monolayer (1 B atom is replaced by 1 Al/Ga in a 2x2x1 monolayer supercell) is one atomic flat like pristine h-BN monolayer and therefore will not display out-of-plane polarization. Note that there 2D materials where doped atoms are protruded from the plane such as Mn doped graphene . However, we find that in a h-BN trilayer Al/Ga doping prefers the protruded phase to the flat structure, and hence these dopants induce a spontaneous polarization along c-direction. Here, we consider a 2x2x1 trilayer supercell and 1 B atom is replaced by 1 Al/Ga at the center layer (see Figure ) of the trilayer to keep the up and down polarization states degenerate in terms of energy. The doping concentration is 4.167%. Based on modern theory of polarization, our calculated P sp 3 for Al (Ga) doped trilayer is 5.186 µC/m 2 (4.216 µC/m 2 ); here thickness of the trilayer is used to obtain the 3D unit of C/m 2 . Interestingly, P sp 3 of the trilayer is slightly lower than that of our doped bulk h-BN, Table , because the protrusion height h of the trilayers (0.609 Å for Al and 0.757 Å for Ga) is smaller compared to their bulk counterparts. This is also evident in the larger interlayer Al/Ga-N bond lengths (2.315 Å for Al-N and 2.530 Å for Ga-N), resulting in lower ICOHP oop Al/Ga-N values (-1.535 eV for Al-N and -0.829 eV for Ga-N), compared to their bulk counterparts as shown in Table . It is worth noting that ICOHP oop Al/Ga-N values ( -0.131 eV for Al-N and -0.110 eV for Ga-N) of the trilayer flat structure are considerably smaller than those of the protruded structure, indicating that the protrusion facilitates the formation of strong interlayer Al/Ga-N bonds. Moreover, the computed FE switching energy barrier ( ∆E ) for Al (Ga) doped tri- ) and electronic (e elc 31 and e elc 32 ) parts of transverse piezoelectric stress coefficients e31 and e32 in C/m 2 unit are calculated by the self-consistent response to a finite electric field ε (LCALCEPS-tag in VASP). The piezoelectric coefficients obtained from DFPT calculations are shown in the parenthesis. Transverse piezoelectric strain coefficients d31 and d31 are in pC/N unit. layer is 12.629 meV/atom (23.200 meV/atom), which is also essentially the same as that of the bulk counterpart (see ∆E in Table ). The ∆E for the doped trilayers are also quite comparable with that of other 2D FE materials, e.g. 31.202 meV/atom for VOCl 2 monolayer and 60 meV per unit cell for In 2 Se 3 monolayer . Therefore, we propose that low concentration doping of Al/Ga in h-BN can realize ferroelectricity down to nanoscale, which would offer significant benefits to FE memory technology. Of course, the interaction between such ultra-thin FE films and substrate or electrodes, effect of epitaxial strain, and FE domain dynamics etc need to be understood. Note that epitaxial tensile (compression) strain can reduce (increase) the FE switching barrier with enhanced (reduced) piezo-response . However, these are beyond the scope of this article.