Conductive Surface Layer Research Articles

Ordered growth of neurons on diamond

Diamond has a number of unique properties that make it an attractive electronic and bio-electronic material. Here we show the ordered growth of mammalian neurons, the principal electrogenic cells of the nervous system, on diamond. Proteins were specifically patterned on diamond surfaces by micro-contact printing. Mouse cortical neurons were then cultured on these substrates. Neuron adhesion and outgrowth was specific for those areas of the diamond that had been stamped with laminin, resulting in ordered growth of high resolution. Neurons survived in culture for the duration of the experiment, and laminin patterns were stable for at least 1 week in culture. The relative biocompatibility of diamond and the suitability of neuron interfacing with the hydrogen surface conductivity layer make this an interesting model for the formation of defined neuronal networks and for implants.

Ac conductance of surface layer in lithium tetraborate single crystals

ac conductance for the electrode effect in Li2B4O7 single crystal was investigated by use of a coplanar electrode applied on the surface of a (001) plate. A coplanar electrode in this material more clearly shows conduction of the electrode effect than a conventional parallel planar electrode. The electrode effect in ac conductance is likely to be controlled by the surface layer, which is a poorly conductive depletion layer possibly filled with vacancies of lithium ions. We found that the surface layer is not locally distributed near the electrodes, but, rather, on the broad area of the surface (001) plane of the material. So we conclude that the electrode effect in ac conduction of Li2B4O7 single crystal is mainly due to the poor conductive surface layer distributed over the whole surface of the (001) plane and is not a secondary phase formed by reaction with the electrode material.

Microelectrodes in solid state ionics

The great potential of microelectrode measurements in several fields of solid state ionics is discussed and exemplified: (i) Microelectrodes are perfectly suited to investigate inhomogeneous bulk conductivities in ionic solids. This is demonstrated for a SrTiO 3 single crystals with a non-stoichiometry profile. (ii) Microelectrode experiments are helpful in determining the distribution of grain boundary properties in ceramics with highly resistive grain boundaries. Data obtained on a SrTiO 3 polycrystal are shown as an example. (iii) Experiments with microelectrodes allow the detection and quantification of highly conductive grain boundaries; the applicability of this measurement mode is demonstrated for AgCl. (iv) Microelectrodes can serve as model electrodes with well-defined three phase boundaries and interfaces. Investigations on Sr-doped LaMnO 3 microelectrodes, for example, yielded an improved understanding of electrochemical processes in solid oxide fuel cells. (v) Microelectrodes are very sensitive to highly conductive surface layers and therefore suited to investigate ionic materials with enhanced surface conductivity (e.g. AgCl). (vi) Several other applications of microelectrodes in solid state ionics are briefly touched upon.

Cryogenic operation of surface-channel diamond field-effect transistors

Cryogenic operation of field-effect transistors (FETs) fabricated on hydrogen-terminated (H-terminated) diamond surface conductive layers is investigated. 5-μm gate-length metal-insulator-semiconductor FETs (MISFETs) is fabricated using CaF 2 film as a gate insulator. The MISFETs operate successfully even at 4.4 K. At low temperature, the contact between source/drain electrode and H-terminated diamond surface cannot maintain ohmic characteristics, because the thermal activation energy of the carriers is not high enough to overcome the barrier height at the interfaces between the source electrode and the H-terminated diamond. Estimated channel mobility increases from 63 cm 2/V-s to 137 cm 2/V-s and the maximum transconductance increases from 10.5 mS/mm to 14.5 mS/mm, as the temperature decreases from 300 K to 4.4 K, indicating reduced phonon scattering of the channel.

Electrochemical activation of electrodes for amperometric detection of nitric oxide

An open question in the literature of nitric oxide detection was investigated: does electrochemical activation account for the enhanced properties of certain presumed chemically-modified electrodes? Uniform electrodes of graphite, iridium, palladium, platinum, and ruthenium were exposed to potential cycling and then tested for amperometric response to nitric oxide to identify principles that govern electrochemical activation of nitric oxide electrodes. These electrodes were compared to similar electrodes that were not cycled. Only cycled graphite and ruthenium showed significantly increased responses. Graphite demonstrated enhanced performance after exposure to cycling potentials at which oxygen, CO 2, and soluble carbonates form, suggesting that erosion of the electrode enhanced its response by increasing the surface area accessible to nitric oxide. This may explain the performance of carbon fibers cycled to the same potentials in solutions containing metalloporphyrins. The response of ruthenium was enhanced after cycling to less extreme potentials at which soluble species do not form and at which a metallic conductive oxide, RuO 2, could lay down a stable, adherent layer on the electrode surface. Cycled ruthenium also exhibited a much greater increase in capacitance after cycling, consistent with the formation of a conductive surface layer.

Fabrication of diamond in-plane-gated field effect transistors using oxygen plasma etching

Diamond dual in-plane-gated field effect transistors with very low gate leakage current have been fabricated on an undoped hydrogen-terminated diamond p-type surface using oxygen plasma etching. Adjusting the threshold voltage optimally by one side gate, lateral electric field from the other side gate modulates the channel conductance. The oxygen plasma etching of 60 nm in depth fully isolated the channel of the hydrogen-terminated diamond surface conductive layer from the side gates resulting very low gate leakage current (<1 pA at −60 V) at room temperature. This feature provides a necessary condition for the fabrication of diamond single-hole transistors operated at room temperature.

High performance diamond MISFETs using CaF 2 gate insulator

A cut-off frequency of 15 GHz and a maximum frequency of oscillation of 20 GHz are realized in a 0.4-μm gate diamond metal–insulator–semiconductor field-effect transistor (MISFET). The cut-off frequency is the highest value for diamond FETs ever reported. The RF characteristics of the MISFETs are higher than those of metal–semiconductor FETs at the same gate lengths. The CaF 2 gate insulator improves the carrier mobility according to the Hall measurement system. The mobility increases in the surface conductive layer result in high RF performance. The source–gate passivation of CaF 2 results in the high DC transconductance because of the reduction of series resistances. A cut-off frequency of more than 30 GHz is expected with the gate minimization and the CaF 2 passivation of source–gate and gate–drain spacings.

Fabrication of in-plane gate transistors on hydrogenated diamond surfaces

The highly conductive surface layer induced in diamond by hydrogen termination has been used to fabricate in-plane gate transistors. The conductive channel has been separated from the Ohmic gate contacts by insulating thin lines, obtained by using a combination of electron-beam lithography with surface oxidation. Oxidized lines of about 100 nm show excellent blocking properties, with leakage current of 0.3 pA/μm at 100 V and room temperature. In-plane transistor properties are reported for operation at 77 K and room temperature with good saturation characteristic and complete pinch-off.

Surface conductivity on hydrogen terminated diamond

Hydrogen terminated diamond exhibits surface conductivity of a p-type character. However, hydrogen termination itself is not sufficient to promote this effect and it has been shown by various authors that an aqueous layer containing adsorbates of a particular electronegativity is required to promote electron out diffusion from the surface, thus creating a population of holes in the vicinity of the surface. This paper describes detailed electrical measurements on this surface conductive layer in various environments. The stability of this transport mechanism with particular attention to device applications is discussed.

Diamond diodes and transistors

Over the past few years a variety of diamond electron devices have been fabricated, analysed and simulated. This includes Schottky diodes on boron-doped p+ diamond substrates, boron/nitrogen pn-junction diodes, bipolar transistors based on this pn-junction and field effect transistors (FETs) with boron delta-doped channels and hydrogen-related surface conductive layers. Many of the fabricated devices considered here represent the current state-of-the-art in this field. This includes the operation of diamond Schottky diodes at temperatures of up to 1000 °C, as well as diamond FET devices with a cut-off frequency of 30 GHz and channel current densities of 300 mA mm−1. Simulations show that diamond boron delta-doped FETs might yield an RF-output power density of up to 30 W mm−1.

Field emission from diamond particles studied by scanning field emission microscopy

Field emission properties from diamond particles (DPs) are studied. The DPs with thin chemically vapor deposited (CVD) diamond overcoat, dispersed onto metal substrate, essentially exhibit negative electron affinity (NEA). Field emission, approximately 1 mA/cm 2 under a macroscopic electric field of 3.5 kV/mm are observed. Microscopic electrical properties were studied by scanning tunneling microscopy/spectroscopy. Most parts of the DP surface exhibit narrow gap and p-type characteristics. The localized regions, which have wide gap like bulk diamond properties, are randomly distributed near the top of DP. The field emission current distribution depicted by scanning field emission microscopy (SFEM) show that the electron emission is originating from a localized region on the selected DPs. We found, through SFEM measurement, some favorable field emission spots (“hot spots”) where measured emission current is several orders higher than that of the other DPs (“normal spots”). Field emission spectroscopy (FES) results suggest that a poorly conducting layer is present along the electron path from the metal electrode to vacuum. We propose two models for field emission from “hot spots”, which involve two main mechanisms. One is electron injection from the metal substrate to the DP, which is attributed to the electric field enhancement at intrinsic non-doped diamond (i-diamond) layer sandwiched between the metal substrate and the surface conductive layer (p-diamond) of the CVD diamond overcoat on the DP. The other is electron emission at the top site of NEA DP through the local i-diamond region or the depletion region of the p-diamond, which is caused by the applied electric field.

Characterization of Sub-Micron In-Plane Devices in H-Terminated Diamond

The highly conductive p-type surface layer that can be induced in diamond by hydrogen termination is very attractive for electrical applications and in particular for the design of in-plane devices. In this paper we will present our work on the characterization of sub-micron in-plane transistors. Two different approaches have been analyzed for the design of the transistor gate. First of all the properties of Al gates on H-terminated diamond will be discussed, focusing on their capacitance–voltage characteristics. As a result of this study it will be shown how the p-type surface layers in H-terminated diamond behaves like a two-dimensional system. However, due to the poor channel control by the Al gates another approach was followed. The gate region is separated from the channel by a thin oxidized line. The channel current is controlled by the voltage applied to the gate, which varies the space charge region width that is formed at the oxidized line. Sub-micron gate structures were generated by using e-beam lithography and a plasma oxidation treatment. The transistor characteristics of the in-plane diamond structures are presented in this paper.

Propagating brush discharge initiation of dust layers – A new test method

In many powder handling processes, powder can be deposited onto a metal or conductive surface layers. If the powders are very good insulators, the deposited layer can build up electrostatic charges to the level of local electrostatic breakdown creating a propagating brush discharge. An explosion hazard can exist if the powder fines cling to the insulating layer in process. The occurrence of a discharge on the insulating layer, either by internal electrostatic breakdown or by moving a grounded object toward the insulating layer, can loft the dust into the air. This discharge may also have sufficient energy to ignite the dust cloud causing catastrophic explosion results. A new test method has been developed to simulate this occurrence. Its goal is to determine the electrostatic discharge energy in the propagating discharge required to ignite the lofted dust. Propagating brush discharge having energies less then 30 millijoules have ignited dust deposits on insulating surfaces backed up by grounded conductive material. A method description and some test results are presented in this paper.

Effect of Cl- Ionic Solutions on Electrolyte-Solution-Gate Diamond Field-Effect Transistors

Diamond field-effect transistors (FETs) operate in electrolyte solutions having a wide pH range of 1–13. The FETs have been fabricated using a p-type surface conductive layer, where the diamond surface is exposed directly to the electrolyte solutions. From the drain current-gate voltage (Ids–Vgs) characteristics of the FETs, it appears that the threshold voltages of the FETs are independent of the pH value of the solution. In Cl- ionic solutions, however, the threshold voltages shift approximately 30 mV with a one-order-of-magnitude change of molar concentration of Cl- ions. This sensitivity of the FET to Cl- ion's concrentration is observed in the 10-1–10-6 M range of potassium chloride (KCl) solutions.

DC and RF characteristics of 0.7-μm-gate-length diamond metal–insulator–semiconductor field effect transistor

A 0.7-μm-gate-length metal–insulator–semiconductor field effect transistor (MISFET) was fabricated on a hydrogen-terminated diamond surface conductive layer. The maximum transconductance of 100 mS/mm was obtained by DC measurement. The cut-off frequency of 11 GHz and the maximum frequency of oscillation of 18 GHz were achieved for the fabricated MISFET biased at VGS=0 V and VDS=−12 V. These are the highest values for diamond MISFETs ever reported. In the MISFET, high-frequency small-signal equivalent circuit analysis is carried out at VGS=0 V and VDS=−3, −5, −8, −10 and −12 V. The analysis indicates that the reduction of parasitic resistance between the source and gate is necessary for realizing higher output power.

Applications of Diamond Surfaces to Electronics and Bioelectronics.

Hydrogen-terminated diamond surface shows p-type conduction even if it is not intentionally doped. Metal-semiconductor (MES) and metal-insulator-semiconductor (MIS) field-effect transistors have been fabricated on hydrogen-terminated diamond surface conductive layers, and their high-frequency performance is demonstrated. Nano-fabrication on a hydrogen-terminated diamond surface is performed by controlling surface adsorbates, using an atomic force microscope (AFM) technique. Insulated areas are successfully obtained by changing hydrogen termination to oxygen termination. Single hole transistors are also fabricated by AFM oxidation, and are operated at liquid nitrogen temperature (77 K). Polycrystalline diamond FET is operated in electrolyte solution. A perfect pinch-off and saturated current-voltage characteristics have been obtained for bias voltages within the potential window. The threshold voltages are almost constant in the electrolytes with different pH values of 7−13. Using this pH insensitive surface, ion selective regions can be fabricated to form transistor-based biosensors.

Modelling the electric field at the seafloor due to a non-uniform ionospheric current

Electric fields occurring at the seafloor during geomagnetic disturbances drive geomagnetically induced currents (GIC) in submarine cables, possibly leading to problems in the operation of the equipment and systems connected to the cable. To be able to estimate the GIC risk on the cables, it is necessary to know the magnitude of the electric field at the seafloor, which can be conveniently determined from the magnetic data recorded at the surface. In this paper, we consider the theoretical models associated with non-uniform source fields existing at high latitudes. The case of a two-dimensional ionospheric current distribution is summarized together with numerical examples, and formulas for the general three-dimensional ionospheric–magnetospheric current system are derived in detail. The seawater is modelled by a highly conducting surface layer, and the basement is characterized by a surface impedance. In the 2D case, the expression for the seafloor electric field is either an inverse Fourier transform over the wave number or a spatial convolution. Corresponding formulas are also obtained in the 3D case, and both expressions are then double integrals. Three-dimensional numerical computations are therefore much more difficult, and they will be the topic of a future paper. This paper demonstrates the applicability of the method to the estimation of the impact of geomagnetic disturbances on submarine cables.

Potential applications of surface channel diamond field-effect transistors

In order to realize high frequency and high power diamond devices, diamond FETs on the hydrogen-terminated diamond surface conductive layer have been fabricated. The fabricated diamond MESFETs show high breakdown voltage and output capability of 1 W mm −1. High transconductance diamond MESFET utilizing a self-aligned gate FET fabrication process has been operated in high frequency for the first time. In the 2 μm gate MESFETs, the obtained cut off frequency f T and maximum frequency of oscillation f max are 2.2 and 7 GHz, respectively. It is expected that the diamond MESFET with 0.5 μm gate length fabricated by self-aligned gate process shows 8 GHz of f T and 30 GHz of f max.

Reversible Switching of the Surface Conductance of Hydrogenated CVD Diamond Films

Chemical vapor deposited (CVD) diamond films have a controversial history regarding their surface electronic properties. Hydrogenation is known to induce a p-type conductive surface layer, which is not present on non-hydrogenated samples. The enhanced surface conductance can decrease significantly after annealing under high vacuum conditions at as low as 200 °C (a temperature which is sufficiently low to ensure that the hydrogen termination remains intact). Although the hydrogen is necessary for the surface conductance, the surface can be made poorly conductive without removing the hydrogen termination. We have performed scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to better understand the origin of the enhanced surface conductance. Our STS experiments confirm that hydrogenation induces the appearance of a conductive surface layer, which can change considerably after high vacuum annealing. In this paper, we will discuss the conditions under which the surface conductivity can be restored. In particular we study the conductance changes during plasma hydrogenation and after exposing it to atmospheric conditions. Topographical STM scans confirm that the surface structure is not altered at low annealing temperatures.

Shallow junction formation on p-like CdTe crystals by indium diffusion using excimer laser annealing

Excimer laser annealing has been investigated to form a shallow junction on p-like CdTe crystals by indium diffusion. By evaporating a thin layer of indium on the CdTe crystals in a physical vapor deposition system without heating the crystals, and then irradiating them with an excimer laser, we could produce a highly conductive thin surface layer on the crystal. These layers showed n-type conductivity in Hall measurements, and an increase on the donor-acceptor pair band, possibly due to indium incorporation, was also observed in the photoluminescent spectrum. The best value of resistivity, carrier concentration and electron mobility of the layer thus obtained was 5 10−3 cm, 9 1018 cm−3, and 138 cm2/Vs, respectively. The carrier transport properties of the shallow p-n junction formed were then investigated by means of current-voltage measurements at different temperatures. Furthermore, applicability of this junction in a gamma-ray detector was also investigated.