Temperature Dependence Of Capacitance Research Articles

Phase structure and enhanced piezoelectric properties in (1-x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-x(Bi0.5Na0.42Li0.08)0.9Sr0.1ZrO3 lead-free piezoelectric ceramics

The piezoelectric properties of KNN lead-free piezoelectric ceramics could be greatly enhanced by forming multiphase coexistence. In this work, binary system (1-x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-x(Bi0.5Na0.42Li0.08)0.9Sr0.1ZrO3 [(abbreviated as (1-x)KNNS-xBNLSZ] ceramics with rhombohedral-tetragonal (r-T) phase boundary was designed and synthesized using the conventional solid-state sintering method, and effects of BNLSZ contents on their micrograph, phase structure and electrical properties were also investigated. According to phase diagram from the results of temperature-dependent capacitance and dielectric constant, the ceramics exhibit the R-T phase coexistence in the composition range of 3.5%≤x<4.5%, and an enhanced dielectric, ferroelectric, and piezoelectric behavior was obtained at such a phase boundary zone. As a result, the ceramics with x=0.04 exhibit optimum electrical properties of d33~461pC/N, kp~46%, tan δ~0.03, Pr~16.9μC/cm2, and Ec ~9kV/cm, together with a Curie temperature (TC) of ~228°C. Such a good comprehensive performance obtained in this present work is due to the R-T phase transition and enhanced ɛrPr. It was believed that this ceramic system would promote the development of KNN-based lead-free ceramics.

Al2O3/ $\beta $ -Ga2O3(-201) Interface Improvement Through Piranha Pretreatment and Postdeposition Annealing

In this letter, we report on the improvement of atomic layer deposited (ALD) Al2O3/ $\beta $ –Ga2O3 (-201) interface quality through piranha pretreatment and postdeposition annealing (PDA). The high quality interface is verified via the temperature dependent capacitance–voltage ( $C$ – $V$ ) and photo-assisted (deep UV) $C$ – $V$ measurements, considering its ultra wide bandgap of 4.8 eV for $\beta $ -Ga2O3. A low $C$ – $V$ hysteresis of 0.1 V from the measurement frequency of 1 kHz to 1 MHz is obtained, compared with the hysteresis of 0.45 V without piranha optimization. An average interface trap density $({\mathrm{ D}}_{\mathrm {it}})$ of $2.3 \times 10^{11}$ cm $^{\mathrm {-2}}\cdot $ eV $^{\mathrm {-1}}$ is extracted from the photo $C$ – $V$ measurements. Piranha pretreatments and PDA turn out to be an effective way to improve the ALD Al2O3/ $\beta $ –Ga2O3 (-201) interface for future high quality Ga2O3 metal–oxide–semiconductor field-effect transistors.

Development of Some Alternative Chalcogenide Materials, Thin Films and Diodes for Fabrication of Low Cost Solar Cells

There has been an immense interest in the development of alternatives materials and fabricate electronic devices which provide affordable solar energy generation as well as opto electronic energy transformation applications. Lot of efforts has, therefore, been put in to explore cost- effective, non-toxic and abundant materials. Chalcogenide materials quite recently have attracted attention particularly in the thin film technology due to their applications as non-linear optical materials, infrared sensors and solar energy convertors. Tin chalcogenide Sn(S,Se,Te) binary and ternary materials offer a wide range of optical band gaps suitable for various optical and optoelectronic applications. These materials have exhibited flexibility to possess variable band gaps which are suitable to tap different regions of solar spectrum which have made these materials as potential candidates for applications in photovoltaic solar cell devices. Besides, these materials have also been used as sensor and laser applications, thin film polarizer and thermo electric materials.The work to be presented in the meeting has been focused at the analysis of some Sn(S,Se,Te) binary and ternary thin films for their use as absorber layer in solar cell fabrications. The emphasis shall be made to analyze the change in the band gap, resistivity, absorption coefficient of the undertaken materials and thin films with variation in their elemental compositions and crystal structures. The undertaken materials and thin films were analyzed on the basis of structural, morphological, electrical and optical properties. SnSe and SnS demonstrated orthorhombic crystal structure while SnSe2 possessed hexagonal structure. SnTeSe, however, showed transition from hexagonal to orthorhombic crystal structures with increase in the substrate temperature from 300K to 550K. The electrical and optical analysis of the undertaken films showed change in the optical band gap, resistivity and absorption coefficient values. The films exhibited band gap energies spanning from the visible to near infrared regions. The Schottky barrier diodes of the deposited films were formed and their characteristics analyzed for the temperature dependent current voltage (I-V) and capacitance voltage (C-V) behavior. The current-voltage and capacitance-voltage characteristics of the diodes confirmed that the diodes formed on the deposited films are of good quality. The ideality factors decreased and barrier heights increased with increase in the substrate temperatures in all diodes. The temperature dependence of the barrier heights and ideality factors has been explained on the basis of “barrier inhomogenities” existing over the interface. Further, the variations between the experimentally observed values to the theoretically generated values have been explained on the basis of Gaussian distribution function which indicated the possibility of the presence of the tunneling current component in the electrical transport mechanism existing over the interface. The breakdown voltage phenomenon in the undertaken diodes provided negative temperature coefficient and depicted soft breakdown due to “defect assisted tunneling” over the interface.

Interface state density and dielectric properties of Au/n-GaP Schottky diode

The temperature and frequency dependent capacitance and conductance measurements of Au/n-GaP Schottky diode were carried out in the 200–400 K and 1 kHz–1 MHz range, under different bias voltages. The obtained interface state density (Nss) and the carrier life time (τ) were evaluated from the measured data. The values of Nss and τ depend strongly on the applied voltage and temperature. The effect of the interface states of the device under different conditions was also analyzed. The series resistance (Rs) of the device was evaluated under different frequencies and temperature conditions using Nicollian and Goetzberger method. The dielectric parameters like dielectric constant (ε′), dielectric loss (ε″), loss tangent (tan δ), and ac electric conductivity (σac) were analyzed under complex permittivity (ε*) formalism. The variations of ε′ and ε″ with frequency and temperature are attributed to the interfacial polarization and the temperature induced restructuring and reordering of interface state density. The activation energy of the interface states shows three different linear regions corresponding to activation energy values of 0.013, 0.034, and 0.061 eV.

N and p-type properties in organo-metal halide perovskites studied by Seebeck effects

Organo-metal halide perovskites can exhibit co-existed electrical polarizations and semiconducting properties respectively from organic and inorganic components. Here, we find that the Seebeck coefficient can be changed between positive and negative values when the concentration of chloride ions is varied between single-halide (CH3NH3PbI3) and mixed-halide structures (CH3NH3PbIxCl3−x). This indicates that varying the concentration of chloride ions can tune the semiconducting properties between the n-type and p-type regimes in the organo-metal halide perovskites. Our temperature-dependent capacitance measurement shows that increasing temperature can cause a change on internal electrical polarization. As a result, we can propose that the internal polarization functions as the underlying mechanism responsible for large temperature-dependent Seebeck coefficients in organo-metal halide perovskites operating between n-type and p-type regimes.

Ferroelectric and flexible barrier resistive switching of epitaxial BiFeO3 films studied by temperature-dependent current and capacitance spectroscopy

The resistive switching of epitaxial ferroelectric BiFeO3 thin films was investigated by the combined current and capacitance spectroscopy, in addition with the piezoresponse force microscopy and conducting atomic force microscopy. A strong correlation between the resistance states and the capacitance states was found in the temperature range from 77 to 300 K, revealing the role of interface traps. It is demonstrated that the trap-assisted current transport contributes to the large ON current of the ferroelectric and flexible barrier resistive switching. The combination of ferroelectric polarization reversal, electromigration of oxygen vacancies, and interface traps leads to the nonvolatile memory performance with the ON/OFF ratio of 102 for >105 s and the endurance of over 8000 cycles at room temperature.

Deep defects and the attempt to escape frequency in organic photovoltaic materials

Trap states are well-known to plague organic photovoltaic devices and their characterization is essential for continued progress. This letter reports on both the deep trap profiles and kinetics of trap emission, studied through temperature dependent capacitance measurements. Three polymer based systems relevant to photovoltaics, namely, P3HT:PC60BM, PTB7:PC70BM, and PCDTBT:PC70BM were investigated. Each polymer showed a markedly different deep trap profile, varying in shape from a nearly constant density of states to a sharp Gaussian. In contrast, the frequency of trap emission was similar for each—ca. 108−109 Hz—indicating a universal value and similar trapping mechanisms despite the differences in energetic distribution. The latter result is important in the light of range of conflicting values reported, or higher value (1012 Hz) typically borrowed from crystalline inorganic materials.

The interface state density characterization by temperature-dependent capacitance–conductance–frequency measurements in Au/Ni/n-GaN structures

We have fabricated the Au/Ni/n-GaN structures and measured their capacitance–frequency (C–f) and conductance (G/w)-angular frequency (w) characteristics in the temperature range of 60–320 K. The C–f curves for different reverse bias voltages have shown a behavior almost independent of the bias voltage at frequencies above 300 kHz at each measurement temperature. We have calculated the temperature-dependent interface state density, Nss, values from the G/w versus w curves. The Nss value for the Ni/n-GaN interface ranges from 3.36 × 1011 cm−2 eV−1 at 0.0 V to 2.92 × 1011 cm−2 eV−1 at 0.40 V for 60 K, and 6.63 × 1011 cm−2 eV−1 at 0.0 V to 3.87 × 1011 cm−2 eV−1 at 0.40 V for 320 K. That is, the interface state density value increases with increasing temperature. It has been seen that the values of Nss obtained from the G/w versus w curves of the device are lower than the given values for metal/n-type GaN interface in the literature.

Optical absorption of the anthracene and temperature-dependent capacitance–voltage characteristics of the Au/anthracene/n-Si heterojunction in metal–organic-semiconductor configuration

An anthracene film has been deposited on an n-type silicon to fabricate an Au/anthracene/n-Si junction device. The band gap of the anthracene film has been determined from the optical measurement as Eg=1.65eV. After the fabrication of the Au/anthracene/n-Si junction device, temperature dependent capacitance–voltage characteristics in the range of 160–300K were studied to obtain the junction parameters of the device. The diffusion potential, barrier height, Fermi energy level and donor concentration parameters have been determined from the linear 1/C2–V curves with reverse bias at all temperatures. Both Fermi energy level and the barrier height increased with the increasing temperature. Temperature-dependence of the barrier height has been attributed to inhomogeneous barrier, traps and interface states. The ionized donor concentrations have varied with the temperature in an unsystematic manner due to the trapping/de-trapping of the charges at various temperatures.

Characterization of composite PVA–Al2O3 thin films prepared by dip coating method

Composite poly (vinyl alcohol)–aluminum oxide (PVA–Al2O3) thin films were successfully prepared on precleaned glass plates by dip coating method. The presence of metal–oxide (M–O) bond in the films was identified by Fourier transform infrared spectroscopy. The X-ray diffraction patterns revealed that both the as-grown and annealed films were predominately amorphous in nature. The scanning electron micrograph images showed that Al2O3 particles were distributed on the film surface and the size of the grains increased upon annealing. The energy dispersive X-ray spectroscopy analysis confirmed the presence of Al and O elements in the prepared films. The optical transmittance was found to be about 80 % for the as-grown film, whereas it was found to be increasing with the increase of annealing temperature and the band gap energy (3.74–3.78 eV) value decreased with the increase of annealing temperature. The frequency and temperature dependence of capacitance, dielectric constant, dielectric loss and AC conduction properties were investigated and the results showed enhanced dielectric properties. The obtained results indicated that the higher transmittance, wide band gap energy with high refractive index and high dielectric constant with low dielectric loss of the prepared composite PVA–Al2O3 thin films could be used as high-k layer in thin film transistors and optoelectronic devices.

Junction temperature measurement of light-emitting diodes using temperature-dependent capacitance

Junction temperature, as one of the most important properties of light-emitting diodes (LEDs), has great impact on LEDs’ power efficiency, luminosity, reliability, life-time, and so on. Precise measurement of junction temperature for LED device is quite important in the research of chip’s fabrication, device packaging and related applications. In this paper, we propose a new approach to measure the junction temperature of LEDs by using temperature-dependent capacitance. The capacitance of white LEDs at room temperature is measured and found to be decreased first and then increased with an increasing reverse bias. Equivalent model using vertical and horizontal capacitances connected in parallel is proposed to qualitatively explain the variation of capacitance under different reverse bias. Result obtained from the model fitting agrees well with the experimental result. The capacitance-temperature (C-T) curve of white LEDs under different reverse bias is measured and analysed. Results show that the capacitance of LEDs is sensitive to temperature at all biases. Under a reverse voltage of 0.5 V, the capacitance has the maximal response of 1.971 pF/℃ and a good linear temperature-dependent property. The C-T curve is used as the calibration for the measurement of junction temperature. By monitoring the change of capacitance of the working LEDs and comparing it with the C-T curve, the junction temperature of the LED device is successfully measured. The junction temperature of a white LED obtained by the proposed C-T method is compared with that by tranditional forward voltage method, and they are in good agreement. The C-T method is also used to measure the real-time junction temperatures of white LEDs under a constant current of 350 mA and a constant voltage of 3.2 V, respectively. In both conditions, the junction temperature of an LED needs approximately 110 sec to rise from room temperature to a steady value, and subsequently needs approximately 500 sec to fall back to room temperature after the LED is turned off. Compared with traditional methods, C-T method only needs to measure one calibration and this calibration can be applied to LEDs working at any current and voltage. Therefore, C-T method is a simple and flexible alternative to the existing technique of temperature measurement in electronic device.

Effect of temperature on the current (capacitance and conductance)–voltage characteristics of Ti/n-GaAs diode

In this study, Ti/n-GaAs Schottky barrier diode has been fabricated by DC magnetron sputtering. The current–voltage, capacitance–voltage, and conductance–voltage characteristics of Ti/n–GaAs diode have been investigated in the temperature range of 80–320 K. The ideality factor and barrier height values have been calculated from the forward current–voltage characteristics. The variation of the diode parameters with the sample temperature has been attributed to the presence of the lateral inhomogeneities of the barrier height. The temperature dependent capacitance–voltage characteristics have been measured to calculate the carrier concentration, diffusion potential, barrier height, and temperature coefficient of the barrier height (α = −0.65 meV K−1). The fact that the temperature coefficient of the barrier height changes from metal to metal has been ascribed to the chemical nature of the contact metal or metal electronegativity.

Polarization of Bi2Te3 thin film in a floating-gate capacitor structure

Metal-Oxide-Semiconductor (MOS) capacitors with Bi2Te3 thin film sandwiched and embedded inside the oxide layer have been fabricated and studied. The capacitors exhibit ferroelectric-like hysteresis which is a result of the robust, reversible polarization of the Bi2Te3 thin film while the gate voltage sweeps. The temperature-dependent capacitance measurement indicates that the activation energy is about 0.33 eV for separating the electron and hole pairs in the bulk of Bi2Te3, and driving them to either the top or bottom surface of the thin film. Because of the fast polarization speed, potentially excellent endurance, and the complementary metal–oxide–semiconductor compatibility, the Bi2Te3 embedded MOS structures are very interesting for memory application.

Origin of ferroelectric phase transition in SbSClxI1-x mixed crystals

The measurements of SbSCl x I 1-x(x = 0.2) temperature dependent capacitance were carried out. The temperature of ferroelectric phase transition TC ≈340 K was measured experimentally. TC of SbSCl x I 1-x was calculated theoretically in anharmonic and harmonic approximations. TC was calculated in anharmonic approximation using temperature dependence of mean potential energy of Sb atoms as a function of the soft B1u symmetry normal coordinate along c(z)-axis. Moreover, TC was calculated in harmonic approximation using temperature dependence of vibrational thermodynamic functions (Helmholc free energy). TC dependence from unit cell parameters a, b and from mixture composition x was carried out.

An Electrothermal Model for Empirical Large- Signal Modeling of AlGaN/GaN HEMTs Including Self-Heating and Ambient Temperature Effects

Accurate modeling of electrothermal effects of GaN electronic devices is critical for reliability design and assessment. In this paper, an electrothermal model for large signal equivalent circuit modeling of AlGaN/GaN HEMTs including self-heating and ambient temperature effects is presented. To accurately describe the effect of ambient temperature, two separate electrothermal networks (I diss , R diss , and C diss for self-heating, and I amb , R amb , and C amb for ambient temperature effect) are used to describe drain-source current slump due to self-heating and ambient temperature effects, respectively. A temperature-dependent thermal resistance and thermal capacitance model is proposed and implemented in the electrothermal network. The extraction of the thermal parameters is fulfilled by using numerical finite-element method. Single tone on wafer load-pull measurements at two operating frequencies (3 and 14 GHz) are carried out for verification purposes. The results show that good agreements on fundamental output power, the second and third harmonics output power, and power added efficiency have been achieved between simulations and measurements over a wide range of -55 °C to 175 °C.

Electrical characterization of Bi1.50-xYxZn0.92Nb1.5O6.92 varactors

The electrical properties of yttrium doped bismuth zinc niobium oxide (BZN) pyrochlore ceramics are explored by means of temperature dependent electrical conductivity dielectric constant and capacitance spectra in the frequency range of 0–3 GHz. It is observed that the doped BZN exhibit a conductivity type conversion from intrinsic to extrinsic as the doping content increased from 0.04 to 0.06. The thermal energy bandgap of the intrinsic type is 3.45 eV. The pyrochlore is observed to exhibit a dielectric breakdown at 395 K. In addition, a negative capacitance (NC) spectrum with main resonance peak position of 23.2 MHz is detected. The NC effect is ascribed to the increased polarization and the availability of more free carriers in the device. When the NC signal amplitude is attenuated in the range of 0–20 dBm at 50 MHz and 150 MHz, wide tunability is monitored. Such characteristics of the Y-doped BZN are attractive for using them to cancel the positive parasitic capacitance of electronic circuits. The canceling of parasitic capacitance improves the high frequency performance of filter inductors and reduces the common mode noise of the resonance signal.

Electrical characteristics of Au/Ti/n-GaAs contacts over a wide measurement temperature range

We have fabricated Au/Ti/n-GaAs/In Schottky barrier diodes using the magnetron dc sputter technique. The capacitance–temperature (C–T) measurements with bias voltage as a parameter and the current–voltage (I–V) and capacitance–voltage (C–V) measurements have been made in the temperature range of 60–300 K. The temperature-dependent capacitance measurements have been made at 1.0 MHz. The capacitance versus temperature curve at each bias voltage has four regions with slopes different from each other. The capacitance decreases with a decrease in temperature at each bias voltage. Such a temperature-dependent behavior could be attributed to modulation of the space charge region caused by the emission of deep-level impurities or interface states. The carrier concentration calculated in the −1.0 to −2.0 V range of C−2–V plots was close to the value of 7.43 × 1015 cm−3 given by the manufacturer around room temperature. The ideality factor value from the I–V characteristics has remained almost unchanged between 1.07 and 1.10 in the temperature range of 150–300 K, indicating that the current across the device obeys the thermionic emission current model quite well over the whole bias range at temperatures above 150 K. Therefore, the conventional Richardson plot in this temperature range has given a Richardson constant of 8.21 A (cm K)−2, within experimental error, which is in very close agreement with the theoretical value of 8.16 A (cm K)−2 for n-type GaAs. Again, it has been seen that the ideality factor with the values of 1.10 at 150 K and 1.22 at 60 K does not show a considerable decrease. The experimental parameters show that the Au(90 nm)/Ti(10 nm)/n-GaAs contact is a good candidate for electronic device applications.

Electrical conductivity and capacitance spectra of Bi1.37Sm0.13Zn0.92Nb1.50O6.92 pyrochlore ceramic in the range of 0–3 GHz

In this work, the electrical properties of samarium-doped bismuth niobium zinc oxide ( Sm -doped BZN ) pyrochlore ceramics are investigated by means of temperature dependent electrical conductivity and capacitance spectroscopy in the frequency range of 0–3 GHz. It was observed that the novel dielectric Sm - BZN ceramic exhibits a temperature and electric field dependent dielectric breakdown. When measured at 300 K, the breakdown electric field is 1.12 kV/cm and when heated the breakdown temperature is ~ 420 K. The pyrochlore is thermally active above 440 K with conductivity activation energy of 1.37 eV. In addition, the room temperature capacitance spectra reflected a resonance–antiresonance switching property at 53 MHz when subjected to an AC signal of low power of 5 dBm. Furthermore, when the Sm - BZN ceramics are used as microwave cavity and tested in the frequency range of 1.0–3.0 GHz, the cavity behaves as low pass filter with wide tunability up to a frequency of 1.91 GHz. At this frequency it behaves as a band rejection filter that blocks waves of 1.91 GHz and 2.57 GHz frequencies. These properties of the Sm -doped BZN are promising as they indicate the usability of the ceramics in digital electronic circuits as resonant microwave cavities suitable for the production of low pass/rejection band filters.

Impact of copper back contact in CdTe solar cells: study of defects by temperature-dependent capacitance–voltage measurements

Copper diffusion from back contact creates semishallow and deep-level defects in n+-CdS/p-CdTe solar cells. This study analyzes the impact of copper as a function of annealing temperatures during back contact formation. The observed defect levels, formed after copper diffusion, were identified by employing the temperature-dependent capacitance–voltage (C-V) characteristics at reverse bias in the dark. Theoretical background involving recombination centers, defect density and the role of impurity defects (copper-related defects) suggests that the temperature-dependent C-V profiling is a suitable technique to investigate the copper-related defects. Samples, annealed at 160°C for 30 min, show only one energy level, whereas samples annealed at 280ºC for 30 min reveal two distinct deep levels. From the reported observations, the trap levels were identified as copper-related defects. Variation in annealing temperature can impact the defect formation process: substitutional impurities of copper (160°C) and deep levels (280°C).

AlxGa1−xAs minority carrier lifetime enhancement at low temperatures

DX-centers in AlxGa1−xAs are known to reduce the minority carrier lifetime. Thus, DX-centers influence various semiconductor device characteristics. However, at temperatures below 100 K, we observed an unexpected improvement in the short circuit current density of an investigated Al0.37Ga0.63As solar cell. Using temperature-dependent capacitance measurements taken on a similar Al0.37Ga0.63As n−p+-diode, we correlated this behavior with the persistent photoconductivity effect. This effect derives from the suppressed silicon-DX-center generation inside the n-doped material at low temperatures.