Capacitance-frequency Measurements Research Articles

Investigation of trap processes in polycrystalline silicon thin film transistors by ac measurement

We report a method for characterizing trap generation–recombination processes in polycrystalline silicon thin film transistors. A small ac voltage was superimposed on a dc gate voltage and the ac current was measured at the source. A theoretical model was developed, whereby n-channel thin film transistors were analyzed. A resonant peak in the imaginary part of the ac current and a corresponding step in the real part were found at frequency of 147 Hz. Our data indicate that the ac response is dominated by a single trap level. By combining the ac current measurement with a low frequency capacitance measurement, we have determined the trap energy, capture cross section, and trap density to be, respectively, 0.91 eV above the valence band, 3.1×10−21 cm2, and 5.7×1015 cm−3.

The interface state energy distribution from capacitance–frequency characteristics of gold/n-type Gallium arsenide Schottky barrier diodes exposed to air

A study on gold/n-type Gallium arsenide (Au/n-GaAs) Schottky barrier diode (SBD) parameters with and without thin native oxide layer fabricated on n-type GaAs was made. The native oxide layer for metal/oxide/semiconductor (MIS) SBD was obtained by exposing the chemically cleaned GaAs surface to clean room air for 30 days, before metal evaporation. The values of 1.089 and 0.730 eV for the ideality factor and barrier height of the reference Au/n-GaAs SBD were obtained, respectively, and the values of 1.427 and 0.671 eV for the MIS and SBD, respectively. We calculated the density distribution and time constant of the interface states from the capacitance–frequency measurements using the Schottky capacitance spectroscopy method. The interface state density N ss of the diodes has an exponential rise with bias from the midgap towards the top of the conduction band; for example, from 1.20×10 10 cm −2 eV −1 in ( E c−0.730) eV to 3.41×10 12 cm −2 eV −1 in ( E c−0.470) eV for reference diode, and from 1.47×10 10 cm −2 eV −1 in ( E c−0.671) eV to 1.68×10 13 cm −2 eV −1 in ( E c−0.411) eV for the MIS diode.

Electrical characterization of acceptor levels in Be-implanted GaN

We have investigated electrically the acceptor levels that are present in Be-implanted GaN. Slight p-type conductivity was attained in undoped GaN films by Be implantation and subsequent annealing at 1050 °C with a SiO2 encapsulation layer. Capacitance-frequency measurements showed a typical dispersion effect characteristic of deep acceptors in fabricated Schottky diodes. Thermal admittance spectroscopy measurements revealed a discrete deep level located at ∼231 meV above the valence band. This energy level is in reasonable agreement with the frequency dependence of the capacitance in view of the impurity transition frequency. Therefore, this energy level can most probably be assigned to a Be-related deep acceptor.

Instrumentation for direct, low frequency scanning capacitance microscopy, and analysis of position dependent stray capacitance

We describe instrumentation for scanning capacitance microscopy (SCM), using an atomic force microscope, that is designed to make calibrated, low frequency measurements of tip–sample capacitance and spreading resistance of thin dielectric films. We also characterize spatial variations in stray capacitance Cstray that must be understood before such measurements can be interpreted. Existing SCM circuits are generally optimized for measuring dC/dV, and not for calibrated low frequency measurements of absolute capacitance. Our circuit uses a commercially available current amplifier and low frequency (∼5 kHz) lock-in detection. This circuit adds an inverted, constant amplitude current to suppress the stray displacement current from the large (∼0.5 pF) stray capacitance Cstray between the sample and the mm-sized cantilever–probe assembly. The capacitance noise spectrum is ∼0.35 aF/Hz and is flat down to 1 Hz measurement frequency, with an integrated noise <5 aF integrated over a 1–160 Hz bandwidth. We have also used this instrumentation to identify and characterize significant (>1 fF) variations in Cstray that must be understood in order to accurately measure aF-level variations in the nm-scale tip–sample capacitance. We find that Cstray varies with lateral probe position. This is due to tilting of the probe assembly as the piezoelectric scanner tube bends during scanning. We also find that Cstray varies significantly with probe–assembly height. This causes topography related artifacts in capacitance images of rough surfaces. However, we show that stray capacitance artifacts can be mostly eliminated by properly characterizing position and height dependent variations in Cstray and subtracting them from measured capacitance data.

The determination of the interface-state density distribution from the capacitance-frequency measurements in Au/n-Si schottky barrier diodes

The Au/n-Si Schottky barrier diodes (SBDs) with 200-µm (sample D200) and 400-µm (sample D400) bulk thicknesses have been fabricated. The ideality factor and the barrier height have been calculated from the forward-bias current-voltage (I-V) characteristics of D200 and D400 SBDs. The energy distribution of the interface states and relaxation time are found from the capacitance-frequency (C-f) characteristics. The density of interface state and relaxation times have a (nearly constant) slow exponential rise with bias in the range of Ec -0.77 and Ec -0.47 eV from the midgap toward the bottom of the conductance band. Furthermore, the energy distribution of the interface states obtained from C-f characteristics has been compared with that obtained from the forward-bias I-V characteristics.

Annealing temperature dependence of electric conduction and capacitance dispersion in nitrogen-implanted GaAs

The annealing behavior of nitrogen-implanted GaAs samples has been investigated by secondary ion mass spectroscopy, current-voltage (I-V) and capacitance-frequency (C-F) measurements. The I-V data show that the conductivity of as-implanted samples is dominated by variable-range hopping between defect states below 300 K. The implanted layer becomes highly resistive after annealing. The activation energy of the resistance is found to increase from 0.2 eV for as-implanted samples to 0.71 eV for 950°C-annealed samples. Significant capacitance dispersion is observed over frequency for implanted samples. Based on a proposed equivalent circuit, the high-frequency capacitance dispersion is shown to be the result of resistance-capacitance (RC) time constant effects. The increase of activation energy of the resistance can be explained by the creation of deep traps by high temperature annealing. Traps at 0.69 eV and 0.82 eV are detected for 700°C and 950°C-annealing, respectively.

Carrier depletion by defects levels in relaxed In0.2Ga0.8As/GaAs quantum-well Schottky diodes

An increase in leakage current accompanied by a drastic carrier depletion is found for InGaAs/GaAs Schottky diodes when the InGaAs thickness is larger than its critical thickness. Due to drastic carrier depletion, free-carrier concentration around the InGaAs region for relaxed samples cannot be obtained from capacitance–voltage data but from resistance–capacitance time constant effect observed in capacitance–frequency measurement. A trap at 0.33 to 0.49 eV is observed for relaxed samples by deep-level transient spectroscopy. The resistance caused by carrier depletion has an activation energy close to that of the trap, supporting that the carrier depletion is caused by capture from the trap.

Observation of abnormal capacitance-frequency behavior in In0.12Ga0.88As/GaAs p-i-n superlattice grown at low temperature

Capacitance-frequency measurement is used to study In0.12Ga0.88As/GaAs p-i-n superlattice, with superlattice layer grown at 300 °C by molecular-beam epitaxy. Three traps are observed, and their parameters are consistent with those obtained from deep-level transient spectroscopy. Among these three traps, the trap (Ea=0.45±0.02 eV, σ=6±4×10−17 cm2) shows an abnormal increase of capacitance with increasing frequency, similar to that observed from the 0.66 eV trap in low-temperature grown GaAs p-i-n structure, suggesting that it is created by the low-temperature growth and is a generation-recombination center. This result also shows that the capacitance-frequency measurement is effective in studying the generation-recombination centers.

Capacitance Spectroscopy of Defects in a-SI:H/c-SI Heterostructures

AbstractWe investigate defects at the interface in heterodiodes of hydrogenated amorphous silicon and monocrystalline silicon by frequency and temperature dependent capacitance measurements. The interpretation of the experimental results is supported by numerical simulations of capacitance experiments via transient calculations of defect charging and decharging in the diodes. A defined variation of waver surface treatments prior to amorphous silicon deposition shows a clear correlation of interface defects determined by capacitance measurements with current-voltage characteristics.

Structural, optical and electrical characteristics of yttrium oxide films deposited by laser ablation

Yttrium oxide films deposited by laser ablation on Si(100) substrates have been evaluated for possible application as high dielectric constant insulator layers. The structural, optical and electrical characteristics of these films have been determined by means of x-ray diffraction, atomic force microscopy, Auger electron spectroscopy, ellipsometry, infrared transmittance, and capacitance and current versus voltage (C-V and I-V) as a function of the deposition parameters. It has been found that the crystallinity of these films is strongly dependent on the substrate temperature during the deposition. At low deposition temperatures (350 °C) the films present an amorphous component and a polycrystalline part that is highly oriented in the (100) direction perpendicular to the surface of the film. As the deposition temperature is increased up to 650 °C, the amorphous part of the films is reduced drastically and the dominant orientation of the polycrystalline part becomes the (111) orientation. The refractive index at 630 nm was found to be in the range between 1.91 and 1.95, having its maximum value for films deposited at 450 °C. Similar behavior was observed for the dielectric constant κ, measured from the high frequency capacitance measurements, with a maximum value of 15 obtained for the above mentioned deposition temperature as well. The average roughness of the films decreases with substrate temperature from ∼30 Å to less than 5 Å in the range of temperatures studied.

Dielectric dispersion in Barium Strontium Titanate thin film capacitors with iridium electrodes

Dielectric dispersion or relaxation results in a frequency dependent loss in capacitance. This reduces the charge storage capacity under DRAM operating conditions. In this work, the dielectric dispersion behavior of Barium Strontium Titanate (BST) thin films (compositions: Ba0.5Sr0.5TiO3 and Ba0.4Sr0.6TiO3) deposited by RF magnetron sputtering on iridium bottom electrodes was investigated. Using capacitance-frequency measurements on BST films of various thicknesses, electrode-dielectric interface effects were separated from the bulk of the dielectric film. For the BST films used in this study, the dispersion in Ba0.4Sr0.6TiO3 films was interface dominated while that in Ba0.5Sr0.5TiO3 was bulk controlled. Various multi-layered BST capacitors with combinations of the above compositions and different individual layer thicknesses were studied to verify the accuracy of the model developed for the films of each composition.

Effect of annealing on physical properties of conducting poly(ortho-anisidine) Langmuir—Blodgett films

Langmuir—Blodgett (LB) films of poly(ortho-anisidine) (POAS) have been fabricated at pH 1 of the subphase, where the doping during the monolayer formation is essential for high quality of the ultrathin films. POAS LB films have been systematically characterized using low-angle X-ray diffraction, UV—Vis and FTIR spectroscopic, current—voltage and capacitance—frequency measurements, respectively. The effect of annealing on the physical properties of POAS LB films has been studied as a function of temperature (from 60 to 150 °C).It has been shown that annealing causes changes in structural, optical and electrical characteristics of POAS LB films. The decrease in thickness (from 24 to 18.1 Å) of each monolayer of POAS film has been observed for annealing at 150 °C. The value of conductivity of POAS LB film has been shown to be varying from 0.4 to 10 −8 S cm −1 as a function of annealing temperature. The continuous shift of the UV absorption bands from 840 to 590 nm with annealing temperature is arising due to the simultaneous change of charge defects in POAS LB films. Differential scanning calorimetry studies show a complete evaporation of dopant ions occurring at 180 °C in the powder form of POAS, whereas complete removal of dopants ions for LB films have been observed for annealing at 150 °C. The Cole—Cole plot obtained at 80 °C for POAS LB film shows a mark deviation from a semicircle which indicates the operation of complex behaviour in the POAS LB films. It also shows the typical behaviour of interfacial polarization caused by the contact effect of POAS LB films with chromium metal of interdigited electrodes for annealing at 150 °C.

Anomalous degradation in silicon solar cells subjected to high-fluence proton and electron irradiations

Distinct from the well-known logarithmic degradation in electrical performances of a crystalline silicon solar cell, an anomalous degradation of short-circuit current density (Isc) was observed in a cell irradiated by energetic protons and electrons at high fluence. From results of proton irradiations with various energies (0.4–10 MeV) and high frequency (1 MHz) capacitance measurements, the anomalous drop of Isc is found to be caused by (1) the p-type substrate changes into the intrinsiclike layer (Fermi level shift) by the irradiations, followed by an extension of the depletion layer, and (2) the drift length of the minority carrier becomes shorter than the depletion layer.

Electrical and structural properties of refractory metal Mo/poly(3-methylthiophene) Schottky barrier diodes

We present the results of a study of Mo/poly(3-methylthiophene)/Pt Schottky diodes. The Mo contact is sputtered onto electrochemically deposited poly(3-methylthiophene) (P3MT). Grazing incidence X-ray diffraction (GIXD) analysis shows that P3MT is amorphous. X-ray photoelectron spectroscopy (XPS) studies reveal that the Mo/P3MT interface is extremely diffuse with a great Mo penetration in the P3MT layer. However, no chemical reaction is observed. The rectification ratio of these devices at ±1 V is 620. The capacitance–frequency measurements show that the capacitance is frequency dependent. This behavior is explained on the basis of the amorphous nature of the polymer. The value of the density of states near the Fermi level N 0 deduced from the capacitance measurements at low frequencies is 1.2×10 17 cm −3 eV −1.

Low frequency capacitance measurements of photovoltaic cells based on microcrystalline chlorophylla

Low-frequency capacitance measurements have been made to characterize aluminium/microcrystalline chlorophylla/silver Schottky barrier photovoltaic cells. The cell shows a voltage-independent geometric capacitance at frequencies above 10Hz in dark. Linear Schottky plots are obtained only at low frequencies, ∼0.1 Hz, or under illumination of the cell with adequate intensity. This is explained by the fact that the depletion layer is mainly due to low-mobility trapped charges that are not able to follow the variations of the applied voltage at high frequencies. The light is believed to detrap and mobilize these trapped charges. From linear Schottky plots, various depletion parameters have been calculated in dark and under illumination.

RuO2/TiN-Based Storage Electrodes for (Ba, Sr)TiO3 Dynamic Random Access Memory Capacitors

Sputtered (Ba, Sr)TiO3 (BST) thin film capacitors have been fabricated with thick RuO2/TiN-based storage electrodes and poly-Si contact plugs, and the electrical properties of the storage electrodes have been studied. The electrode height was higher than 450 nm and the contact size was 0.8×0.8 µ m2. Resistance of the storage electrodes including contact plugs can be evaluated from the dispersion observed in capacitance-frequency measurements. TiN oxidation at the RuO2/TiN interface and native oxide at the TiN/Si contact contribute to the electrode resistance of RuO2/TiN electrodes. With increasing BST deposition temperature, the thickness of oxidized TiN in RuO2/TiN electrodes increases and the electrode resistance increases correspondingly. A Ru layer inserted at the RuO2/TiN interface, a TiN/TiSi2/Si junction and rapid thermal annealing in N2 ambient of the TiN layer are effective ways to reduce the resistance of RuO2/TiN-based electrodes.

A method for the determination of barrier heights from the capacitance-voltage characteristics of a Schottky junction containing bulk deep traps

In Schottky barrier diodes, appreciable concentrations of deep levels in the space charge region influence high frequency capacitance measurements. The barrier height, calculated by the usual analysis of 1 C 2 vs V plot, exhibits large steps at temperatures corresponding to the relaxation of the traps occurring during the voltage sweeps. In this work we develop a model which takes into account the partial ionization of the deep levels, to determine the correct parameters of the Schottky barrier by temperature dependent capacitance-voltage measurements. The influence of two kinds of bulk traps, already detected by DLTS, is clearly evidenced in the 1 C 2 vs V intercept and taken into account for the calculation of the true barrier height and trap concentrations. The correlation coefficient of the linear regression is very significantly improved if this model is used. The analysis is applied to Ti GaAs Schottky diode prepared by ion beam sputtering.

Surface potential fluctuations in metal–oxide–semiconductor capacitors fabricated on different silicon carbide polytypes

The standard deviation σs of the surface potential fluctuations affecting metal–oxide–silicon carbide capacitors has been extracted from low frequency conductance and capacitance measurements. These fluctuations may become very large for thermal oxides and aluminum-doped epilayers. They must induce large errors in usual metal–oxide–semiconductor characterization techniques and are most probably the main reason for observing an activated transport in silicon carbide inversion layers.

Effect of temperature on photoconduction and low frequency capacitance measurements on β-CuPc photovoltaic cells

The electrical properties of In/β-CuPc-PVAc/Au (where CuPc is copper phthalocyanine and PVAc is polyvinyl-acetate) devices in the dark and under illumination have been studied. The temperature dependence of the photocurrent was attributed to a high trap density. Electron traps located at 0.25 eV below the conduction band have been observed. Schottky barrier parameters of the cell in the dark and under different monochromatic light intensities have been determined by a low frequency differential capacitance method. Light intensity increased significantly the space change density, which is a major concern as a possible limiting factor for the performance of organic solar cells.

Monitoring of photodegradation and recovery of a-Si : H p-i-n solar cells by capacitance measurements

In this paper we report an application of forward bias, low frequency capacitance measurements in order to relate the electrical properties of the intrinsic material to the solar cell performance after photodegradation and thermal recovery. The capacitance vs. frequency technique (C-v-f) is based on the measurement of the variation of the total charge trapped in the gap defect states of the intrinsic layer during a low frequency voltage perturbation. As expected, after illumination the efficiencies of the tested devices decreased. These variations were found to be only weakly related to the initial electrical performance, but strongly dependent on the initial density of states of the intrinsic layers. Efficiency degradation of the investigated solar cells has been quantitatively related to a reduced mobility-lifetime product and, lesser, to a variation of the density of defects. The defects detected by the C-v-f technique are positioned around 0.55 eV from the band-edge and do not act as recombination centers but, rather, as free carrier traps. Thus, due to their energy position they do not interfere with steady-state current-voltage (I-V) measurements and with recombination mechanisms. Employing either the I-V or the C-v-f technique, a more complete picture of the defect distribution in the gap of a-Si : H has been drawn and the defects behaviour, after light soaking and annealing, has been investigated.