  Recent studies suggests that in disordered ultrathin films superconducting
(SC) state may be intrinsically inhomogeneous. Here we investigate the nature
of SC state in ultrathin Nb films, of thickness $d$ ranging from 1.2 nm to 20
nm, which undergo a transition from amorphous to polycrystalline structure at
the thickness $d \simeq 3.3$ nm. We show that the properties of SC state are
very different in polycrystalline and amorphous films. The upper critical field
($H_{c2}$) is orbitally limited in the first case, and paramagnetically limited
in the latter. The magnetic field induced superconductor-metal transition is
observed, with the critical field approximately constant or decreasing as a
power-law with the film conductance in polycrystalline or amorphous films,
respectively. The scaling analysis indicates distinct scaling exponents in
these two types of films. Negative contribution of the SC fluctuations to
conductivity exists above $H_{c2}$, particularly pronounced in amorphous films,
signaling the presence of fluctuating Cooper pairs. These observations suggest
the development of local inhomogeneities in the amorphous films, in the form of
proximity-coupled SC islands. An usual evolution of SC correlations on cooling
is observed in amorphous films, likely related to the effect of quantum
fluctuations on the proximity-induced phase coherence.
