First-principles studies of a two-dimensional electron gas at the interface of polar/polar LaAlO3/KNbO3 superlattices

We explored the possibility of producing a two-dimensional electron gas (2DEG) in polar/polar (LaAlO3)m/(KNbO3)n perovskite superlattices that have N type and P type interfaces using the first-principles electronic structure calculations. Two different kinds of LaAlO3/KNbO3 superlattices were constructed, namely stoichiometric NP superlattice (NP-SL) and non-stoichiometric NN superlattice (NN-SL). We discovered that the NP-SL undergoes a transition from an insulating to a metallic state when LaAlO3 has more than 3 unit cells. This reveals the completely spin-polarized two-dimensional hole gas (2DHG), as well as 2DEG with an interfacial charge carrier density of n ∼ 1013 cm−2 and an electron effective mass of 0.240me (for 5 unit cells of LaAlO3). In comparison, the NN-SL is intrinsically metallic, and when LaAlO3 has 4.5 unit cells, the structure shows a 2DEG with a larger density (n ∼ 1014 cm−2) and a smaller electron effective mass (0.185me). In addition, the charge carrier properties are highly sensitive to the number of LaAlO3 unit cells in the NP-SL model, while the size effect of LaAlO3 is negligible for the NN-SL one. Our results demonstrate that electronic reconstruction at the interfaces of the stoichiometric structure can produce both the 2DHG and 2DEG, whereas extra electrons are introduced to form solely the 2DEG at the non-stoichiometric structure interfaces. This research provides fundamental insights into the different interfacial electronic properties and the primary mechanism responsible for the formation of polar/polar heterojunction LaAlO3/KNbO3 superlattices.