Control of TiN oxidation upon atomic layer deposition of oxides.

The interfaces of a physical-vapor deposited (PVD)-TiN electrode with atomic-layer deposited (ALD) HfO2 layers were studied using photoelectron spectroscopy with high kinetic energies of photoelectrons enabling nondestructive in-depth chemical profiling and phase analysis. Our results reveal the presence of only TiNxOy at the TiN/ALD-HfO2 interface with no measurable traces of the TiO2 phase. By contrast, the interface formed by ALD of HfO2 on top of PVD TiN contains both TiO2 and TiNxOy compounds and may be compared to an HfO2/TiN interface with intentional ALD TiO2 interlayer (IL) formation prior to HfO2 growth. Pre-growth of ALD Al2O3 IL drastically reduces the TiO2 and TiNxOy amounts present at the HfO2/TiN interface, which can be ascribed to oxygen scavenging from the initially oxidized TiN surface by energetically more favorable Al-O bonds. The present study demonstrates that the amount of TiO2 phase can be effectively controlled, i.e., increased or decreased, during the ALD process enabling engineering of vacancy-mediated processes.

Redistribution of valence and conduction band states depending on the method of modification of SiO2 structure.

The effect of introducing porosity and the insertion of methyl groups in SiO4 tetrahedra on the distribution of valence and conduction band states of SiO2 was studied using high-resolution near edge X-ray absorption fine structure spectroscopy (NEXAFS) and soft X-ray photoelectron spectroscopy (XPS). Alignment of NEXAFS spectra and valence band XPS spectra in a common energy scale was performed using binding energies of the initial levels obtained by XPS. It was established that the insertion of methyl groups into SiO4 tetrahedra leads to a significant shift of the top of the valence band EV to smaller binding energies due to the reduction of the electronegativity of the nearest surrounding neighbors of the Si atoms, while introducing porosity changes the position of EV only slightly. The position of the bottom of the conduction band is affected by neither the introduction of porosity nor the insertion of methyl groups. The formation of the π*C[double bond, length as m-dash]C state near the Fermi level, caused by the presence of porogen residues in the structure of organosilicate glass (OSG) and responsible for the leakage currents, was revealed. It was found that high level porosity in OSG films induces a significant variation of Si-O-Si valence angles. A number of Si-O dangling bonds were found in the surface layers of por-SiO2, while methyl groups effectively passivated these dangling bonds in OSG films. The obtained results are important for understanding the regularities of electronic structure formation in SiO2-based low-k dielectrics, which is necessary for the reduction of energy dissipated in semiconductor integrated circuits (ICs).

Re-distribution of oxygen at the interface between γ-Al2O3 and TiN.

Interface of TiN electrode with γ-Al2O3 layers was studied using near edge X-ray absorption fine structure, conventional X-ray photoelectron spectroscopy and photoelectron spectroscopy with high energies. Despite the atomic-layer deposited Al2O3 being converted into thermodynamically-stable polycrystalline cubic γ-phase by high-temperature (1000 or 1100 °C) anneal, our results reveal formation of a thin TiNxOy (≈1-nm thick) interlayer at the interface between γ-Al2O3 film and TiN electrode due to oxygen scavenging from γ-Al2O3 film. Formation of the TiO2 was not observed at this interface. As environmental effect, a strong oxidation resulting in formation of a TiO2(1.4 nm)/TiNxOy(0.9 nm) overlayers on the top of the TiN electrode is traced. Development of O-deficiency of γ-Al2O3 is observed and related to the polarization anisotropy due to the preferential orientation of spin states involved in the X-ray absorption in the plane parallel to the surface. Investigation of the TiN electrode reveals the predominantly "stretched" octahedra in its structure with the preferential orientation relative the interface with γ-Al2O3. This anisotropy can be correlated with ≈200 meV electron barrier height increase at the O-deficient TiN/γ-Al2O3 interface as compared to the TiN/γ-Al2O3 barrier formed under abundant oxidant supply condition as revealed by internal photoemission of electrons from TiN into the oxide.

Local electronic structure and nanolevel hierarchical organization of bone tissue: theory and NEXAFS study.

Theoretical and experimental investigations of native bone are carried out to understand relationships between its hierarchical organization and local electronic and atomic structure of the mineralized phase. The 3D superlattice model of a coplanar assembly of the hydroxyapatite (HAP) nanocrystallites separated by the hydrated nanolayers is introduced to account the interplay of short-, long- and super-range order parameters in bone tissue. The model is applied to (i) predict and rationalize the HAP-to-bone spectral changes in the electronic structure and (ii) describe the mechanisms ensuring the link of the hierarchical organization with the electronic structure of the mineralized phase in bone. To check the predictions the near-edge x-ray absorption fine structure (NEXAFS) at the Ca 2p, P 2p and O 1s thresholds is measured for native bone and compared with NEXAFS for reference compounds. The NEXAFS analysis has demonstrated the essential hierarchy induced HAP-to-bone red shifts of the Ca and P 2p-to-valence transitions. The lowest O 1s excitation line at 532.2 eV in bone is assigned with superposition of core transitions in the hydroxide OH-(H2O) m anions, Ca2+(H2O) n cations, the carboxyl groups inside the collagen and [PO4]2- and [PO4]- anions with unsaturated P-O bonds.

Comparative study of the X-ray reflectivity and in-depth profile of a-C, B4C and Ni coatings at 0.1-2 keV.

The use of soft X-rays near the carbon edge of absorption (270-300 eV) greatly enhances studies in various branches of science. However, the choice of reflecting coatings for mirrors operating in free-electron and X-ray free-electron laser (FEL and XFEL) beamlines in this spectral range is not so evident and experimental justifications of the mirror efficiency are rather limited. In the present paper it is demonstrated experimentally that the reflectivity of B4C- and Ni-coated grazing-incidence mirrors is high enough for their operation in FEL or XFEL beamlines near the carbon K-edge of absorption. The minimal reflectivity of both mirrors proves to exceed 80% near the carbon absorption edge at a grazing angle of 0.6°. An in-depth profile of the chemical elements composing the reflecting coatings is reconstructed based on analysis of a set of reflectivity curves measured versus the grazing angle at different photon energies in the soft X-ray spectral region. This allows us to predict correctly the mirror reflectivity at any X-ray energy and any grazing angle.