Dark Capacitance Research Articles

Junction capacitance study of an oxygen impurity defect exhibiting configuration relaxation in amorphous silicon–germanium alloys deposited by hot-wire CVD

We report the observation of light induced electron capture in oxygen contaminated (∼5×1020cm−3) hydrogenated amorphous silicon–germanium alloys grown by hot-wire chemical vapor deposition (HWCVD). By examining the time evolution of dark capacitance after 1.2eV photoexcitation, we are able to estimate the free energy barrier (⩾0.8eV) for the release of electrons into the conduction band. Such a large thermal barrier, for a defect whose optical threshold is centered (∼1.35eV) so close to the band-gap (1.5eV), indicates significant configurational relaxation once the oxygen impurity state is occupied with photoexcited electrons.

Carrier Transport Mechanisms of p-CuPc / n-Si Heterojunction

p-CuPc / n-Si heterojunction cells have been fabricated by thermal evaporation of p-type CuPc thin films onto n-Si single crystal wafers. Current – Voltage and Capacitance – Voltage measurements have been performed to determine some electrical properties of these structures. Rectifying properties have been obtained, which are definitely of the diode type. The analysis of dark Current – Voltage characteristics of cells under test at several temperatures reveals some junction parameters such as: rectification ratio, the series resistance, the shunt resistance, the diode ideality factor, the potential barrier height and the reverse activation energy. The forward current involves thermionic emission as well as space charge limited current at low and high forward bias voltages respectively. The reverse current is probably limited by generation – recombination mechanism. The dark Capacitance – Voltage behavior indicates an abrupt heterojunction, with homogeneous distribution of impurities inside the space charge region. Values of conversion efficiency as high as 1.29 % and open voltage of 0.36 V were evaluated from the loaded I-V characteristics at input power of 50mW/cm2.

Impedance spectroscopy of unetched CdTe/CdS solar cells—equivalent circuit analysis

A detailed study of electric and structural properties has been carried out on CdTe/CdS solar cells which deliberately were not subjected to etching by a nitric-phosphoric (NP) or bromine-methanol (Br-Me) acids, conventionally employed for the formation of Te-rich layer before back contacting. In the previous work [J. Appl. Phys. 101, 014505 (2007)] we have shown that cells that were not etched provide more extensive information on sample/material properties than the etched ones, as analyzed by admittance spectroscopy. Although seemingly being able to describe the distribution of defect energy levels, the admittance spectroscopy approach has a significant drawback because the underlying theoretical formulation does not take into account the frequency-dependent contribution from the back contact together with its influence on the trap contributions. In this work we use an alternative methodology for analysis of impedance data measured in dark conditions, which applies an equivalent circuit model to the experimental spectra. In particular, a complete model consisting of 10–12 elements is suggested, which describes all the sets of data taken at different temperatures, unambiguously separating the respective roles of p-n junction parameters, defect trap levels, back contact, as well as spatial inhomogeneities within the cell. It is essential that the values of the parameters used to describe ac response from trap levels and that from the back contact are found to be consistent with admittance and I-V measurements. In addition, the temperature dependence of the dark conductance (GJ) and capacitance (CJ) of the main p-n junction, as well as temperature dependence of back contact resistance (RB), were obtained and analyzed. It was found that GJ(T) follows exp(T/T0) behavior which is characteristic of temperature-assisted tunneling, while CJ(T) agrees well with values of the high-frequency capacitance of the cell CHF(T). The T dependence of RB is found to follow activation behavior defined by a Schottky barrier with a height of (0.545±0.015) eV, that being close to the value obtained from dark I-V measurements.

Diffusion length variation in photovoltaic cells with Bridgman-grown CuInSe 2 substrates

In a group of CuInSe 2-CdS-ZnO photovoltaic cells, where the absorber was a layer cut from Bridgman-grown p-type CuInSe 2 ingots, electron diffusion lengths at room temperature ( L n) were estimated by the photocurrent–capacitance method. Dark capacitance measurements were also made on the same cells against reverse bias and from Mott-Schottky plots, slope concentrations p MS were determined at a reverse bias of 1.5 V. In a plot of L n against p MS, it was found that, despite much scatter in the experimental points, there was an apparent trend of L n decreasing by about an order of magnitude with increase of p MS from 10 16 to 10 17 cm − 3 . Detailed proposals were then made to explain this trend. These were reduction of lifetime via Shockley-Read trapping with mobility decrease by impurity scattering, shunt resistance lowering by light and optical penetration depth reduction at shorter illumination wavelengths.

InP-based quantum cascade detectors in the mid-infrared

We present two InP-based quantum cascade detectors (QCDs) in the mid-infrared wavelength range. Their narrow band detection spectra are centered at 5.3 and 9μm. A vertical intersubband transition followed by a carefully designed extraction cascade, which is adapted to the LO-phonon energy, leads to 10K responsivities R of 3.2 and 9.0mA∕W and background limited detectivities DBLIP* of 2×108 and 3×109 Jones, for the 5.3 and the 9μm devices, respectively. Detection has been observed up to device temperatures of 300K (RT), albeit reasonable performance is restricted to temperatures below 150K (5.3μm) and 70K (9μm). Designed for zero bias operation, QCDs do not produce any dark current and therefore do not suffer from dark current noise and capacitance saturation at long integration times, making them ideal devices for large focal plane arrays.

Electrical characterization of novel Si solar cells

A new approach for hybrid metal-insulator-semiconductor Si solar cells is adopted by the Institute of Fundamental Problems for High Technology, Ukrainian Academy of Sciences. In this technique, the porous silicon layers are created on both sides of single crystal wafers by chemical etching before an improved MIS cell preparation process. Using this technique, the solar cells with efficiencies above 15% have been obtained under AM1.5 condition (under 100 mW/cm 2 illumination at 25 °C). In this work the dark current–voltage–temperature ( I– V– T) characteristics of these cells are studied over a temperature range between 79 and 400 K. The dark capacitance–voltage ( C– V) and conductance–voltage ( G– V) behaviours of these cells at 100 kHz are analysed for the same temperature range. At room temperature C– V and G– V measurements are also made under illumination levels changing five folds. The diode ideality factors calculated from the dark I– V characteristics are significantly larger than unity and exhibit strong temperature dependence. The analysis of the temperature tendencies of diode ideality factor in these cells indicated that the current transport mechanism consists of both the trap-assisted tunnelling and the thermionic emission. While the temperature decreases, the barrier height calculations from I– V– T and C– V– T measurements showed that ϕ B( I − V) decreases whereas ϕ B( C − V) increases. The difference in ϕ B( I − V) and ϕ B( C − V) values and in their temperature dependence are explained by a model assuming Gaussian distribution of barrier heights. All the results are analysed after series resistance corrections.

Current transport mechanisms and photovoltaic properties of tetraphenylporphyrin/n-type silicon heterojunction solar cell

Au/tetraphenylporphyrin (TPP)/n-type silicon (n-Si)/Al heterojunction solar cell was constructed and prepared by growing TPP film on n-Si wafer using thermal evaporation technique, the dark current–voltage and dark capacitance–voltage characteristics of the cell were measured over temperature range from 291 to 373 K. The parameters and mechanisms of conduction of heterojunction diode have been studied, current–voltage characteristics indicated an ohmic conduction at voltages < 100 mV and at voltages > 1.3 V. Space charge-limited conduction and multi-step tunneling mechanisms occur consequentially at (0.11–1.25) V. The capacitance–voltage measurements showed that the diode is linearly graded junction and the width of depletion layer, impurity gradient and built in voltage were estimated. The current–voltage characteristics under illumination have also been investigated and photovoltaic properties of Au/TPP/n-Si/Al solar cell were evaluated.

High Speed Heterostructure Metal-Semiconductor-Metal Photodetectors

A. Colaa,∗, B. Nabet, Xiying Chen and F. Quaranta CNR-IMM sez. di Lecce, via Arnesano 73100 Lecce, Italy Electrical and Computer Engineering Dept., Drexel University 318 Commonwealth, Philadelphia, PA, 19104, USA In this work we review the properties of a class of metal−semiconductor– metal photodetectors based on heterojunction structures. Particularly, an AlGaAs/GaAs device is detailed in which the absorption region is in the GaAs layer, and a two-dimensional electron gas is formed at the heterointerface due to δ-doping of the widegap material. This heterostructure metal– semiconductor–metal photodetector also contains an AlGaAs distributed Bragg reflector that forms a resonant cavity for detection at 850 nm. The beneficial effect of the two-dimensional electron gas in the GaAs absorption layer in terms of speed and sensitivity is demonstrated by comparing samples with and without doping in the AlGaAs layer. The design and the physical properties of the grown epitaxial structure are presented, together with the static and dynamic characteristics of the device in time domain. In particular, photocurrent spectra exhibit a 30 nm wide peak at 850 nm, and time response measurements give a bandwidth over 30 GHz. A combination of very low dark current and capacitance, fast response, wavelength selectivity, and compatibility with high electron mobility transistors makes this device suitable for a number of application areas, such as Gigabit and 10 Gigabit Ethernet, wavelength division multiplexing, remote sensing, and medical applications.

GaAs/Ge solar cell AC parameters under illumination

The ac parameters of GaAs/Ge solar cell were measured under illumination at different cell temperatures using impedance spectroscopy technique. They are compared with the dark measurements. It is found that the cell capacitance is higher and cell resistance is lower under illumination than in dark for all cell terminal voltages. The cell capacitances at the corresponding maximum power point voltage (terminal) do not vary with temperature where as the cell resistance decreases. The cell capacitance under illumination is estimated from the dark cell capacitance and it is in good agreement with the measured illumination data.

Mott-Schottky Analysis of Nanoporous Semiconductor Electrodes in Dielectric State Deposited on SnO2 ( F ) Conducting Substrates

This paper analyzes the dark capacitance of nanostructured electrodes in the dielectric state, with particular emphasis on electrodes deposited over a transparent conducting substrate of It is shown that at those potentials where the nanostructure is in the dielectric state, the capacitance is controlled by the contact The partial or total covering of the substrate by a dielectric medium causes a modification of the Mott-Schottky plot of the bare substrate. We provide a mapping of the various Mott-Schottky curves that will appear depending on the film characteristics. If the dielectric nanoparticles completely block part of the substrate surface, the slope of the Mott-Schottky plot increases (with the same apparent flatband potential) as an effect of area reduction. The covering of a significant fraction of the surface by a thin dielectric layer shifts the apparent flatband negatively. Measurements on several nanostructured electrodes show that the capacitance contribution of the semiconductor network in the dielectric state is very low, indicating that the field lines penetrate little into the network, not much further than the first particle. The different surface covering observed for rutile-anatase and pure anatase colloids is explained by lattice matching rules with the substrate. By comparing different electrodes, the Helmholtz capacitance at the interface was calculated and the apparent flatband potential was corrected for the effect of band unpinning. © 2003 The Electrochemical Society. All rights reserved.

Analysis of interface states of n-InP MIS structures based on bias dependence of capacitance and photoluminescence intensity

Bias dependence of the integrated room temperature photoluminescence (PL) intensity and capacitance measurements, are used to study the interface properties of metal–insulator–semiconductor (MIS) structures realized on indium phosphide (InP). Boron nitride (BN) insulating films have been grown by a radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD) technique. The capacitance–voltage ( C– V g) measurements are performed in the dark and under illumination. The Terman's method was first used to analyze the interface sates distribution. The results proved to be non-reliable. Indeed low (≤5×10 11 cm −2 eV −1) and even negative values were obtained particularly from the dark capacitance data. The interface states density is also determined from PL intensity variations versus applied voltage. The obtained distributions of interface states are U shaped with a minimum, nearly 10 12 cm −2 eV −1, located around 0.4 eV below the conduction band minimum. This technique, which is frequency independent and less sensitive to the leakage currents in the insulator, leads to more reliable results.

Studies of neutron irradiation of avalanche photodiodes using 252Cf

Results on the radiation hardness of photodiodes to fast neutrons are presented. Four photodiodes (three avalanche photodiodes from two manufacturers, and one PIN photodiode) were exposed to neutrons from a 252 Cf source at Oak Ridge National Laboratory. The effects of this radiation on many parameters such as gain, intrinsic dark current, quantum efficiency, noise, capacitance, and voltage and temperature coefficients of the gain for these devices for fluences up to ∼2×10 13 neutrons/cm 2 are shown and discussed. While degradation of APDs occurred during neutron irradiation, they remained photosensitive devices with gain.

Effect of interface states on the performance of GaAs p+-i far-infrared detectors

Interface states have been shown to have an appreciable effect on the performance of p-GaAs multilayer (p+-i-p+-i-…) homojunction interfacial work function internal photoemission (HIWIP) far-infrared detectors. In this article, a comparison of detector performance was made of p-GaAs HIWIP detectors with different interface state densities, with the emphasis on the detector’s dark current, noise, and capacitance characteristics.

Dark C– V and I– V characteristics of silicon multi-junctions prepared by liquid-phase epitaxy

The dark capacitance–voltage ( C– V) and current–voltage ( I– V) characteristics of p–n Si multi-junctions are measured over a reasonable temperature range. Analysis of data suggests that multistep tunneling is the current conduction mechanism due to a high interface density of charges.

Characterization of ITO—PPHT—metal contacts (PPHT=poly(3-phenylhydrazone thiophene)) using electrical and capacitance measurements

The electrical properties of thin film sandwich devices (M/PPHT/ITO) have been studied in a quantitative manner. Here, M represents Al or In; PPHT stands for poly(3-phenylhydrazone thiophene); and ITO is indium—tin oxide. These devices were studied by analyzing the current—voltage curves and capacitance measurements at different frequencies and biasing d.c. voltages in air. These devices showed the rectification effect, and forward bias corresponds to a negative voltage at the metal (Al or In) with respect to ITO. The device with In contact forms a Schottky junction at the In/PPHT interface, while the device with Al contact passed much smaller currents under a given bias and failed to behave quantitatively as a Schottky junction like the device with In contact. Various evaluated electrical parameters indicate that the Al—PPHT junction is deteriorated by an emerged interposed zone of Al 2O 3. The dark capacitance measurements of the devices were studied in various frequency ranges and under different d.c. bias conditions. These measurements reveal that the standard Schottky model cannot be applied to such metal/organic interfaces. Three different contributions to the admittance may be distinguished. A surface charge layer associated with a high capacitive term extends over a few angstroms of the semiconductor. This capacitance is only slightly dependent upon superimposed d.c. voltages. The space charge region next to the surface charge layer extends over approximately 100–400 nm. The corresponding capacity is almost independent of any d.c. imposed voltage. At the same time the resistance of the space charge region is highly dependent on external d.c. voltage. Finally, the rest of the metal—PPHT layer shows properties of bulk materials.

Theoretical study of a GaAs lateral p-i-n photodetector

We present a numerical study of a high speed GaAs lateral p-i-n (LPIN) photodiode. The LPIN is a planar structure composed of interdigitated p+ and n+ wells. A metal–semiconductor–metal (MSM) photodiode with identical finger spacing and geometry is simulated for comparison. When pulsed with an 827 nm optical source at 0.68 mW/cm2, the lateral p-i-n exhibited improved frequency performance and responsivity compared to the MSM. The dark current and capacitance are similar in magnitude between the two devices. Based on these results, it is concluded that a lateral p-i-n can be an attractive alternative to standard MSM photodetectors.

Uniform and high performance of monolithically integrated 1×12 array of planar GaInAs photodiodes

In this article, we describe the fabrication of a monolithically integrated 1/spl times/12 array of GaInAs/InP planar photodiodes, which has highly uniform characteristics in dark current, capacitance and crosstalk capacitance, quantum efficiency, and the frequency bandwidth at 3-dB reduction with a deviation of /spl plusmn/1%. Besides, each diode on the array exhibits an extremely low dark current of 75 pA, a low capacitance of /spl sim/2.3 pF and a crosstalk capacitance between adjacent diodes of /spl sim/0.36 pF, a high quantum efficiency of 95% at 1.3 /spl mu/m and 89% at 1.53 /spl mu/m, the 3-dB frequency of >2 GHz, and a small 1/f noise component over a wide operating voltage range. Also, the diode on the array has a negligible degradation after the burn-in test of -20 V, 200/spl deg/C, and 20 h.

An integrated, crystalline organic waveguide-coupled InGaAs photodetector

We describe a novel, integrated organic-inorganic waveguide/photodetector with an internal quantum efficiency of (53/spl plusmn/15%). Low-loss (2.7/spl plusmn/0.3 dB/cm at /spl lambda/=1.064 μm for 10-μm wide waveguides) crystalline organic waveguides are grown directly onto the pre-processed InGaAs photodetector by organic molecular beam deposition (OMBD) without affecting either the dark current or capacitance of the photodiode. The polarization-preserving nature of the waveguides suggests that a patterned substrate significantly influences the organic molecular orientation during vacuum deposition.

Electrochemical deposition and characterisation of CdTe thin films from an ethylene-glycol-based bath

CdTe films were deposited on Ni and conducting glass (SnO,) substrates from an ethyleneglycol-based bath by galvanostatic and potentiostatic methods. The film composition and electrical properties depend on parameters such as working electrode potential current density, deposition temperature, substrate type and post-deposition treatments. It is possible to improve the grain size and stoichiometry of the film by post-deposition heat treatment in air. The conductivity type was determined from the photocurrent-working electrode potential behaviour of the film. Dark capacitance measurements in a 0.5 M H,SO, solution at kHz showed a linear behaviour, from which the flatband potential V,= -0.365 V vs. a saturated calomel electrode (SCE) and the doping density N, = 1.35 x 10 cm-' were determined. @ 1997 by John Wiley & Sons, Ltd.

Probing resonant tunneling and charge accumulation via capacitance measurements in [111]-oriented [formula omitted] MQW and superlattices

The features present in the capacitance and conductance-voltage characteristics of [111]-oriented InGaAs GaAs MQW and SL, under illumination or dark conditions, are described. The different peaks observed in a p- i- n diode with intrinsically negative average electric field in the MQW region are attributed to sequential tunneling of electrons and holes between adjacent wells. The capacitance characteristics and their evolution with voltage provide clear evidence for a long-range (out-of-well) screening of the piezoelectric fields by the photogenerated carriers accumulated at the extremes of the MQW region. This effect is shown to be consistent with a self-consistent solution of the Schrödinger and Poisson equations. The voltage corresponding to zero average electric field is detected by a feature in the dark device capacitance in a seven period MQW structure and in a 40 period [111] strained superlattice, for which changes in capacitance and conductance associated with miniband formation are shown. Δn ≠ 0 transitions in the quantum wells are observed by photocapacitance spectroscopy even under negative average field conditions, where the photocurrent response cannot be detected. X-ray measurements have been used to obtain the period thickness and the average In content.