M. W. Denhoff

Growth and characterization of Si1−xGex and Ge epilayers on (100) Si

Two approaches to the growth of high‐quality epitaxial Ge epilayers on (100) Si have been investigated. The first consisted of compositional‐grading Si1−xGex layers and the use of strained‐layer superlattices as dislocation filters. In general, this method produced unsatisfactory results, due to the difficulty in achieving good epitaxial growth in the Ge concentration interval 30%−70%. The second approach consisted of simply depositing pure Ge directly on (100) Si. Excellent epitaxial films with dislocation densities of less than 107 cm−2 and smooth morphology were obtained after optimization of the growth parameters. The initial growth temperature and post‐growth annealing were found to be critical in obtaining good epitaxial material.

Epitaxial Y1Ba2Cu3O7 thin films on CeO2 buffer layers on sapphire substrates

Pulsed laser deposition has been used to deposit Y1Ba2Cu3O7 layer on CeO2 buffer layers on (11_02) sapphire. Both layers are epitaxial with the 〈110〉 direction of the CeO2 layer aligned with the 〈2_021〉 direction of the sapphire substrate. The c‐axis Y1Ba2Cu3O7 layer has its 〈100〉 direction alligned with the 〈110〉 direction of the CeO2. Cross‐sectional transmission electron microscopy shows the epitaxy to be coherent and the interfaces to be abrupt at an atomic level. The best films have a critical current of 9 × 106 A/cm2 at 4.2 K and lower microwave surface resistance than copper at 77 K and at a frequency of 31 GHz.

Misfit dislocation structure at a Si/SixGe1−x strained-layer interface

The misfit dislocation structure at a Si/Si0.75Ge0.25 strained‐layer interface has been characterized by transmission electron microscopy. Through weak‐beam imaging it is found that partial dislocation in the form of extended nodes exist in the misfit dislocation network. The density of nodes as observed by microscopy compares favorably with the estimate of the density of charged interface states derived from capacitance‐voltage measurements.

Contact resistivity of some magnesium/silicon and magnesium silicide/silicon structures

Transmission line model and end resistance measurements were made to determine the contact resistivity of Mg and Mg2Si contacts to Si doped n‐type in the range 1018–1020 cm−3. The data are consistent with a barrier height of 0.4 eV for Mg and 0.52 eV for Mg2Si. The morphology, structure, and composition were studied using transmission electron microscopy.

From Understanding Cellular Function to Novel Drug Discovery: The Role of Planar Patch-Clamp Array Chip Technology

All excitable cell functions rely upon ion channels that are embedded in their plasma membrane. Perturbations of ion channel structure or function result in pathologies ranging from cardiac dysfunction to neurodegenerative disorders. Consequently, to understand the functions of excitable cells and to remedy their pathophysiology, it is important to understand the ion channel functions under various experimental conditions – including exposure to novel drug targets. Glass pipette patch-clamp is the state of the art technique to monitor the intrinsic and synaptic properties of neurons. However, this technique is labor intensive and has low data throughput. Planar patch-clamp chips, integrated into automated systems, offer high throughputs but are limited to isolated cells from suspensions, thus limiting their use in modeling physiological function. These chips are therefore not most suitable for studies involving neuronal communication. Multielectrode arrays (MEAs), in contrast, have the ability to monitor network activity by measuring local field potentials from multiple extracellular sites, but specific ion channel activity is challenging to extract from these multiplexed signals. Here we describe a novel planar patch-clamp chip technology that enables the simultaneous high-resolution electrophysiological interrogation of individual neurons at multiple sites in synaptically connected neuronal networks, thereby combining the advantages of MEA and patch-clamp techniques. Each neuron can be probed through an aperture that connects to a dedicated subterranean microfluidic channel. Neurons growing in networks are aligned to the apertures by physisorbed or chemisorbed chemical cues. In this review, we describe the design and fabrication process of these chips, approaches to chemical patterning for cell placement, and present physiological data from cultured neuronal cells.

Annealing studies of highly doped boron superlattices

Coevaporation of B2 O3 during silicon molecular‐beam epitaxy at growth temperatures (TG ) varying from 540 to 800 °C has been used to prepare superlattice structures (pipi’s) of varying boron concentration (3×1018 –3×1020 B cm−3). The superlattices were subsequently subjected to various annealing procedures and the layers were examined by secondary ion mass spectrometry, electrochemical profiling, and cross‐sectional transmission electron microscopy. A significant redistribution of boron was observed even before annealing for TG >700 °C and high boron concentrations. In addition, significant oxygen was incorporated for TG ≤700 °C, with a growth rate of 0.5 nm s−1 and a B2 O3 flux of 2×1013 cm−2 s−1. After annealing, the boron diffusion coefficients were determined for the layers and found to vary significantly with TG.

Charge-carrier induced barrier-height reduction at organic heterojunction

In order to provide an accurate theoretical description of current density voltage (J-V) characteristics of an organic heterojunction device over a wide range of electric fields at various temperatures, it is proposed that an accumulation of charge carriers at the heterojunction will lead to a reduction in the barrier height across the heterojunction. Two well-known hole transporting materials, 4,4,4-Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (MTDATA) and N,N-diphenyl-N,N-bis(1-naphthyl)(1,1-biphenyl)-4,4diamine (NPB) were used to fabricate unipolar heterojunction devices. It is found that the J-V characteristics depends strongly on applied bias. The simulated J-V characteristics of the heterojunction device, with the modified injection model, are found to be in excellent agreement with the experimental data.

A novel silicon patch‐clamp chip permits high‐fidelity recording of ion channel activity from functionally defined neurons

We report on a simple and high‐yield manufacturing process for silicon planar patch‐clamp chips, which allow low capacitance and series resistance from individually identified cultured neurons. Apertures are etched in a high‐quality silicon nitride film on a silicon wafer; wells are opened on the backside of the wafer by wet etching and passivated by a thick deposited silicon dioxide film to reduce the capacitance of the chip and to facilitate the formation of a high‐impedance cell to aperture seal. The chip surface is suitable for culture of neurons over a small orifice in the substrate with minimal leak current. Collectively, these features enable high‐fidelity electrophysiological recording of transmembrane currents resulting from ion channel activity in cultured neurons. Using cultured Lymnaea neurons we demonstrate whole‐cell current recordings obtained from a voltage‐clamp stimulation protocol, and in current‐clamp mode we report action potentials stimulated by membrane depolarization steps. Despite the relatively large size of these neurons, good temporal and spatial control of cell membrane voltage was evident. To our knowledge this is the first report of recording of ion channel activity and action potentials from neurons cultured directly on a planar patch‐clamp chip. This interrogation platform has enormous potential as a novel tool to readily provide high‐information content during pharmaceutical assays to investigate in vitro models of disease, as well as neuronal physiology and synaptic plasticity. Biotechnol. Bioeng. 2010;107:593–600.

Structural properties of ultrathin arsenic‐doped layers in silicon

We have grown δ‐doped layers in Si by low‐energy As‐ion implantation during molecular beam epitaxy. The layers were investigated using cross‐sectional transmission electron microscopy, secondary‐ion mass spectrometry, Rutherford backscattering, and electrical measurements. The δ‐doped layers were between 3.5 and 5.5 nm thick, and showed perfect epitaxy with 50–80% of the incorporated As on substitutional sites. Layers doped at concentrations from 1×1013 cm−2 to 8×1013 cm−2 had bulk‐like mobilities and spanned the metal to insulator transition.

Boron redistribution in doping superlattices grown by silicon molecular beam epitaxy using B2O3

Coevaporation of B2O3 during silicon molecular beam epitaxy has been used to prepare heavily doped superlattices (pipi’s). Full activation up to 3×1020 cm−3 (100 times the solid solubility limit) was obtained at growth temperatures below 700 °C. Significant boron redistribution has been observed into the undoped layers when the dopant level in the intentionally doped layers exceeds the solid solubility limit and the growth temperature is greater than 700 °C. Oxygen was not incorporated into the lattice for growth temperatures above 700 °C when using B2O3 as the source of boron, a Si growth rate for 0.5 nm s−1, and a B2O3 arrival rate of ∼2×1013 cm−2 s−1.