Alexander A. Demkov

A silicon-based photocathode for water reduction with an epitaxial SrTiO3 protection layer and a nanostructured catalyst

The rapidly increasing global demand for energy combined with the environmental impact of fossil fuels has spurred the search for alternative sources of clean energy. One promising approach is to convert solar energy into hydrogen fuel using photoelectrochemical cells. However, the semiconducting photoelectrodes used in these cells typically have low efficiencies and/or stabilities. Here we show that a silicon-based photocathode with a capping epitaxial oxide layer can provide efficient and stable hydrogen production from water. In particular, a thin epitaxial layer of strontium titanate (SrTiO3) was grown directly on Si(001) by molecular beam epitaxy. Photogenerated electrons can be transported easily through this layer because of the conduction-band alignment and lattice match between single-crystalline SrTiO3 and silicon. The approach was used to create a metal-insulator-semiconductor photocathode that, under a broad-spectrum illumination at 100 mW cm(-2), exhibits a maximum photocurrent density of 35 mA cm(-2) and an open circuit potential of 450 mV; there was no observable decrease in performance after 35 hours of operation in 0.5 M H2SO4. The performance of the photocathode was also found to be highly dependent on the size and spacing of the structured metal catalyst. Therefore, mesh-like Ti/Pt nanostructured catalysts were created using a nanosphere lithography lift-off process and an applied-bias photon-to-current efficiency of 4.9% was achieved.

Fermi level pinning by defects in HfO2-metal gate stacks

Various mechanisms for the Fermi level pinning of p-gate metals on HfO2 are analyzed in detail. It is found that for Fermi energies below the Si valence band, HfO2 can oxidize Si by creating positively charged O vacancies. The band bending due to this vacancy concentration can account for the observed Fermi level pinning of p metals on HfO2.

Two-dimensional growth of high-quality strontium titanate thin films on Si

Most semiconductor materials such as Si, Ge, and GaAs are subject to oxidation when exposed to oxidants. This results in difficulties in the heterointegration of epitaxial oxides on these semiconductors. Even though certain oxides may be thermodynamically stable when placed in contact with semiconductors, direct epitaxy of these oxides encounters kinetic difficulties due to the loss of epitaxy caused by the formation of an amorphous oxide at the interface. In this article, we address some important issues on the heteroepitaxy of oxides on semiconductors and show a stepped growth method that utilizes the kinetic characteristics of the growth process to suppress the oxidation of the substrate surface and thereby achieve oxide films with a high degree of crystallinity. The epitaxy of high-quality SrTiO3 (STO) thin films directly on Si was achieved. The chemical and structural properties of the STO/Si interface were evaluated in situ using reflection high-energy electron diffraction, x-ray photoelectron spect...

Materials fundamentals of gate dielectrics

Preface. 1: Materials and Physical Properties of High-K Oxide Films Ran Liu. 2: Device Principles of High-K Dielectrics Kurt Eisenbeiser. 3: Thermodynamics of Oxide Systems Relevant to Alternative Gate Dielectrics Alexandra Navrotsky and Sergey V. Ushakov. 4: Electronic Structure and Chemical Bonding in High-K Transition Metal and Lanthanide Series Rare Earth Alternative Gate Dielectrics: Applications to Direct Tunneling and Defects at Dielectric Interfaces Gerald Lucovsky. 5: Atomic Structure, Interfaces and Defects of High Dielectric Constant Gate Oxides J. Robertson and P.W. Peacock. 6: Dielectric Properties of Simple and Complex Oxides from First-Principles U.V. Waghmare and K.M. Rabe. 7: IVb Transition Metal Oxides and Silicates: An Ab Initio Study Gian-Marco Rignanese. 8: The Interface Phase and Dielectric Physics for Crystalline Oxides on Semiconductors Rodney Mckee. 9: Interfacial Properties of Epitaxial Oxide/Semiconductor Systems Y. Liang and A.A. Demkov. 10: Functional Structures Matt Copel. 11: Mechanistic Studies of Dielectric Growth on Silicon Martin M. Frank and Yves J. Chabal. 12: Methodology for Development of High-k Stacked Gate Dielectrics on III-V Semiconductors Matthias Passlack. Index

The interface of epitaxial SrTiO3 on silicon: in situ and ex situ studies

The formation of interfacial layers between silicon and the overgrown epitaxial SrTiO3 as a function of the growth temperature has been studied in detail using x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. Models for the chemical compositions and atomic bonding states have been established. With a good understanding of the mechanisms of the interfacial layer formations, the molecular-beam epitaxy growth process can be well controlled to form high-quality, single-crystalline oxide films, as well as a desired interface between the grown oxide and silicon substrate. The epitaxial relationship has been found to be SrTiO3 (001)∥Si(001), and SrTiO3 〈100〉∥Si〈110〉.

Challenges for the integration of metal gate electrodes

Integration challenges for metal gate electrodes including the presence of Fermi level pinning and the impact of interface chemistry on the effective metal work function are discussed. Gate stack thermal instabilities are explored, and for the first time results using tantalum-carbon based electrodes are presented.

Investigating Alternative Gate Dielectrics: A Theoretical Approach

We describe several applications of first-principles computational methods based on density functional theory (DFT) to the study of potential gate dielectric materials. First we investigate the stability of binary alkaline earth oxides in contact with Si and SiO 2 . In particular, we consider the case of SrO, which is important for the epitaxial growth of the SrTiO 3 perovskite structure on the Si (001) surface. Then we discuss the energetics of the SrTiO 3 (001) surface. We conclude with a brief discussion of the structure and electronic properties of crystalline ZrO 2 and HfO 2 .

Optical properties of bulk and thin-film SrTiO3 on Si and Pt

We have studied the optical properties (complex dielectric function) of bulk SrTiO3 and thin films on Si and Pt using spectroscopic ellipsometry over a very broad spectral range, starting at 0.03 eV [using Fourier transform infrared (FTIR) ellipsometry] to 8.7 eV. In the bulk crystals, we analyze the interband transitions in the spectra to determine the critical-point parameters. To interpret these transitions, we performed band structure calculations based on ab initio pseudopotentials within the local-density approximation. The dielectric function was also calculated within this framework and compared with our ellipsometry data. In the FTIR ellipsometry data, we notice a strong lattice absorption peak due to oxygen-related vibrations. Two longitudinal optic (LO) phonons were also identified. In SrTiO3 films on Si, the refractive index below the band gap decreases with decreasing thickness because of the increasing influence of the amorphous interfacial layer between the SrTiO3 film and the Si substrate....

Mechanism of cleaning Si(100) surface using Sr or SrO for the growth of crystalline SrTiO3 films

A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800 °C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(or SrO)→SiO(g)+O+Sr(or SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and passivating the newly exposed Si surface, preventing further etching and the formation of pits in the substrate.A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800 °C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(or SrO)→SiO(g)+O+Sr(or SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppresse...

Magnetoelectric coupling and electric control of magnetization in ferromagnet/ferroelectric/normal-metal superlattices

Ferromagnet/ferroelectric/normal-metal superlattices are proposed to realize the large room-temperature magnetoelectric effect. Spin-dependent electron screening is the fundamental mechanism at the microscopic level. We also predict an electric control of magnetization in this structure. The naturally broken inversion symmetry in our tricomponent structure introduces a magnetoelectric coupling energy of PM(2). Such a magnetoelectric coupling effect is general in ferromagnet/ferroelectric heterostructures, independent of particular chemical or physical bonding, and will play an important role in the field of multiferroics.