1-octadecene monolayers on Si(111) hydrogen-terminated surfaces: effects of substrate doping

We have studied the electronic properties, in relation to their structural properties, of monolayers of 1-octadecene attached on a hydrogen-terminated (111) silicon surface. The molecules are attached using the free-radical reaction between C=C and Si-H activated by an ultraviolet illumination. We have compared the structural and electrical properties of monolayers formed on silicon substrate of different types (n-type and p-type) and different doping concentrations from low-doped (~1E14 cm-3) to highly doped (~1E19 cm-3) silicon substrates. We show that the monolayers on n-, p- and p+ silicon are densely packed and that they act as very good insulating films at a nanometer thickness with leakage currents as low as ~1E-7 A.cm-2 and high quality capacitance-voltage characteristics. The monolayers formed on n+-type silicon are more disordered and therefore exhibit larger leakage current densities (>1E-4 A.cm-2) when embedded in a silicon/monolayer/metal junction. The inferior structural and electronic properties obtained with n+-type silicon pinpoint the important role of surface potential and of the position of the surface Fermi level during the chemisorption of the organic monolayers.Comment: 33 pages, 8 figures, to be published J. Appl. Phy

Gate Stack Dielectric Degradation of Rare-Earth Oxides Grown on High Mobility Ge Substrates

We report on the dielectric degradation of Rare-Earth Oxides (REOs), when used as interfacial buffer layers together with HfO2 high-k films (REOs/HfO2) on high mobility Ge substrates. Metal-Oxide-Semiconductor (MOS) devices with these stacks,show dissimilar charge trapping phenomena under varying levels of Constant- Voltage-Stress (CVS) conditions, which also influences the measured densities of the interface (Nit) and border (NBT) traps. In the present study we also report on C-Vg hysteresis curves related to Nit and NBT. We also propose a new model based on Maxwell-Wagner instabilities mechanism that explains the dielectric degradations (current decay transient behavior) of the gate stack devices grown on high mobility substrates under CVS bias from low to higher fields, and which is unlike to those used for other MOS devices. Finally, the time dependent degradation of the corresponding devices revealed an initial current decay due to relaxation, followed by charge trapping and generation of stress-induced leakage which eventually lead to hard breakdown after long CVS stressing.Comment: 19pages (double space), 7 figures, original research article, Submitted to JAP (AIP

Study of X-ray Radiation Damage in Silicon Sensors

The European X-ray Free Electron Laser (XFEL) will deliver 30,000 fully coherent, high brilliance X-ray pulses per second each with a duration below 100 fs. This will allow the recording of diffraction patterns of single complex molecules and the study of ultra-fast processes. Silicon pixel sensors will be used to record the diffraction images. In 3 years of operation the sensors will be exposed to doses of up to 1 GGy of 12 keV X-rays. At this X-ray energy no bulk damage in silicon is expected. However fixed oxide charges in the insulating layer covering the silicon and interface traps at the Si-SiO2 interface will be introduced by the irradiation and build up over time. We have investigated the microscopic defects in test structures and the macroscopic electrical properties of segmented detectors as a function of the X-ray dose. From the test structures we determine the oxide charge density and the densities of interface traps as a function of dose. We find that both saturate (and even decrease) for doses between 10 and 100 MGy. For segmented sensors the defects introduced by the X-rays increase the full depletion voltage, the surface leakage current and the inter-pixel capacitance. We observe that an electron accumulation layer forms at the Si-SiO2 interface. Its width increases with dose and decreases with applied bias voltage. Using TCAD simulations with the dose dependent parameters obtained from the test structures, we are able to reproduce the observed results. This allows us to optimize the sensor design for the XFEL requirements

Gold nanoparticle-pentacene memory-transistors

We demonstrate an organic memory-transistor device based on a pentacene-gold nanoparticles active layer. Gold (Au) nanoparticles are immobilized on the gate dielectric (silicon dioxide) of a pentacene transistor by an amino-terminated self-assembled monolayer. Under the application of writing and erasing pulses on the gate, large threshold voltage shift (22 V) and on/off drain current ratio of ~3E4 are obtained. The hole field-effect mobility of the transistor is similar in the on and off states (less than a factor 2). Charge retention times up to 4500 s are observed. The memory effect is mainly attributed to the Au nanoparticles

Frequency dispersion reduction and bond conversion on n-type GaAs by in situ surface oxide removal and passivation

The method of surface preparation on n-type GaAs, even with the presence of an amorphous-Si interfacial passivation layer, is shown to be a critical step in the removal of accumulation capacitance frequency dispersion. In situ deposition and analysis techniques were used to study different surface preparations, including NH4OH, Si-flux, and atomic hydrogen exposures, as well as Si passivation depositions prior to in situ atomic layer deposition of Al2O3. As–O bonding was removed and a bond conversion process with Si deposition is observed. The accumulation capacitance frequency dispersion was removed only when a Si interlayer and a specific surface clean were combined

Design and Tests of the Silicon Sensors for the ZEUS Micro Vertex Detector

To fully exploit the HERA-II upgrade,the ZEUS experiment has installed a Micro Vertex Detector (MVD) using n-type, single-sided, silicon micro-strip sensors with capacitive charge division. The sensors have a readout pitch of 120 micrometers, with five intermediate strips (20 micrometer strip pitch). The designs of the silicon sensors and of the test structures used to verify the technological parameters, are presented. Results on the electrical measurements are discussed. A total of 1123 sensors with three different geometries have been produced by Hamamatsu Photonics K.K. Irradiation tests with reactor neutrons and Co-60 photons have been performed for a small sample of sensors. The results on neutron irradiation (with a fluence of 1 x 10^{13} 1 MeV equivalent neutrons / cm^2) are well described by empirical formulae for bulk damage. The Co-60 photons (with doses up to 2.9 kGy) show the presence of generation currents in the SiO_2-Si interface, a large shift of the flatband voltage and a decrease of the hole mobility.Comment: 33 pages, 25 figures, 3 tables, accepted for publication in NIM

Box traps on an atom chip for one-dimensional quantum gases

We present the implementation of tailored trapping potentials for ultracold gases on an atom chip. We realize highly elongated traps with box-like confinement along the long, axial direction combined with conventional harmonic confinement along the two radial directions. The design, fabrication and characterization of the atom chip and the box traps is described. We load ultracold ($\lesssim1 \mu$K) clouds of $^{87}$Rb in a box trap, and demonstrate Bose-gas focusing as a means to characterize these atomic clouds in arbitrarily shaped potentials. Our results show that box-like axial potentials on atom chips are very promising for studies of one-dimensional quantum gases.Comment: 9 pages 4 figure

Direct Observation of Site-specific Valence Electronic Structure at Interface: SiO2/Si Interface

Atom specific valence electronic structures at interface are elucidated successfully using soft x-ray absorption and emission spectroscopy. In order to demonstrate the versatility of this method, we investigated SiO2/Si interface as a prototype and directly observed valence electronic states projected at the particular atoms of the SiO2/Si interface; local electronic structure strongly depends on the chemical states of each atom. In addition we compared the experimental results with first-principle calculations, which quantitatively revealed the interfacial properties in atomic-scale.Comment: 4 pages, 3 figure