Elder A. de Vasconcelos

Potential of a simplified measurement scheme and device structure for a low cost label-free point-of-care capacitive biosensor

A simplified measurement scheme and device structure aiming at developing a low cost, label-free, point-of-care capacitive biosensor were investigated. The detection principle is the increase of low frequency capacitance between two planar Al electrodes observed after antibody-antigen interaction. The electrodes, deposited on oxidized Si wafers, were covered with an antibody layer, with and without using self-assembled thiol monolayer. Immunoglobulin G (IgG) and cardiac troponin T (TnT) were used as analytes to asses this proposal. The device was able to detect successfully TnT levels in the range 0.07 to 6.83ng/mL in human serum from patients with cardiac diseases and in the range 0.01ng/mL to 5ng/mL for TnT in phosphate buffer saline. An equivalent circuit model able to reproduce the general behavior of experimental capacitance versus frequency curves was presented. The investigated features that have potential to reduce costs and simplify measurements were: use of single, low frequency (1kHz) measurement signal, within the range of low cost portable capacitance meters; employment of a lower cost electrode material, aluminum, instead of gold electrodes; and use of simple and miniaturized planar two-electrodes arrangement, thus making a portable system for point-of-care applications.

Potential of High-purity Polycrystalline Silicon Carbide for Thermistor Applications

This report concerns with a new material for thermistors, which consists of an undoped, polycrystalline, ultrahigh-purity chemical vapor deposition silicon carbide wafer. The resistance-temperature characteristic was measured from 25°C to 365°C. A good fit to the thermistor equation was observed and the thermistor B constant was 4845 K. These wafers are thermally and chemically stable, homogeneous and pure. They might be useful for fabrication of sensitive thermistors with stability and reproducibility of characteristics.

Correlation between dopant reduction and interfacial defects in low-energy x-ray-irradiated MOS capacitors

We discuss effects associated with dopant reduction and interface defects generated in p-type metal-oxide semiconductor (MOS) capacitors exposed to doses of low-energy x-ray radiation up to 500 Mrad (Si). The dependence on dose, size, oxide charge and interface trapped charge density suggests that the dopant-reduction behaviour is influenced by the oxide and interface charges and their behaviour during and after irradiation. In particular, the interfacial defect generation dynamics suggests that mechanisms associated with tunnelling of carriers at the interface may be involved.

Dynamic Photocurrent Images of a Gas Sensing Surface

A high-speed surface photovoltage technique which uses a numerically processed parallel lock-in amplifier was applied to obtain olfactory images. This high-speed system has measurement time short enough to allow the observation of the time evolution of gas response patterns. Representative images are presented and some of their characteristic features, such as the influence of gas type and the direction of gas flow, are discussed. Possible directions for further improvement of the system are also discussed. The capability to obtain dynamic photocurrent images is an important step towards real-time visualization and further development of olfactory camera technology.

High-temperature thin-catalytic gate devices for combustion emissions control

4H- and 6H-SiC Schottky diodes responding down to 5 ppm of NO and NO2 gases at temperatures up to 450oC were fabricated. Upon exposure to gas, the forward current of the devices changes due to variations in the Schottky barrier height. For NO gas, the response follows a simple Langmuir adsorption model. Device parameters were evaluated from linear conductance/current versus conductance plots, with improved accuracy. The linearity of these plots is an indication that the devices are not being affected by interfacial oxide layers or pinning of the Fermi level. 4H-SiC devices showed slightly superior stability and sensitivity for this application.

Reliability physics study for semiconductor-polymer device development

Abstract We discuss several aspects of the reliability physics of silicon–polyaniline heterojunctions, such as degradation effects induced by local heating, charge trapping and temperature changes. The results further confirm the quality of the devices electrical characteristics and their suitability for radiation and gas sensors applications.

A versatile technique to transfer multi-walled carbon nanotubes membranes to surfaces

We present a simple and versatile technique to fabricate and transfer multi-walled carbon nanotube (MWCNT) membranes to virtually any substrate, including devices and a large variety of materials with different shapes. First, a mesoporous silica (Mobil Composite material MCM-41)/MWCNT composite is deposited by spin coating on SiO2. Immersion of the wafer in HF solution, releases a floating MWCNT membrane that strongly adheres to any surface that makes contact with it. The process does not require functionalization or surfactants, thus avoiding contamination and additional cleaning steps, and has potential to be extended to allow the fabrication and transfer of membranes of other carbon materials, including other CNT types, fullerenes and graphene.

Thermal Lens Technique for the Determination of SiC Thermo-Optical Properties

In this paper we combine the use of photo-thermal techniques, Thermal lens (TL) spectrometry, Photoacoustic and heat capacity measurements to determine the optical path dependence with temperature (ds/dT) of a polycrystalline 3C-SiC sample. Results obtained for the polycrystalline sample with the TL technique show that ds/dT is negative at room temperature. This means that the thermal lens formed in 3C-SiC acts as a divergent lens when light impinges the sample. Our measurements, demonstrate that photo-thermal techniques can be used to obtain thermal parameters in circumstances where other techniques cannot be used, for example, in harsh environments.

Post-irradiation dopant passivation in MOS capacitors exposed to high doses of x-rays

We study post-irradiation dopant passivation in p-type silicon metal-oxide-semiconductor capacitors exposed to high doses of low-energy x-rays. The passivation presents a logarithmic temporal behaviour which can be detected within less than one hour and continues to evolve for several thousand hours after irradiation. The time delay between irradiation and passivation increases with radiation dose and a change in the anneal curve slope of oxide charge is associated with the onset of passivation. These observations are discussed in terms of tunnelling electrons from the silicon substrate, which may interact with traps in the oxide and trigger the passivation by releasing hydrogen-related species.

X-Ray Radiation Response of Epitaxial and Nonepitaxial n-6H–SiC Metal-Oxide-Semiconductor Capacitors

We investigated total interface trap generation in epitaxial and nonepitaxial n-type 6H–SiC metal-oxide-semiconductor (MOS) capacitors by the photo-capacitance–voltage method (Jenq technique) for doses of low energy X-rays up to 20 Mrad (Si). Interface trap generation is low for both epitaxial and nonepitaxial SiC MOS capacitors and they show considerable radiation hardness, even without being specially processed to be radiation-hard. Characteristic features of the photo-capacitance–voltage curves and trapped charge are briefly discussed.