Nanosecond laser induced single atom deposition with nanometer spatial resolution using a STM

Abstract

Nanosecond laser pulses, with 2.33 eV photon energy and ∼0.6 MW/cm2 radiation flux, have been used to initiate a transient increase of tunneling current between a W tip and a Si sample surface in an ultrahigh vacuum scanning tunneling microscope (STM) apparatus. As the laser power is increased to ∼2.5 MW/cm2, single atom transfer from the tip to a silicon surface occurs. For both polarities, the laser induced tunneling current is linear with laser pulse energy up to ∼0.6 MW/cm2. A transient tunneling current up to 15 μA has been observed. The similarity of the laser induced transient tunneling for both polarities, and hence its independence on material, suggest that the same mechanism is operative in both directions of tunneling. Both ballistic electron tunneling and band bending effects have been considered in the analysis of the electron transfer. It is proposed, however, that pulse laser heating of the tip causes this transient increase of the tunneling current due to a transient thermal expansion, reducing the tip-sample tunneling distance. The increase in tunneling current may lead to additional Nottingham heating of the tip apex. At a laser flux of 2.5 MW/cm2, single atom transfer between the W tip and the silicon surface occurs. The number of atoms transferred can be controlled by the laser flux, and the transfer process is virtually independent of the tip-sample bias polarity. Since a maximum tip temperature of 650 K is estimated during the pulse, W atom transfer must occur under the influence of strong W-Si chemical interaction. The speed of the pulse laser atom transfer (8 ns) exceeds by orders of magnitude the transfer speed that could be achieved by pulsing the STM piezodrive. © 1996 American Institute of Physics.