  Phase engineering by strains in 2D semiconductors is of great importance for
a variety of applications. Here, we present a study of strain induced
ferroelectric (FE) transition on bismuth oxyselenide (Bi$_2$O$_2$Se) films, a
high-performance (HP) semiconductor for next-generation electronics.
Bi$_2$O$_2$Se is non-FE at ambient. Upon a loading force $\gtrsim 400$ nN,
piezoelectric force responses exhibit butterfly loops on magnitude and
180$^\textrm{o}$ phase switching. By carefully ruling out extrinsic factors,
these features are attributed to a transition to FE phase. The transition is
further proved by the appearance of a sharp peak on optical second harmonic
generation under an uniaxial strain. Fundamentally, solids with paraelectric at
ambient and FE under strains are scarce. FE transition is discussed with the
help of first-principle calculations and theoretical simulations. The switching
of FE polarization acts as a knob for Schottky barrier engineering at contacts
and serves as basis for a memristor with a huge switching ratio of 10$^6$. Our
work endows a new degree of freedom to a HP electronic/optoelectronic
semiconductor and the integration of FE and HP semiconductivity paving the way
for multiple exciting functionalities, including HP neuromorphic computation
and bulk piezophotovoltaic.

Materials Science