Hosung Seo

Epitaxial integration of ferromagnetic correlated oxide LaCoO3 with Si (100)

We have grown epitaxial strained LaCoO3 on (100)-oriented silicon by molecular beam epitaxy using a relaxed epitaxial SrTiO3 buffer layer. Superconducting quantum interference device magnetization measurements show that, unlike the bulk material, the ground state of the strained LaCoO3 on silicon is ferromagnetic with a TC of 85 K. First principles calculations suggest that a ferromagnetic ground state can be stabilized in LaCoO3 by a sufficiently large biaxial tensile strain with the transition accompanied by a partial untilting of the CoO6 octahedra.

Quantum decoherence dynamics of divacancy spins in silicon carbide.

Long coherence times are key to the performance of quantum bits (qubits). Here, we experimentally and theoretically show that the Hahn-echo coherence time of electron spins associated with divacancy defects in 4H–SiC reaches 1.3 ms, one of the longest Hahn-echo coherence times of an electron spin in a naturally isotopic crystal. Using a first-principles microscopic quantum-bath model, we find that two factors determine the unusually robust coherence. First, in the presence of moderate magnetic fields (30 mT and above), the 29Si and 13C paramagnetic nuclear spin baths are decoupled. In addition, because SiC is a binary crystal, homo-nuclear spin pairs are both diluted and forbidden from forming strongly coupled, nearest-neighbour spin pairs. Longer neighbour distances result in fewer nuclear spin flip-flops, a less fluctuating intra-crystalline magnetic environment, and thus a longer coherence time. Our results point to polyatomic crystals as promising hosts for coherent qubits in the solid state.

Atomic layer deposition of photoactive CoO/SrTiO3 and CoO/TiO2 on Si(001) for visible light driven photoelectrochemical water oxidation

Cobalt oxide (CoO) films are grown epitaxially on Si(001) by atomic layer deposition (ALD) using a thin (1.6 nm) buffer layer of strontium titanate (STO) grown by molecular beam epitaxy. The ALD growth of CoO films is done at low temperature (170–180  °C), using cobalt bis(diisopropylacetamidinate) and water as co-reactants. Reflection high-energy electron diffraction, X-ray diffraction, and cross-sectional scanning transmission electron microscopy are performed to characterize the crystalline structure of the films. The CoO films are found to be crystalline as-deposited even at the low growth temperature with no evidence of Co diffusion into Si. The STO-buffered Si (001) is used as a template for ALD growth of relatively thicker epitaxial STO and TiO2 films. Epitaxial and polycrystalline CoO films are then grown by ALD on the STO and TiO2 layers, respectively, creating thin-film heterostructures for photoelectrochemical testing. Both types of heterostructures, CoO/STO/Si and CoO/TiO2/STO/Si, demonstrate water photooxidation activity under visible light illumination. In-situ X-ray photoelectron spectroscopy is used to measure the band alignment of the two heterojunctions, CoO/STO and CoO/TiO2. The experimental band alignment is compared to electronic structure calculations using density functional theory.

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.

Charge transfer in Sr Zintl template on Si(001)

The formation of the half monolayer (ML) Sr Zintl template layer on Si(001) is investigated in a combined experimental and theoretical work consisting of in situ reflection high energy electron diffraction, in situ x-ray photoelectron spectroscopy (XPS), and density functional theory. Starting with clean 2 × 1 reconstructed Si(001), we demonstrate that Sr deposition leads to a charge transfer from the metal to the Si substrate resulting in the disappearance of the asymmetry of Si dimers—an essential structural change that enables direct perovskite epitaxy on Si, and likely, other semiconductors. XPS reveals an unexpected shift to higher binding energy of the Si 2p core-level components, including the bulk. This unusual behavior is attributed to final state effects using first principles calculations. As measured by ultraviolet photoelectron spectroscopy, the deposition of 0.5 ML of Sr lowers the work function of the system by 1.35 eV, and is in good agreement with our theoretical calculations.

Using Zintl-Klemm intermetallics in oxide-semiconductor heteroepitaxy

We propose using the Zintl-Klemm (Z-K) bonding to engineer transition layers that provide wetting between ionic oxides and covalent semiconductors to ensure two-dimensional epitaxial growth. Using density functional theory to test this concept, we consider the thermodynamics of wetting at the GaAs/SrTiO3 interface, and identify Sr aluminide SrAl2 as the Z-K wetting layer. We discuss the atomic structure and bonding at the interface, and estimate the conduction band discontinuity to be 0.6 eV, in good agreement with recent experiment.

Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies.

Spin defects in wide-band gap semiconductors are promising systems for the realization of quantum bits, or qubits, in solid-state environments. To date, defect qubits have only been realized in materials with strong covalent bonds. Here, we introduce a strain-driven scheme to rationally design defect spins in functional ionic crystals, which may operate as potential qubits. In particular, using a combination of state-of-the-art ab-initio calculations based on hybrid density functional and many-body perturbation theory, we predicted that the negatively charged nitrogen vacancy center in piezoelectric aluminum nitride exhibits spin-triplet ground states under realistic uni- and bi-axial strain conditions; such states may be harnessed for the realization of qubits. The strain-driven strategy adopted here can be readily extended to a wide range of point defects in other wide-band gap semiconductors, paving the way to controlling the spin properties of defects in ionic systems for potential spintronic technologies.

First-principles study of the growth thermodynamics of Pt on SrTiO3 (001)

Using density functional theory, we investigate the growth mode of Pt (001) on SrTiO3 (001) (STO) and explore the thermodynamic wetting conditions at this interface. The authors calculate the surface energy of Pt (001) to be 2.45 J/m2 and that of TiO2-terminated STO (001) to range from 1.30–2.06 J/m2, depending on the chemical environment. The calculated interface energy is 0.37 J/m2 higher than that of the STO (001) surface across the entire thermodynamically allowed range, suggesting that Pt (001) would grow on the STO (001) surface as Volmer–Weber three-dimensional islands. Using Young’s equation, we calculate the contact angle between a Pt (001) island and STO (001) to be between 98.7° and 100.6°.Using density functional theory, we investigate the growth mode of Pt (001) on SrTiO3 (001) (STO) and explore the thermodynamic wetting conditions at this interface. The authors calculate the surface energy of Pt (001) to be 2.45 J/m2 and that of TiO2-terminated STO (001) to range from 1.30–2.06 J/m2, depending on the chemical environment. The calculated interface energy is 0.37 J/m2 higher than that of the STO (001) surface across the entire thermodynamically allowed range, suggesting that Pt (001) would grow on the STO (001) surface as Volmer–Weber three-dimensional islands. Using Young’s equation, we calculate the contact angle between a Pt (001) island and STO (001) to be between 98.7° and 100.6°.

InP-Based Quantum-Dot Infrared Photodetectors With High Quantum Efficiency and High-Temperature Imaging

We report a room temperature operating InAs quantum-dot infrared photodetector grown on InP substrate. The self-assembled InAs quantum dots and the device structure were grown by low-pressure metalorganic chemical vapor depositon. The detectivity was 6 times 1010 cm Hz1/2/W at 150 K and a bias of 5 V with a peak detection wavelength around 4.0 mum and a quantum efficiency of 48%. Due to the low dark current and high responsivity, a clear photoresponse has been observed at room temperature. A 320 times 256 middle wavelength infrared focal plane array operating at temperatures up to 200 K was also demonstrated. The focal plane array had 34 mA/W responsivity, 1.1% conversion efficiency, and noise equivalent temperature difference of 344 mK at 120 K operating temperature.

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.