Epitaxial integration of nanowires in microsystems by local micrometer-scale vapor-phase epitaxy

Free-standing epitaxially grown nanowires provide a controlled growth system and an optimal interface to the underlying substrate for advanced optical, electrical, and mechanical nanowire device connections. Nanowires can be grown by vapor-phase epitaxy (VPE) methods such as chemical vapor deposition (CVD) or metal organic VPE (MOVPE). However, VPE of semiconducting nanowires is not compatible with several microfabrication processes due to the high synthesis temperatures and issues such as cross-contamination interfering with the intended microsystem or the VPE process. By selectively heating a small microfabricated heater, growth of nanowires can be achieved locally without heating the entire microsystem, thereby reducing the compatibility problems. The first demonstration of epitaxial growth of silicon nanowires by this method is presented and shows that the microsystem can be used for rapid optimization of VPE conditions. The important issue of the cross-contamination of other parts of the microsystem caused by the local growth of nanowires is also investigated by growth of GaN near previously grown silicon nanowires. The design of the cantilever heaters makes it possible to study the grown nanowires with a transmission electron microscope without sample preparation.     

Creep stress concentrations in mis-matched weldments—starting points for early creep damage and creep cracking

The material creep properties of weldments, i.e. the minimum creep strain rate, the creep rupture strength and the creep ductility, do in general differ from those of the parent material. It is often assumed that if the weld material creep strength is higher than that of the parent, the life of the weldment will also be higher. This is not always true. In fact, when subjected to a prescribed displacement, e.g. a circumferential weldment in a pipe subjected to internal pressure, a creep hard weld develops a stress concentration which may result in a reduced life expectancy of the weldment system. Similarly, the properties of the heat affected zone often differ from those of the parent and weld material properties which may be one reason for early Type IV cracking. In addition, the weld preparation geometry, the width of the HAZ and other geometric parameters influence the stress distribution. The present paper summarizes some of the effects of material property variations in weldments and their impact on the creep life expectancy. It is shown that the stress concentrations resulting from material property mis-matching may be high enough to produce premature creep damage in the weldment, or will cause creep cracking starting from initial defects.     

Position-controlled interconnected InAs nanowire networks

We demonstrate here a method for controlled production of complex self-assembled three-dimensional networks of InAs nanowires on a substrate, based on sequentially seeded epitaxial nanowire structures, or "nanotrees". A position-controlled array of trunk nanowires is first produced using lithographically defined Au particles as seeds. With these wires positioned along the proper crystallographic directions with respect to each other, nanotree branches grow toward neighboring trunks, connecting them together. Finally, we investigate the crystal structure of the interconnected nanotrees, demonstrating that branch growth after the contact with the second trunk has an epitaxial relationship to that trunk.     

Au-free epitaxial growth of InAs nanowires

III-V nanowires have been fabricated by metal-organic vapor-phase epitaxy without using Au or other metal particles as a catalyst. Instead, prior to growth, a thin SiOx layer is deposited on the substrates. Wires form on various III-V substrates as well as on Si. They are nontapered in thickness and exhibit a hexagonal cross-section. From high-resolution X-ray diffraction, the epitaxial relation between wires and substrates is demonstrated and their crystal structure is determined.     

Particle-assisted Ga(x)In(1-x)P nanowire growth for designed bandgap structures

Non-tapered vertically straight Ga(x)In(1-x)P nanowires were grown in a compositional range from Ga(0.2)In(0.8)P to pure GaP in particle-assisted mode by controlling the trimethylindium, trimethylgallium and hydrogen chloride flows in metal-organic vapor phase epitaxy. X-ray energy dispersive spectroscopy in transmission electron microscopy revealed homogeneous radial material composition in single nanowires, whereas variations in the material composition were found along the nanowires. High-resolution x-ray diffraction indicates a variation of the material composition on the order of about 19% measuring an entire sample area, i.e., including edge effects during growth. The non-capped nanowires emit room temperature photoluminescence strongly in the energy range of 1.43-2.16 eV, correlated with the bandgap expected from the material composition.     

Realizing lateral wrap-gated nanowire FETs: controlling gate length with chemistry rather than lithography

An important consideration in miniaturizing transistors is maximizing the coupling between the gate and the semiconductor channel. A nanowire with a coaxial metal gate provides optimal gate-channel coupling but has only been realized for vertically oriented nanowire transistors. We report a method for producing laterally oriented wrap-gated nanowire field-effect transistors that provides exquisite control over the gate length via a single wet etch step, eliminating the need for additional lithography beyond that required to define the source/drain contacts and gate lead. It allows the contacts and nanowire segments extending beyond the wrap-gate to be controlled independently by biasing the doped substrate, significantly improving the subthreshold electrical characteristics. Our devices provide stronger, more symmetric gating of the nanowire, operate at temperatures between 300 and 4 K, and offer new opportunities in applications ranging from studies of one-dimensional quantum transport through to chemical and biological sensing.     

Properties of deep Cu levels in GaP

Optical properties of two Cu-induced deep acceptorlike levels in GaP with binding energies E_A = 0.50 ± 0.01 eV and E_B 0.7 eV have been investigated. Data are reported here from purely optical techniques, based on detection of photoluminescence. A detailed spectral analysis of the broad Cu-related 1.65 eV-emission reveals a moderate Frank-Condon shift Δ_FC 105 ± 15 meV for the A-center. Accurate spectral data for optical cross section σ_pA⁰(hν) and σ_nA⁰(hν) were measured for the 0.50 eV A-center. Optical cross sections σ_pB⁰(hν) and σ_nB⁰(hν) could also be measured via luminescence, in spite of the fact that the B-center appears to have a completely nonradiative recombination.     

A theoretical study of the electronic properties of intercalated graphite

We present the results of electronic structure calculations for first and second stages of lithium intercalated graphite (LiC₆ and LiC₁₂). The various stages of Li intercalated graphite all have hexagonal symmetry, where different carbon layers are stacked with C-atoms directly on top of each other (AIAI…), as opposed to natural graphite where the C-layers are staggered (ABAB…). All our calculations have been performed within the framework of the extended tight binding method with Gaussian type basis sets. From the orbital and total densities of states, we conclude that Li-2s electrons are transferred into carbon π-bands. This results in shifting the Fermi level into the region of high density of states (compared with pure graphite) and, hence, to observed metallic behavior. The calculated density of states for LiC₆ and LiC₁₂ is 0.25 and 0.12/(eV C-atom), respectively. Recall that for pure graphite the value is nearly zero and for copper it is 0.29. We also found it instructive to obtain the electronic structure of LiC₆ and LiC₁₂ based on a rigid band model.     

Is copyright law steering the right course?

The Whelan decision, a landmark copyright decision, has questioned the legality of reusing the abstractions and structures underlying software. The author argues that this is a flawed decision that could harm software-engineering practice. She discusses copyright versus patent protection, describes the Whelan case, and critiques the decision. She addresses the legal handling of software in general, arguing that it more reasonably ought to be protected by patent than by copyright