  Recent experimental evidences point to rotation symmetry breaking
superconductivity in doped Bi$_2$Se$_3$, where the relevant order parameter
belongs to a two-component odd-parity representation $E_u$ of the crystal point
group. The $E_u$ channel admits two possible phases, the chiral phase, that
breaks time-reversal symmetry, and nematic phase, that well explains the
reported rotation symmetry breaking. In weakly anisotropic three-dimensional
systems the nematic phase is the stable one. We study the stability of the
nematic phase versus the chiral phase as a function of the anisotropy of the
system and the thickness of the sample and show that in the strongly
anisotropic case or in thin slabs the chiral phases is favoured. For the
extreme 2D limit composed by a single layer of Bi$_2$Se$_3$ the system hosts
two chiral Majorana modes flowing at the boundary of the system.
