Kristy J. Kormondy

Quasi-two-dimensional electron gas at the epitaxial alumina/SrTiO3 interface: Control of oxygen vacancies

In this paper, we report on the highly conductive layer formed at the crystalline γ-alumina/SrTiO3 interface, which is attributed to oxygen vacancies. We describe the structure of thin γ-alumina layers deposited by molecular beam epitaxy on SrTiO3 (001) at growth temperatures in the range of 400–800u2009°C, as determined by reflection-high-energy electron diffraction, x-ray diffraction, and high-resolution electron microscopy. In situ x-ray photoelectron spectroscopy was used to confirm the presence of the oxygen-deficient layer. Electrical characterization indicates sheet carrier densities of ∼1013u2009cm−2 at room temperature for the sample deposited at 700u2009°C, with a maximum electron Hall mobility of 3100u2009cm2V−1s−1 at 3.2u2009K and room temperature mobility of 22u2009cm2V−1s−1. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator.

Microstructure and ferroelectricity of BaTiO3 thin films on Si for integrated photonics

Significant progress has been made in integrating novel materials into silicon photonic structures in order to extend the functionality of photonic circuits. One of these promising optical materials is BaTiO3 or barium titanate (BTO) that exhibits a very large Pockels coefficient as required for high-speed light modulators. However, all previous demonstrations show a noticable reduction of the Pockels effect in BTO thin films deposited on silicon substrates compared to BTO bulk crystals. Here, we report on the strong dependence of the Pockels effect in BTO thin films on their microstructure, and provide guidelines on how to engineer thin films with strong electro-optic response. We employ several deposition methods such as molecular beam epitaxy and chemical vapor deposition to realize BTO thin films with different morphology and crystalline structure. While a linear electro-optic response is present even in porous, polycrystalline BTO thin films with an effective Pockels coefficient r effxa0=xa06 pm V-1, it is maximized for dense, tetragonal, epitaxial BTO films (r effxa0=xa0140 pm V-1). By identifying the key structural predictors of electro-optic response in BTO/Si, we provide a roadmap to fully exploit the linear electro-optic effect in novel hybrid oxide/semiconductor nanophotonic devices.

Scavenging of oxygen from SrTiO3 during oxide thin film deposition and the formation of interfacial 2DEGs

SrTiO3 is a widely used substrate for the growth of other functional oxide thin films. The reactivity of the substrate with respect to the film during deposition, particularly with regard to redox reactions, has typically been glossed over. We demonstrate by depositing a variety of metals (Ti, Al, Nb, Pt, Eu, and Sr) and measuring the in situ core level spectra of both the metal and SrTiO3 that, depending on the oxide formation energy and work function of the metal, three distinct types of behavior occur in thin metal films on SrTiO3 (100). In many cases, there will be an interfacial layer of oxygen-deficient SrTiO3 formed at the interface with the overlying film. We discuss how this may affect the interpretation of the well-known two-dimensional electron gas present at the interface between SrTiO3 and various oxides.

Quasi-two-dimensional electron gas at the interface of γ-Al2O3/SrTiO3 heterostructures grown by atomic layer deposition

We report the formation of a quasi-two-dimensional electron gas (2-DEG) at the interface of γ-Al2O3/TiO2-terminated SrTiO3 (STO) grown by atomic layer deposition (ALD). The ALD growth of Al2O3 on STO(001) single crystal substrates was performed at temperatures in the range of 200–345u2009°C. Trimethylaluminum and water were used as co-reactants. In situ reflection high energy electron diffraction, ex situ x-ray diffraction, and ex situ cross-sectional transmission electron microscopy were used to determine the crystallinity of the Al2O3 films. As-deposited Al2O3 films grown above 300u2009°C were crystalline with the γ-Al2O3 phase. In situ x-ray photoelectron spectroscopy was used to characterize the Al2O3/STO interface, indicating that a Ti3+ feature in the Ti 2p spectrum of STO was formed after 2–3 ALD cycles of Al2O3 at 345u2009°C and even after the exposure to trimethylaluminum alone at 300 and 345u2009°C. The interface quasi-2-DEG is metallic and exhibits mobility values of ∼4 and 3000u2009cm2 V−1 s−1 at room temperature...

Epitaxy of polar semiconductor Co3O4 (110): Growth, structure, and characterization

The (110) plane of Co3O4 spinel exhibits significantly higher rates of carbon monoxide conversion due to the presence of active Co3+ species at the surface. However, experimental studies of Co3O4 (110) surfaces and interfaces have been limited by the difficulties in growing high-quality films. We report thin (10–250u2009A) Co3O4 films grown by molecular beam epitaxy in the polar (110) direction on MgAl2O4 substrates. Reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction, and transmission electron microscopy measurements attest to the high quality of the as-grown films. Furthermore, we investigate the electronic structure of this material by core level and valence band x-ray photoelectron spectroscopy, and first-principles density functional theory calculations. Ellipsometry reveals a direct band gap of 0.75u2009eV and other interband transitions at higher energies. A valence band offset of 3.2u2009eV is measured for the Co3O4/MgAl2O4 heterostructure. Magnetic measurements show the si...

Spectrum and phase mapping across the epitaxial γ-Al2O3/SrTiO3 interface

Epitaxial heterostructures of γ−Al2O3/SrTiO3, grown by atomic layer deposition (ALD) and molecular beam epitaxy, have been characterized by advanced electron microscopy techniques, including aberration-corrected negative-Cs imaging, electron-energy-loss near-edge fine-structure analysis, and off-axis electron holography. Analysis of two-dimensional spectrum maps from samples that previously showed highly conductive interfacial layers revealed partial reduction of the Ti oxidation state in the SrTiO3 layer from Ti4+ to Ti3+, which was confined to within ∼1–2 unit cells of the interface. Electron holography of an ALD-grown sample revealed a phase profile within the SrTiO3 layer that rose sharply over a distance of about 1u2009nm moving away from the interface. Taken together, these results suggest a strong connection between reduction of oxidation state, which could be caused by oxygen vacancies and the quasi-two-dimensional electron gas present at the γ−Al2O3/SrTiO3 interface.

Large positive linear magnetoresistance in the two-dimensional t 2g electron gas at the EuO/SrTiO 3 interface

The development of novel nano-oxide spintronic devices would benefit greatly from interfacing with emergent phenomena at oxide interfaces. In this paper, we integrate highly spin-split ferromagnetic semiconductor EuO onto perovskite SrTiO3 (001). A careful deposition of Eu metal by molecular beam epitaxy results in EuO growth via oxygen out-diffusion from SrTiO3. This in turn leaves behind a highly conductive interfacial layer through generation of oxygen vacancies. Below the Curie temperature of 70u2009K of EuO, this spin-polarized two-dimensional t2g electron gas at the EuO/SrTiO3 interface displays very large positive linear magnetoresistance (MR). Soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) reveals the t2g nature of the carriers. First principles calculations strongly suggest that Zeeman splitting, caused by proximity magnetism and oxygen vacancies in SrTiO3, is responsible for the MR. This system offers an as-yet-unexplored route to pursue proximity-induced effects in the oxide two-dimensional t2g electron gas.

An EELS signal-from-background separation algorithm for spectral line-scan/image quantification

Background removal is an important step in the quantitative analysis of electron energy-loss structure. Existing methods usually require an energy-loss region outside the fine structure in order to estimate the background. This paper describes a method for signal-from-background separation that is based on subspace division. The linear space is divided into two subspaces. The signal is recovered from a linear subspace containing no background information, and the other subspace containing the background is discarded. This method does not rely on any signal outside the energy-loss range of interest and should be very helpful for multiple linear least-squares (MLLS) regression analysis on experimental signals with little or no available smooth pre-edge region or with overlapping pre-edge features. Use of the algorithm is demonstrated with several practical applications, including closely overlapping core-loss spectra and zero-loss peak removal. Tests based on experimental data indicate that the algorithm has similar or better performance relative to conventional pre-edge power-law fitting methods in applications such as MLLS regression for electron energy-loss near-edge structure.

Piezoelectric modulation of nonlinear optical response in BaTiO3 thin film

We study the nonlinear optical response in a strained thin film ferroelectric oxide BaTiO3 using piezoelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (001) as a variable strain substrate and La-doped SrTiO3 as a conductive buffer layer. The rotation-anisotropic second harmonic intensity profile shows hysteretic modulation corresponding to the strain variation from the inverse piezoelectric response of the substrate. An enhancement of 15% is observed at 1.2u2009kV/cm, while a control sample shows negligible change as a function of piezovoltage. Reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, and high-resolution scanning transmission electron microscopy reveal the epitaxial interface. X-ray diffraction and piezoresponse force microscopy confirm tetragonal distortion and ferroelectricity of the BaTiO3 overlayer. Our results suggest a promising route to enhance the performance of nonlinear optical oxides for the development of future nano-opto-mechanical devices.

Microstructure and ferroelectricity of barium titanate thin films on Si for integrated photonics

Significant progress has been made in integrating novel materials into silicon photonic structures to extend the functionality of photonic circuits. One of these promising optical materials, BaTiO3 (BTO), exhibits a large Pockels coefficient as required for high-speed light modulators. Here, we employ several deposition methods such as molecular beam epitaxy and chemical vapor deposition to realize BTO thin films with different morphology and crystalline structure. By identifying the key structural predictors of electro-optic response in BTO/Si, we provide a roadmap to fully exploit the linear electro-optic effect in novel hybrid oxide/semiconductor nanophotonic devices.