Deactivation of electrically supersaturated Te-doped InGaAs grown by MOCVD

Achieving and sustaining the highest doping level possible in InGaAs is critical for the reduction of contact resistance in future microelectronic applications. Tellurium (Te) is a very promising n-type dopant with high reported n-type doping densities. However, the stability of this dopant during post-growth thermal processing is unknown. Supersaturated Te-doped InGaAs layers were grown by MOCVD at 500 °C. The electrically active concentration of Te doping was 4.4 × 1019 cm−3 as grown. The thermal stability of the Te was investigated by studying the effect of post-growth annealing between 550 and 700 °C on the electrical activation. At all temperatures, the electrical activation decreased from its starting electron concentration of 4.4 × 1019 cm−3 down to 6–7 × 1018 cm−3. The rate of deactivation was measured at each temperature, and the activation energy for the deactivation process was determined to be 2.6 eV. The deactivation could be caused by either Te–Te clustering or a Te-point defect reaction. HAADF-STEM images showed no visible clustering or precipitation after deactivation. Based on previous ab initio calculations that suggest the VIII population increases as the Fermi level moves toward the conduction band, it is proposed that formation of isolated point defect complexes, possibly a Te–VIII complex, is associated with the deactivation process

Temperature-Dependent Self-Assemblies of C60 on (1X2)- Pt(110): A STM/DFT Investigation

Thermal self-assembly of C60 on (1 7 2)-Pt(110) following room-temperature deposition has been studied by means of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) calculations. Two distinct C60 chemisorption phases have been identified and characterized as a function of the annealing temperature. After a thermal treatment at 700 K, islands of a C60 quasi-hexagonal lattice form. These islands are characterized by the highest surface density so far reported for a two-dimensional surface-supported fullerene phase (the surface area per molecule is 87.0 \uc52), with intermolecular nearest-neighbor distances equal to 9.6 \ub1 0.1 \uc5. Embedded nanowires of fullerene dumbbell dimers (with an intermolecular distance equal to 9.2 \ub1 0.1 \uc5) occasionally nucleate within this high-density phase following a \u201cmolecular zip\u201d mechanism. Highly site selective chemisorption driven by the particularly strong overlayer 12substrate bonding is proposed to be responsible for the first reported example of surface-templated chemical bond formation between fullerene molecules. After annealing at 850 K, an oblique C60 chemisorption phase forms. This is characterized by a tetramolecular basis associated to each lattice point of the two-dimensional superstructure and by a peculiar bright 12dim contrast in STM images acquired at positive sample bias values. The combination of high- resolution STM images with LEED data and DFT calculations leads to the conclusion that the (1 7 2) substrate reconstruction is lifted at the fullerene 12platinum interface and also provides a detailed description of the molecular bonding sites and orientations found within the phase. It is proposed that the main factor ruling the interconversion of chemisorption phases is the variation of substrate atom mobility as a function of temperature

Bottom-up Assembly of Single-Domain Titania Nanosheets on (1x2)-Pt(110)

A bottom-up route towards the synthesis of titania nanosheets is explored, alternative to the exfoliation of layered titanates. Nanosheets are assembled from the constituent elements and epitaxially matched to a suitable substrate: (1x2)-Pt110. Their basic lepidocrocite structure is modulated at the nanoscale due to coincidence with the substrate. Density functional calculations reveal the structure details of the nanosheet, which is also shown to be in close relationship with a (001)-oriented anatase bilayer

Ultrathin TiO2 Films on (1x2)-Pt(110): a LEED, Photoemission, STM, and Theoretical Investigation

The preparation and characterization of fully oxidized TiO2 ultrathin films obtained by reactive deposition of Ti in an O2 background on the (1 72)-Pt(110) reconstructed surface is described in details. The structure, the electronic properties, and the morphology of the epitaxial films giving rise to a (14 74) coincidence superstructure are discussed on the basis of low-energy electron diffraction, photoemission (both core and valence band), angle-scanned X-ray photoelectron diffraction, scanning tunneling microscopy data, and density functional theory calculations. We show that the oxide overlayer is a stoichiometric lepidocrocite-like single-domain nanosheet. This can be thought of as originating from a (100) oriented anatase bilayer which spontaneously restructure by a uniaxial relative sliding of one single layer with respect to the other by half a unit cell. According to the results of theoretical calculations, the process is self-driven by the spatial confinement, whereas a minor role is played by the interaction with the substrate. The occurrence of the (14 74) coincidence between the overlayer and the substrate is fully rationalized on the basis of the reported data

The use of surgical central venous catheters in very low-birth weight neonates (under 1000 g)

Since 1985 we have been using central venous catheters (CVC) for Parenteral Nutrition (NPT) in the Neonatal Intensive care Unit (NICU) of our University. The CVC's are put in place surgically. In the last 5 years much greater use has been made of CVC in patients with very low birth weight (VLBW), absence of vascular access and important gastrointestinal problems. In the period between January 1985 and December 1990 this technique was used in 154 neonates applying 185 CVC's. Fifty one (33.1%) of these patients weighed less than 1000 g (Min 400) when the catheter was put in place. All catheters (Nutricath, Vygon I.D. 0.8 mm) used were inserted in the NICU. In the VLBW group the catheter was inserted in the external jugular vein in 11 cases (21.6%) and in the internal jugular vein in 40 cases (78.4%). Using the surgical technique we had a 100% success rate in this group. The mean duration of CVC was 23.5 days (range 1-113), 32 CVC's (62.7%) were removed as no longer required, 15 (29.4%) because patients died and 4 (7.8%) because of accidental dislodgement. In this period we had 7 cases (13.7%) of atrial thrombosis (AT) and 2 episodes of catheter-related sepsis (3.9%); however these complications were all successfully treated. In three cases a serious bradycardia was observed at sedation: this was solved with intensive care