  Emission of high energy gamma rays via the non-linear inverse Compton
scattering process (ICS) in interactions of ultra-intense laser pulses with
thin solid foils is studied using particle-in-cell simulations. It is shown
that the angular distribution of the ICS photons has a forward-oriented
two-directional structure centred at an angle $\vartheta = \pm 30^\circ$, a
value predicted by a theoretical model based on a standing wave approximation
to the electromagnetic field in front of the target, which only increases at
the highest intensities due to faster hole boring, which renders the
approximation invalid. The conversion efficiency is shown to exhibit a
super-linear increase with the driving pulse intensity. In comparison to
emission via electron-nucleus bremsstrahlung, it is shown that the higher
absorption, further enhanced by faster hole boring, in the targets with lower
atomic number strongly favours the ICS process.
