  The evolution of the vortex glass (VG) phase transition and vortex creep with
decreasing film thickness is studied in ultrathin, polycrystalline niobium
films, with thickness in the range 7.4 nm to 44 nm, using current-voltage
characteristics measurements in perpendicular magnetic field. Standard methods,
including scaling laws, allow to identify VG transition in the thickest film,
while in thinner films creep produces large uncertainty in the putative VG
transition temperature and scaling exponents. Using strong pinning theory we
perform analysis of the creep, and extract the dependence of the activation
energy for vortex pinning on temperature, magnetic field, and film thickness.
This analysis provides more information on vortex dynamics than the standard
evaluation of critical current density. The results reveal two distinct regimes
of pinning, which we propose to identify with $\delta l$ or $\delta T_c$-types
of pinning (due to spacial fluctuation of mean free path $l$ or spacial
fluctuation of superconducting transition temperature $T_c$, respectively). In
the thickest film $\delta l$ pinning is observed, but with the decrease of film
thickness the second pinning regime appears, and becomes dominant in the
thinnest film. We link these pinning regimes with the structural disorder due
to grain boundaries, which produce charge carrier scattering in the thickest
film, but with decreasing film thickness gradually evolve into amorphous
inclusions, producing fluctuations in the $T_c$.
