Polarity-manipulation based on nanoscale structural transformation on strained 2D MgO

Abstract

Strain induced nanoscale structural transformation is demonstrated in this paper to have the ability of triggering polarity flipping in a wide bandgap system of MgxZn1−xO/MgO/Al2O3. Relaxation dynamics of semiconductor components under large compressive pressures up to 13.7 GPa were studied by a combination of theoretical analysis and experimental characterizations including in situ reflection high-energy electron diffraction and high-resolution transmission electron microscopy. The gigantic force between MgZnO and ultrathin-MgO/Al2O3 delayed the structural transformation of MgZnO from six-fold cubic to four-fold wurtzite into the second monolayer, and consequently flipped the polarity of the film deposited on relaxed MgO. Additionally, dislocation-induced strain relaxation was suggested to happen around 1 nm thick cubic MgO grown on Al2O3, instead of the previous well-accepted concept that wurtzite structures can be inherited from the oxygen sub-lattice of sapphire substrates below the critical thickness. Finally, the structural transformation method employing an ultrathin-MgO interfacial layer was demonstrated to be a suitable technique for accommodating the large lattice mismatch comparing with the dislocation-relaxation mechanism achieving a UV photodetector with four orders of rejection ratio of the UV-to-visible photoresponse.