A. Ourmazd

Scaling the Si MOSFET: from bulk to SOI to bulk

Scaling the Si MOSFET is reconsidered. Requirements on subthreshold leakage control force conventional scaling to use high doping as the device dimension penetrates into the deep-submicrometer regime, leading to an undesirably large junction capacitance and degraded mobility. By studying the scaling of fully depleted SOI devices, the important concept of controlling horizontal leakage through vertical structures is highlighted. Several structural variations of conventional SOI structures are discussed in terms of a natural length scale to guide the design. The concept of vertical doping engineering can also be realized in bulk Si to obtain good subthreshold characteristics without large junction capacitance or heavy channel doping. >

Time-resolved serial crystallography captures high-resolution intermediates of photoactive yellow protein

Serial femtosecond crystallography using ultrashort pulses from x-ray free electron lasers (XFELs) enables studies of the light-triggered dynamics of biomolecules. We used microcrystals of photoactive yellow protein (a bacterial blue light photoreceptor) as a model system and obtained high-resolution, time-resolved difference electron density maps of excellent quality with strong features; these allowed the determination of structures of reaction intermediates to a resolution of 1.6 angstroms. Our results open the way to the study of reversible and nonreversible biological reactions on time scales as short as femtoseconds under conditions that maximize the extent of reaction initiation throughout the crystal. Structural changes during a macromolecular reaction are captured at near-atomic resolution by an x-ray free electron laser. Watching a protein molecule in motion X-ray crystallography has yielded beautiful high-resolution images that give insight into how proteins function. However, these represent static snapshots of what are often dynamic processes. For photosensitive molecules, time-resolved crystallography at a traditional synchrotron source provides a method to follow structural changes with a time resolution of about 100 ps. X-ray free electron lasers (XFELs) open the possibility of performing time-resolved experiments on time scales as short as femtoseconds. Tenboer et al. used XFELs to study the light-triggered dynamics of photoactive yellow protein. Electron density maps of high quality were obtained 10 ns and 1 µs after initiating the reaction. At 1 µs, two intermediates revealed previously unidentified structural changes. Science, this issue p. 1242

Oxygen‐related thermal donors in silicon: A new structural and kinetic model

A structural model for the oxygen‐related thermal donors produced at moderate temperatures (<500 °C) is presented, where electrical activity commences with clusters containing five or more oxygen atoms and arises from a silicon atom at the center of the cluster. The donor activity of a cluster is terminated upon the ejection of this central silicon atom in order to bring about stress relaxation. A large number of electrically active donor species are predicted, differing only in the number of oxygen atoms each species contains. This structural model satisfies the stringent symmetry requirements of electron paramagnetic resonance (EPR), explains the lack of hyperfine interaction with Si29 or O17 nuclei and the large multiplicity in the number of donor species observed by infrared spectroscopy and EPR. The proposed clusters are shown to be embryonic forms of the much larger rodlike defects examined in detail by electron microscopy. Chemical reaction theory is used to deduce the kinetic consequences of the s...

An approach to quantitative high-resolution transmission electron microscopy of crystalline materials

Abstract We describe how lattice images may be used to measure the variation of the projected potential in crystalline solids in any projection, with no knowledge of the imaging conditions. This approach is applicable to many solids with atoms residing entirely on coherent lattices, in which interfacial topography or changes in composition are of interest. We present atomic-level topographic maps of Si/SiO 2 interfaces in plan-view, and compositional maps of Si/GeSi/Si quantum wells in cross-section. We conclude with a detailed discussion of the capabilities and limitations of this approach.

Quantitative chemical lattice imaging: theory and practice

Abstract We describe how the composition of materials may be quantitatively mapped with near-atomic resolution and sensitivity. The procedure combines chemical lattice imaging (which sensitively records the compositional information) with vector pattern recognition (which efficiently extracts and quantifies the local information content). We describe the theoretical underpinnings which allow the local information content to be related to the local sample composition, and the procedure by which this may be realized in practice. Single and double atom substitutions in individual atomic columns of semiconductors can thus be detected at about 60% and 90% confidence levels, respectively. Potential complications due to nonlocal effects (Fresnel fringing, dynamical scattering), geometrical sample imperfections (thickness changes, surface roughness), radiation damage, and photographic nonlinearities are shown to be negligible.

Does luminescence show semiconductor interfaces to be atomically smooth

Luminescence spectra from quantum wells are routinely interpreted in terms of atomically smooth and atomically abrupt interfaces. Here we show that this interpretation is inconsistent with photoluminescence, photoluminescence excitation, and quantitative microscopic (chemical lattice imaging) results. We argue that the discussion of interfacial roughness in terms of ‘‘an island size’’ is too naive. A full characterization of an interface requires the description of a ‘‘roughness spectrum,’’ specifying the amplitude of the interfacial corrugation versus corrugation wavelength over the relevant length scale.

Femtosecond structural dynamics drives the trans/cis isomerization in photoactive yellow protein.

Visualizing a response to light Many biological processes depend on detecting and responding to light. The response is often mediated by a structural change in a protein that begins when absorption of a photon causes isomerization of a chromophore bound to the protein. Pande et al. used x-ray pulses emitted by a free electron laser source to conduct time-resolved serial femtosecond crystallography in the time range of 100 fs to 3 ms. This allowed for the real-time tracking of the trans-cis isomerization of the chromophore in photoactive yellow protein and the associated structural changes in the protein. Science, this issue p. 725 The trans-to-cis isomerization of a key chromophore is characterized on ultrafast time scales. A variety of organisms have evolved mechanisms to detect and respond to light, in which the response is mediated by protein structural changes after photon absorption. The initial step is often the photoisomerization of a conjugated chromophore. Isomerization occurs on ultrafast time scales and is substantially influenced by the chromophore environment. Here we identify structural changes associated with the earliest steps in the trans-to-cis isomerization of the chromophore in photoactive yellow protein. Femtosecond hard x-ray pulses emitted by the Linac Coherent Light Source were used to conduct time-resolved serial femtosecond crystallography on photoactive yellow protein microcrystals over a time range from 100 femtoseconds to 3 picoseconds to determine the structural dynamics of the photoisomerization reaction.

Ge-Si layered structures : artificial crystals and complex cell ordered superlattices

We report the first successful synthesis of ordered GeSi superlattices grown on (001) Si substrates by molecular beam epitaxy. Two types of structures were prepared and characterized: superlattices with one‐dimensional periodicity of one unit cell (GeGeSiSi...) and complex cell superlattices made up of sublayers of pure Si and alternating bilayers of Ge and Si. In the first case, the artificial stacking in the [001] direction results in a vertical array of alternating Ge and Si monolayers parallel to the (110) or (110) planes. In spite of the lattice mismatch of 4.2%, Rutherford backscattering and channeling experiments indicate high quality crystallinity in both types of structures. Long‐range order is deduced from the electron diffraction patterns that exhibit characteristic superlattice reflections and from high resolution lattice imaging. The precise deposition control on the scale of a fraction of a monolayer should allow band structure engineering in this and in other related systems and in turn ta...

High quality narrow GaInAs/InP quantum wells grown by atmospheric organometallic vapor phase epitaxy

A series of GaInAs/InP quantum wells from 10 to 135 A has been grown by atmospheric organometallic vapor phase epitaxy using pressure balancing techniques. These wells exhibit strong exciton peaks at 4 K and have quantized energy shifts of up to 326 meV. These energy shifts are compared with two simple finite well models (Kronig–Penney and envelope function approximation) using a conduction‐band offset of Vc≊40% ΔEg(GaInAs) and are in close agreement with the latter model. The full width half‐maximum linewidths indicate an average interface roughness of ≊1 monolayer.

Room temperature 0.1 /spl mu/m CMOS technology with 11.8 ps gate delay

We report a room temperature, 0.1 /spl mu/m CMOS technology on bulk Si substrates that delivers a record ring-oscillator gate delay of 11.8 psec at 2.5 V. Frequency dividers at 2.0 V operate with input frequencies exceeding 8.5 GHz. Feature sizes obey g-line lithography design rules except at the gate level. The high speed CMOS performance and the good subthreshold and drain characteristics for both NMOS and PMOS devices are obtained through the implementation of vertical doping engineering and the reduction of parasitics.<<ETX>>