Semiconductor-Metal Transition Caused by Enhanced Polarization in Two-Dimensional Quintuple-Layers Al2o3 Materials

Two-dimensional (2D) quintuple-layer (QL) Al2O3 with intrinsic out-of-plane polarization exhibit many interesting physical properties. We systematically study the structural and electronic properties of QL-Al2O3 layers using the first-principles calculations. Our results show that polarized QL-Al2O3 is semiconductor with an indirect bandgap. For few QLs-Al2O3 system, its electronic properties can be either semiconductor or metal, depending on the stacking mode of each QL-Al2O3. When the stacked 2QLs-Al2O3 and 3QLs-Al2O3 system possess the same polarization direction, they behave the metallicity with the presence of 2D electron gas and 2D hole gas at two separate surfaces. The metallicity originates from directly interlayer band alignment and from indirectly two-step interfacial charge transfer mechanism with an enhanced surface charge. The limited surface polarized charges make the saturated polarization strength in QLs-Al2O3 system. While for the stacked 2QLs-Al2O3 and 3QLs-Al2O3 system with opposite polarization direction, the disappeared polarization and interfacial charge transfer make them keep the semiconductor. The revealed semiconductor-metal transition mechanism in stacked QLs-Al2O3 materials is also applicable to other 2D polarized van der Waals materials and heterostructures.