By analyzing the temperature ͑T͒ and density ͑n͒ dependence of the measured conductivity ͑͒ of twodimensional ͑2D͒ electrons in the low-density ͑ϳ10 11 cm −2 ͒ and temperature ͑0.02-10 K͒ regimes of highmobility ͑1.0 and 1.5ϫ 10 4 cm 2 / Vs͒ Si metal-oxide-semiconductor field-effect transistors, we establish that the putative 2D metal-insulator transition is a density-inhomogeneity-driven percolation transition where the density-dependent conductivity vanishes as ͑n͒ ϰ ͑n − n p ͒ p , with the exponent p ϳ 1.2 being consistent with a percolation transition. The "metallic" behavior of ͑T͒ for n Ͼ n p is shown to be well described by a semiclassical Boltzmann theory, and we observe the standard weak localization-induced negative magnetoresistance behavior, as expected in a normal Fermi liquid, in the metallic phase.The so-called two-dimensional ͑2D͒ metal-insulator transition ͑MIT͒ has been a subject 1,2 of intense activity and considerable controversy ever since the pioneering experimental discovery 3 of the 2D MIT phenomenon in Si metaloxide-semiconductor field-effect transistors ͑MOSFETs͒ by Kravchenko and Pudalov some 15 years ago. The apparent MIT has now been observed in almost all existing 2D semiconductor structures, including Si MOSFETs, 3,4 electrons, 5-7 and holes [8][9][10][11] in GaAs/AlGaAs, and electrons in Si/SiGe ͑Refs. 12 and 13͒ systems. The basic phenomenon refers to the observation of a carrier density-induced qualitative change in the temperature dependence of the resistivity ͑n , T͒, where n c is a critical density separating an effective "metallic" phase ͑n Ͼ n c ͒ from an "insulating" phase ͑n Ͻ n c ͒, exhibiting d / dT Ͼ 0͑Ͻ0͒ behavior typical of a metal ͑insulator͒.The high-density metallic behavior ͑n Ͼ n c ͒ often manifests in a large ͑by 25% for electrons in GaAs/AlGaAs heterostructures to factors of 2-3 in Si MOSFETs͒ increase in resistivity with increasing temperature in the lowtemperature ͑0.05-5 K͒ regime where phonons should not play much of a role in resistive scattering. The insulating regime, at least for very low ͑n Ӷ n c ͒ densities and temperatures, seems to be the conventional activated transport regime of a strongly localized system. The 2D MIT phenomenon occurs in relatively high-mobility systems, although the mobility values range from 10 4 cm 2 / Vs ͑Si MOSFET͒ to 10 7 cm 2 / Vs͑GaAs/ AlGaAs͒ depending on the 2D system under consideration. The 2D MIT phenomenon is also considered to be a low-density phenomenon although, depending on the 2D system under consideration, the critical density n c differs by 2 orders of magnitude ͑n c ϳ 10 11 cm −2 in 2D Si and ϳ10 9 cm −2 in high-mobility GaAs/AlGaAs heterostructures͒. The universal features of the 2D MIT phenomenon are ͑1͒ the existence of a critical density n c distinguishing an effective high-density metallic ͑d / dT Ͼ 0 for n Ͼ n c ͒ phase from an effective low-density insulating ͑d / dT Ͻ 0 for n Ͻ n c ͒ phase, and ͑2͒ while the insulating phase for n Ͻ n c seems mostly to manifest the conventional activated transport be...
Optical lithography based on microfabrication techniques was employed to fabricate one-dimensional nanogaps with micrometre edge lengths in silicon. These one-dimensional nanogaps served as a platform on which organic/nanoparticle films were assembled. Characterization of the gaps was performed with high-resolution TEM, SEM, and electrical measurements. Novel self-assembling attachment chemistry, based on the interaction of silicon with a diazonium salt, was used to iteratively build a multi-layer nanoparticle film across a 7 nm gap. By using nanoparticles capped with an easily displaced ligand, a variable conductive path was created across the 1D nanogap. Electrical measurements of the gap showed a dramatic change in the I(V) characteristics after assembly of the nanoparticle film.
We present an electron spin resonance (ESR) approach to characterize shallow electron trapping in band-tail states at Si/SiO2 interfaces in metal-oxide-semiconductor (MOS) devices and demonstrate it on two MOS devices fabricated at different laboratories. Despite displaying similar low temperature (4.2 K) peak mobilities, our ESR data reveal a significant difference in the Si/SiO2 interface quality of these two devices, specifically an order of magnitude difference in the number of shallow trapped charges at the Si/SiO2 interfaces. Thus, our ESR method allows a quantitative evaluation of the Si/SiO2 interface quality at low electron densities, where conventional mobility measurements are not possible.
This article details a simple four-step procedure to create a one-dimensional nanogap on a buried oxide substrate that relies on conventional photolithography performed on a stack of silicon/silicon oxide/silicon, metal evaporation, and hydrofluoric acid oxide removal. Once the nanogap was fabricated it was bridged with an assembly of 1,8-octanedithiol and 5 nm Au nanoparticles capped with a sacrificial dodecylamine coating. Before assembly, characterization of the nanogaps was performed through electrical measurements and SEM imaging. Post assembly, the resistance of the nanogaps was evaluated. The current increased from 70 fA to 200 microA at +1 V bias, clearly indicating a modification due to nanoparticle molecule assembly. Control experiments without nanoparticles or octanedithiol did not show an increase in current.
We present measurements of silicon ͑Si͒ metal-oxide-semiconductor ͑MOS͒ nanostructures that are fabricated using a process that facilitates essentially arbitrary gate geometries. Stable Coulomb-blockade behavior showing single-period conductance oscillations that are consistent with a lithographically defined quantum dot is exhibited in several MOS quantum dots with an open-lateral quantum-dot geometry. Decreases in mobility and increases in charge defect densities ͑i.e., interface traps and fixed-oxide charge͒ are measured for critical process steps, and we correlate low disorder behavior with a quantitative defect density. This work provides quantitative guidance that has not been previously established about defect densities and their role in gated Si quantum dots. These devices make use of a double-layer gate stack in which many regions, including the critical gate oxide, were fabricated in a fully qualified complementary metal-oxide semiconductor facility.
Transmission electron microscopy studies of the microstructure of AuNiGe ohmic contact to n-type GaAs Microstructure analysis and contact resistance measurements of alloyed AuNiGe contacts to GaAs were performed to assist in the development of low resistance Ohmic contacts for metalsemiconductor field-effect transistor (MESFET) devices. The contact metals were prepared by sequential deposition of 100 nm of Au-27 at. % Ge, 35 nm Ni, and 50 nm Au onto sputter-cleaned GaAs wafers in which conducting channels were formed by Si doping to a level of about 1 X 10 18 cm -3. The contact resistances were determined by the transmission line method. Analysis of the substrate and the film microstructure was carried out by x-ray diffraction, Auger electron spectroscopy (AES), and x-ray photoelectron spectroscopy (XPS). A strong correlation between the contact resistance and the film microstructure was observed. Low resistances were observed when NiAs compounds containing Ge were in contact with GaAs and the {3-AuGa phase was concentrated near the top of the contact. High resistances were measured when free Au, the a-AuGa phase, or a NiGe compound were present. The temperature dependence of the contact resistance and the kinetics of compound formation were found to be influenced by the deposition sequence. Deposition of 5 nm Ni as a first layer significantly enhanced the formation of the NiAs compounds containing Ge, resulting in low contact resistance at a lower alloying temperature. However, a further increase in the thickness of the first Ni layer to 10
We report on the fabrication and performance of a novel single ion Geiger mode avalanche (SIGMA) diode detector that senses single ions with ∼100% detection efficiency at room temperature for 250 keV protons. The SIGMA diode detector utilizes Geiger mode operation of avalanche photodiodes, which can be sensitive to single electron-hole (e-h) pairs induced by the ion stopping. The SIGMA diode detector takes advantage of a complementary metal oxide semiconductor foundry allowing for future integration with silicon nanostructures to build novel single atom modified devices. SIGMA diode detector offers potential improvement in current integrated ion detector approaches that have noise floors in the order of 103 e-h pairs.
Articles you may be interested inErratum: "Comparison of the submicron particle analysis capabilities of Auger electron spectroscopy, time-offlight secondary ion mass spectrometry, and scanning electron microscopy with energy dispersive x-ray spectroscopy for particles deposited on silicon wafers with one micron thick oxide layers" [J.Comparison of the submicron particle analysis capabilities of Auger electron spectroscopy, time-of-flight secondary ion mass spectrometry, and scanning electron microscopy with energy dispersive x-ray spectroscopy for particles deposited on silicon wafers with 1 μm thick oxide layers Development of a multifunctional surface analysis system based on a nanometer scale scanning electron beam: Combination of ultrahigh vacuumscanning electron microscopy, scanning reflection electron microscopy, Auger electron spectroscopy, and xray photoelectron spectroscopy Rev.Timeofflight secondary ion mass spectrometry of insulators with pulsed charge compensation by lowenergy electrons J.Particulate contamination can result in a significant yield loss during semiconductor device fabrication. As device design rule dimensions decrease the critical defect size also decreases, resulting in the need to analyze smaller defects. Current manufacturing requirements include analysis of sub-0.5-m defects, with analysis of sub-0.1-m defects expected in the near future. This article investigates the particle analysis capabilities of Auger electron spectroscopy, time-of-flight secondary ion mass spectrometry, and energy dispersive x-ray spectroscopy during scanning electron microscopy ͑SEM/EDS͒. In order to evaluate each method carefully, a standard set of samples was prepared and analyzed. These samples consist of 0.5-, 0.3-, and 0.1-m Al and Al 2 O 3 deposited on 1-in. Si wafers. Although all the methods observed an Al signal, a semiquantitative gauge of capability based on the relative strengths of particle versus substrate signal is provided. The dependence of the sample-to-substrate signal on primary electron energy is examined for both EDS and Auger analyses. The ability to distinguish metallic Al particles from Al oxide particles for the three techniques is also discussed.
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