K. van Benthem

Bulk electronic structure of SrTiO3: Experiment and theory

Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3. Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states. The experimentally determined indirect band gap energy is 3.25 eV, while the direct band gap energy is 3.75 eV. The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV. The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV. The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 3.25 eV up to 15 eV. The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV. The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV. Interband transitions from the O 2s band to the conduction bands appear at 26 eV. A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons.

The effect of interfacial layer properties on the performance of Hf-based gate stack devices

The influence of Hf-based dielectrics on the underlying SiO2 interfacial layer (IL) in high-k gate stacks is investigated. An increase in the IL dielectric constant, which correlates to an increase of the positive fixed charge density in the IL, is found to depend on the starting, pre-high-k deposition thickness of the IL. Electron energy-loss spectroscopy and electron spin resonance spectra exhibit signatures of the high-k-induced oxygen deficiency in the IL consistent with the electrical data. It is concluded that high temperature processing generates oxygen vacancies in the IL responsible for the observed trend in transistor performance.

Aberration-corrected scanning transmission electron microscopy: from atomic imaging and analysis to solving energy problems.

The new possibilities of aberration-corrected scanning transmission electron microscopy (STEM) extend far beyond the factor of 2 or more in lateral resolution that was the original motivation. The smaller probe also gives enhanced single atom sensitivity, both for imaging and for spectroscopy, enabling light elements to be detected in a Z-contrast image and giving much improved phase contrast imaging using the bright field detector with pixel-by-pixel correlation with the Z-contrast image. Furthermore, the increased probe-forming aperture brings significant depth sensitivity and the possibility of optical sectioning to extract information in three dimensions. This paper reviews these recent advances with reference to several applications of relevance to energy, the origin of the low-temperature catalytic activity of nanophase Au, the nucleation and growth of semiconducting nanowires, and the origin of the eight orders of magnitude increased ionic conductivity in oxide superlattices. Possible future directions of aberration-corrected STEM for solving energy problems are outlined.

Valence electron energy loss study of Fe-doped SrTiO3 and a Σ13 boundary: electronic structure and dispersion forces

Valence electron energy loss spectroscopy in a dedicated scanning transmission electron microscope has been used to obtain the interband transition strength of a sigma13 tilt grain boundary in SrTiO3. In a first step the electronic structure of bulk SrTiO3 has been analysed quantitatively by comparing VEELS spectra with vacuum ultraviolet spectra and with ab initio density of states calculations. The electronic structure of a near sigma13 grain boundary and the corresponding dispersion forces were then determined by spatially resolved VEELS. Also the effects of delocalization of the inelastic scattering processes were estimated and compared with results from the literature.

Progress in the preparation of cross-sectional TEM specimens by ion-beam thinning

Abstract In transmission electron microscopy (TEM), often the preparation of samples is the most critical part. Specimens have to have disk geometries of 3 mm diameter laterally, and they have to be transparent for the electron beam vertically. Therefore, a specimen thickness in the range of some 1–10 nm has to be achieved by the preparation process. While shrinking the specimen dimensions, care has to be taken to recover the materials properties in the nm-regime. We report and shortly discuss some TEM specimen preparation techniques mainly used in the Stuttgart TEM specimen preparation laboratory. Furthermore, we demonstrate how more advanced techniques lead to a more reliable preparation of weakly-bonded metal/SrTiO3 interfaces. In addition, the advantage of low-voltage ion-milling is demonstrated by a case study for bulk SrTiO3. As a result, low-voltage ion polishing as a final step in the TEM specimen preparation by conventional ion-thinning turns out to significantly increase the specimen quality. In...

Si/SiO2 and SiC/SiO2 Interfaces for MOSFETs – Challenges and Advances

Silicon has been the semiconductor of choice for microelectronics largely because of the unique properties of its native oxide (SiO2) and the Si/SiO2 interface. For high-temperature and/or high-power applications, however, one needs a semiconductor with a wider energy gap and higher thermal conductivity. Silicon carbide has the right properties and the same native oxide as Si. However, in the late 1990’s it was found that the SiC/SiO2 interface had high interface trap densities, resulting in poor electron mobilities. Annealing in hydrogen, which is key to the quality of Si/SiO2 interfaces, proved ineffective. This paper presents a synthesis of theoretical and experimental work by the authors in the last six years and parallel work in the literature. High-quality SiC/SiO2 interfaces were achieved by annealing in NO gas and monatomic H. The key elements that lead to highquality Si/SiO2 interfaces and low-quality SiC/SiO2 interfaces are identified and the role of N and H treatments is described. More specifically, optimal Si and SiC surfaces for oxidation are identified and the atomic-scale processes of oxidation and resulting interface defects are described. In the case of SiC, we conclude that excess carbon at the SiC/SiO2 interface leads to a bonded Si-C-O interlayer with a mix of fourfold- and threefold-coordinated C and Si atoms. The threefold coordinated atoms are responsible for the high interface trap density and can be eliminated either by H-passivation or replacement by N. Residual Si-Si bonds, which are partially passivated by H and N remain the main limitation. Perspectives for the future for both Si- and SiC-based MOSFETs are discussed.

Electronic structure investigations of Ni and Cr films on (100)SrTiO3 substrates using electron energy-loss spectroscopy

Abstract Metal/ceramic interfaces betweef thin nickel and chromium films and the TiO2 terminated (100) surface of SrTiO3 were studaed by various transmission electron microscopy techniques to investigate the bonding behaviour between film and substrate. For the Ni/SrTiO3 interface, a semi-coherent interface structure betweef three-dimensionally grown Ni islands and the SrTiO3 substrate was observed. Froe electron energy-loss spectroscopy measurements and high-resolution transmission electron microscopy studies, the formation of a two-dimensional NiO layer at the interface was concluded, which dominates the adhesion between the Ni film and the ceramic substrate. At the Cr/SrTiO3 interface, Cr–O bonds were found, possessing an ionic-bonding contribution. Comparing the bonding characteristics of Ni and Cr on the (100) surface of SrTiO3, a stronger adhesion of Cr than for Ni is proposed.

Core-hole effects on the ELNES of absorption edges in SrTiO3

Near-edge structures of absorption edges in electron energy-loss spectra (ELNES) of SrTiO(3) were calculated and compared to experimental inelastic electron scattering data. The goal of this study was to investigate final-state effects on the electronic structure. Two theoretical approaches were applied: density-functional theory with a band-structure supercell method and a real-space multiple-scattering cluster approach. Within both techniques, the Z+1 approximation was used to model the core hole generated by the inelastic scattering process. For the band-structure calculations, supercells of (SrTiO(3))(n)(n=1,4,8,16) composition with three-dimensional periodic boundary conditions were applied. The influence of supercell size and shape on calculated site- and symmetry-projected local densities of unoccupied states is assessed quantitatively. Relevant convergence criteria are the length scale set by the spatial extension of the valence-electron screening cloud around the core hole, and the interaction energy of neighbouring core hole centres. For a sufficiently large supercell size, the Z+1 approximation yields a reasonable description of the local densities of unoccupied states probed by the energy losses of inelastically scattered electrons of the Ti L(3)-, O K- and Sr L(3)-absorption edges. The quantitative equivalence of ELNES information extracted from the multiple-scattering cluster calculations and the band-structure supercell calculations is demonstrated. Discrepancies between theoretical and experimental results are discussed.

Advances in EELS spectroscopy by using new detector and new specimen preparation technologies

First results obtained with a Gatan UHV Enfina system, which was attached to a VG HB 501 UX dedicated STEM, are reported. The Enfina system is based on a CCD detector and offers, compared to the previously used photodiode array, a narrower point‐spread function, higher sensitivity, and faster read‐out capabilities. These improvements are demonstrated with electron energy‐loss measurements on various oxides, such as Al2O3, TiO2 and SrTiO3. It is shown that a better energy resolution is achieved and that acquisition of high‐energy absorption edges with a reasonable signal‐to‐noise ratio becomes possible. Furthermore, we report on the influence of the TEM specimen quality on the energy‐loss spectra. Thin amorphous layers at the specimen surfaces, which are induced by ion‐milling processes, can modify specific electron energy‐loss near‐edge structure features. We found that for the investigated ceramics the use of low‐energy ion‐milling systems is highly recommended, since the loss of energy‐loss near‐edge structure details by the presence of the amorphous layers is considerably reduced. This is especially true for very thin specimens.

Increased thermal conductivity polycrystalline diamond for low-dissipation micromechanical resonators

This paper reports an investigation of microcrystalline diamond (MCD) films deposited under different conditions to increase thermal conductivity and therefore mechanical quality factor (Q) in micromechanical resonators. Through a study of different deposition conditions, we demonstrate a three-fold increase in thermal conductivity and quality factor. Quality factor measurements were conducted on double ended tuning fork resonators, showing Q = 241,047 at fn = 246.86 kHz after annealing, the highest Q reported for polycrystalline diamond resonators. We further present a study of the unique microstructure of hot filament chemical vapor deposition (HFCVD) diamond films and relate growth conditions to observed microstructural defects.