On the profile of frequency and voltage dependent interface states and series resistance in (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructures by using current–voltage (I–V) and admittance spectroscopy methods

Cataloged from PDF version of article.In order to explain the experimental effect of interface states (N-ss) and series resistance (R-s) of device on the non-ideal electrical characteristics, current-voltage (I-V), capacitance-voltage (C-V) and conductance-voltage (G/omega-V) characteristics of (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructures were investigated at room temperature. Admittance measurements (C-V and G/omega-V) were carried out in frequency and bias voltage ranges of 2 kHz-2 MHz and (-5 V)-(+5 V), respectively. The voltage dependent R-s profile was determined from the I-V data. The increasing capacitance behavior with the decreasing frequency at low frequencies is a proof of the presence of interface states at metal/semiconductor (M/S) interface. At various bias voltages, the ac electrical conductivity (sigma(ac)) is independent from frequencies up to 100 kHz, and above this frequency value it increases with the increasing frequency for each bias voltage. In addition, the high-frequency capacitance (C-m) and conductance (G(m)/omega) values measured under forward and reverse bias were corrected to minimize the effects of series resistance. The results indicate that the interfacial polarization can more easily occur at low frequencies. The distribution of N-ss and R-s is confirmed to have significant effect on non-ideal I-V. C-V and G/omega-V characteristics of (Ni/Au)/Al0.22Ga038N/AlN/GaN heterostructures. (C) 2011 Elsevier Ltd. All rights reserved

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

Temperature dependent negative capacitance behavior in (Ni/Au)/AlGaN/AIN/GaN heterostructures

Cataloged from PDF version of article.The temperature dependent capacitance voltage (C-V) and conductance voltage (G/omega-V) characteristics of (Ni/Au)/Al(0.22)Ga(0.78)N/AlN/GaN heterostructures were investigated by considering the series resistance (R(s)) effect in the temperature range of 80-390 K. The experimental results show that the values of C and G/omega are strongly functioning of temperature and bias voltage. The values of C cross at a certain forward bias voltage point (similar to 2.8 V) and then change to negative values for each temperature, which is known as negative capacitance (NC) behavior. In order to explain the NC behavior, we drawn the C vs I and G/omega vs I plots for various temperatures at the same bias voltage. The negativity of the C decreases with increasing temperature at the forward bias voltage, and this decrement in the NC corresponds to the increment of the conductance. When the temperature was increased, the value of C decreased and the intersection point shifted towards the zero bias direction. This behavior of the C and G/omega values can be attributed to an increase in the polarization and the introduction of more carriers in the structure. R(s) values increase with increasing temperature. Such temperature dependence is in obvious disagreement with the negative temperature coefficient of R or G reported in the literature. The intersection behavior of C-V curves and the increase in R(s) with temperature can be explained by the lack of free charge carriers, especially at low temperatures. (C) 2010 Elsevier B.V. All rights reserve

Tunneling current via dislocations in schottky diodes on AlInN/AlN/GaN heterostructures

The forward current-voltage-temperature characteristics of (Ni/Au)-Al 0.83In0.17N/AlN/GaN heterostructures were studied in a temperature range of 80-375 K. The temperature dependences of the tunneling saturation current (It) and tunneling parameters (E0) were obtained. Weak temperature dependence of the saturation current and the absence of temperature dependence of the tunneling parameters were observed in this temperature range. The results indicate that in the temperature range of 80-375 K, the mechanism of charge transport in the (Ni/Au)-Al0.83In 0.17N/AlN/GaN heterostructure is performed by tunneling among dislocations intersecting the space-charge region. A model is used for nonuniform tunneling along these dislocations that intersect the space-charge region. The dislocation density that was calculated from the current-voltage characteristics, according to a model of tunneling along the dislocation line, gives the value 7.4 × 108 cm-2. This value is close in magnitude to the dislocation density that was obtained from the x-ray diffraction measurements value of 5.9 × 108 cm-2. These data show that the current flows manifest a tunneling character, even at room temperature. © 2009 IOP Publishing Ltd

The effect of insulator layer thickness on the main electrical parameters in (Ni/Au)/AlxGa1-xN/AIN/GaN heterostructures

(Ni/Au)Alx Ga1-x N/AlN/GaN(x = 0.22) heterostructures with and without a passivation layer of the SiNx were fabricated in order to see the effect of the insulator layer on the main electrical parameters such as zero-bias barrier height (BH) (φBO), ideality factor (n), series resistance (Rs) of the structure, and the interface state density (Nss). Some of these parameters were determined from both I-V and admittance (C-V and G/ω-V) measurements at room temperature and at 1 MHz and were compared. The experimental results show that the value of N ss in a Schottky contact without passivation is nearly 1 order of magnitude larger than that in a Schottky contact with SiNx passivation layers. Also, the values of Rs increase with the increasing thickness of the passivation layer. In the forward bias region, the negative values of capacitance are an attractive result of this study. This negative capacitance disappears in presence of the passivation layer. Copyright © 2010 John Wiley & Sons, Ltd

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

Electrical characterization of MS and MIS structures on AlGaN/AlN/GaN heterostructures

The forward and reverse bias I-V, C-V, and G/ω-V characteristics of (Ni/Au) Schottky barrier diodes (SBDs) on the Al 0.22Ga 0.78N/AlN/GaN high-electron-mobility-transistor (HEMTs) without and with SiN x insulator layer were measured at room temperature in order to investigate the effects of the insulator layer (SiN x) on the main electrical parameters such as the ideality factor (n), zero-bias barrier height ( B0), series resistance (R s), interface-state density (N ss). The energy density distribution profiles of the N ss were obtained from the forward bias I-V characteristics by taking into account the voltage dependence of the effective barrier height ( e) and ideality factor (n V) of devices. In addition, the N ss as a function of E c-E ss was determined from the low-high frequency capacitance methods. It was found that the values of N ss and R s in SBD HEMTs decreases with increasing insulator layer thickness. © 2010 Elsevier Ltd. All rights reserved

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