Santosh Raghavan

Long-term retention in organic ferroelectric-graphene memories

Long-term stability of high- and low-resistance states in full-organic ferroelectrically gated graphene transistors is an essential prerequisite for memory applications. Here, we demonstrate high retention performance for both memory states with fully saturated time-dependence of the graphene channel resistance. This behavior is in contrast with ferroelectric-polymer-gated silicon field-effect-transistors, where the gap between the two memory states continuously decreases with time. Before reaching saturation, the current decays exponentially as predicted by the retention model based on the charge injection into the interface-adjacent layer. The drain current saturation attests to a high quality of the graphene/ferroelectric interface with low density of charge traps.

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.

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.

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

Magnetism and local structure in low-dimensional Mott insulating GdTiO3

Author(s): Zhang, Jack Y.; Jackson, Clayton A.; Raghavan, Santosh; Hwang, Jinwoo; Stemmer, Susanne | Abstract: Cation displacements, oxygen octahedral tilts, and magnetism of epitaxial, ferrimagnetic, insulating GdTiO3 films sandwiched between cubic SrTiO3 layers are studied using scanning transmission electron microscopy and magnetization measurements. With decreasing GdTiO3 film thickness, structural (GdFeO3-type) distortions are reduced, concomitant with a reduction in the Curie temperature. Ferromagnetism persists to smaller deviations from the cubic perovskite structure than is the case for the bulk rare-earth titanates. The results indicate that the ferromagnetic ground state is controlled by the narrow bandwidth, exchange and orbital ordering, and only to second order depends on the amount of the GdFeO3-type distortion.

Carrier density independent scattering rate in SrTiO3-based electron liquids.

We examine the carrier density dependence of the scattering rate in two- and three-dimensional electron liquids in SrTiO3 in the regime where it scales with Tn (T is the temperature and n ≤ 2) in the cases when it is varied by electrostatic control and chemical doping, respectively. It is shown that the scattering rate is independent of the carrier density. This is contrary to the expectations from Landau Fermi liquid theory, where the scattering rate scales inversely with the Fermi energy (EF). We discuss that the behavior is very similar to systems traditionally identified as non-Fermi liquids (n < 2). This includes the cuprates and other transition metal oxide perovskites, where strikingly similar density-independent scattering rates have been observed. The results indicate that the applicability of Fermi liquid theory should be questioned for a much broader range of correlated materials and point to the need for a unified theory.

Ferroelectric transition in compressively strained SrTiO3 thin films

We report the temperature dependent capacitance-voltage characteristics of Pt/SrTiO3 Schottky diodes fabricated using compressively strained SrTiO3 thin films grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates. The measurements reveal a divergence of the out of plane dielectric constant of SrTiO3 peaked at ∼140 K, implying a ferroelectric transition. A Curie-Weiss law fit to the zero-bias dielectric constant suggests a Curie temperature of ∼56 K. This observation provides experimental confirmation of the theoretical prediction of out of plane ferroelectricity in compressively strained SrTiO3 thin films grown on LSAT substrate. We also discuss the roles of the field-dependent dielectric constant and the interfacial layer in SrTiO3 on the extraction of the Curie temperature.

Au-gated SrTiO3 field-effect transistors with large electron concentration and current modulation

We report the fabrication of low-leakage rectifying Pt and Au Schottky diodes and Au-gated metal-semiconductor field effect transistors (MESFETs) on n-type SrTiO3 thin films grown by hybrid molecular beam epitaxy. In agreement with previous studies, we find that compared to Pt, Au provides a higher Schottky barrier height with SrTiO3. As a result of the large dielectric constant of SrTiO3 and the large Schottky barrier height of Au, the Au-gated MESFETs are able to modulate ∼1.6 × 1014 cm−2 electron density, the highest modulation yet achieved using metal gates in any material system. These MESFETs modulate current densities up to ∼68 mA/mm, ∼20× times larger than the best demonstrated SrTiO3 MESFETs. We also discuss the roles of the interfacial layer, and the field-dependent dielectric constant of SrTiO3 in increasing the pinch off voltage of the MESFET.

Structure and optical band gaps of (Ba,Sr)SnO3 films grown by molecular beam epitaxy

Epitaxial growth of (BaxSr1−x)SnO3 films with 0 ≤ x ≤ 1 using molecular beam epitaxy is reported. It is shown that SrSnO3 films can be grown coherently strained on closely lattice and symmetry matched PrScO3 substrates. The evolution of the optical band gap as a function of composition is determined by spectroscopic ellipsometry. The direct band gap monotonously decreases with x from to 4.46 eV (x = 0) to 3.36 eV (x = 1). A large Burnstein-Moss shift is observed with La-doping of BaSnO3 films. The shift corresponds approximately to the increase in Fermi level and is consistent with the low conduction band mass.

Conduction band edge effective mass of La-doped BaSnO3

BaSnO3 has attracted attention as a promising material for applications requiring wide band gap, high electron mobility semiconductors, and moreover possesses the same perovskite crystal structure as many functional oxides. A key parameter for these applications and for the interpretation of its properties is the conduction band effective mass. We measure the plasma frequency of La-doped BaSnO3 thin films by glancing incidence, parallel-polarized resonant reflectivity. Using the known optical dielectric constant and measured electron density, the resonant frequency determines the band edge electron mass to be 0.19 ± 0.01. The results allow for testing band structure calculations and transport models.