F.D. McDaniel

Spontaneous coordinated activity in cultured networks: Analysis of multiple ignition sites, primary circuits, and burst phase delay distributions

All higher order central nervous systems exhibit spontaneous neural activity, though the purpose and mechanistic origin of such activity remains poorly understood. We quantitatively analyzed the ignition and spread of collective spontaneous electrophysiological activity in networks of cultured cortical neurons growing on microelectrode arrays. Leader neurons, which form a mono-synaptically connected primary circuit, and initiate a majority of network bursts were found to be a small subset of recorded neurons. Leader/follower firing delay times formed temporally stable positively skewed distributions. Blocking inhibitory synapses usually resulted in shorter delay times with reduced variance. These distributions are characterizations of general aspects of internal network dynamics and provide estimates of pair-wise synaptic distances. The resulting analysis produced specific quantitative constraints and insights into the activation patterns of collective neuronal activity in self-organized cortical networks, which may prove useful for models emulating spontaneously active systems.

Nonrelativistic plane-wave Born-approximation calculations of direct Coulomb M-subshell ionization by charged particles

Expressions used in the calculation of screened hydrogenic M-subshell and total M-shell cross sections for direct Coulomb ionization in the nonrelativistic plane-wave Born approximation have been numerically integrated and tabulated for a range of heavy charged-particle velocity and target-binding energy parameters. Explicit expressions are given for the calculation of x-ray production cross sections from ionization cross sections, Coster-Kronig transition rates, and fluorescence yields. The present results are in disagreement with those of Hansteen, Johnsen, and Kocbach.

Rapid isothermal annealing of As-, P-, and B-implanted silicon

Single‐crystal silicon wafers have been implanted with As, P, and B to doses of 1×1013–1×1016/cm2 and given a transient anneal using a Varian IA‐200 Rapid Isothermal Annealer. The system uses infrared radiation to heat the wafers to temperatures in excess of 1000 °C for times on the order of 10 sec. Sheet resistance and Hall measurements have been used to determine the effect of the anneal on the electrical properties of the wafers. Rutherford backscattering and secondary ion mass spectroscopy have been used to measure lattice damage and dopant profiles before and after annealing. As and P are lost during the anneal unless the wafer is capped. Complete activation can be achieved with very little dopant diffusion. Residual damage is minimal in (100) oriented wafers that had been implanted with As. However, for (111) wafers damage is less in (111) wafers implanted to doses ≥5.0×1015/cm2, than in (111) wafers implanted to doses ≤5.0×1015/cm2. The diffusion of As during this transient anneal has been modeled ...

Triply-ionized B2 molecules from a tandem accelerator

Abstract Beams of 10B3+2, 11B23+, and 10B11B3+ ions have been observed to emerge from a tandem accelerator. B2− mo into the accelerator, and positive ions emerging from the machine were analyzed for mass per charge and total energy. For 10B11B, intensities of singly-, doubly-, and triply-charged molecules were measured as a function of N2 gas pressure in the accelerator terminal stripper canal. These intensities were found to exhibit the same qualitative behavior for all charge states, with the number of triply-charged molecules a factor of ~ 2 × 10−4 less than for the doubly-charged molecules. No quadruply-ionized molecules were seen. The observation of particles corresponding to the breakup products of the multiply-ionized molecules indicates that these species were decaying in flight, and are apparently metastable with lifetimes of ~ μs. Comparisons are made between these observations and molecular orbital calculations.

Narrow band UV emission from direct bandgap Si nanoclusters embedded in bulk Si.

We report on the formation of UV emitting Si nanoclusters (NCs) in Si, using a two stage Au implantation technique. These Si NCs, with an average size of 2 nm, show photoluminescence at room temperature, over a narrow band of about 100 meV with a peak of emission near 3.3 eV. With emission lifetimes in the range of 1.5-2.5 ns, the transitions seem to come from excitonic recombinations across a quasi-direct gap. Since the structures are below the surface, there is no adverse effect of oxidation resulting in a shift in emission wavelength. On the other hand, an annealing at 500 °C has been found to result in a significant increase in the emission intensity. This is due to localized plasmon induced electric field enhancement in Au nano-islands in the vicinity.

Rapid isothermal anneal of 75As implanted silicon

Silicon wafers implanted with 75As have been annealed with a Varian IA‐200 isothermal annealer. The anneal occurs in vacuum using radiation from a resistively heated sheet of graphite. The anneal quality depends on the graphite heater temperature and exposure time. If the anneal time is too short implantation damage remains and if the time is too long measurable losses of As occur causing the sheet resistance to increase. The loss of As can be prevented by depositing 0.05 μm of SiO2 on the wafer before annealing.

The Top Silicon Layer of SOI Formed by Oxygen Ion Implantation

High dose oxygen ion implantation has been used to form a buried oxide layer in Czochralski grown silicon. Wafers were implanted with 300 keV O2+ to a total dose of 1.32 × 1018 ions cm-2. A 0.5 m thick SiO2 layer is formed beneath a 0.17 ¿m thick top Si layer. Epitaxial films were grown on both annealed and unannealed wafers. Samples were subsequently annealed at 1150°C for times from 10 to 240 minutes in either Ar or N2. The highest quality epitaxial layers were obtained with substrates that were annealed after implantation, but prior to epitaxial growth for 2 hrs at 1150°C followed by 4 hrs at 1150°C after epitaxial growth. RBS channeling shows that the top 300 nm of these films have <110> channel backscattering yields lower than any SOI produced to date. The buried oxide plus epitaxial process is a leading candidate for VLSI applications.

Fast diffusion of As in polycrystalline silicon during rapid thermal annealing

The diffusion of As in polycrystalline silicon films subjected to rapid thermal annealing has been studied using sheet resistance and Rutherford backscattering. The polycrystalline Si films were deposited on oxidized silicon wafers, implanted with As, and annealed with a Varian IA‐200 isothermal annealer. Infrared radiation from a resistively heated sheet of graphite heats the wafer, in a vacuum, to temperatures >1000 °C for times on the order of a few seconds. The rate of diffusion and rate of loss of As from the polycrystalline Si is much faster than the diffusion rate and loss rate in single crystal Si annealed with identical conditions. Diffusion prior to grain growth agrees with previously reported results for As in polycrystalline Si. However, grain growth appears to enhance As diffusion.

Characterization of Ion Implanted Silicon Annealed with a Graphite Radiation Source

A Varian IA-200 isothermal annealer has been used to anneal ion implantation damage and electrically activate dopants in As and B implanted silicon with minimal redistribution of the impurity. The anneal occurs in vacuum using infrared radiation from a resistively heated sheet of graphite. Good quality anneals of /sup 11/B implanted samples have been obtained with minimal boron diffusion. Arsenic implanted wafers have also be annealed with excellent results except for a substantial loss of As which occurs unless the wafer has an SiO/sub 2/ cap. The anneal quality of (100) wafers was independent of dose, however, the anneal quality of the (111) wafers improved with increasing As dose.

Anomalous charge collection from silicon-on-insulator structures

An increased single event upset cross-section was found in some silicon-on-insulator (SOI) devices in recent experiments. In order to investigate this unexpected increase we performed both broad beam and microbeam experiments on thermal oxide capacitors. Charge collection was measured using ion beam induced charge collection (IBICC) and time resolved IBICC under different biasing conditions. Lateral charge collection profiles were recorded across the top electrode. We found that the collected charge strongly depends on the applied bias and the oxide thickness. Lateral non-uniformity was observed for low bias conditions. In this paper we will give a qualitative explanation for the charge collection mechanism in SOI devices.