AVS 66 Session OX+EM+MI+SS-WeM: Electronic and Magnetic Properties of Complex Oxide Surfaces and Interfaces

Wednesday, October 23, 2019 8:00 AM in Room A220-221
Wednesday Morning

Session Abstract Book
(310KB, Apr 26, 2020)
Time Period WeM Sessions | Abstract Timeline | Topic OX Sessions | Time Periods | Topics | AVS 66 Schedule

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8:00 AM OX+EM+MI+SS-WeM-1 Charge Transfer in Lanthanum Ferrite-Strontium Nickelate Superlattices
Le Wang, Zhenzhong Yang, Mark Bowden (Pacific Northwest National Laboratory); John William Freeland (Argonne National Laboratory); Yingge Du, Scott. A Chambers (Pacific Northwest National Laboratory)

Charge transfer at oxide interfaces can drive emergent phenomena that do not occur in the bulk, thereby significantly enriching our fundamental understanding of these material systems and their applications. Designing oxide heterostructures and seeking new and novel interfacial phenomena has been an active area of research for some time. We have synthesized a series of [(LaFeO3)m/(SrNiO3-d)n]z ([(LFO)m/(SNO)n]z) superlattices (SLs) (z = 7 to 21) by oxide molecular beam epitaxy on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) (001) substrates. In situ RHEED patterns and x-ray diffraction measurements reveal a high degree of structural quality in the SLs. X-ray photoemission spectroscopy (XPS) shows that the Fe is Fe4+ in the (LFO1/SNO1)21 SL. However, the Fe 2p binding energy shifts to lower values with increasing LFO layer thickness in (LFOm/SNO1)z SLs, suggesting that the volume averaged Fe valence decreases. Fe L-edge X-ray absorption spectroscopy (XAS) measurements corroborate the XPS results, indicating that Fe is 4+ for the (LFO1/SNO1)21 SL and mostly 3+ for the (LFO5/SNO1)10 SL. On the other hand, Ni L-edge XAS shows that Ni valence is Ni3+ for the (LFO1/SNO1)21 SL as is also true for insulating NdNiO3, suggesting that the Ni layers in this SL are insulating, which is consistent with our in-plane transport measurements. However, for the (LFO5/SNO1)10 SL, the Ni valence is larger than 3+. The measured energy shifts suggest that Ni is close to 4+. The thicker LFO layer in the (LFO5/SNO1)10 SL may result in a larger band offset and create a potential well to trap the holes in the Ni layer, inducing the formation of Ni4+. Our ongoing studies are probing the impact of the SNO layer thickness on material structure as well as the evolution of the Fe and Ni valences in (LFO5/SNOn)z SLs. Additional planned experimental and theoretical investigations will address how charge transfer from Fe to Ni occurs at the LFO/SNO interface, and how to stabilize the unusual high 4+ valence in Fe4+ and Ni4+ by means of interfacial engineering.

8:20 AM OX+EM+MI+SS-WeM-2 Self-healing Growth of LaNiO3 on Mixed-terminated (LaAlO3)0.3-(Sr2AlTaO6)0.7
Friederike Wrobel, Hawoong Hong, Seyoung Cook, Tassie Andersen, Deshun Hong, Changjiang Liu, Anand Bhattacharya, Dillon Fong (Argonne National Laboratory)

Epitaxial LaNiO3 (LNO) thin films and superlattices are known to be antiferromagnetic and weakly insulating for LNO thicknesses of 2 unit cells but paramagnetic and metallic for higher LNO thicknesses [1]. The quality of the single-crystal substrate surface, and in particular the chemical composition of the surface, is known to be a key factor governing the quality of the deposited thin film. For SrTiO3 (001) substrates, there are well-established preparation methods to ensure that the surface is TiO2-terminated and atomically smooth; the only features that appear with atomic force microscopy are the regular steps and terraces associated with crystal miscut. SrTiO3 is therefore often preferred as a substrate over other materials like (LaAlO3)0.3-(Sr2AlTaO6)0.7 (LSAT), whose surface composition is harder to control. Interestingly, for unknown reasons, the highest quality LaNiO3 thin films have been grown on mixed-terminated, untreated LSAT (001) substrates [2, 3]. At present, very few detailed studies have been conducted regarding the precise influence of the substrate on thin film growth behavior due to the need for an in-situ, atomic-scale characterization technique. Exploiting an in-situ, oxide molecular beam epitaxy (MBE) chamber at the Advanced Photon Source, we were able to monitor the deposition of thin films of LNO on LSAT (001) substrates with different surface compositions. Both non-resonant and resonant (Sr K-edge) X-ray scattering measurements were conducted at several points during the growth process. We observed the formation of atomically smooth, high-quality LNO films regardless of the initial substrate surface composition, suggesting that any excess, non-stoichiometric material on the initial LSAT substrate rises to the surface during deposition. With atomic layer-by-atomic layer MBE under the right conditions, we can therefore achieve self-healing growth behavior of complex oxides on top of mixed-terminated substrates. We will discuss details of the in-situ growth measurements and the methods used to determine the atomic and chemical structures.

1. Frano, A., et al., Orbital Control of Noncollinear Magnetic Order in Nickel Oxide Heterostructures. Physical Review Letters, 2013. 111(10): p. 106804.

2. Liu, C., et al., Counter-thermal flow of holes in high-mobility LaNiO3 thin films. Physical Review B, 2019. 99(4): p. 041114.

3. Wrobel, F., et al., Comparative study of LaNiO3/LaAlO3 heterostructures grown by pulsed laser deposition and oxide molecular beam epitaxy. Applied Physics Letters, 2017. 110(4): p. 041606.

8:40 AM Invited OX+EM+MI+SS-WeM-3 Optoelectronics with Oxides and Oxide Heterostructures
Alexander Demkov (University of Texas at Austin)

Si photonics is a hybrid technology combining semiconductor logic with fast broadband optical communications and optical information technologies. With the increasing bandwidth requirement in computing and signal processing, the inherent limitations in metallic interconnection are seriously threatening the future of traditional IC industry. Silicon photonics can provide a low-cost approach to overcome the bottleneck of the high data rate transmission by replacing the original electronic integrated circuits with photonic integrated circuits. The development has proceeded along several avenues including mounting optical devices based on III-V semiconductors and/or LiNbO3 (LNO) on Si chips, incorporation of active optical impurities into Si, and utilization of stimulated Raman scattering in Si. All these approaches have had limited success. Recently, another path to Si photonics through epitaxial integration of transition metal oxide films was demonstrated when an effective electro-optic (Pockels) coefficient of BaTiO3 ( BTO) films epitaxially grown on Si via an SrTiO3 buffer was reported to be an order of magnitude larger than that in commercially-available LNO modulators. More generally, epitaxial growth of SrTiO3 on Si(001) enables monolithic integration of many functional perovskite oxides on Si, including ferroelectric BTO, ferromagnetic LaCoO3, photocatalytic TiO2 and CoO, and many others.

In this talk, I will focus on two materials systems integrated on Si (001) and well-suited for implementation in the next-generation optical technologies: SrTiO3/LaAlO3 quantum wells and Pockels-active BTO thin film heterostructures. Both materials systems are promising for use in a wide variety of optical and electro-optical devices central to integrated photonic technologies, including quantum cascade lasers, photodetectors, electro-optic modulators and switches. The resulting devices achieve refractive index tuning with power consumption many orders of magnitude less than previously reported. Taken together, these two approached will hopefully open the door for the development of new kinds of optical and electro-optical devices for use in integrated photonics technologies.

9:20 AM Invited OX+EM+MI+SS-WeM-5 Medard W. Welch Award Lecture: Defect-Mediated Coupling of Built-in Potentials at Buried Interfaces Involving Epitaxial Complex Oxides
Scott. A Chambers (Pacific Northwest National Laboratory)

Semiconductor-based devices are of broad importance, not only in electronics, but also in energy technology. Internal electric fields dictate the flow of charge that occurs both laterally and vertically. The associated potential profiles can be approximated from electronic transport data, and also calculated via Poisson-Schrodinger modeling, provided the properties of the constituent materials and interface structures are sufficiently well understood. These approaches work well for heterostructures involving, for instance, III-V semiconductors. However, when complex oxides are involved, they become unreliable because of poorly understood defects that can be present. There is, therefore, a critical need for new methods to enable the determination of band-edge profiles in heterostructures involving these materials.

The SrTiO3/Si(001) interface has been a prototypical system for understanding the materials physics and electronic structure of crystalline oxides on semiconductors. Thinner films (a few unit cells, u.c.) are known to result in flat-band heterojunctions in which the valence (conduction) band offset is large (small). However, we have recently found that thicker films (~30 u.c.) of SrNbxTi1-xO3 (0 ≤ x ≤ 0.2) on intrinsic Si(001) result in completely different electronic structures. Transport data suggest sharp upward band bending in the Si, leading to hole gas formation at the interface, and a large (~2 eV) built-in potential in the SNTO, along with surface depletion. We have probed these buried interfaces using hard x-ray photoelectron spectroscopy (HAXPES). The resulting core-level spectra exhibit unusual features not seen in thinner films, and not credibly ascribed to secondary phases or many-body effects. In order to interpret these line shapes, we hypothesize that they result from large built-in potentials within the system. We have developed an algorithm to extract these potential profiles by fitting heterojunction spectra to linear combinations of spectra from phase-pure, flat-band materials, summed over layers within the probe depth, each with a binding energy characteristic of the potential at each depth. This approach leads to excellent agreement with experiment and band-edge profiles completely consistent with those from transport data. Moreover, we find that the built-in potentials extracted from HAXPES on the Si side of the interface are in quantitative agreement with those resulting from solving Poisson’s equation using the SIMS profile for in-diffused oxygen from the STO. Oxygen is a shallow donor in Si, and assuming 100% donor ionization, along with the 18O SIMS depth profile, leads to near-perfect agreement with HAXPES.
10:00 AM BREAK - Complimentary Coffee in Exhibit Hall
11:00 AM OX+EM+MI+SS-WeM-10 Spin Transport Studies on Epitaxial Ultrathin SrIrO3 Films Grown using Pulsed Laser Deposition (PLD)
M S Ramachandra Rao (Indian Institute of Technology, India); Kanikrishnan Sethupathi, T. Suraj, Sreya Suresh (Indian Institute of Technology Madras, India)
Generation, transportation and detection of spin in a controlled manner are very important in spintronic devices for efficient switching mechanisms in a magnetic tunnel junction (MTJ)1,2,3. The conversion of charge current to spin current is known as spin Hall effect (SHE) and the inverse phenomenon is inverse SHE (ISHE). The detection of spin is usually realized by the ISHE. Spin Hall magnetoresistance (SMR) arises due to the combined effect of SHE and ISHE in a bilayer heterostructures consisting of a ferromagnetic (or ferrimagnetic) insulator (FMI) and a normal metal (NM). Magnetic Proximity Effect (MPE) observed in YIG/Pt complicates the spin transport scenario with additional effects such as Anomalous Hall effect (AHE) which attenuates the SMR signal.4,5. Our group has successfully studied Ga: ZnO insertion on Bi: YIG/Pt and observed SMR signals6. Correlated metal oxide SrIrO3 is a potential candidate which can replace Pt because of its high spin-orbit coupling.

We have deposited SrIrO3 thin films with thickness from 3 nm to 10 nm using pulsed laser deposition on LSAT(001) substrate with LaMnO3 as a buffer layer. Detailed structural measurements have been carried out to understand the strain induced in the bilayer system by varying oxygen partial pressure during deposition of LaMnO3. Also, spin transport studies have been carried out since LaMnO3 is an A-type antiferromagnet. Angle-dependent magnetic measurements performed on these bilayer samples shows SMR, as well as anisotropic magnetoresistance (AMR) and the AMR part, is dominating over SMR. A particular thickness of SrIrO3 shows a crossover from negative magnetoresistance to positive magnetoresistance. I will be presenting our experimental observations on spin transport studies carried out in these bilayers.

References

  1. Matthias Althammer et. al., Phys. Rev. B 87, 224401 (2013).
  2. T. Kikkawa et. al., Phys. Rev. Lett. 110, 067207 (2013).
  3. E. Saitoh et. al., Appl. Phys. Lett. 88, 182509 (2006).
  4. H. Nakayama et al., Phys. Rev. Lett. 110, 206601 (2013).
  5. V. Castel et al., Appl. Phys. Lett. 101, 132414 (2012).
  6. Matthias Althammer et.al.Appl. Phys. Lett. 110, 052403 (2017).
11:20 AM OX+EM+MI+SS-WeM-11 Structural and Dielectric Characterization of Epitaxial Entropy-Stabilized Oxide Thin Films
George Kotsonis, Jon-Paul Maria (Pennsylvania State University)

The emergence of entropy-stabilized oxides (ESOs) represents a new paradigm for complex oxide engineering. The large configurational entropy of ESOs facilitates mixing of chemically dissimilar cations in significant proportions. ESO research continues to intensify as the oxide community works toward a thorough understanding of structure-property-synthesis relationships. Due to inherent metastability, high energy, non-equilibrium synthesis techniques are well suited for ESO fabrication. In particular, laser ablation has excelled at producing high quality epitaxial ESO thin films, which provide a platform for fundamental characterization.

We present the growth and characterization of Ba(Ti0.2Sn0.2Zr0.2Hf0.2Nb0.2)O3 and similar Barium-based perovskite structured ESO thin films grown by laser ablation. Crystal structure, surface morphology, and optical properties are characterized by X-ray diffraction, atomic force microscopy, and ellipsometry respectively. Epitaxial thin film capacitor structures were fabricated to characterize the frequency, voltage, and temperature dependence of electrical properties.

By exploiting the entropy-stabilized nature of ESOs, we demonstrate the incorporation of significant amounts of aliovalent cation pairs (e.g. Sc3+Ta5+) in hopes of producing nano-polar regions supporting a dispersive dielectric response similar to relaxor ferroelectrics. Additionally, we explore compositional space in search of a phase boundary between a high-symmetry ESO phase and a lower symmetry end-member. Compositions at such a boundary may exhibit phase instability and enhanced dielectric functionality similar to compositions at or near a morphotropic phase boundary. The compositional degrees of freedom available in ESO systems provide new avenues for property tuning and studying the effects of extreme chemical disorder on dielectric properties.

11:40 AM OX+EM+MI+SS-WeM-12 Oxygen Vacancy-Mediated Epitaxy: TiO2(111)/Al2O3(0001) and Ferromagnetic Cr2O3(0001)/TiO2(111)
Chad Ladewig, Fatima Anwar, Jeffry Kelber (University of North Texas); Syed Shah, Peter A. Dowben (University of Nebraska-Lincoln)

The formation of all-oxide heterostructures comprising multiferroic oxides interfaced with appropriate semiconducting substrates is a promising path towards low power, voltage-switchable spintronics, including non-volatile memory and multi-functional logic devices. At the same time, the necessary scaling of film thicknesses to the nm range can induce structures and properties sharply different than those of the bulk. We report here in situ XPS, LEED, EELS and ex-situ MOKE data on the growth and properties of Cr2O3(0001) on TiO1.7(111) on Al2O3(0001). The data indicate that the presence of O vacancies during film growth can mediate the further growth of oxides with unusual structures and properties. These data show that (a) O vacancies during initial stages of film growth yield a TiO2 film of an unusual crystallographic orientation and structure; and that (b) this leads to growth of an epitaxial Cr2O3 layer exhibiting magnetic ordering above the expected Néel temperature of thin film chromia - indicative of a strained chromia lattice due to epitaxial growth on a substrate with a lattice constant of 5.1 Å, compared to the bulk chromia lattice constant of 4.9 Å. Molecular beam epitaxy (MBE) of Ti at 500 K in 10-6 Torr O2 on Al2O3(0001) initially yields TiO1.7(111) with the structure of corundum phase Ti2O3 (a = b=5.1 Å). Further deposition and annealing in O2 results in stoichiometric TiO2(111), but with the same lattice structure and orientation as Ti2O3(111), and with a total thickness of 5 nm. This is sharply different from the generally observed growth of TiO2(001) on Al2O3(0001). MBE of ~ 1 monolayer of Cr on TiO2(111) yields hexagonally-ordered Cr2O3 and the formation of titania oxygen vacancies. MOKE measurements confirm that this chromia layer is magnetically ordered at 280 to 315 K, likely antiferromagnetically ordered, with exchange bias coupling to the TiO1.7(111) substrate. O vacancies in the TiO2(111) lattice exhibit weak ferromagnetic behavior, as is evident in the In-plane MOKE, enhancing the canting of the magnetism away from the thin film normal, which is expected for the Cr2O3(0001) alone. These data demonstrate that careful control of initial growth conditions and film stoichiometry during oxide MBE can template the subsequent growth of stoichiometric oxide heterostructures with non-bulk like structures and properties.

Acknowledgement: Work at UNL was supported in part by the Semiconductor Research Corporation (SRC) as task 2760.002 and NSF through ECCS 1740136.

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12:00 PM OX+EM+MI+SS-WeM-13 Incorporation of Ti into Epitaxial Films of Magnetite
Tiffany Kaspar, Steven Spurgeon, Daniel Schreiber, Sandra Taylor, Mark Bowden, Scott. A Chambers (Pacific Northwest National Laboratory)
Magnetite, Fe3O4, exhibits metallic conductivity via electron hopping between Fe2+ and Fe3+ occupying octahedral sites in the spinel lattice. As Ti4+ is doped into the octahedral sites of magnetite (the titanomagnetite series), an equal fraction of Fe3+ is reduced to Fe2+ to maintain charge neutrality. The site occupancies of Fe2+ and Fe3+ determine the transport properties of the titanomagnetite series; the end-member ulvöspinel, Fe2TiO4, exhibits p-type semiconducting transport properties. The Fe2+/Fe3+ site occupancy remains controversial, but is likely in part a function of the lattice strain induced by doping smaller Ti4+ into the lattice. Here, we have deposited titanomagnetites and ulvöspinel as well-defined epitaxial thin films on MgO, MgAl2O4, and Al2O3 substrates by oxygen-plasma-assisted molecular beam epitaxy. The incorporation of Ti into the magnetite lattice is found to depend strongly on deposition conditions and substrate orientation. We have characterized the crystalline structure, phase segregation, and surface morphology with XRD, STEM/EDS, APT, and AFM, and related these to the kinetic and thermodynamic factors determined by the deposition conditions. The Fe valence state is evaluated with in situ XPS. The impact of film structure and Fe oxidation state on the electrical transport properties of the films will be discussed.
Session Abstract Book
(310KB, Apr 26, 2020)
Time Period WeM Sessions | Abstract Timeline | Topic OX Sessions | Time Periods | Topics | AVS 66 Schedule