 molecules which also reveal molecular resolution within the BEEM images.
References:
[1] Kaiser, W. J.,Bell, L. D., Phys. Rev. Lett. 60, 1406 (1988)
[2] Bannani, A., Bobisch, C. and Moeller, R., Ballistic electron transport through individual molecules, accepted for publication in Science
[3] Bell, L.D., Hecht, M.H., Kaiser, W. J., Phys. Rev. Lett. 64, 2679 (1990)
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