R. Lal

Conducting polymer-based biosensors

Abstract The electronic conductivity of polymers such as polyaniline exhibits a strong dependence on the redox and protonation state of the polymer. This property has been utilised to develop a generic biosensor concept. The concept involves immobilization of the appropriate enzyme in the conducting polymer matrix. The enzyme catalyzed reaction of the biomolecule results in a change in redox potential and/or pH of the micro environment in the polymer matrix. This change in the micro environment triggers a change in the electronic conductivity which is monitored. Results shall be presented which provide proof of the above concept. The systems chosen for investigation are glucose/glucose oxidase, urea/urease, neutral lipid/lipase and hemoglobin/pepsin.

Deposition and properties of cadmium oxide films by activated reactive evaporation

Activated reactive evaporation (ARE) is a useful technique for the deposition of compound semiconductor films at low temperatures. High quality transparent and conducting oxides and hard coatings of transition metal compounds have been successfully deposited using this technique. However, owing largely to the complexity introduced by plasma, the microscopic deposition mechanism of the process is not yet fully understood. Based upon experimental data of cadmium oxide films deposited by ARE, it is proposed in this paper that the reaction forming CdO is heterogenous. It is also found that the microscopic deposition process and thereby the structural and electrical properties of CdO films are primarily influenced by the chamber pressure and the substrate temperature. The oxygen percentage in the chamber is found to have comparatively less influence on film properties. The data demonstrate that film resistivities as low as 2 × 104Ω cm with about 85% transparency can be obtained using this techinique.

High-field effects in silicon nitride passivated GaN MODFETs

This paper presents a detailed study of high-field effects in GaN MODFETs. Degradation of DC characteristics and change of flicker noise due to hot electron and high-reverse current stresses in Si/sub 3/N/sub 4/ passivated GaN MODFETs have been investigated. The authors observe that during hot electron stress, electron trapping in the barrier layer and interface state creation occur. These cause a positive shift of V/sub t/, reduce I/sub D/, skew the transfer characteristics, and degrade g/sub m/. Flicker noise (1/f) measurements show that after hot electron stress, the scaled drain current noise spectrum (S/sub I(D)//I/sub D//sup 2/) decreases in depletion, but increases only slightly in strong accumulation, corroborating the creation of interface states but only a small creation of transition-layer tunnel traps that contribute to 1/f noise. During high-reverse current stress, electron trapping dominates for the first 50-60 s and then hole trapping and trap creation begin to manifest. However, there still is net electron trapping under the gate after one hour of stress. The degradation processes bring about a positive shift of V/sub t/, degrade I/sub D/ and g/sub m/, and increase reverse leakage. After high-reverse current stress, S/sub I(D)//I/sub D//sup 2/ increases substantially in strong accumulation, indicating the creation of transition layer tunnel traps.

Structural and electrical properties of cadmium oxide films deposited by the activated reactive evaporation technique

Abstract Transparent and conducting films of cadmium oxide were deposited on glass substrates by the activated reactive evaporation (ARE) technique for the first time. The sublimating nature of cadmium coupled with the reduction in ambient pressure due to oxygen consumption during deposition causes loss of control, which can be mitigated by introducing oxygen to maintain the pressure. The effect of boat temperature and substrate temperature on the structural and electrical properties of the films is reported. These properties are found to be strongly dependent on substrate temperature whereas the boat temperature has little influence. Films are polycrystalline even when substrate temperature is as low as 30°C. As the substrate temperature increases, the films exhibit increasing preferred orientation in (100) direction and a reduction in lattice strain. Simultaneously, the mobility and carrier concentration also improve. The ARE process enables deposition of excellent transparent and conducting films with conductivity about 2500 Ω -1 cm -1 which is as good as the best reported for CdO films.

Photoluminescent, wide-bandgap a-SiC:H alloy films deposited by Cat-CVD using acetylene

Hydrogenated amorphous silicon/carbon films (a-Si-C:H) are deposited from a silane and acetylene gas mixture by the catalytic chemical vapour deposition (Cat-CVD) technique. It is observed that under certain conditions of total gas pressure and filament temperature (TF), the optical bandgap varies non-linearly with the acetylene to silane (C2H2/SiH4) ratio, having a maximum value of 3.6 eV for a C2H2/SiH4 ratio ≥0.8. However, the deposition rate drastically reduces with an increase in acetylene fraction. FTIR spectra indicate that the total hydrogen content is lower compared to samples deposited by PECVD using similar gas mixtures, with hydrogen being preferentially attached to carbon rather than silicon atoms. The photoluminescence (PL) spectra of these films show PL in the visible spectral region at room temperature. The films with larger bandgap (>2.5 eV) exhibit PL at room temperature, with the emission having peak energy in the range 2.0–2.3 eV.

Effect of electrolyte exposure on silicon dioxide in electrolyte-oxide-semiconductor structures

Abstract The effect of hydration and ion penetration in silicon dioxide has been studied by fabricating metal-oxide-semiconductor capacitors on oxide exposed to acidic solutions with and without bias. Changes in oxide properties due to exposure have been probed using avalanche injection. Oxide charge build-up and interface state generation in the capacitors have been monitored. The measurements show that before avalanche injection the capacitance-voltage plots for the various electrolyte exposure conditions are similar. However, oxide integrity degradation is different for cathodic bias compared with anodic bias or no bias. This is due to penetration of protons or hydroxyl ions in the oxide, producing hydrogen-related defects that effect trapping and interface state generation.

Electrochemistry of polyaniline Langmuir-Blodgett films

Abstract Langmuir–Blodgett (LB) manipulation of polyaniline has been variously employed to obtain organized assemblies for use in molecule-based electronic devices. However, the electrochemical characteristics of polyaniline LB films which have a direct implication on the proposed device applications have not been given much attention. We report here, electrochemical characterization of multilayer LB films of polyaniline prepared by employing N -methylpyrrolidone as the processing solvent. The films have been transferred onto a variety of substrates such as quartz, platinum, platinum sputtered on quartz, and conducting tin oxide on glass. The electrochemistry of polyaniline LB films has been studied using cyclic voltammetry coupled with a quartz crystal microbalance (QCM). Cyclic voltammograms were obtained for films transferred at different surface pressures, for films containing different number of layers. The electrochemical characteristics of LB multilayers were compared with those of electrochemically deposited films and were used to infer the packing behaviour of polymer chains in the LB film structure. It was found that multilayer LB films are electroactive but the kinetics of counter ion transport in these films is slower than that observed in electrochemically deposited films. The multilayer LB films also show poor electrochromic switching behaviour.

Facile fabrication of microfluidic systems using electron beam lithography

We present two fast and generic methods for the fabrication of polymeric microfluidic systems using electron beam lithography: one that employs spatially varying electron-beam energy to expose to different depths a negative electron-beam resist, and another that employs a spatially varying electron-beam dose to differentially expose a bi-layer resist structure. Using these methods, we demonstrate the fabrication of various microfluidic unit structures such as microchannels of a range of geometries and also other more complex structures such as a synthetic gel and a chaotic mixer. These are made without using any separate bonding or sacrificial layer patterning and etching steps. The schemes are inherently simple and scalable, afford high resolution without compromising on speed and allow post CMOS fabrication of microfluidics. We expect them to prove very useful for the rapid prototyping of complete integrated micro/nanofluidic systems with sense and control electronics fabricated by upstream processes.

The DNA SET: a novel device for single-molecule DNA sequencing

We present the principle of the dnaSET, a novel device for single-molecule sequence analysis of DNA strands. The central idea is to measure the variations in current through a single electron transistor (SET) as a target DNA molecule slithers through a nanoeye integrated between the SETs floating island and gate. We present a basic but illustrative simulation of the operation, and the results therefrom lead us to conclude that the DNA SET has a single-nucleotide resolution that enables it to read off the individual bases in a DNA strand. The versatility of this idea makes it suitable for incorporation in other bio-assay tools, and thus the DNA/protein SET promises to be a powerful direct sensing device for a wide range of applications.

Modeling, Simulation, and Design Guidelines for Piezoresistive Affinity Cantilevers

This paper presents design guidelines for piezoresistive affinity cantilevers for operation in liquid environments. For the first time, we consider the interdependence of various functional elements (such as biological, mechanical, and electrical) of the cantilever, their dependence on material choice, microfabrication processes, and geometry, and the resultant effects on the mechanical and electrical sensitivities of the cantilever. The cantilever design guidelines that include material selection as well as determination of geometrical dimensions are proposed. As an example, we have designed and simulated a multilayer piezoresistive silicon nitride affinity cantilever for performance in a liquid environment under constraints imposed by microfabrication and electrical and mechanical considerations. Systematic steps toward optimization of geometrical dimensions include initial analytical estimates of geometrical dimensions, followed by finite-element modeling and analysis of such cantilevers under the applied surface stress. Simulation studies brought forth the limitation on maximum obtainable ΔR/R as well as the nonlinear behavior of the cantilever which was not observed in analytical estimates.