STM Fabrication and Characterization of Nanodots on Silicon Surfaces


Using a scanning tunneling microscope we have fabricated, characterized and modified nanometer-sized structures on clean and oxidized silicon surfaces.  Our approach involves field evaporation from either an Al- or Au-coated tungsten STM tip.  This has the advantage of allowing imaging of the structures subsequent to fabrication, with the same tip.  Application of a short voltage pulse to a tip held in close proximity to the surface produces nanodots with a probability and dot size which depend on the size and polarity of the pulse. 





Fig. 1.  Schematic of nanometer-size dot formation by field evaporation from Au coated STM tip


For Au-coated tips, smaller dots are produced with negative pulses.  The probability of dot formation rises sharply above a threshold voltage, which in turn depends nearly linearly on the oxide thickness for a constant STM set point current and voltage [1].  The associated threshold field strength also increases with oxide thickness, but shows only rough agreement with the standard model for field evaporation.  Tunneling spectra from Au nanodots show an increased probability of interface state-associated negative differential resistivity (NDR) from those measured above the neighboring oxidized surface, indicating that the dots act to increase the area probed by the STM tip [2].  We have also demonstrated the modification of existing nanodots, via the application of additional, larger voltage pulses of both polarities.  Negative pulses allow us to increase the size of a dot, while positive pulses allow “erasure” of dots.



Fig. 2.  Left:  STM image of Au dots (approx. 10 nm dia. x 1.2 nm ht.) deposited on oxidized Si(100) by application of -8V, 10 msec

 pulses to the tip.  Right:  Same Au dots after modification by application of +10 v pulse (left, dot erased) and  –10 v pulse (right,  dot enlarged).




 [1] J. Y. Park, R. J. Phaneuf, and E. D. Williams, Surf. Sci. 470, L69 (2000).


[2] J. Y. Park, R. J. Phaneuf, and E. D. Williams, J. Vac. Sci. Technol. B 19, 1158 (2001).



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