Need R F, Marshall P B, Weschke E, Grutter A J, Gilbert D A, Arenholz E, Shafer P, Stemmer S, Wilson S D
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
Materials Department, University of California, Santa Barbara, California 93106, USA.
Phys Rev Mater. 2018 Sep;2(9). doi: 10.1103/physrevmaterials.2.093801.
Charge transfer in oxide heterostructures can be tuned to promote emergent interfacial states, and accordingly, has been the subject of intense study in recent years. However, accessing the physics at these interfaces, which are often buried deep below the sample surface, remains difficult. Addressing this challenge requires techniques capable of measuring the local electronic structure with high-resolution depth dependence. Here, we used linearly polarized resonant x-ray reflectometry (RXR) as a means to visualize charge transfer in oxide superlattices with single unit cell precision. From our RXR measurements, we extract valence depth profiles of SmTiO (SmTO)/SrTiO (STO) heterostructures with STO quantum wells varying in thickness from five SrO planes down to a single SrO plane. At the polar-nonpolar SmTO/STO interface, an electrostatic discontinuity leads to approximately half an electron per areal unit cell transferred from the interfacial SmO layer into the neighboring STO quantum well. We observe this charge transfer as a suppression of the absorption peaks that minimizes contrast with the neighboring SmTO layers at those energies and leads to a pronounced absence of superlattice peaks in the reflectivity data. Our results demonstrate the sensitivity of RXR to electronic reconstruction in single unit cell layers, and establish RXR as a powerful means of characterizing charge transfer at buried oxide interfaces.
氧化物异质结构中的电荷转移可以通过调节来促进新出现的界面态，因此，近年来一直是深入研究的主题。然而，要研究这些通常深埋在样品表面以下的界面处的物理性质仍然很困难。应对这一挑战需要能够以高分辨率深度依赖性测量局部电子结构的技术。在这里，我们使用线性偏振共振X射线反射测量法（RXR），以单胞精度可视化氧化物超晶格中的电荷转移。通过我们的RXR测量，我们提取了SmTiO（SmTO）/SrTiO（STO）异质结构的价态深度分布，其中STO量子阱的厚度从五个SrO平面变化到单个SrO平面。在极性-非极性SmTO/STO界面处，静电不连续性导致每面单位晶胞大约有半个电子从界面SmO层转移到相邻的STO量子阱中。我们将这种电荷转移观察为对吸收峰的抑制，这使得在这些能量下与相邻SmTO层的对比度最小化，并导致反射率数据中明显没有超晶格峰。我们的结果证明了RXR对单胞层中电子重构的敏感性，并将RXR确立为表征埋藏氧化物界面电荷转移的有力手段。
