Kurt D. Fredrickson

Carrier density modulation in a germanium heterostructure by ferroelectric switching

The development of non-volatile logic through direct coupling of spontaneous ferroelectric polarization with semiconductor charge carriers is nontrivial, with many issues, including epitaxial ferroelectric growth, demonstration of ferroelectric switching and measurable semiconductor modulation. Here we report a true ferroelectric field effect-carrier density modulation in an underlying Ge(001) substrate by switching of the ferroelectric polarization in epitaxial c-axis-oriented BaTiO3 grown by molecular beam epitaxy. Using the density functional theory, we demonstrate that switching of BaTiO3 polarization results in a large electric potential change in Ge. Aberration-corrected electron microscopy confirms BaTiO3 tetragonality and the absence of any low-permittivity interlayer at the interface with Ge. The non-volatile, switchable nature of the single-domain out-of-plane ferroelectric polarization of BaTiO3 is confirmed using piezoelectric force microscopy. The effect of the polarization switching on the conductivity of the underlying Ge is measured using microwave impedance microscopy, clearly demonstrating a ferroelectric field effect.

Atomic and electronic structure of the ferroelectric BaTiO3/Ge(001) interface

In this study, we demonstrate the epitaxial growth of BaTiO3 on Ge(001) by molecular beam epitaxy using a thin Zintl template buffer layer. A combination of density functional theory, atomic-resolution electron microscopy and in situ photoemission spectroscopy is used to investigate the electronic properties and atomic structure of the BaTiO3/Ge interface. Aberration-corrected scanning transmission electron micrographs reveal that the Ge(001) 2 × 1 surface reconstruction remains intact during the subsequent BaTiO3 growth, thereby enabling a choice to be made between several theoretically predicted interface structures. The measured valence band offset of 2.7 eV matches well with the theoretical value of 2.5 eV based on the model structure for an in-plane-polarized interface. The agreement between the calculated and measured band offsets, which are highly sensitive to the detailed atomic arrangement, indicates that the most likely BaTiO3/Ge(001) interface structure has been identified.

Surface electronic structure for various surface preparations of Nb-doped SrTiO3 (001)

High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to study the surface electronic structure of Nb-doped SrTiO3 (STO) single crystals prepared using a variety of surface preparations. ARPES measurements show that simple degreasing with subsequent anneal in vacuum is not an adequate surface preparation of STO, rather, preparations consisting of etching with buffered HF or HCl, and to a lesser extent, simple water leaching resulted in surfaces with much less disorder. A non-dispersing, mid-gap state was found ∼800 meV above the top of the valence band for samples which underwent etching. This mid-gap state is not present for vacuum-annealed and water-leached samples, as well as for STO thin films grown using molecular beam epitaxy. Theoretical modeling using density functional theory suggests that this mid-gap state is not related to the SrO- and TiO2-terminated surfaces, but rather, is due to a partial hydrogenation of the STO surface that occurs during etching.

Wetting at the BaTiO3/Pt interface

Using density functional theory, we analyze the wetting conditions for Pt on the (001) surface of ferroelectric BaTiO3 (BTO). We estimate the surface energy of (100), (110), and (111) Pt to be 2.42, 2.49, and 2.00 J/m2, respectively. We find the BTO surface energy to vary between 0.26 and 2.28 J/m2 depending on termination, polarization, and chemical environment. The interface energy between TiO2-terminated out-of-plane polarized BTO and (100) Pt is found to be between 1.64 and 2.62 J/m2, indicating that (100) Pt cannot wet BTO for this interface. A similar result is found for an interface with (110) Pt. Cross-sectional transmission electron microscopy of Pt films grown on BTO by molecular beam epitaxy with a low flux at high deposition temperature shows Volmer-Weber islands, consistent with first principles calculations.

Mechanism of oxidation protection of the Si(001) surface by sub-monolayer Sr template

We investigate theoretically the oxidation stability of the Si(001) (2 × 1) reconstructed surface passivated by Sr. Using density functional theory, we find that the Sr surface with ½ monolayer of Sr is protected against oxidation. The presence of Sr delays the oxidation of the surface dimer, and even when the dimer is oxidized, O does not react with the back-bond, preventing the unwanted vertical growth of SiO2. We also show that ¼ monolayer of Sr protects the Si surface in a different way. In the presence of ¼ monolayer of Sr, O atoms are attracted to the Sr-Si dimer complexes, thus preventing the formation of SiO2.

Theoretical modeling and experimental observations of the atomic layer deposition of SrO using a cyclopentadienyl Sr precursor

First-principle calculations are used to model the adsorption and hydration of strontium bis(cyclopentadienyl) [Sr(Cp)2] on TiO2-terminated strontium titanate, SrTiO3 (STO), for the deposition of strontium oxide, SrO, by atomic layer deposition (ALD). The Sr(Cp)2 precursor is shown to adsorb on the TiO2-terminated surface, with the Sr atom assuming essentially the bulk position in STO. The C–Sr bonds are weaker than in the free molecule, with a Ti atom at the surface bonding to one of the C atoms in the cyclopentadienyl rings. The surface does not need to be hydrogenated for precursor adsorption. The calculations are compared with experimental observations for a related Sr cyclopentadienyl precursor, strontium bis(triisopropylcyclopentadienyl) [Sr(iPr3Cp)2], adsorbed on TiO2-terminated STO. High-resolution x-ray photoelectron spectroscopy and low-energy ion scattering spectroscopy show adsorption of the Sr precursor on the TiO2-terminated STO after a single precursor dose. This study suggests that ALD gro...

Two-Dimensional Electron Gas at Oxide Interfaces

In this chapter, we provide an overview of the growing field of the two-dimensional electron gas in oxide heterostructures. The discovery of the high mobility electron gas at the oxide-oxide interface has spurred subsequent investigations which draw from the large body of work on polar oxide surfaces and thin films. We discuss the three main mechanisms of electronic reconstruction, oxygen vacancy formation, and cation exchange in order to address the question, “How can the interface between two insulators be conducting?” Throughout the chapter, in addition to the model LaAlO3/SrTiO3 system, we provide the reader with a sampling of what has been learned from other oxide heterostructures through both experiment and theory.

Spin-polarized, orbital-selected hole gas at the EuO/Pt interface

Using density functional theory, we explore the magnetic behavior of a EuO/Pt heterostructure. The calculations suggest that the heterostructure could be used as a spin filter, as the Schottky barriers in the spin-up and spin-down channels are calculated to be 1.42 and 2.18 eV, respectively. We discover that the interfacial and second layer of EuO, both have a reduced magnetic moment, while the remainder of the oxide maintains bulk magnetization. These first two layers support a localized mid-gap electronic state that protects the remainder of EuO from losing charge into the large work function Pt, which in turn results in the magnetic moment reduction. Our calculations are consistent with recent experimental results of Barbagallo et al. [Phys. Rev. B 84, 075219 (2011)].