  We study the fate of the surface states of Bi$_2$Se$_3$ under disorder with
strength larger than the bulk gap, caused by neon sputtering and nonmagnetic
adsorbates. We find that neon sputtering introduces strong but dilute defects,
which can be modeled by a unitary impurity distribution, whereas adsorbates,
such as water vapor or carbon monoxide, are best described by Gaussian
disorder. Remarkably, these two disorder types have a dramatically different
effect on the surface states. Our soft x-ray ARPES measurements combined with
numerical simulations show that unitary surface disorder pushes the Dirac state
to inward quintuplet layers, burying it below an insulating surface layer. As a
consequence, the surface spectral function becomes weaker, but retains its
quasiparticle peak. This is in contrast to Gaussian disorder, which smears out
the quasiparticle peak completely. At the surface of Bi$_2$Se$_3$, the effects
of Gaussian disorder can be reduced by removing surface adsorbates using neon
sputtering, which, however, introduces unitary scatterers. Since unitary
disorder has a weaker effect than Gaussian disorder, the ARPES signal of the
Dirac surface state becomes sharper upon sputtering.

Mesoscale and Nanoscale Physics;Strongly Correlated Electrons