  Impurity-induced quasiparticle bound states on a pair-breaking surface of a
d-wave superconductor are theoretically described, taking into account
hybridization of impurity- and surface-induced Andreev states. Further a theory
for effects of surface disorder (of thin impurity surface layer) on the
low-bias conductance of tunnel junctions is developed. We find a threshold
$n_c$ for surface impurity concentration $n_S$, which separates the two regimes
for surface impurity effects on the zero-bias conductance peak (ZBCP). Below
the threshold, surface impurities do not broaden the ZBCP, but effectively
reduce its weight and generate impurity bands. For low $n_S$ impurity bands can
be, in principle, resolved experimentally, being centered at energies of bound
states induced by an isolated impurity on the surface. For larger $n_S$
impurity bands are distorted, move to lower energies and, beginning with the
threshold concentration $n_S=n_c$, become centered at zero energy. With
increasing $n_S$ above the threshold, the ZBCP is quickly destroyed in the case
of strong scatterers, while it is gradually suppressed and broaden in the
presence of weak impurity potentials. More realistic cases, taking into account
additional broadening, not related to the surface disorder, are also
considered.
