  We present the results of detailed theoretical investigations of changes in
local density of total electronic surface states in 2D anisotropic atomic
semiconductor lattice in vicinity of impurity atom for a wide range of applied
bias voltage. We have found that taking into account changes in density of
continuous spectrum states leads to the formation of a downfall at the
particular value of applied voltage when we are interested in the density of
states above the impurity atom or even to a series of downfalls for the fixed
value of the distance from the impurity. The behaviour of local density of
states with increasing of the distance from impurity along the chain differs
from behaviour in the direction perpendicular to the chain.
