Sirong Lu

Atomic layer deposition of crystalline SrHfO3 directly on Ge (001) for high-k dielectric applications

The current work explores the crystalline perovskite oxide, strontium hafnate, as a potential high-k gate dielectric for Ge-based transistors. SrHfO3 (SHO) is grown directly on Ge by atomic layer deposition and becomes crystalline with epitaxial registry after post-deposition vacuum annealing at ∼700 °C for 5 min. The 2 × 1 reconstructed, clean Ge (001) surface is a necessary template to achieve crystalline films upon annealing. The SHO films exhibit excellent crystallinity, as shown by x-ray diffraction and transmission electron microscopy. The SHO films have favorable electronic properties for consideration as a high-k gate dielectric on Ge, with satisfactory band offsets (>2 eV), low leakage current (<10−5 A/cm2 at an applied field of 1 MV/cm) at an equivalent oxide thickness of 1 nm, and a reasonable dielectric constant (k ∼ 18). The interface trap density (Dit) is estimated to be as low as ∼2 × 1012 cm−2 eV−1 under the current growth and anneal conditions. Some interfacial reaction is observed betwee...

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 400–800 °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 ∼1013 cm−2 at room temperature for the sample deposited at 700 °C, with a maximum electron Hall mobility of 3100 cm2V−1s−1 at 3.2 K and room temperature mobility of 22 cm2V−1s−1. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator.

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 200–345 °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 300 °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 345 °C and even after the exposure to trimethylaluminum alone at 300 and 345 °C. The interface quasi-2-DEG is metallic and exhibits mobility values of ∼4 and 3000 cm2 V−1 s−1 at room temperature...

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 1 nm 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.

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.

ELNES spectrum unmixing and mapping for oxide/oxide interfaces.

1. School of Engineering for Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287 2. Department of Physics, University of Texas at Austin, Austin, TX 78712 3. Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712 4. LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe AZ 85287 5. Department of Physics, Arizona State University, Tempe, AZ 85287

Integration of ferroelectric BaTiO3 with Ge: The role of a SrTiO3 buffer layer investigated using aberration-corrected STEM

The integration of semiconductors with ferroelectrics having a controlled polarization direction is an ongoing and challenging topic of research. In this work, BaTiO3 (BTO)/SrTiO3 (STO) heterostructures were grown by molecular beam epitaxy either directly with STO substrates or by using 2-nm-thick STO buffer layers on Ge(001) substrates. Sharp, chemically abrupt interfaces and c-axis-oriented BTO films for both types of heterostructures were observed using aberration-corrected scanning transmission electron microscopy and elemental mapping. Anti-phase boundaries as well as ⟨100⟩ misfit dislocations were present in the BTO/STO samples, with the offsets of the dislocation cores varying by distances between 1 and 5 nm away from the BTO/STO interface. Conversely, misfit dislocations were not observed in the BTO/STO/Ge structure although vertical anti-phase boundaries were still common. Overall, the results emphasize the benefits of identifying a suitable buffer layer to ensure the growth of a high quality mat...

ELNES analysis of γ-Al 2 O 3 /SrTiO 3 and LaTiO 3 /SrTiO 3 interfaces

1. School of Engineering for Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287 2. Department of Physics, University of Texas at Austin, Austin, TX 78712 3. Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712 4. Le-Roy Eyring Center for Solid State Science, Arizona State University, Tempe AZ 85287 5. Department of Physics, Arizona State University, Tempe, AZ 85287

Characterization of Two-Dimensional Electron Gas at the y-Al 2 O3/SrTiO 3 Interface

1. School of Engineering for Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, USA 2. Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA 3. Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, USA 4. Le-Roy Eyring Center for Solid State Science, Arizona State University, Tempe 85287-1704, USA 5. Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, USA