Enhanced donor binding energy close to a semiconductor surface

Phys Rev Lett. 2009 Apr 24;102(16):166101. doi: 10.1103/PhysRevLett.102.166101. Epub 2009 Apr 21.


We measured the ionization threshold voltage of individual impurities close to a semiconductor-vacuum interface, where we use the STM tip to ionize individual donors. We observe a reversed order of ionization with depth below the surface, which proves that the binding energy is enhanced towards the surface. This is in contrast to the predicted reduction for a Coulombic impurity in the effective mass approach. We can estimate the binding energy from the ionization threshold and show experimentally that in the case of silicon doped gallium arsenide the binding energy gradually increases over the last 1.2 nm below the (110) surface.