Voltage-independent Capacitance Research Articles

Enhanced microwave dielectric and electrical properties of Zn substituted Mg2TiO4 ceramics for RF/microwave applications

The structural, microstructural, electrical and microwave dielectric properties of Mg2Ti1−xZnxO4 (x = 0.01–0.05) ceramics prepared by solid-state reaction method have been reported. The X-ray diffraction patterns and Raman spectra have showed the presence of pure Mg2TiO4 (MMTO) along with some fraction of MgTiO3 (MTO) phase. It is observed that the cubic structure of MMTO is deformed and evaluated to become pseudo cubic structure with Zn substitution. The surface morphology of the sintered MMTO ceramics at 1350 °C exhibited uniform and dense microstructures. Temperature dependent dielectric response of Mg2Ti1−xO4Znx ceramics with x = 0.03, showed stable dielectric response in the temperature range from RT to 300 °C. The overall enhancement in dielectric response of the samples is attributed to the compositional heterogeneity due to substitution of Zn2+ in the Ti4+ site. The complex impedance spectra revealed two parallel RC combination circuit [(RgCg) (RgbCgb)] elements in series. The dispersion of AC conductivity followed Jonscher’s power law, and non-overlapping small polaron hole hopping assisted conductivity. Mg2Ti1−xZnxO4 ceramics with x = 0.03, exhibited best structural, dielectric and microwave dielectric property with uniform and dense microstructure, moderate dielectric constant (er) ~ 23 and quality factor (Q × fo) ~ 165 THz at 8.89 GHz. A voltage independent capacitance for the sample at all the measured frequencies have observed. The obtained properties of the ceramics are promising for dielectric resonator, microwave integrated circuit and type-I capacitor applications.

Localization and Characterization of a Degraded Site in Crystalline Silicon Photovoltaic Cells Exposed to Acetic Acid Vapor

To examine the mechanisms of degradation owing to liberated organic acid from polymer materials in crystalline silicon (c-Si) photovoltaic (PV) modules, we attempt to characterize the degraded site in a PV cell exposed to acetic acid vapor by means of ac impedance spectroscopy. The location was electrically determined at the interface between the front electrodes and emitter of a silicon wafer. Concerning this interface, two distinct electric characteristics were identified as follows: 1) the contact in this interface can rectify the current in the same direction as the p-n junction of a p-type PV cell, and 2) any linear relationship in the Mott-Schottky plot was not confirmed in the capacitance component placed in this interface, unlike in the case of that located in the p-n junction. From these characteristics, the inclusion of a layer with a voltage-independent capacitance in this interface, an inhomogeneous depth profile of impurities within the near-surface of the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -Si layer of this contact, and/or the existence of peculiar surface/interface states in this interface were deduced. It is concluded that a crucial electrical feature involved in the degradation of c-Si PV cells/modules under acidic conditions is verified with regard to these characteristics.

Optical control of capacitance in a metal-insulator-semiconductor diode with embedded metal nanoparticles.

This paper describes a metal-insulator-semiconductor (MIS) capacitor with flat capacitance voltage characteristics and a small quadratic voltage capacitance coefficient. The device characteristics resemble a metal-insulator-metal diode except that here the capacitance depends on illumination and exhibits a strong frequency dispersion. The device incorporates Fe nanoparticles (NPs), mixed with SrF2, which are embedded in an insulator stack of SiO2 and HfO2. Positively charged Fe ions induce dipole type traps with an electronic polarization that is enhanced by photogenerated carriers injected from the substrate and/or by inter nanoparticle exchange of carriers. The obtained characteristics are compared with those of five other MIS structures: two based on Fe NPs, one with and the other without SrF2 sublayers. Additionally, devices contain Co NPs embedded in SrF2 sublayers, and finally, two structures have no NPs, with one based on a stack of SiO2 and HfO2 and the other which also includes SrF2. Only structures containing Fe NPs, which are incorporated into SrF2, yield a voltage independent capacitance, the level of which can be changed by illumination. These properties are essential in radio frequency/analog mixed signal applications.

Experimental study of charge transport mechanisms in a hybrid metal/organic/inorganic device

An hybrid metal/organic/inorganic semiconductor heterostructure was built under ultrahigh vacuum conditions (UHV) and characterized in situ. The aim was to investigate the influence of thin film layers of the organic material Dimethyl-3,4,9,10-perylenetetracarboxylic diimide (DiMe-PTCDI) on the electrical response of organic-modified $\mathrm{Ag}∕\mathrm{Ga}\mathrm{As}$ Schottky diodes. The device was tested by combining photoemission spectroscopy (PES) with atomic force microscopy (AFM) and electrical measurements (current-voltage I-V, capacitance-voltage C-V, impedance and charge transient spectroscopies QTS). The energy level alignment through the heterostructure was deduced. This allows us to consider electrons acting as majority carriers injected over a barrier by thermionic emission as a primary event in the charge transport. For thin organic layers (below $8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thickness) most of the characterization techniques show an island-like growth leading to formation of voids. The coverage of the ${\mathrm{H}}^{+}\mathrm{Ga}\mathrm{As}$ substrate as a function of the nominal thickness of DiMe-PTCDI was assessed via C-V measurements assuming a voltage independent capacitance of the organic layer. The frequency response of the device was evaluated through C-V and impedance measurements in the range $1\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}\text{\ensuremath{-}}1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. The almost independent behavior of the capacitance in the measured frequency range confirmed the assumption of a near geometrical capacitor, which was used for modeling the impedance of the device. QTS measurements performed on the heterostructure showed the presence of two relaxations induced by deposition of the organic layer. The first one is attributed to a deep trap at the metal/organic interface, while the second one has very small activation energy $(\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ which is probably due to disorder at the organic film. With such information a fit of the I-V characteristics of DiMe-PTCDI organic modified diodes based on the analytical expressions of a trapped charge limited current regime (TCLC) was intended. (Some figures are available in color only in the on-line version.)

Influence of trapped and interfacial charges in organic multilayer light-emitting devices

The influence of trapped and interfacial charges on the device characteristics of organic multilayer light-emitting devices is investigated. We have studied devices consisting of 20 nm copper phthalocyanine as buffer and hole-injection layer, 50 nm N,N′-di(naphthalene-1-yl)-N,N′ diphenyl-benzidine (NPB) as hole-transport layer, and 65 nm tris(8-hydroxyquinolinato)aluminum (Alq3) as electron transport and emitting layer sandwiched between a high-work-function metal and a semitransparent Ca electrode. Current–voltage measurements show that the device characteristics in negative bias direction and at low positive bias are influenced by charges trapped within the organic layers. This is manifested by a strong dependence of the current on the direction and speed of the voltage sweep in this range. Low-frequency capacitance–voltage and static charge measurements reveal a voltage-independent capacitance in negative bias direction and a significant increase between 0 and 2 V, indicating the presence of negative interfacial charges at the NPB/Alq3 interface. Transient experiments show that the delay time of electroluminescence under forward bias conditions is controlled by the buildup of internal space charges rather than by charge-carrier transport through the organic layers.

Influence of trapped and interfacial charges in organic multilayer light-emitting devices

Trapped and interfacial charges have significant impact on the performance of organic light-emitting devices (OLEDs). We have studied devices consisting of 20 nm copper phthalocyanine (CuPc) as the buffer and hole-injection layer, 50 nm N, N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) as the hole transport layer, and 65 nm tris(8-hydroxyquinolinato)aluminum (Alq3) as the electron transport and emitting layer sandwiched between a high-work-function metal and a semitransparent Ca electrode. Current-voltage measurements show that the device characteristics in the negative bias direction and at low positive bias below the built-in voltage are influenced by trapped charges within the organic layers. This is manifested by a strong dependence of the current in this range on the direction and speed of the voltage sweep. Low-frequency capacitance-voltage and static charge measurements reveal a voltage-independent capacitance in the negative bias direction and a significant increase between 0 and 2 V in the given device configuration, indicating the presence of negative interfacial charges at the NPB/Alq3 interface. Transient experiments show that the delay time of electroluminescence at low voltages in these multilayer devices is controlled by the buildup of internal space charges, which facilitates electron injection, rather than by charge-carrier transport through the organic layers. To summarize, our results clearly demonstrate that the tailoring of internal barriers in multilayer devices leads to a significant improvement in device performance.

Studies on electrical and photoelectrical behaviour of ITO/ArV/In Schottky barrier device

The electrical and photoelectrical properties of aryl viologen (ArV), chemically known as 1,1′-diphenyl-4,4′-bipyridinium dichloride, in the form of thin film, sandwiched between ITO and In electrode were studied. The current–voltage ( J– V) characteristics in dark show the rectification effect due to the formation of Schottky barrier at In–ArV interface. The diode quality factor of the device, greater than unity, indicates the recombination of electron-hole in depletion region. Ohmic conduction in low voltage range and space charge limited conduction (SCLC) controlled by an exponential distribution of traps above the valence band edge, for higher voltage region have been observed. Various electrical parameters were calculated from the analysis of J– V and capacitance–voltage ( C– V) characteristics at different temperatures and discussed in details. At higher frequencies, the device exhibit voltage independent capacitance, which is explained in terms of the extremely slow kinetics of space charge and low mobility of charge carriers. The photoaction spectra of the device and absorption spectra of the ArV thin film reveal that the fraction of light, which is absorbed near or within the diffusion length of exciton, is responsible for producing the free charge carriers. The photovoltaic parameters were calculated from the J– V characteristics under illumination through ITO and discussed in detail.

A.c. impedance of frozen junction polymer light-emitting electrochemical cells

The structure of the frozen p—i—n junction in polymer light-emitting electrochemical cells (LECs) is studied by measurements of the a.c. impedance of LEC devices. Impedance plots of frozen junction LECs show characteristics that are typical of polymer light-emitting diodes. The data demonstrate the ‘freeze-out’ of ionic mobility. The frequency-independent and voltage-independent capacitance of the frozen junction supports the conclusion that the insulating ‘i’ in the p—i—n junction occupies most of the thickness of the LEC polymer layer.

Electrical behavior of low-power RF magnetron-sputtered indium tin oxide films on silicon substrate

Indium tin oxide (ITO) films were deposited onto p-Si substrates by RF magnetron sputtering. Small sputtering powers (≤ 28 W) were supplied to a one-inch diameter target, and a low substrate temperature (< 80°C) was maintained during sputtering. The electrical behavior of the ITO films was explored. The relationship (carrier mobility) ∝ (carrier concentration) −0.64 suggests an impurity scattering mechanism for the transport of charge carriers. A linear current-voltage characteristic and a voltage-independent capacitance indicate that the low-power sputtering process does not damage the ITO/p-Si interface.

The interpretation of non-linear Schottky barrier C -2-V characteristics

An analysis has been made of the non-linear C -2-V characteristics exhibited by n-CdTe Schottky barriers. The empirical data are shown to be explained satisfactorily assuming the existence of deep traps at 0.8 eV below the conduction band. The shallow donor concentration inferred from this analysis is close to that calculated by a simpler method which treats the real junction as an equivalent ideal Schottky diode in parallel with a voltage independent capacitance. The latter procedure is shown to be invalid in this instance and an explanation is given for its apparent success.

Electrical properties of n-n ZnSe/GaAs heterojunctions

ZnSe/GaAs n-n heterojunctions were prepared by heteroepitaxial growth of ZnSe on GaAs from the vapor phase. The electrical properties of the heterojunctions were found to be strongly dependent on the ZnSe resistivity. Heterojunctions with semi-insulating (10 9–10 11 Ω cm) ZnSe showed symmetric current density versus voltage ( J-V) characteristics and a voltage-independent capacitance. Rectification was shown by n-n heterojunctions prepared with conductive (5–25 Ω cm) ZnSe. J-V and capacitance versus voltage measurements indicated that the n-n structure behaves like a lattice-matched Schottky barrier with a barrier height of 0.7 eV. These findings were compared with recent theoretical models of the electronic structure of the ZnSe/GaAs interface.

Digital implementation of the Q-C method for MOS measurements

Computer controlled hardware and software needed for digital implementation of the Q-C method are described. Incorporation of a program that generates ideal data permits debugging and complete verification of the analysis routines. An error sensitivity check shows the parameters that must be measured most carefully are the device area, oxide capacitance, and the voltage independent capacitance in series with the MOS capacitor. The digital technique was found to provide advantages in convenience, speed, accuracy and versatility when implementing the Q-C method.

The polarization impedance of the C|PbCl2 and Pt|PbCl2 interfaces

Complex admittance measurements of electrochemical cells of the type Pb|PbCl2|M (M=Pt, C) have been performed in the frequency range 0.1 Hz to 1 kHz, and in the temperature range 380 to 530 K. The M|PbCl2 interfaces can be represented by a parallel combination of a capacitance, a resistance, and a diffusional impedance. Different theoretical models for the interface capacitance are reviewed. The observed, nearly voltage-independent capacitance is ascribed to an inner-layer capacitance. The resistance is strongly correlated to the deposition voltage. Possible origins of the diffusion process are considered; diffusion of neutral crystal components within the electrode material or diffusion of electronic charge carriers within the electrolyte is ruled out.

The polarization impedance of the C|PbCl 2 and Pt|PbCl 2 interfaces

Complex admittance measurements of electrochemical cells of the type Pb|PbCl 2|M (M=Pt, C) have been performed in the frequency range 0.1 Hz to 1 kHz, and in the temperature range 380 to 530 K. The M|PbCl 2 interfaces can be represented by a parallel combination of a capacitance, a resistance, and a diffusional impedance. Different theoretical models for the interface capacitance are reviewed. The observed, nearly voltage-independent capacitance is ascribed to an inner-layer capacitance. The resistance is strongly correlated to the deposition voltage. Possible origins of the diffusion process are considered; diffusion of neutral crystal components within the electrode material or diffusion of electronic charge carriers within the electrolyte is ruled out.