Indentation effects on atommanipulation on Si(111) surfaces investigated by STM

We have used a scanning tunneling microscope (STM) to investigate the processes of Si atom removal during indentation on p- and n-type Si(111)-7×7 surfaces. Si adatoms can be removed for both polarities, and removed preferably at positive sample biases for the p-type substrate, and at negative sample biases for the n-type substrates. There is a greater probability of adatom removal for the n-type substrate than for the p-type, which we suggest may be due to interaction of the Si adatoms with the dopants, or to the electric field generated by contact potential effects. The current variation as a function of the bias voltage shows Schottkytype rectification characteristics. Stepwise current drops are also observed when the tip is retracted from the contact area. These are attributed not to quantized conductance but to an atom-by-atom decrease of the size of the contact. This clearly indicates that we can estimate the contact size by dividing the maximum current at the contact by the height of the staircases.