On the profile of frequency dependent dielectric properties of (Ni/Au)/GaN/Al0.3Ga0.7N heterostructures

Cataloged from PDF version of article.The voltage (V) and frequency (f) dependence of dielectric characteristics such as dielectric constant (epsilon'), dielectric loss (epsilon ''), dielectric loss tangent (tan (5) and real and imaginary part of electrical modulus (M' and M '') of the (Ni/Au)/GaN/Al0.3Ga0.7N heterostructures have been investigated by using experimental admittance spectroscopy (capacitance-voltage (C-V) and conductance-voltage (G/w-V)) measurements at room temperature. Experimental results show that the values of the epsilon', epsilon '', tan delta and the real and imaginary parts of the electric modulus (M' and M '') obtained from the C and G/w measurements were found to be strong function of frequency and applied bias voltage especially in depletion region at low frequencies. These changes in dielectric parameters can be attributed to the interfacial GaN cap layer, interface polarization and a continuous density distribution of interface states and their relaxation time at metal/semiconductor interface. While the values of the epsilon' decrease with increasing frequencies, tan delta, M' and M '' increase with the increasing frequency. Also, the dielectric loss (epsilon '') have a local maximum at about frequency of 100 kHz. It can be concluded that the interface polarization can occur more easily at low frequencies with the number of interface states located at the metal/semiconductor interface. (C) 2010 Elsevier Ltd. All rights reserved

The profile of temperature and voltage dependent series resistance and the interface states in (Ni/Au)/Al0.3Ga0.7N/AlN/GaN heterostructures

Cataloged from PDF version of article.The profile of the interface state densities (N(ss)) and series resistances (R(s)) effect on capacitance-voltage (C-V) and conductance-voltage (G/omega-V) of (Ni/Au)/Al(x)Ga(1-x)N/AlN/GaN heterostructures as a function of the temperature have been investigated at 1 MHz. The admittance method allows us to obtain the parameters characterizing the metal/semiconductor interface phenomena as well as the bulk phenomena. The method revealed that the density of interface states decreases with increasing temperature. Such a behavior of N(ss) can be attributed to reordering and restructure of surface charges. The value of series R(s) decreases with decreasing temperature. This behavior of R(s) is in obvious disagreement with that reported in the literature. It is found that the N(ss) and R(s) of the structure are important parameters that strongly influence the electrical parameters of (Ni/Au)/Al(x)Ga(1-x)N/AlN/GaN(x = 0.22) heterostructures. In addition, in the forward bias region a negative contribution to the capacitance C has been observed, that decreases with the increasing temperature. Copyright (C) 2010 John Wiley & Sons, Ltd

The behavior of the I-V-T characteristics of inhomogeneous (Ni/Au)-Al0.3Ga0.7N/AlN/GaN heterostructures at high temperatures

Cataloged from PDF version of article.We investigated the behavior of the forward bias current-voltage-temperature (I-V-T) characteristics of inhomogeneous (Ni/Au)-Al0.3Ga0.7N/AlN/GaN heterostructures in the temperature range of 295-415 K. The experimental results show that all forward bias semilogarithmic I-V curves for the different temperatures have a nearly common cross point at a certain bias voltage, even with finite series resistance. At this cross point, the sample current is temperature independent. We also found that the values of series resistance (R-s) that were obtained from Cheung's method are strongly dependent on temperature and the values abnormally increased with increasing temperature. Moreover, the ideality factor (n), zero-bias barrier height (Phi(B0)) obtained from I-V curves, and R-s were found to be strongly temperature dependent and while Phi(B0) increases, n decreases with increasing temperature. Such behavior of Phi(B0) and n is attributed to Schottky barrier inhomogeneities by assuming a Gaussian distribution (GD) of the barrier heights (BHs) at the metal/semiconductor interface. We attempted to draw a Phi(B0) versus q/2kT plot in order to obtain evidence of the GD of BHs, and the values of (Phi) over bar (B0)=1.63 eV and sigma(0)=0.217 V for the mean barrier height and standard deviation at a zero bias, respectively, were obtained from this plot. Therefore, a modified ln(I-0/T-2)-q(2)sigma(2)(0)/2(kT)(2) versus q/kT plot gives Phi(B0) and Richardson constant A(*) as 1.64 eV and 34.25 A/cm(2) K-2, respectively, without using the temperature coefficient of the barrier height. The Richardson constant value of 34.25 A/cm(2) K-2 is very close to the theoretical value of 33.74 A/cm(2) K-2 for undoped Al0,3Ga0,7N. Therefore, it has been concluded that the temperature dependence of the forward I-V characteristics of the (Ni/Au)-Al0.3Ga0.7/AlN/GaN heterostructures can be successfully explained based on the thermionic emission mechanism with the GD of BHs. (c) 2007 American Institute of Physics

On the profile of frequency dependent dielectric properties of (Ni/Au)/GaN/Al0.3Ga0.7N heterostructures

The voltage (V) and frequency (f) dependence of dielectric characteristics such as dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tan δ) and real and imaginary part of electrical modulus (Μ′ and M″) of the (Ni/Au)/GaN/Al0.3Ga 0.7N heterostructures have been investigated by using experimental admittance spectroscopy (capacitance-voltage (C-V) and conductance-voltage (G/w-V)) measurements at room temperature. Experimental results show that the values of the ε′, ε″, tan δ and the real and imaginary parts of the electric modulus (M′ and M″) obtained from the C and G/w measurements were found to be strong function of frequency and applied bias voltage especially in depletion region at low frequencies. These changes in dielectric parameters can be attributed to the interfacial GaN cap layer, interface polarization and a continuous density distribution of interface states and their relaxation time at metal/semiconductor interface. While the values of the ε ′ decrease with increasing frequencies, tan δ,M′ and M″ increase with the increasing frequency. Also, the dielectric loss (ε″) have a local maximum at about frequency of 100 kHz. It can be concluded that the interface polarization can occur more easily at low frequencies with the number of interface states located at the metal/semiconductor interface. © 2010 Elsevier Ltd. All rights reserved

Optically trapped bacteria pairs reveal discrete motile response to control aggregation upon cell–cell approach

Aggregation of bacteria plays a key role in the formation of many biofilms. The critical first step is cell–cell approach, and yet the ability of bacteria to control the likelihood of aggregation during this primary phase is unknown. Here, we use optical tweezers to measure the force between isolated Bacillus subtilis cells during approach. As we move the bacteria towards each other, cell motility (bacterial swimming) initiates the generation of repulsive forces at bacterial separations of ~3 μm. Moreover, the motile response displays spatial sensitivity with greater cell–cell repulsion evident as inter-bacterial distances decrease. To examine the environmental influence on the inter-bacterial forces, we perform the experiment with bacteria suspended in Tryptic Soy Broth, NaCl solution and deionised water. Our experiments demonstrate that repulsive forces are strongest in systems that inhibit biofilm formation (Tryptic Soy Broth), while attractive forces are weak and rare, even in systems where biofilms develop (NaCl solution). These results reveal that bacteria are able to control the likelihood of aggregation during the approach phase through a discretely modulated motile response. Clearly, the force-generating motility we observe during approach promotes biofilm prevention, rather than biofilm formation

The Vibrio cholerae Minor Pilin TcpB Initiates Assembly and Retraction of the Toxin- Coregulated Pilus

Type IV pilus (T4P) systems are complex molecular machines that polymerize major pilin proteins into thin filaments displayed on bacterial surfaces. Pilus functions require rapid extension and depolymerization of the pilus, powered by the assembly and retraction ATPases, respectively. A set of low abundance minor pilins influences pilus dynamics by unknown mechanisms. The Vibrio cholerae toxin-coregulated pilus (TCP) is among the simplest of the T4P systems, having a single minor pilin TcpB and lacking a retraction ATPase. Here we show that TcpB, like its homolog CofB, initiates pilus assembly. TcpB co-localizes with the pili but at extremely low levels, equivalent to one subunit per pilus. We used a micropillars assay to demonstrate that TCP are retractile despite the absence of a retraction ATPase, and that retraction relies on TcpB, as a V. cholerae tcpB Glu5Val mutant is fully piliated but does not induce micropillars movements. This mutant is impaired in TCP-mediated autoagglutination and TcpF secretion, consistent with retraction being required for these functions. We propose that TcpB initiates pilus retraction by incorporating into the growing pilus in a Glu5-dependent manner, which stalls assembly and triggers processive disassembly. These results provide a framework for understanding filament dynamics in more complex T4P systems and the closely related Type II secretion system

Bacterial Chemotaxis in an Optical Trap

An optical trapping technique is implemented to investigate the chemotactic behavior of a marine bacterial strain Vibrio alginolyticus. The technique takes the advantage that the bacterium has only a single polar flagellum, which can rotate either in the counter-clock-wise or clock-wise direction. The two rotation states of the motor can be readily and instantaneously resolved in the optical trap, allowing the flagellar motor switching rate to be measured under different chemical stimulations. In this paper the focus will be on the bacterial response to an impulsive change of chemoattractant serine. Despite different propulsion apparati and motility patterns, cells of V. alginolyticus apparently use a similar response as Escherichia coli to regulate their chemotactic behavior. Specifically, we found that the switching rate of the bacterial motor exhibits a biphasic behavior, showing a fast initial response followed by a slow relaxation to the steady-state switching rate . The measured can be mimicked by a model that has been recently proposed for chemotaxis in E. coli. The similarity in the response to the brief chemical stimulation in these two different bacteria is striking, suggesting that the biphasic response may be evolutionarily conserved. This study also demonstrated that optical tweezers can be a useful tool for chemotaxis studies and should be applicable to other polarly flagellated bacteria

Ecological influences on the behaviour and fertility of malaria parasites

BACKGROUND: Sexual reproduction in the mosquito is essential for the transmission of malaria parasites and a major target for transmission-blocking interventions. Male gametes need to locate and fertilize females in the challenging environment of the mosquito blood meal, but remarkably little is known about the ecology and behaviour of male gametes. METHODS: Here, a series of experiments explores how some aspects of the chemical and physical environment experienced during mating impacts upon the production, motility, and fertility of male gametes. RESULTS AND CONCLUSIONS: Specifically, the data confirm that: (a) rates of male gametogenesis vary when induced by the family of compounds (tryptophan metabolites) thought to trigger gamete differentiation in nature; and (b) complex relationships between gametogenesis and mating success exist across parasite species. In addition, the data reveal that (c) microparticles of the same size as red blood cells negatively affect mating success; and (d) instead of swimming in random directions, male gametes may be attracted by female gametes. Understanding the mating ecology of malaria parasites, may offer novel approaches for blocking transmission and explain adaptation to different species of mosquito vectors

Frequency and gate voltage effects on the dielectric properties of Au/SiO2/n-Si structures

To determine the dielectric constant (?'), dielectric loss (?¨), loss tangent (tan ?), the ac electrical conductivity (?ac) and the electric modulus of Au/SiO2/n-Si structure, the measurement admittance technique was used. Experimental results show that the values of ?', ?¨, tan ?, ?ac and the electric modulus show fairly large frequency and gate bias dispersion especially at low frequencies due to the interface charges and polarization. An increase in the values of the ?' and ?¨ were observed with both a decrease in frequency and an increase in frequency. The ?ac is found to increase with both increasing frequency and voltage. In addition, the experimental dielectrical data have been analyzed considering electric modulus formalism. It can be concluded that the interface charges and interfacial polarization have strong influence on the dielectric properties of metal-insulator-semiconductor (MIS) structures especially at low frequencies and both in depletion and accumulation regions. © 2008 Elsevier B.V. All rights reserved