O-terminated Interface Research Articles

Interfacial atomic structures, energetics and band offsets of Ge:ZrO2 interfaces

Future field effect transistors may use germanium as a high mobility channel material instead of silicon, and high dielectric constant (κ) oxides such as ZrO2 and HfO2 instead of SiO2 as the gate dielectric. First principles calculations of the polar (100) and nonpolar (110) Ge:ZrO2 interfaces are presented. A number of interface configurations that satisfy the valence bonding requirements are constructed and their relaxed structures, total energies, interface electronic states, and band offsets are calculated. For the polar (100) interfaces, the results are quite similar to those of (100) Si:ZrO2. There are numerous semiconducting O-terminated interfaces. The most stable O-terminated interface for a 1×1 surface unit cell has three coordinated oxygen sites. The interface with a tenfold coordinated Zr is the most stable metal-terminated interface, but it is metallic which makes this interface not useful for devices. The band offsets of the O-terminated interfaces have a relatively narrow range, whereas the Zr-terminated band offsets exhibit a range of 0.9eV.

First-principles study on the adhesion nature of the α-Al2O3(0001)/Ni(111) interface

First-principles calculations of the α-Al2O3(0001)/Ni(111) interface have been performed, and are compared with our previous results of the α-Al2O3(0001)/Cu(111) interface (Yang et al 2004 Phil. Mag. Lett. 84 425, Yang et al 2005 Phil. Mag. 85 2961). Interface models where the Ni atoms are located on top of the interface O atoms have been examined. For the O-terminated interface, the adhesion is caused by the strong O-2p/Ni-3d orbital hybridization and ionic interactions. For the Al-terminated interface, the adhesion originates from the image-like electrostatic and Ni–Al hybridization interactions. Charge transfer occurs from Al2O3 to Ni, which is opposite to that in the O-terminated interface. The degree of orbital hybridization and adhesive energies are more significant for the Al2O3/Ni interfaces than for the Al2O3/Cu interfaces, because of more reactivity of Ni.

Ab initiostudy ofAg∕Al2O3andAu∕Al2O3interfaces

Structural stability, adhesion, and chemical bonding of the $\mathrm{Ag}(111)∕\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) and $\mathrm{Au}(111)∕\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) interfaces are investigated by an ab initio approach based on density functional theory. The interfaces are shown to have different stable structures of ${\mathrm{Al}}_{2}$, Al, or O termination depending on the chemical potential of aluminum or oxygen atom. A link to thermodynamic factors, i.e., the partial pressure of oxygen gas or the activity of aluminum, is established based on the ab initio thermodynamics developed recently. For condition applicable to sessile drop experiments, the O-terminated interface could exist for the $\mathrm{Ag}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ system but be hard to observe for the $\mathrm{Au}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interfaces, consistent with the known experiments. The ${\mathrm{Al}}_{2}$ termination is possible for the $\mathrm{Au}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interface at relatively low ${\mathrm{O}}_{2}$ pressure or high Al activity but may be hard to form for the $\mathrm{Ag}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interface. Works of adhesion ${W}_{\mathrm{ad}}$ of the stoichiometric interfaces are calculated to be $0.33\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ in generalized gradient approximation (GGA) and $0.59\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ in local density approximation (LDA) for the $\mathrm{Ag}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interface, $0.29\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ in GGA and $0.58\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ in LDA for the $\mathrm{Au}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interface, in reasonable agreement with measured data.

First-principles study on the tensile strength and fracture of the Al-terminated stoichiometric α-Al2O3(0001)/Cu(111) interface

The first-principles tensile tests have been applied to the Al-terminated stoichiometric α-Al2O3(0001)/Cu(111) interface by using the ab initio pseudo-potential method based on the density-functional theory. Firstly, the Cu/Al and Cu/Cu interlayers have been examined by the rigid-type tensile test. The interlayer potential curves derived from the first-principles calculations are well fitted by the universal binding-energy relation (UBER) curves. The Cu–Al adhesion is weaker than the back Cu–Cu adhesion. Secondly, the relaxed-type tensile test has revealed the tensile strength and features of interfacial fracture. The ideal tensile strength is about 10 GPa, and the local Young's modulus is about 40 GPa, which means that the Cu/Al interface is quite weak and soft compared with the bulk regions and the O-terminated interface. The failure is initiated from the charge depletion region near the interfacial O atoms when the interlayer stretching exceeds about 30%, and the behaviour of electrons and ions indicates no strong Cu–O bond compared with substantial Cu–Al interactions. The present ab initio data are useful for the construction of effective interatomic potentials at the interface.

アルミナ／銅界面の準連続体解析

The atomistic scale tensile test of mixed O-terminated and Al-terminated α-Al2O3(0001)/Cu(111) interface has been examined by quasicontinuum method. The quasicontinuum method has been developed for large-scale atomistic simulations. In this approach, the number of degrees of freedom is much reduced in comparison to molecular dynamics simulation, so that the computational time is significantly decreased. In the Al2O3/Cu interface, the O-terminated interface has quite large adhesive energy, and the Cu-O bond is much stronger than the Cu-Cu bond. The Al-terminated interface has quite a small adhesive energy, and the Cu-Al bond is much smaller than the Cu-Cu bond. In the tensile test, adhesive energy of mixed interface of O-terminated and Al-terminated is the mean value of that of O-terminated interface and Al-terminated interface.

Fermi level pinning and Hf–Si bonds at HfO2: Polycrystalline silicon gate electrode interfaces

Doped polycrystalline Si (poly-Si) gate electrodes on HfO2 films on Si substrates are found not to cause as large shifts in the flat band voltage as those of SiO2 on Si. This effect has been attributed to a weak pinning of the Fermi level at the top poly-Si HfO2 interface. The effect is shown to be consistent with the formation of Hf–Si bonds at an otherwise O-terminated oxide interface. Vacancies, divacancies, and substitutional Si atoms are introduced into models of oxygen-terminated Si–HfO2 (100) interfaces and the resulting Hf–Si bonds are found to create a metallic interface with the Fermi level pinned at about 0.3 eV below the Si conduction band-edge.

HRTEM and EELS characterization of atomic and electronic structures in Cu/α-Al 2O 3 interfaces

Interfacial atomic structures of Cu/Al 2O 3(0 0 0 1) and Cu/Al 2O 3( 1 1 2 ¯ 0 ) prepared by the pulsed-laser deposition technique were characterized by high-resolution transmission electron microscopy (HRTEM). It was found that (1 1 1) and (0 0 1) planes of Cu were epitaxially oriented to Al 2O 3(0 0 0 1) and Al 2O 3( 1 1 2 ¯ 0 ) planes, respectively. Chemical bonding states at the interfaces were analysed by electron energy-loss spectroscopy (EELS). In oxygen–K edge energy-loss near-edge structure (O–K ELNES) of the Cu/Al 2O 3(0 0 0 1) and Cu/Al 2O 3( 1 1 2 ¯ 0 ) interfaces, a shoulder peak appeared at the lower energy side of the main peak. This indicates that Cu–O interactions were formed across these Cu/Al 2O 3 interfaces. In fact, the simulated HRTEM images based on the O-terminated interface models agreed well with the experimental ones. It can be concluded that the O-terminated interfaces were formed in the present Cu/Al 2O 3 interfaces.

Chemical bonding at the Al-terminated stoichiometric α-Al2O3(0001)/Cu(111) interface

The adhesion of an Al-terminated stoichiometric Al2O3(0001)/Cu(111) interface has been examined using a first-principles calculation based on density-functional theory, and compared with an O-terminated interface and other oxide/Cu interfaces. The interface adhesion originates from the electrostatic interactions and hybridization interactions between Cu and Al2O3 surface dangling orbitals, with the latter appearing to be substantial. Charge transfer occurs from Al2O3 to Cu, which is opposite to that in the O-terminated interface and larger than that in the MgO/Cu interface.

First-Principles Characterization of Atomic Structure of Al<SUB>2</SUB>O<SUB>3</SUB>(0001)/Cu Nano-Hetero Interface

Atomic structures characterization of Al 2 O 3 (0001)/Cu nano-hetero interfaces has been performed by the first-principles pseudopotential method and in cooperation with HRTEM observations. The physical properties of the interfaces depend strongly on the interface stoichiometry. Bonding nature of the O-rich (O-terminated) interface is explained as strong covalent and ionic interactions, whereas that of the stoichiometric (Al-terminated) interface is weak covalent and electrostatic image interactions. The O-terminated interface has quite larger adhesive energy than that of the stoichiometric one. Recent HRTEM observations of the Al 2 O 3 (0001)/Cu interface have confirmed the O-terminated interlace. However, the observed incoherent interface is not the same as an ideal coherent interface obtained by the first-principles. We explain the relationship between the present coherent interface and the practical incoherent one.

Study of electronic structure in Co/Al 2O 3/Co heterojunctions from first principles

The electronic structure and magnetic properties of Co/Al 2O 3/Co with O-terminated and Al-terminated interface models of different thicknesses are investigated by first-principles discrete variational method with the local-spin-density approximation. Our calculations results show that the magnetic moment of interface Co layer is enhanced for Al-terminated and weakened for O-terminated interface compared with that of bulk Co. For O-terminated interface models, spin polarization at Fermi energy of Co layer at interface exhibits negative and becomes positive for O layer at interface. In contrast, both Co and Al layers at interface possess negative SP for the Al-terminated interface models. We have also found that TMR ratio of Al-terminated interface models is much larger than that of O-terminated interface. In addition, the change of SP with the thickness of insulating layer is in a similar way as that of magnetic moment.

First-Principles Study of Mechanical Properties of Alumina-Copper Nano-Coating Interfaces

ABSTRACTMechanical properties of alumina-copper nano-coating interfaces have been studied by the first-principles calculations. We have applied the rigid-type first-principles tensile tests for O-rich (O-terminated) and stoichiometric (Al-terminated) interfaces and for back Cu1st-Cu2nd interlayers. The O-terminated interface is twice as strong as the Cu1st-Cu2nd interlayer whereas the Al-terminated interface is twice as weak as the Cu1st-Cu2nd interlayer. We have performed the fitting of interlayer potentials by universal binding-energy relation (UBER). The interlayer potential of the Cu-Al interface is well reproduced by UBER in whole region, although those of Cu-O interface and Cu1st-Cu2nd interlayer are partially reproduced.

Atomic and Electronic Structures of Ni/YSZ(111) Interface

Thin Ni films were deposited on the (111) surface of YSZ at 1073 K by a pulsed laser deposition technique. The interfacial atomic structure of Ni/YSZ was investigated by high-resolution transmission electron microscopy (HRTEM). It was found that Ni was epitaxially oriented to the YSZ surface, and the following orientation relationship (OR) was observed: (111) N ι ∥ (111) Y S Z , [110] N i ∥ [110] Y S Z . Geometrical coherency of the Ni/YSZ system was also evaluated by the coincidence of reciprocal lattice points (CRLP) method. It was found that the most coherent OR predicted by CRLP method was (705) N i ∥ (111) Y S Z , [010] N i ∥ [110] Y S Z , which was not consistent with the experimentally observed OR. To understand the detailed atomic structure, HRTEM image simulations were performed. However, simulated images based on both O-terminated and Zr-terminated interface models were quite similar to the experimental image, and thus it was hard to determine which model is comparable with the actual interface only by the HRTEM image simulations. In order to clarify the termination layer at the interface, electronic structures of the Ni/YSZ interface were investigated by electron energy-loss spectroscopy. It was found that significant differences were observed in O-K edge spectra between the interface and the YSZ crystal interior, and the spectrum from the interface showed similar features to the reference spectrum of bulk NiO. This indicates that the Ni-O interaction occurs at the interface to terminate the oxygen {111} plane of YSZ at the Ni/YSZ interface. In addition, the density of Ni-O bonds across the interface in the experimental OR was larger than that in the most coherent OR predicted by CRLP method, which also suggests that the on-top Ni-O bonds stabilize the Ni/YSZ(111) interface.

Atomic and electronic structures of Cu/a-Al2O3 interfaces prepared by pulsed-laser deposition

Interfacial atomic structures of Cu/Al2O3(0001) and Cu/Al2O3 systems prepared by a pulsed-laser deposition technique have been characterized by using high-resolution transmission electron microscopy (HRTEM). It was found that Cu metals were epitaxially oriented to the surface of Al2O3 substrates, and the following orientation relationships (ORs) were found to be formed: (111)Cu⁄⁄(0001)Al2O3, cu⁄⁄Al2O3 in the Cu/Al2O3(0001) interface and (001)Cu⁄⁄Al2O3, Cu⁄⁄[0001]Al2O3 in the Cu/Al2O3 interface. Geometrical coherency of the Cu/Al2O3 system has been evaluated by the coincidence of reciprocal lattice points method, and the result showed that the most coherent ORs were (111)Cu⁄⁄(0001)Al2O3, Cu⁄⁄[11Ī00]Al2O3 and Cu⁄⁄Al2O3, [111]Cu⁄⁄[0001]Al2O3, which are equivalent to each other. These ORs were not consistent with the experimentally observed ORs, and it was possible that crucial factors to determine the ORs between Cu and Al2O3 were not only geometrical coherency, but also other factors such as chemical bonding states. Therefore, to understand the nature of the interface atomic structures, the electronic structures of the Cu/Al2O3 interfaces have been investigated by electron energy-loss spectroscopy. It was found that the pre-edge at the lower energy part of the main peak appeared in the O-K edge spectra at the interface region in both the Cu/Al2O3(0001) and Cu/Al2O3 systems. This indicates the existence of Cu—O interactions at the interface. In fact, HRTEM simulation images based on O-terminated interface models agreed well with the experimental images, indicating that O-terminated interfaces were formed in both systems. Since the overlapped Cu atomic density in the experimental ORs were larger than that in the most coherent OR, it is considered that the on-top Cu—O bonds stabilize the O-terminated Cu/Al2O3 interfaces.

Adhesion, atomic structure, and bonding at theAl(111)/α−Al2O3(0001)interface: A first principles study

We have performed a series of ab initio calculations to determine the atomic structure, ideal work of adhesion $({\mathcal{W}}_{\mathrm{ad}\mathrm{}}),$ and bonding character of the $\mathrm{Al}(111)/\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ interface. Six candidate interface geometries were considered, including Al and O terminations of the oxide. Minimization of the Hellman-Feynman forces resulted in substantial changes to the atomic structure of the metal near the interface, wherein some atoms adopted positions consistent with a continuation of the oxide's Al-sublattice crystal structure across the interface. Consequently, the lowest-energy structures (i.e., having the largest ${\mathcal{W}}_{\mathrm{ad}\mathrm{}})$ are those that facilitate this ``oxide extension'' mechanism. By applying several methods of analysis we have thoroughly characterized the electronic structure and have determined that Al-O bonds constitute the primary interfacial bonding interaction. These bonds are very similar to the cation-anion bonds found in the oxide bulk and are mainly ionic, yet maintain a small amount of covalent character. In addition, there is evidence of metal-cation bonding at the optimal Al-terminated interface. Taking into account recent theoretical and experimental evidence suggesting an Al termination of the clean oxide surface, our calculations predict ${\mathcal{W}}_{\mathrm{ad}\mathrm{}}=1.36{\mathrm{J}/\mathrm{m}}^{2}$ [local density approximation (LDA)] and 1.06 ${\mathrm{J}/\mathrm{m}}^{2}$ [generalized gradient approximation (GGA)] for the optimal Al-terminated structure, which are in good agreement with the experimental value of 1.13 ${\mathrm{J}/\mathrm{m}}^{2}$ as scaled to 0 K. These values are approximately an order of magnitude smaller than what is found for the optimal O-terminated interface: 10.70 ${\mathrm{J}/\mathrm{m}}^{2}$ (LDA) and 9.73 ${\mathrm{J}/\mathrm{m}}^{2}$ (GGA). Although cleavage preferentially occurs at the interface for the Al termination, strong bonding at the O-terminated interface favors cleavage within the metal.

Structural and electronic properties ofCo/Al2O3/Comagnetic tunnel junction from first principles

A detailed first-principles study of the atomic and electronic structure of the $\mathrm{Co}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}/\mathrm{Co}$ magnetic tunnel junction has been performed in order to elucidate the key features determining the spin-dependent tunneling. The atomic structure of the multilayer with the O- and Al-terminated interfaces between fcc Co(111) and crystalline $\ensuremath{\alpha}$-${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ has been optimized using self-consistent spin-polarized calculations within density-functional theory and the generalized gradient approximation. We found that the relaxed atomic structure of the O-terminated interface is characterized by a rippling of the Co interfacial plane, the average Co-O bond length being 2.04 \AA{} which is within 5% of that in bulk CoO. The corresponding electronic structure is influenced by the covalent bonding between the O $2p$ and Co $3d$ orbitals resulting in exchange-split bonding and antibonding states and an induced magnetic moment of 0.07${\ensuremath{\mu}}_{B}$ on the interfacial oxygen atoms. The Al-terminated interface contains Co-Al bonds with an average bond length of 2.49 \AA{} compared to 2.48 \AA{} in bulk CoAl. Due to charge transfer and screening effects the Co interfacial layer acquires a negative charge which results in a reduced magnetic moment of $1.15{\ensuremath{\mu}}_{B}$ per Co atom. We found that the electronic structure of the O-terminated $\mathrm{Co}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}/\mathrm{Co}$ tunnel junction exhibits negative spin polarization at the Fermi energy within the first few monolayers of alumina but it eventually becomes positive for distances beyond 10 \AA{}.

Bonding and Cohesive Properties of Cobalt/Alumina Magnetic Tunnel Junctions

AbstractMagnetic tunnel junctions (MTJs) are promising candidates for applications in spintronic devices such as magnetic random access memories, read heads and sensors. Previous studies have revealed the critical influence of the chemistry and structure of the ferromagnet metal/alumina interface on both sign and magnitude of the tunneling current in MTJs. We have performed a detailed first-principles study of the structure and bonding at the cobalt/alumina interface in order to understand the relationship between magnetic properties, local bonding environment and cohesion at the interface. Both Al- and 0- terminated interfaces were considered and full geometry optimization of the structure was done by selfconsistent spin-polarized calculations within density functional theory and the generalized gradient approximation. We found that the relaxed atomic structure of the 0-terminated interface is characterized by a rippling of the Co interfacial plane, the average Co-O bond length being 2.04 Å which is within 5% of that in bulk CoO and it is characterized by the covalent bonding between the 0 2p and Co 3d orbitals. The Al-terminated interface contains Co-Al bonds with an average bond length of 2.49 Å compared to 2.48 Å in bulk CoAl and it is characterized by metallic behaviour of the first aluminium layer. As a consequence of different character of bonding, the cohesion at the O-terminated interface is stronger as that at the Al-terminated interface, the work of separation being equal to 6.35 J/m2 and 3.64 J/m2 respectively.