Session P30: Advances in Complex Oxide Film Growth

Oxygenation Dependence of the Resistance Upturn in $\mathbf{LaAlO_{3}}$/$\mathbf{SrTiO_{3}}$ Heterostructures

Among the phenomena exhibited by LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures, the appearance of a low-temperature resistance upturn has attracted much recent debate [1-4]. This phenomenon has been observed to co-occur with both nonlinear Hall effect and anisotropic magnetoresistance, and attributed to either Kondo effect, multiband conduction, or localization. It is primarily seen in samples grown by pulsed laser deposition, and is sensitive to film thickness, growth condition and electrostatic gating, all of which could affect the oxygen content. In this work, we study the effect of post-growth oxygenation on the resistance upturn in samples grown by molecular beam epitaxy, and how the Hall conductance and magnetoresistance are related to the occurrence of the upturn. X-ray photoelectron spectroscopy is used to monitor the valence states of Ti ions, in an effort to correlate them with the conduction and magnetic properties. Our results are analyzed in terms of oxygen vacancies in the presence of polar charge transfer, and the effect of these vacancies on the resistance upturn in LAO/STO heterostructures.\break [1] S. Das {\em et al}., PRB 90 (2014). [2] V. K. Guduru {\em et al}., PRB 88 (2013). [3] Z. Huang {\em et al}., PRB 88 (2013). [4] C. Bell {\em et al}., APL 94 (2009).   

Controlling Kondo Scattering at the Conducting Oxide Interfaces via Lattice Mismatch and Growth Oxygen Pressure

The interface magnetism, such as Kondo effect and ferromagnetism at the conducting interfaces between nonmagnetic oxides, has attracted great attention in recent years. In this report, we show that the interfacial Kondo scattering is enhanced by large lattice mismatch and high growth oxygen pressure. For the (001) LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ interface with 3.0{\%} lattice mismatch, the sheet resistance upturn appears around 40 K when the growth oxygen pressure $P_{\mathrm{O2}}$ is beyond 1 mTorr. By contrast, for the (001) (La$_{\mathrm{0.3}}$Sr$_{\mathrm{0.7}})$(Al$_{\mathrm{0.65}}$Ta$_{\mathrm{0.35}})$O$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ interface with 1.0{\%} lattice mismatch, no resistance upturn is observed until $P_{\mathrm{O2}}$ is increased to 100 mTorr. Moreover, the magnetoresistance data confirm the resistance upturn is caused by Kondo scattering. We propose that the interface disorders, which can be induced by a large lattice mismatch and high $P_{\mathrm{O2}}$, are important for forming localized Ti$^{\mathrm{3+}}$ ions. These Ti$^{\mathrm{3+}}$ ions can be spin-polarized and scatter electrons that are confined near the interface by high $P_{\mathrm{O2}}$. This explains why the stronger magnetic interaction is observed at the SrTiO$_{\mathrm{3}}$-based interfaces with the higher $P_{\mathrm{O2}}$ and larger lattice mismatch, paving the way for manipulating the interface magnetism at the functional oxide interface.   

Metal-insulator transition at lanthanum aluminate-strontium titanate interface induced by oxygen plasma treatment.

The formation of two-dimensional electron gas (2DEG) at lanthanum aluminate (LAO)-strontium titanate (STO) interface, as well as the 2DEG's unique characters in metal-insulator transition, have evoked widespread interest. Highly insulating interfaces are obtained for the structures with LAO thickness below 3 unit cell (uc) and abrupt transition from an insulating to conducting interface was observed for samples with thicker LAO layers. For 3uc LAO/STO samples, reversible nanoscale control of the metal-insulator transition was implemented by a conductive AFM writing. Our research furtherly discovered a very stable metal-insulator transition can be achieved by oxygen plasma (OP) treatment for samples with thicker LAO layers. AFM imaging and XPS measurement demonstrated the low energy OP treatment altered only the surface bonds, which confirmed the importance of surface properties in the heterostructures. Then microscale Hall bars were patterned at the interface and imaged by electrostatic force microscope. Their transport and magnetic properties were measured. This research will promote deeper understanding about the interfacial metal-insulator transition mechanism and open new device opportunities. This work is supported by the Department of Energy Grant No. DE-SC-0010399 and National Science Foundation Grant No. NSF-1454950.   

On the Growth of Complex Oxides by Molecular Beam Epitaxy

Functional materials based on complex oxides in thin film form offer new and exciting strategies for meeting many of our outstanding energy challenges through systematic control of layer sequencing, strain, etc. However, the synthesis of such oxide films can be a major challenge even when utilizing reactive molecular-beam epitaxy (MBE), a powerful deposition technique that allows the construction of materials atomic plane by atomic plane. To understand the fundamental physics of oxide growth by reactive MBE, we present \textit{in situ} surface x-ray diffraction results on the growth of SrTiO$_{\mathrm{3}}$ and SrO-SrTiO$_{\mathrm{3}}$ thin films on (001)-oriented SrTiO$_{\mathrm{3}}$ substrates. For homoepitaxy, we compare sequential deposition (alternating Sr and Ti monolayer doses) with that of co-deposition of Sr and Ti, both in a background of oxygen pressure, and observe drastically different growth pathways due to the presence of a TiO$_{\mathrm{2}}$ double layer. For heteroepitaxial growth of Ruddlesden-Popper SrO-SrTiO$_{\mathrm{3}}$ films, we find that layers rearrange dynamically, resulting in layer sequences distinct from the shutter sequence. In general, the starting surface structure and composition, in combination with local thermodynamic considerations, strongly influence our ability to atomically construct new complex oxides.   

Self-regulating MBE growth of stoichiometric BaSnO$_{3}$ films \textit{via} reactive radical mechanism

Growth of thin films comprising of element with low oxidation potential such as Sn often requires reactive oxidants such as ozone or high-pressure oxygen plasma. By utilizing the chemistry of highly reactive radical of Sn, we will present on the growth of phase-pure, epitaxial BaSnO$_{3}$ films using a hybrid molecular beam epitaxy (MBE) approach with scalable growth rates. The notable finding was that Sn radicals are very reactive to yield phase-pure BaSnO$_{3}$ films even in molecular oxygen [1]. In this approach, we use hexamethylditin (HMDT) as a tin source, a solid effusion cell for Ba and either molecular oxygen or an rf oxygen plasma source. Phase-pure BaSnO$_{3}$ films were grown at 900 $^{0}$C, and oxygen pressure of 5x10$^{-6}$ Torr as a function Sn:Ba ratio. \textit{In-situ} time-dependent RHEED intensity oscillations were observed establishing a layer-by-layer growth mode and a critical thickness of $\approx $1 nm for strain relaxation [1]. Rutherford backscattering spectrometry and lattice constant determined using high-resolution X-ray diffraction was used to optimize cation stoichiometry. ``MBE growth window'' was identified where films show bulk-like lattice parameter (4.116 {\AA}) over a wide-range of cation flux ratios. A correlation between RHEED patterns, stoichiometry, and surface morphology was established. [1] A. Prakash \textit{et al.}, J. Vac. Sci. {\&} Technol. A \textbf{33}, 060608 (2015).   

\textbf{Single phase growth of materials with high polar mismatch at the interface}

Growing thin films on polar surfaces has always been a challenge. An even greater challenge though, has been growing thin films on polar surfaces with polar mismatch at the interface. The divergence of surface free energy inhibits appearance of a sharp interface in these materials. Here for the first time, we report successful growth of LaNiO$_{3}$ on SrTiO$_{3}$ in (111) direction with correct phase. We show that by controlling the substrate's electronic and stoichiometric properties along with careful tune of thermodynamic parameters during the growth, it is possible to grow a single phase thin film of LaNiO$_{3}$. Using RHEED, LEED, STM and TEM, we show that the interface is sharp and there is no structural discontinuity, and the surface of the film preserves its symmetry during the growth. In addition, angle-resolved core-level photoelectron spectroscopy confirms the single phase growth.   

Surface structural reconstruction of SrVO$_{\mathrm{3}}$ thin films on SrTiO$_{\mathrm{3\thinspace }}$(001)

Paramagnetic metallic oxide SrVO$_{\mathrm{3\thinspace }}$(SVO) is an itinerant system known to undergo thickness-induced metal-insulator-transition (MIT) in ultrathin film form, which makes it a prototype system for the study of the mechanism behind metal-insulator-transition like structure distortion, electron correlations and disorder-induced localization. We have grown SrVO$_{\mathrm{3}}$ thin film with atomically flat surface through the layer-by-layer deposition by laser Molecular Beam Epitaxy (laser-MBE) on SrTiO$_{\mathrm{3}}$ (001) surface. Low Energy Electron Diffraction (LEED) measurements reveal that there is a ($\surd $2X$\surd $2) R45\textdegree surface reconstruction independent of film thickness. By using LEED-I(V) structure refinement, we determine the surface structure. In combination with X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM), we discuss the implication on the MIT in ultrathin films below 2-3 unit cell thickness.   

Van der Waals Epitaxy of Functional Oxide Heterostructures

In the diligent pursuit of low-power consumption, multifunctional, and environmentally friendly electronics, more sophisticated requirements on functional materials are on demand. Recently, the discovery of 2D layered materials has created a revolution to this field. Pioneered by graphene, these new 2D materials exhibit abundant unusual physical phenomena that is undiscovered in bulk forms. These materials are characterized with their layer form and almost pure 2D electronic behavior. The confinement of charge and heat transport at such ultrathin planes offers possibilities to overcome the bottleneck of present device development in thickness limitation, and thus push the technologies into next generation. Van der Waals epitaxy, an epitaxial growth method to combine 2D and 3D materials, is one of current reliable manufacturing processes to fabricate 2D materials by growing these 2D materials epitaxially on 3D materials. Then, transferring the 2D materials to the substrates for practical applications. In the mean time, van der Waals epitaxy has also been used to create free-standing 3D materials by growing 3D materials on 2D materials and then removing them from 2D materials since the interfacial boding between 2D and 3D materials should be weak van der Waals bonds. In this study, we intend to take the same concept, but to integrate a family of functional materials in order to open new avenue to flexible electronics. Due to the interplay of lattice, charge, orbital, and spin degrees of freedom, correlated electrons in oxides generate a rich spectrum of competing phases and physical properties. Recently, lots of studies have suggested that oxide heterostructures provide a powerful route to create and manipulate the degrees of freedom and offer new possibilities for next generation devices, thus create a new playground for researchers to investigate novel physics and the emergence of fascinating states of condensed matter. In this talk, we use a 2D layered material as the substrate. And we take several oxides as examples to demonstrate a pathway to integrate 3D functional oxides on 2D layered materials.   

Atomic-level sculpting in STEM for studies of thickness dependent structural behavior in oxide thin films

Oxide thin films offer a rich playground for fundamental physics and possible applications due to a wide variety of electronic and magnetic properties and control of chemistry and strain conditions during growth. Scanning Transmission Electron Microscopy (STEM) studies of thin films often demonstrate that film structure is highly dependent on thickness; changes in overall symmetry at a two-atomic-layer scale have been reported. At the same time it was recently shown that crystalline oxide nanostructures can be grown locally during a STEM experiment from amorphous precursors. This method can be used for producing crystalline structures as small as 1-2 nm and the process can be observed \textit{in situ} with atomic resolution. Here we utilize the sculpting approach to study size effects in ultrathin films of oxides as they are being grown inside a microscope. Transitions such as emergence of tilted structure, misfit defects and rotational domains can now be pinpointed with single atomic layer precision. Atomistic molecular dynamics simulations are used to establish theoretical underpinnings of the beam-induced growth mechanism. This work is funded by DMSE of the DOE BES (QH, AB), and by CNMS, which is funded at ORNL by SUFD of the DOE BES.   

Electrical properties of solid-solution SrZr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{\mathrm{3}}$ grown epitaxially on Ge by molecular beam epitaxy

The epitaxial growth of crystalline oxides on semiconductors enables new functionalities to be introduced to semiconductor devices. In particular, dielectric and ferroelectric oxides grown epitaxially on semiconductors provide a pathway to realize ultra-low power logic and memory devices. Here we present electrical characterization of solid-solution SrZr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{\mathrm{3}}$ grown epitaxially on Ge through oxide molecular beam epitaxy. SrZr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{\mathrm{3}}$ is of particular interest since the band offset with respect to the semiconductor can be tuned through Zr content x. We will present current-voltage, capacitance-voltage and piezoforce microscopy characterization of SrZr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{\mathrm{3}}$ -Ge heterojunctions. In particular, we will discuss how the electrical characteristics of SrZr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{\mathrm{3}}$ -Ge heterojunctions evolve with respect to composition, annealing and film thickness.   

First principles study of Al/SrTiO$_{3}$ interface formation

Two-dimensional electron gasses (2DEGs) at the interfaces of oxides have been the subject of much interest in recent years due to their relatively high carrier mobilities and potential for use in all-oxide devices. In particular, the $\gamma $-Al$_{2}$O$_{3}$ ($\gamma $-alumina)-SrTiO$_{3}$ (STO) system has been the focus of much research. It exhibits a 2DEG at the interface with a carrier mobility ranging from 10$^{3}$-10$^{5}$ cm$^{2\, }$V$^{-1}$ s$^{-1}$, depending on the thickness of the STO and how the $\gamma $-alumina film was grown. It is believed that Al atoms steal oxygen from the STO substrate at growth temperature and thus create a conductive channel in STO near the interface. We investigate the initial studies of the interface formation using density functional theory. The results of first principles calculations are compared with those of x-ray photoemission spectroscopy (XPS) performed in situ on thin Al films deposited on STO by molecular beam epitaxy. Analysis of the Al 2p XPS spectrum shows one layer of aluminum is fully oxidized during growth at 40\textdegree C and 4 layers of aluminum are fully oxidized during growth at 600\textdegree C. Furthermore, the Ti 2p XPS spectrum shows the titanium atoms are reduced, which is consistent with the presence of oxygen vacancies in STO.