Surface passivation is of great technological importance due to the increasing miniaturisation of
electronic devices. It has been known for many years that under certain conditions surface states
can form; when they do so in a quantum well (QW) the result is an unbound (i.e., evanescent) state
in the QW. Such surface states are generally undesirable, so a good physical understanding of them
is important. A simple single- p -orbital valence band model is used with two types of surface
passivation to examine surface states in a QW: (1) an energy upshift added to the terminal atoms;
and (2) explicit passivation by an s -orbital on each end of the QW. These models show these
unbound/evanescent QW states can occur in both models; that in them the wavefunction is bound to the
terminal atoms; and that the existence of these states is connected to the effective valence-band
offset between the terminal atoms and the bulk QW.