Hybrid Molecular Beam Epitaxy of Complex Oxide Heterostructures with Ultrahigh Electron Density

Abstract

Two-dimensional electron gases (2DEG) at complex oxide interfaces have attracted lots of attention for fundamental physics studies and potential applications in novel oxidebased electronics. While most researches focuses on LaAlO3/SrTiO3 heterostructures, many interesting phenomena were also discovered at titanates heterostructures, such as LaTiO3/SrTIO3 and GdTiO3/SrTiO3. In this study, we chose another material system, NdTiO3/SrTiO3, to investigate the 2DEG at the interfaces. NdTiO3 and SrTiO3 thin films and heterostructures were grown using the hybrid molecular beam epitaxy approach. 3E14 cm-2 (0.5 e-/u.c.) was realized at NdTiO3/SrTiO3 interfaces, and precise control of carrier density and metal-to-insulator transition were achieved by intentionally introducing Nd vacancies. Moreover, ultrahigh carrier density (~1E15 cm-2) was discovered by band engineering NdTiO3/SrTiO3 heterostructures. The charge transfer model was proposed to explain such high carrier density. Both experimental results and computational modeling suggest the broken-gap type band offset drives charge transfer from NdTiO3 to SrTiO3. Finally, we will also discuss environmental oxygen effects on electronic transport properties of NdTiO3/SrTiO3.