SEMITIP V6, UniInt2, Example 3: n-type GaAs(110), with extrinsic surface states

Click here for input/output files for Example 3

In many experimental situations a small density of "extrinsic" surface states, arising from defects and/or residual surface contamination, may be present. Such state invariably cause the Fermi-level position to shift towards midgap, thus producing upwards band bending for n-type material or downwards band bending for p-type material. In this example, we consider a surface state density that is distributed uniformly over the band gap, with the states being acceptor-like (negative when occupied by an electron and neutral when empty) for energies above the charge-neutrality level, and donor-like (neutral when occupied by an electron and positive when empty) for energies above the charge-neutrality level. A density of 1013 cm-2eV-1 is assumed, with charge-neutrality level at midgap (0.71 eV). The spectrum shifts to higher voltages (since the Fermi-level moves down in the bandgap) compared to example 1, with the dopant-induced (accumulation) component completely disappearing. Output for the conductance goes to FORT.15, which when plotted appears as:

NOTE: This result has no dependence on the intrisic states of the surface (i.e. resonant with the conduction band), since the Fermi-level is never close at all to those states.