Questions for STM tutorial

Question 1.

At low voltages and temperature the tunneling current is given by

Current Relation

where d is the distance between the tip and sample, Assuming the local barrier height is about 4eV, and if the current is kept constant to within 2%. Show the current sensitivity to distance between the tip and sample.


Question 2

Bias-dependent STM images can probe the occupied and unoccupied states. Here are the STM images of GaAs(110)-2x1surface. Images were obtained by applying (a) +1.9V (b) -1.9V to the sample wtih respect to the tip. The rectangles in the images indicate the corresponding position. And it was suggested that the filled states are localized on the As atoms, while the empty states are localized on the Ga atoms. Draw the GaAs(110)-2x1 surface. and gives a little explanation as well.

GaAs (110)-2x1 surface


Question 3

STM images of Si(111)x777 surface

One of the applications of STM in surface science is to study adsorption. Here are the images of Si(111) surface (a) before and (b) after adsorption of NH3. Both images were obtained with same bias voltage. It appeared that some surface atoms have been removed after adsorption. However, those atoms were observed at a higher bias. Below the two images are the tunneling conductance spectra at different location (shown by the arrows), the tunneling conductance changed quiet obviously. Give explanation for the changes.


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Author: Tit-Wah Hui email: <>
Curator: Dan Thomas email: <>
Last Updated: Mon, Apr 14, 1997 14:24 EST