Field-effect Structure Research Articles

Characterization of biodegradable polymers with capacitive field-effect sensors

In vitro studies of the degradation kinetic of biopolymers are essential for the design and optimization of implantable biomedical devices. In the presented work, a field-effect capacitive sensor has been applied for the real-time and in situ monitoring of degradation processes of biopolymers for the first time. The polymer-covered field-effect sensor is, in principle, capable to detect any changes in bulk, surface and interface properties of the polymer induced by degradation processes. The feasibility of this approach has been experimentally proven by using the commercially available biomedical polymer poly(d,l-lactic acid) (PDLLA) as a model system. PDLLA films of different thicknesses were deposited on the Ta2O5-gate surface of the field-effect structure from a polymer solution by means of spin-coating method. The polymer-modified field-effect sensors have been characterized by means of capacitance–voltage and impedance-spectroscopy method. The degradation of the PDLLA was accelerated by changing the degradation medium from neutral (pH 7.2) to alkaline (pH 9) condition, resulting in drastic changes in the capacitance and impedance spectra of the polymer-modified field-effect sensor.

Characterization of Large Area 4H-SiC and 6H-SiC Capacitive Devices at 600 °C

SiC based capacitive devices have the potential to operate in high temperature, chemically corrosive environments provided that the electrical integrity of the gate oxide and metallization can be maintained in these environments. We report on the performance of large area, up to 8 x 10-3 cm2, field-effect capacitive sensors fabricated on both the 4H and 6H polytypes at 600°C. Large area capacitors improve the signal/noise (S/N) ratio which is proportional to the slope of the capacitance-voltage characteristic. At 600 °C we obtain a S/N ~ 20. The device response is independent of polytype, either 4H or 6H-SiC. These results demonstrate the reliability of our field-effect structure, operating as a simple potentiometer at high temperature.

Stark spectroscopy and radiative lifetimes in single self-assembled CdTe quantum dots

We present studies on Coulomb interactions in single self-assembled CdTe quantum dots. We use a field effect structure to tune the charge state of the dot and investigate the impact of the charge state on carrier wave functions. The analysis of the quantum confined Stark shifts of four excitonic complexes allows us to conclude that the hole wave function is softer than electron wave function, i. e. it is subject to stronger modifications upon changing of the dot charge state. These conclusions are corroborated by time-resolved photoluminescence studies of recombination lifetimes of different excitonic complexes. We find that the lifetimes are notably shorter than expected for strong confinement and result from a relatively shallow potential in the valence band. This weak confinement facilitates strong hole wave function redistributions. We analyze spectroscopic shifts of the observed excitonic complexes and find the same sequence of transitions for all studied dots. We conclude that the universality of spectroscopic shifts is due to the role of Coulomb correlations stemming from strong configuration mixing in the valence band.

Theoretical study of indirect excitons’ lifetime in coupled AlGaN/GaN quantum wells in the presence of an electrostatic trap

The lifetime of electrostatically trapped indirect excitons in a field-effect structure based on coupled AlGaN/GaN quantum wells has been theoretically studied. Within the plane of a double quantum well, indirect excitons are trapped between the surfaces of the AlGaN/GaN heterostructures and a semitransparent metallic top gate. The trapping mechanism has been assumed to be a combination of the quantum confined Stark effect and local field enhancement. In order to study the trapped exciton lifetime, the binding energy of indirect excitons in coupled quantum wells is calculated by finite difference method in the presence of an electric field. Thus, the lifetime of trapped excitons is computed as a function of well width, AlGaN barrier width, the position of double quantum well in the device and applied voltage.

Dynamics of charge leakage from self-assembled CdTe quantum dots

We study the leakage dynamics of charge stored in an ensemble of CdTe quantum dots embedded in a field-effect structure. Optically excited electrons are stored and read out by a proper time sequence of bias pulses. We monitor the dynamics of electron loss and find that the rate of the leakage is strongly dependent on time, which we attribute to an optically generated electric field related to the stored charge. A rate equation model quantitatively reproduces the results.

Anomalous Hall Effect in Field-Effect Structures of (Ga,Mn)As

The anomalous Hall effect in metal-insulator-semiconductor structures having thin (Ga,Mn)As layers as a channel has been studied in a wide range of Mn and hole densities changed by the gate electric field. Strong and unanticipated temperature dependence, including a change of sign, of the anomalous Hall conductance sigma(xy) has been found in samples with the highest Curie temperatures. For more disordered channels, the scaling relation between sigma(xy) and sigma(xx), similar to the one observed previously for thicker samples, is recovered.

Charge storage in self-assembled CdTe quantum dots

We demonstrate storage of photoexcited electrons in a layer of self assembled CdTe quantum dots embedded in a field-effect structure. We study the storage efficiency with respect to the duration of the photoexcitation pulse. The electrons persist in the quantum dots for as long as 9 milliseconds.

Emission properties of single InAs/GaAs quantum dot pairs and molecules grown in GaAs nanoholes

We have studied the lateral coupling between InAs/GaAs quantum dot pairs embedded in a field-effect structure. Quantum dot pairs and molecules have been identified by the correlated evolution of the Coulomb blockade features of each QD in the pair. This behaviour is largely distorted in the presence of resonant coupling states in the QD molecule. Single QD voltage evolution shows a crossover in the lineshape profile, which is associated to Spectral Diffusion processes due to residual charged environment.

Penicillin biosensor based on a capacitive field-effect structure functionalized with a dendrimer/carbon nanotube multilayer

Silicon-based sensors incorporating biomolecules are advantageous for processing and possible biological recognition in a small, reliable and rugged manufactured device. In this study, we report on the functionalization of field-effect (bio-)chemical sensors with layer-by-layer (LbL) films containing single-walled carbon nanotubes (SWNTs) and polyamidoamine (PAMAM) dendrimers. A capacitive electrolyte-insulator-semiconductor (EIS) structure modified with carbon nanotubes (EIS-NT) was built, which could be used as a penicillin biosensor. From atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM) images, the LbL films were shown to be highly porous due to interpenetration of SWNTs into the dendrimer layers. Capacitance–voltage (C/V) measurements pointed to a high pH sensitivity of ca. 55mV/pH for the EIS-NT structures. The biosensing ability towards penicillin of an EIS-NT-penicillinase biosensor was also observed as the flat-band voltage shifted to lower potentials at different penicillin concentrations. A dynamic response of penicillin concentrations, ranging from 5.0μM to 25mM, was evaluated for an EIS-NT with the penicillinase enzyme immobilized onto the surfaces, via constant–capacitance (ConCap) measurements, achieving a sensitivity of ca. 116mV/decade. The presence of the nanostructured PAMAM/SWNT LbL film led to sensors with higher sensitivity and better performance.

Coaction of electric field induced strain and polarization effects inLa0.7Ca0.3MnO3/PMN-PTstructures

The coaction of the electric field induced strain and polarization effect on magnetic and electrical properties in field-effect structures, composing of ${\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$ (LCMO) films and $\text{Pb}({\text{Mg}}_{1/3}{\text{Nb}}_{2/3}){\text{O}}_{3}{\text{-PbTiO}}_{3}$ (PMN-PT) crystal, have been studied. The strain state of the films can continuously be tuned by application of bias electric field. The resistance variation in LCMO film is about $\ensuremath{-}4.34%$ and $+3.48%$ when a negative $(\ensuremath{-}8\text{ }\text{kV}/\text{cm})$ and positive $(+8\text{ }\text{kV}/\text{cm})$ bias field is applied, respectively, at 50 K. With increasing temperature, the field-dependent shuttlelike loop of relative resistance changes to butterflylike hysteresis loop under the coaction of field induced strain and polarization effect. Moreover, a sharp, stable, and reversal magnetization controlling effect caused by electric field has also been found in the structure. The magnetization decreases as much as $\ensuremath{-}8.64%$ when a positive field 9.6 kV/cm is applied at 10 K. The data clearly show that the electric field induced strain effect is dominant for the magnetization, while the polarization is dominant for the effect on electrical transport properties when the LCMO is in ferromagnetic metal state. These results reveal that the electric field can tune the lattice distortion and carry distribution artificially as well as magnetic and electrical properties by using manganite/PMN-PT structure.

Dependence of the magnetic properties of MnGaN epitaxial layers on external electrical field

AbstractInvestigation of the magnetic properties of MnGaN epitaxial layers as a function of external electrical field was performed on the basis of field effect structure. The structure included substrate of n-type GaN, epitaxial layer of n-type MnxGa1-xN, dielectric layer and metal layer acting as field effect device gate. Each Mn atom in MnxGa1-xN contributes 4 net spins due to the electrons occupying energy levels 4F, 4D, 4P and 4G belonging to 3d orbital, and these levels are in the energy band gap and in the top of the valence band of MnxGa1-xN. The position of the Fermi level is determined to be in the energy band gap of the layer of GaN and to be above the level 4F in the layer of MnxGa1-xN. In this way application of external negative voltage on the gate causes change in the number of electrons contributing net spins and the saturation magnetization Msat of MnxGa1-xN changes as well. It was found that Msat changes in the range 1.15 × 10−3–0.7 × 10−3 A μm−1 if the external voltage changes in the interval 0–−5V. The application of this structure for the design of spintronic devices is discussed in this paper.

Conducting channel formation and annihilation in organic field-effect structures

We report measurements of displacement current to study the transient effects of conducting channel formation and annihilation at organic semiconductor/dielectric interfaces. The device structure resembled a typical organic thin-film transistor with either source or drain electrode removed. However, the channel length was very long (1–6 mm) in order to increase the transit time and enhance the displacement current. The devices consisted of a gold electrode contacting a 30 nm thick pentacene thin film, thermally deposited on SiO2 dielectric, with a heavily doped p-type Si substrate serving as a bottom electrode. Electrical measurements were performed by measuring the displacement current running through the grounded gold contact while linearly sweeping the voltage bias applied to the bottom electrode. The processes of conducting channel formation and annihilation were observed as transients in the I-V characteristics. By integrating the displacement current with respect to time, the concentrations of carriers injected into, extracted from, and trapped inside the pentacene film during the voltage sweep cycle were determined. The results are analyzed in terms of a device model that includes the effects of traps.

Coulomb interactions in single charged self-assembled quantum dots: Radiative lifetime and recombination energy

We present results on the charge dependence of the radiative recombination lifetime, $\ensuremath{\tau}$, and the emission energy of excitons confined to single self-assembled InGaAs quantum dots. There are significant dot-to-dot fluctuations in the lifetimes for a particular emission energy. To reach general conclusions, we present the statistical behavior by analyzing data recorded on a large number of individual quantum dots. Exciton charge is controlled with extremely high fidelity through an $n$-type field effect structure, which provides access to the neutral exciton (${X}^{0}$), the biexciton $(2{X}^{0})$, and the positively $({X}^{1+})$ and negatively $({X}^{1\ensuremath{-}})$ charged excitons. We find significant differences in the recombination lifetime of each exciton such that, on average, $\ensuremath{\tau}({X}^{1\ensuremath{-}})/\ensuremath{\tau}({X}^{0})=1.25$, $\ensuremath{\tau}({X}^{1+})/\ensuremath{\tau}({X}^{0})=1.58$, and $\ensuremath{\tau}(2{X}^{0})/\ensuremath{\tau}({X}^{0})=0.65$. We attribute the change in lifetime to significant changes in the single particle hole wave function on charging the dot, an effect more pronounced on charging ${X}^{0}$ with a single hole than with a single electron. We verify this interpretation by recasting the experimental data on exciton energies in terms of Coulomb energies. We directly show that the electron-hole Coulomb energy is charge dependent, reducing in value by 5%--10% in the presence of an additional electron, and that the electron-electron and hole-hole Coulomb energies are almost equal.

Electrical and optical charging of CdTe quantum dots

AbstractWe study two charging mechanisms of CdTe self assembled quantum dots embedded in a field effect structure: A standard electrical injection of carriers from the back contact resulting from an application of a gate voltage and a novel all‐optical effect. The optical charging results from a reduction of the built in electric field by an above barrier illumination, which enables the tunneling of carriers from the back contact. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Biosensor Arrays Based on the Degradation of Thin Polymer Films Interrogated by Scanning Photoinduced Impedance Microscopy

Disposable sensors based on the degradation of thin films as a result of an enzymatic reaction have been developed into efficient enzyme detectors. Film degradation has traditionally been monitored using surface plasmon resonance (SPR), quartz crystal microbalance (QCM), or classical ac impedance measurements. The enzyme detection principle has now been integrated with an array technology derived from a recently developed impedance imaging technique, scanning photoinduced impedance microscopy (SPIM). SPIM is based on photocurrent measurements at field-effect structures. The material under investigation is commonly deposited onto a semiconductor-insulator substrate. In this work, field-effect capacitors were replaced by hydrogenated amorphous silicon (a-Si:H) n-i-p photodiode structures, which have recently been shown to be suitable for SPIM measurements with good lateral resolution. To demonstrate the feasibility of SPIM for the characterization of biosensor arrays, polymer dots of the inert polymer cellulose acetate and an alpha-chymotrypsin-sensitive poly(ester amide) were deposited onto a-Si:H n-i-p/SiO2 structures and their enzymatic degradation was monitored using a laser scanning setup.

Loading indirect excitons into an electrostatic trap formed in coupled GaAs quantum wells

We demonstrate how to load indirect excitons into an electrostatic trap, which is formed in a field-effect structure based on coupled GaAs quantum wells. Within the plane of a double quantum well, indirect excitons are trapped at the perimeter of a SiO 2 area sandwiched between the surface of the GaAs heterostructure and a semi-transparent metallic top gate. The trapping mechanism is well explained by a combination of the quantum confined Stark effect and local field enhancement. We find that the trap can be filled with indirect excitons, as soon as the distance between the laser excitation and the trap is shorter than the effective diffusion length of the indirect excitons.

Micropatterned electrostatic traps for indirect excitons in coupled GaAs quantum wells

We demonstrate an electrostatic trap for indirect excitons in a field-effect structure based on coupled GaAs quantum wells. Within the plane of a double quantum well indirect excitons are trapped at the perimeter of a SiO2 area sandwiched between the surface of the GaAs heterostructure and a semitransparent metallic top gate. The trapping mechanism is well explained by a combination of the quantum confined Stark effect and local field enhancement. We find the one-dimensional trapping potentials in the quantum well plane to be nearly harmonic with high spring constants exceeding 10 keV/cm^2.

Handheld measurement device for field-effect sensor structures: Practical evaluation and limitations

Abstract A user-friendly handheld measurement device in combination with a field-effect-based capacitive electrolyte–insulator–semiconductor (EIS) sensor has been developed. The measurement device allows the characterisation of various chemical sensors and biosensors in both the capacitance–voltage ( C / V ) and the constant-capacitance (ConCap) mode. The developed device has been systematically tested with pH-sensitive EIS sensors in terms of accuracy, reliability and noise behaviour. Many possible applications for this type of semiconductor-based sensor with the associated convenient handheld measurement device can be found in the biotechnology environment. First practical aspects and considerations to apply EIS-based sensors together with the new handheld measurement device for pH measurements in a cell-culture medium will be reported.

Scanning Photoinduced Impedance Microscopy Using Amorphous Silicon Photodiode Structures

Scanning photoinduced impedance microscopy (SPIM) is an impedance imaging technique, which is based on photocurrent measurements at electrolyte-insulator-semiconductor (EIS) and metal-insulator-semiconductor (MIS) field-effect structures. The material to be investigated has to be deposited on top of the insulator (E/I or M/I interface). The lateral resolution of SPIM is limited by the lateral diffusion of minority charge carriers. Therefore, it would be advantageous if semiconductors with a short diffusion length of charge carriers such as amorphous silicon could be employed. However, field-effect capacitors fabricated using amorphous silicon suffered from a large number of interface states, high leakage currents through the insulator, and small photocurrents. In this work, field-effect capacitors were replaced by amorphous hydrogenated silicon photodiode structures (a-Si:H p-i-n/SiO2 or n-i-p/SiO2) as this was expected to result in higher photocurrents and eliminate the necessity of a high-quality insulator. The photodiode structures were shown to be suitable for SPIM measurements. The resolution of photocurrent measurements was found to depend strongly on the frequency of the modulated light and the doping concentration of the amorphous silicon layer closest to the insulator. An equivalent circuit model was developed to simulate this behavior.

Two-dimensional electron systems with strong interaction in silicon MOSFETs

The results of experimental research on a strongly interacting two-dimensional Fermi liquid in silicon field-effect structures are briefly reviewed. By extrapolation of the experimental data from the metallic region it is concluded that the effective mass of the electrons diverges and the total spin polarization vanishes at a finite electron density.