Hans Lueth

Semiconductor-based field-effect structures for chemical sensing

Silicon sensors can be fabricated as small, rugged and reliable chip devices with a broad field of applications in medicine, biotechnology, food analysis and environmental monitoring. Thus, there is an increasing demand in realizing such sensors for the determination of, e.g. chemical and biological quantities in aqueous solutions. By developing semiconductor-based field-effect structures, moreover, their main advantage is due to the combination of both the physical effect as the transducer principle and the deposition of the sensitive layers directly onto the silicon chip. In this work, different sensor types that are originated from the field effect are presented: The capacitive ElS (electrolyte-insulator-semiconductor) sensor is suitable for the pH detection using the capacitance/voltage technique. By immobilizing an additional enzyme layer, e.g. of penicillinase, a biosensor has been realized. Both sensors can be integrated as an EIS sensor array. The utilization of the porous silicon technology offers the possibility of a further miniaturization. The LAPS (light-addressable potentiometric sensor) is based on the identical ElS structure. Here, each measuring point on the surface can be arbitrarily addressed by a probing light. The resulting photocurrent is generated as the sensor signal. This arrangement also allows a two-dimensional mapping of the spatial distribution of ions or molecules.

Field-effect-based multifunctional hybrid sensor module for the determination of both (bio-)chemical and physical parameters

Sensor systems for multi-parameter detection in fluidics usually combine different sensors, which are designed to detect either a physical or (bio-)chemical parameter. Therefore, such systems include a more complicated fabrication technology and measuring set-up. In this work, an ISFET (ion-sensitive field-effect transistor), which is well known as a (bio-)chemical sensor, is utilized as transducer for the detection of both (bio-)chemical and physical parameters. A multifunctional hybrid module for the determination of two (bio-)chemical parameters (pH, penicillin concentration) and three physical parameters (temperature, flow velocity and flow direction) using only two sensor structures, an ion generator and a reference electrode, is realized and its performance has been investigated. Here, a multifunctionality of the sensor system is achieved by means of different sensor arrangements and/or different operation modes. A Ta2O5-gate ISFET was used as transducer for all sensors. A novel time-of-flight type ISFET-based flow-velocity (flow rate) and flow-direction sensor using in-situ electrochemical generation of chemical tracers is presented. Due to the fast response of the ISFET (usually in the millisecond range), an ISFET-based flow sensor is suitable for the measurement of the flow velocity in a wide range. With regard to practical applications, pH measurements with this ISFET were performed in rain droplets.

Novel concept for flow-rate and flow-direction determination by means of pH-sensitive ISFETs

Sensor systems for multi-parameter detection in fluidics usually combine different sensors, which are designed to detect only one physical or (bio-)chemical parameter. In the present work, an ISFET (ion-sensitive field-effect transistor), which is well known as a (bio-)chemical sensor, is utilised for the flow velocity and flow direction measurement for the first time. The proposed flow sensor presents a chemical sensor-actuator system and consists of a H+-ion generator and a pH ISFET that detects the in-situ electrochemically generated H+ ions. By measuring the time of flight, the flow velocity can be determined. Since this measuring method represents a dynamic method, a calibration of the sensor usually is not required, because only relative changes in the sensor output signal are of interest. Moreover, sensor+s drift, temperature instability and sensitivity discrepancy between the various ISFETs are not relevant. The experimental results show good linearity between the measured flow velocity with the ISFET and the delivered flow rate of the pump. Due to the fast response of the ISFET (usually in the millisecond range), an ISFET-based flow sensor is suitable for the measurement of the flow velocity in a wide range. The results of the flow direction measurement with two ISFETs are presented, too.

Pulsed-laser deposition as a novel preparation technique for chemical microsensors

The application of sensitive layers for chemical microsensors consisting of multicomponent compositions and dielectric materials requires specific deposition techniques, since the different chemical and physical properties of the respective components can be significantly disturbed during the deposition process. To avoid this drawback, the pulsed laser deposition technique is suggested as a novel thin film preparation method for such sensor devices.

Influence of Silicon Doping on the SA-MOVPE of InAs Nanowires

The influence of Si-doping on the growth and material characteristics of InAs nanowires deposited by metal-organic vapor phase epitaxy (MOVPE) was investigated. It was observed that above a certain partial pressure ratio, doping has an influence on the morphology. The nanowires exhibit better uniformity but lower height vs. diameter aspect ratio as the supply of the dopant increases. It was consistantly found that the specific conductance of the nanowires also increases. Moreover the electrical measurements showed a transition from semiconducting to metallic behavior in the case of highly doped nanowires.

Thin-film electrodes for trace metal analysis by dc resistance changes

The lateral d.c. resistivity of thin metal films with layer thicknesses of less than 30 nm is increased due to the adsorption of certain particles and is decreased by their desorption. The contribution of the adatoms to the film resistivity can be understood similarly to the effect of foreign atoms in a bulk metal. The magnitude of the resistivity increase is related to the surface coverage of the thin metal film. Using thin metal films of gold as working electrodes in a conventional three-electrode arrangement, a novel electrochemical microsensor, based on the described mechanism of the surface resistivity changes has been developed. The thin film sensor has been prepared by means of process steps of silicon planar technology. With this sensor the trace analysis of heavy metals, such as cadmium, lead, nickel, thallium, and zinc ions as well as cadmium-EDTA complexes in aqueous solutions is possible. The different species could be distinguished from each other due to their characteristic stripping potentials. For the investigated species a linear signal relation has been obtained over a wide range of concentrations from several ppb to some ppm.

Insect chemoreceptors coupled to silicon transistors as innovative biosensors

Applications like the warning about agricultural pest infestations, the detection of spoilt food during storage and transport as well as the monitoring of smoldering fires require highly selective odor sensing techniques. Insect antennae that have been optimized by evolution over million of years are most suitable for such a sensitive and selective detection of certain organic substances in air. The utilization of this highly specialized sense of smell from insects needs in terms of analytical tools, however, an adaptation of the antenna to the microelectronic technique. Therefore, a beetle/FET (field-effect transistor) interface as an innovative biosensor has been developed. This BioFET (biologically sensitive FET) is based on the direct combination of the intact chemoreceptor of an insect with the gate of a FET by means of an electrolyte solution. Depending on the experimental set-up, two different biosensor configurations, namely a whole-beetle BioFET and an isolated-antenna BioFET have been designed. In both configurations, the organic compound that is detected by the beetle initiates a recognition process at its nerve cell membranes, which results in a net potential over the whole insect antenna. Then, this potential drop modified the gate conductivity and consequently, the drain current of the FET. By applying various kinds of insect antennae (e.g. ofthe Colorado potato beetle and the steelblue jewel beetle) different odor concentrations, such as cis- 3-hexen-1-ol, guaiacol and 1-octen can be detected down to the low ppb range.

AlGaN/GaN MBE heterostructures: polarization effects and their implication on electronic properties

The origin of the strong polarization fields in nitride heterostructures is discussed by comparing the symmetry properties of zincblende and wurtzite structures. Some peculiar effects in nitride heterostructure electronic properties, induced by the polarization fields are considered, like the determination of the valence band offset by x-ray photo emission spectroscopy (XPS) and the formation of 2D electron gases in AlGaN/GaN. The Fermi level position at MBE GaN, AlGaN and AlN surfaces has been measured in-situ by XPS. The role played by surface states has been emphasized, experimentally and through self consistent calculations.