  For the high luminosity phase of the Large Hadron Collider to start operation
around 2026, a major upgrade of the ATLAS Inner Tracker (ITk) is in
preparation. Thanks to their low power dissipation and high charge-collection
efficiency after irradiation, thin planar pixel modules are the baseline option
to instrument all, except for the innermost layer of the pixel detector. To
optimise the sensor layout for a pixel cell size of $50\times50\,\mu m^2$, TCAD
simulations are being performed. Charge-collection efficiency, electronic noise
and electrical-field properties are investigated. A radiation-damage model is
employed in TCAD simulations to estimate the performance before- and after
irradiation. The impact of storage time at room temperature for the ITk pixel
detector during maintenance periods are estimated using sensors irradiated up
to a fluence of 5$\times10^{15}\,$n$_\text{eq}$/cm$^2$. Pixel sensors of
$100-150\,\mu m$ thickness, interconnected to FE-I4 read-out chips with pixel
dimensions of $50\times250\,\mu m^2$, are characterised using the testbeam
facilities at the CERN-SPS and DESY. The charge-collection and hit efficiencies
are compared before and after annealing at room temperature for up to one year.
