SEMITIP V6, Uni2, Example 2: extrinsic surface states

Click here for input/output files for Example 2

We illustrate the effect of extrinsic states on the band bending, for the same parameters as used in example 4 of UniInt2. We consider the case of n-type GaAs(110) with a uniform band of surface states with density 1012 cm-2 (roughly 0.01 monolayer) and charge neutrality level at midgap. The sample bias voltage is -1.3 V and the contact potential is 0.43 eV, so that the electrostatic potential of the tip relative to a point deep inside the semiconductor is -1.3 + 0.43 = -0.87 eV. The plot below shown the potential profile into the semiconductor (left) from FORT.11, and along the surface (right) from FORT.12. Since we have extrinsic surface states, the bands bend upwards at the surface. But near the tip this upwards band bending is reduced, due to the applied bias of the tip.

Contour lines for the potential from FORT.20 - FORT.26 are shown below, plotted over a horizontal distance of 60 nm and a vertical distance of 20 nm.
The spacing of the contour lines is deduced from the last few lines of the FORT.16 file: 
 MIN, MAX POTENTIAL VALUES = -0.86999995      0.49671048    
 CONTOUR SPACING =  0.19524434
The sample bias voltage in this example is -1.3 V and the contact potential is 0.43 eV. Thus, the electrostatic potential of the tip relative to a point deep inside the semiconductor is -1.3 + 0.43 = -0.87 eV. This is the minimum potential in the problem, corresponding to the surface of the tip (shown black in the above plot). Six additional potential contours (as specified in the FORT.9 file) are plotted, the first at -0.87+0.1952 eV and each successive one spaced a further 0.1952 eV from the previous. Wavefunctions for this potential profile are displayed in example 4 of UniInt2 and a complete spectrum is shown in example 3 of UniInt2.