Katarina Radulović

Adsorption-desorption noise in nanowire FET biosensors

The adsorption-desorption (AD) noise in nanowire FET biosensors is analyzed, which manifests itself in the form of gate voltage fluctuations resulting from the fluctuations of the number of adsorbed particles. The derived theoretical model of the fluctuations takes into account the mass transfer effects. The numerical calculations show that when the mass transfer is slow, the low-frequency amplitude and the corner frequency of the Lorentzian AD noise spectrum obtained by derived expressions are significantly different compared to the same parameters predicted by the model that neglects mass transport effects. The presented analysis enables good estimation of these parameters from the measured noise spectrum, which is important because they are used as the source of information about the analyte concentration and the kinetics of biomolecular interactions.

Photoacoustic elastic bending method: Study of the silicon membranes

Photoacoustic (PA) amplitude and phase spectra vs the modulation frequency of the excitation optical beam were measured and analyzed by PA elastic bending method. The PA spectra were measured in a frequency range (from 20 to 20000 Hz), for different thicknesses of Si rectangular membranes (from 10 to 100 μm).. The elastic characteristics of the Si chip with square membranes were described by the theory of thin elastic plate, i.e. as an elastic simply supported rectangular plate. The PA signals (sum of the thermodiffusion, thermoelastic and electronic deformation components) were calculated and compared with the experimental ones. These results showed that the PA elastic bending spectra are convenient for investigation the characteristics of micromechanical membraines. The PA elastic bending spectra of the optically driven micro-opto-electro-mechanical systems (MOEMS) enable, for example, to investigate the different electronic and thermal transport characteristics or technological processes in MEMS fabrications, etc.

Boron Redistribution During SOI Wafers Thermal Oxidation

We fabricated silicon-on-insulator (SOI) pressure sensors intended for operation in a wide temperature range. After the thermal oxidation process, the measured sheet resistance of the silicon active layer was higher than expected for the case when the dopant distribution in the active layer is uniform and the layer thickness reduced. The reason was the redistribution of dopant (boron) in the active layer during the high temperature thermal oxidation processes. We developed a model to calculate the boron redistribution in an SOI wafer which was initially uniformly doped. The temperature dependence of hole mobility (and, based on it, the sheet resistance and its temperature dependence) was determined for a calculated impurity profile after oxidation in wet O 2. The experimental temperature dependence of sheet resistance was obtained by measuring the temperature dependence of piezoresistor resistance. The best match between the theoretical and experimental TCR (temperature coefficient of resistance) was achieved by slightly modifying the Aroras model for hole mobility

A contribution of carrier transport processes to the photoacoustic effects in doped narrow gap semiconductors

The contribution of free carrier transport processes to the photoacoustic effect in doped AIVBVI compounds was investigated using the photoacoustic (PA) frequency modulated transmission technique as a valuable tool in the study of thermal and carrier transport processes in semiconductors. The PA signals were measured and analyzed as a function of the modulation frequency in a specially constructed PA cell. The multiparametric fitting procedure was used to obtain some thermal and electronic transport parameters of doped and pure AIVBVI compounds.

Functionalization of plasmonic metamaterials utilizing metal–organic framework thin films

We considered theoretically and experimentally a strategy to functionalize plasmonic metamaterials utilizing either a metal–organic framework (MOF) or inorganic–organic hybrids for application in adsorption-based gas sensing. MOFs are one-dimensional (1D), 2D or 3D crystalline compounds that simultaneously contain metal ions or ion clusters and organic moieties, forming thus porous networks ensuring an increased effective surface for adsorption. Metamaterials can enhance plasmonic sensor performance through metal–dielectric nanocompositing that simultaneously tailors the electromagnetic response and boosts adsorption of the targeted analyte through the use of nanopores. To perform functionalization, it is necessary to integrate one or several layers of MOF nanocrystals with the metamaterial scaffold. The simplest approach is to use dip or drop coating or the layer-by-layer technique. The scaffolds that we considered included freestanding, ultrathin membranes and sandwich structures with nanoaperture arrays. For this investigation, we used a non-aqueous sol–gel route to synthesize vanadium oxyanthracene carboxylate, a novel material with 1D crystal structure. Our results suggest that preferential concentration of analyte within the MOF pores may ensure improved adsorption and thus sensor sensitivity enhancement. Also, one may increase selectivity by introducing nanoparticle fillers or by utilizing other functionalizing materials such as catalysts or ligands.

MEMS accelerometer with all-optical readout based on twin-defect photonic crystal waveguide

A novel design is proposed for a Si accelerometer sensor chip where a stress-tunable photonic crystal is used as the sensitive element to optically measure the deflection of a bulk-micromachined diaphragm with a boss. The photonic crystal may be 1D stack, 2D channel type, microbridge, etc. The photoelasticity effect is used instead of customarily utilized piezoresistivity and thus an all-optical readout is obtained. A feasibility analysis of the proposed structure is performed. 3D finite element modeling is used to determine the mechanical properties of the structure and the transfer matrix method to calculate its electromagnetic behavior. The advantages of the proposed concept are its simplicity, all-optical operation, monolithic integration with the sensor chip and compatibility with contemporary fiber-optic and integrated optics systems and devices.

Photopyropiezoelectric elastic bending method

A method of investigation of semiconductors and metal–semiconductor structures based on two techniques: The elastic bending technique and pyropiezoelectric (PPE) technique was presented. The method was demonstrated on a metal–semiconductor–metal (MSM) structure, which is attached to a PPE detector. Two different ac voltages can be measured: One on the electrodes of MSM structure—the ac-photovoltage, and another on the electrodes of the PPE detector—the PPE voltage. The ac photovoltage is a consequence of the photogenerated plasma processes in the sample (MSM structure). Photogenerated plasma waves in a semiconductor are followed by the thermal and elastic waves (the elastic bending). Then, the pyroelectric voltage is a consequence of the thermal processes and the piezoelectric voltage is a consequence of elastic bending in the sample-PPE detector system. A theoretical model for a metal–semiconductor–metal-pyro(piezo)electric system is given including the space-charge regions and electronic states on the s...

Investigation of the microcantilevers by the photoacoustic elastic bending method

The amplitude of the photoacoustic (PA) elastic bending signals vs. the modulation frequency of the excitation optical beam for the chip with Si cantilevers were measured and analyzed. The experimental PA elastic bending signals of the whole micromechanical structure were measured by using special constructed PA cell (the gas-microphone detection technique with transmission configuration). The PA spectra were measured in a frequency range from 20 to 20000 Hz. The signal in the PA cell without optical excitation (noise) was also measured and analyzed. Experimental results show that the PA measuring system (PA cell width electrets microphone and lock-in amplifier) has signal-noise ratio S/N ~ 30000 at 100 Hz; ~ 3000 at 1000 Hz; ~ 5 at 10000 Hz. The correction of the measured signal, in order to remove the coherent electronic noise as a systematic error, was made. The experimental PA elastic bending signals of the cantilever were compared with the experimental PA elastic bending signals of the Si square membranes. These results showed that the PA elastic bending method is convenient for investigation the characteristics of micromechanical structures as microcantilevers.

Numerical simulation of transient response of chemical and biological micro/nanofabricated sensors operating in multianalyte environments

In different kinds of surface-based chemical and biological micro/nanosensors, interpretation of the experimentally obtained data is performed based on approximate solutions for the sensor response, valid for the case of single-analyte samples. A question arises whether or not the approximate models used for experimental data fitting in the case of a single analyte are also applicable for multianalyte environments. We test the validity of the two-compartment model for approximation of sensor transient response in the case of concurrent binding of multiple analytes on the sensing surface, by comparing the approximate response with the numerical simulation of the dynamics of a real adsorption experiment.

Gradient-index infrared metamaterials based on metal-dielectric submicrometer pillar arrays

This paper presents the design and microfabrication of a two-dimensional metal-dielectric metamaterial structure based on an array of pillars with submicrometer diameters and heights. The diameters of pillars periodically vary along one axis in a sawtooth fashion and are constant along the other. The electromagnetic field distribution within this graded metamaterial was considered utilizing an accurate analytical approach. The pillar arrays were fabricated in photoresist and subsequently covered with a sputter-deposited aluminum film. Structures were defined by direct laser writing in photoresist film. Controlled overexposure has been applied in order to make pillar features smaller than the nominal resolution of the equipment. The structures were characterized by optical and atomic force microscopy and by angle-dependent Fourier Transform infrared spectroscopy. The produced graded frequency-selective surfaces may be used e.g. in sensing.