Jack Y. Zhang

Electrostatic carrier doping of GdTiO3/SrTiO3 interfaces

Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3 are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately ½ electron per surface unit cell at each GdTiO3/SrTiO3 interface. The sheet carrier densities in all structures containing more than one unit cell of SrTiO3 are independent of layer thicknesses and growth sequences, indicating that the mobile carriers are in a high concentration, two-dimensional electron gas bound to the interface. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental results, insights into interfacial band alignments, charge distribution, and the influence of different electrostatic boundary conditions are obtained.

High-mobility BaSnO3 grown by oxide molecular beam epitaxy

High-mobility perovskite BaSnO3 films are of significant interest as new wide bandgap semiconductors for power electronics, transparent conductors, and as high mobility channels for epitaxial integration with functional perovskites. Despite promising results for single crystals, high-mobility BaSnO3 films have been challenging to grow. Here, we demonstrate a modified oxide molecular beam epitaxy (MBE) approach, which supplies pre-oxidized SnOx. This technique addresses issues in the MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO3. We demonstrate room temperature electron mobilities of 150 cm2 V−1 s−1 in films grown on PrScO3. The results open up a wide range of opportunities for future electronic devices.

Nanoscale quantification of octahedral tilts in perovskite films

NiO6-octahedral tilts in ultrathin LaNiO3 films were studied using position averaged convergent beam electron diffraction (PACBED) in scanning transmission electron microscopy. Both the type and magnitude of the octahedral tilts were determined by comparing PACBED experiments to frozen phonon multislice simulations. It is shown that the out-of-plane octahedral tilt of an epitaxial film under biaxial tensile stress (0.78% in-plane tensile strain) increases by ∼20%, while the in-plane rotation decreases by ∼80%, compared to the unstrained bulk material.

Transport in ferromagnetic GdTiO3 / SrTiO3 heterostructures

Epitaxial GdTiO3/SrTiO3 structures with different SrTiO3 layer thicknesses are grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate surfaces by hybrid molecular beam epitaxy. It is shown that the formation of the pyrochlore (Gd2Ti2O7) phase can be avoided if GdTiO3 is grown by shuttered growth, supplying alternating monolayer doses of Gd and of the metalorganic precursor that supplies both Ti and O. Phase-pure GdTiO3 films grown by this approach exhibit magnetic ordering with a Curie temperature of 30 K. The electrical transport characteristics can be understood as being dominated by a conductive interface layer within the SrTiO3.

Growth window and effect of substrate symmetry in hybrid molecular beam epitaxy of a Mott insulating rare earth titanate

The conditions for the growth of stoichiometric GdTiO3 thin films by molecular beam epitaxy (MBE) are investigated. It is shown that relatively high growth temperatures (>750 °C) are required to obtain an MBE growth window in which only the stoichiometric film grows for a range of cation flux ratios. This growth window narrows with increasing film thickness. It is also shown that single-domain films are obtained by the growth on a symmetry-matched substrate. The influence of lattice mismatch strain on the electrical and magnetic characteristics of the GdTiO3 thin film is investigated.

Structural origins of the properties of rare earth nickelate superlattices

NiO6 octahedral tilts in the LaNiO3/SrTiO3 superlattices are quantified using position averaged convergent beam electron diffraction in scanning transmission electron microscopy. It is shown that maintaining oxygen octahedra connectivity across the interface controls the octahedral tilts in the LaNiO3 layers, their lattice parameters, and their transport properties. Unlike films and layers that are connected on one side to the substrate, subsequent LaNiO3 layers in the superlattice exhibit a relaxation of octahedral tilts towards bulk values. This relaxation is facilitated by tilts in the SrTiO3 layers and is correlated with the conductivity enhancement of the LaNiO3 layers in the superlattices relative to individual films.

Correlation between stoichiometry, strain, and metal-insulator transitions of NdNiO3 films

The interplay of film stoichiometry and strain on the metal-insulator transition (MIT) and Hall coefficient of NdNiO3 films grown under different conditions is investigated. Unstrained lattice parameters and lattice mismatch strains are evaluated for films grown under a range of growth pressures and on different substrates. It is shown that both the temperature of the MIT and the Hall coefficient in the metallic phase are highly sensitive to film strain. In films grown with lower oxygen/total growth pressures, very large compressive in-plane strains can be obtained, which can act to suppress the MIT. Both the Hall coefficient and the temperature of the MIT are relatively insensitive to growth pressure, provided that films under the same strain are compared. The results support an itinerant picture of the transition that is controlled by the Ni eg bands, and that is relatively insensitive to changes in film stoichiometry.

Quantum critical behaviour in confined SrTiO3 quantum wells embedded in antiferromagnetic SmTiO3

Quantum phase transitions are driven by quantum fluctuations that alter the nature of the electronic quasiparticles, resulting in phenomena such as non-Fermi liquid behaviour. Oxide heterostructures offer fundamentally new ways of manipulating quantum criticality. Here, we report on non-Fermi liquid behaviour in thin SrTiO3 quantum wells that are embedded in insulating, antiferromagnetic SmTiO3, as a function of temperature, quantum well thickness and SmTiO3 layer thickness in superlattices. Such quantum wells contain very high sheet carrier densities on the order of one electron per pseudocubic planar unit cell. We show that the quantum well thickness is a tuning parameter for non-Fermi liquid behaviour. Increasing the thickness by a single atomic layer and coupling in superlattices recover the Fermi liquid behaviour. The critical exponents, the symmetry of the order parameter, the role of carrier densities and symmetry-lowering distortions are discussed, and the results are compared with those of quantum wells embedded in ferrimagnetic GdTiO3.

Al-doped HfO2/In0.53Ga0.47As metal-oxide-semiconductor capacitors

Hafnium oxide gate dielectrics doped with a one to two percent of aluminum are grown on In0.53Ga0.47As channels by codeposition of trimethylaluminum TMA and hafnium tertbutoxide HTB. It is shown that the addition of TMA during growth allows for smooth, amorphous films that can be scaled to 5 nm physical thickness. Metal-oxide-semiconductor capacitors MOSCAPs with this dielectric have an equivalent oxide thickness of 1 nm, show an unpinned, efficient Fermi level movement and lower interface trap densities than MOSCAPs with HfO2 dielectrics grown by sequential TMA/HTB deposition.

Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO3 quantum wells

PHYSICAL REVIEW B 89, 075140 (2014) Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO 3 quantum wells Jack Y. Zhang, 1 Clayton A. Jackson, 1 Ru Chen, 2 Santosh Raghavan, 1 Pouya Moetakef, 1,* Leon Balents, 3 and Susanne Stemmer 1 Materials Department, University of California, Santa Barbara, California 93106, USA Department of Physics, University of California, Santa Barbara, California 93106, USA Kavli Institute of Theoretical Physics, University of California, Santa Barbara, Santa Barbara, California 93106, USA (Received 10 January 2014; revised manuscript received 13 February 2014; published 28 February 2014) The electrical and structural characteristics of SmTiO 3 /SrTiO 3 /SmTiO 3 and GdTiO 3 /SrTiO 3 /GdTiO 3 het- erostructures are compared. Both types of structures contain narrow SrTiO 3 quantum wells, which accommodate a confined, high-density electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO 3 quantum wells embedded in GdTiO 3 show a metal-to-insulator transition when their thickness is reduced so that they contain only two SrO layers. In contrast, quantum wells embedded in SmTiO 3 remain metallic down to a single SrO layer thickness. Symmetry-lowering structural distortions, measured by quantifying the Sr-column displacements, are present in the insulating quantum wells, but are either absent or very weak in all metallic quantum wells, independent of whether they are embedded in SmTiO 3 or in GdTiO 3 . We discuss the role of orthorhombic distortions, orbital ordering, and strong electron correlations in the transition to the insulating state. DOI: 10.1103/PhysRevB.89.075140 PACS number(s): 71.27.+a, 71.30.+h, 81.07.St I. INTRODUCTION Quantum-confined transition-metal oxides allow for cre- ating new states of matter through manipulation of spin and orbital order, interfacial proximity effects, and reduced dimensionality, and can thus serve to elucidate the physics of two-dimensional, strongly correlated electron systems [1]. For example, narrow, high-electron-density quantum wells of a nonmagnetic band insulator, SrTiO 3 , which are embedded in a Mott insulating ferrimagnet, GdTiO 3 , show ferromagnetism and mass enhancement due to strong electron correlations [2–4]. At the smallest dimensions, when the quantum wells contain just two SrO layers, the electron system abruptly localizes and the resistivity increases by several orders of magnitude [2]. The transition to the insulating state is accompanied by structural distortions of the Ti-O octahedra, which can be experimentally detected by measuring concurrent displacements of the Sr cations [5]. Metal-insulator transitions at reduced thicknesses have also been observed in narrow quantum wells and thin films of many other perovskite materials, such as SrVO 3 [6], LaNiO 3 [7–9], and NdNiO 3 [10]. In general, in many d-electron systems, symmetry breaking of spin and orbital degrees of freedom plays a crucial role in promoting an insulating state in materials that undergo a metal-insulator transition [11]. Transition-metal–oxygen octahedral tilts that reduce the symmetry relative to the parent cubic perovskite structure are modified in quantum wells due to film strain [12,13] and interfacial coherency [3,14–16]. To understand the underlying physics of Mott transitions in confined quantum wells, such as the relative roles of disorder, the interactions among the electrons themselves (strong correlations), and interactions of the carriers with the lattice, it is useful to explore if the localization can be systematically tuned by changing the external parameters of Present address: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD. the system. Towards this goal, we compare the electrical and structural properties of thin SrTiO 3 quantum wells embedded in GdTiO 3 and SmTiO 3 , respectively. We have previously reported on the electrical and structural properties of the structures with GdTiO 3 [2,5], and they are included here for comparison. In both cases, the quantum wells contain a two-dimensional electron gas with sheet carrier densities of close to one electron per (pseudo-)cubic planar unit cell, which is introduced by the charge discontinuity at the interface [2,17]. This sheet carrier density is independent of the film thicknesses. It is important to emphasize that SrTiO 3 is a band insulator in bulk, and has the ideal cubic perovskite structure at room temperature. Therefore, and in contrast to the aforementioned confined correlated metals, such as the nickelates, correlated properties—including magnetism, mass enhancement, and metal-insulator transitions—are induced in a material that does not exhibit Mott physics in the bulk. Both GdTiO 3 and SmTiO 3 are prototypical Mott insulators, with a d 1 electron configuration. SmTiO 3 has the same orthorhombic crystal structure as GdTiO 3 , albeit with slightly smaller octahedral distortions [18]. The two compounds also differ in their low-temperature magnetic properties—GdTiO 3 is ferrimagnetic, while SmTiO 3 is antiferromagnetic [19]. These properties couple with the electron system in the quantum well [4]. Furthermore, they exhibit different orbital ordering, which is antiferro-orbital in GdTiO 3 and ferro-orbital in SmTiO 3 , respectively [20–22]. II. EXPERIMENT All films were grown by hybrid molecular beam epitaxy (MBE) [23,24] on (001) (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 )O 3 (LSAT, ˚ substrates. Electrical measurements were carried a = 3.86 A) out on GdTiO 3 /SrTiO 3 /GdTiO 3 and SmTiO 3 /SrTiO 3 /SmTiO 3 quantum well structures that contained a single SrTiO 3 quan- tum well. The GdTiO 3 and SmTiO 3 layers were 10 nm thick. The thicknesses of the SrTiO 3 quantum wells are specified in ©2014 American Physical Society