Differential Capacitance Technique Research Articles

Characterization of a segmented printed circuit board (PCB) as a standard for absorbed dose to water from alpha-emitting radionuclides.

Our previous work introduced and evaluated a standard for surface absorbed dose rate per unit radioactivity to water from unsealed alpha-emitting radionuclides used in targeted radionuclide therapy (TRT). An overall uncertainty over 4.0% at k=1 was reported for the absorbed dose to air measurements, which was partially attributed to the rotational alignment uncertainty in the geometrical setup. A printed circuit board (PCB) with a segmented guard was constructed to align the extrapolation chamber (EC) and the source plates using a differential capacitance technique. The PCB EC aimed to enhance the repeatability of the ionization current measurements. The PCB EC was evaluated using a thin film 210Po source. The measured absorbed dose to air cavity was compared with the Monte Carlo (MC) calculations. Using the extrapolation method, the surface absorbed dose rate to water was calculated. The PCB EC was constructed with a 4.50mm diameter collector surrounded by four sectors and a guard electrode. The sectors were isolated for rotational alignment and later connected to the guard for ionization current measurements. A bridge circuit measured differential capacitance between opposing sectors, and a hexapod motion stage rotated the source substrate to minimize the differential capacitance. The EC was evaluated using a 210Po source with a 3.20mm diameter and 1.253 Ci radioactivity. MC simulations were performed to calculate the , , and correction factors. Ionization current measurements were performed for air gaps in the 0.3-0.525mm range and surface absorbed dose rate to water was calculated. Rotational offsets of up to 3.0° were found and the current repeatability was found to increase with the absorbed dose to air uncertainty calculated to be ∼2.0%. Using the capacitance method, the effective EC diameter was measured to be 4.53mm. The recombination, polarity, and electrometer corrections were reported to be within 1.00% across all measurement trials. The MC-calculated correction factors were calculated to be much larger than the recombination and polarity correction factors. The average , , and corrections were calculated to be 1.063, 0.9402, and 2.136, respectively. The MC-calculated absorbed dose to air was found to overestimate the absorbed dose by over 4.00% when compared with the measured absorbed dose to air. The surface absorbed dose rate to water was calculated to be Gy/s/Bq with an overall uncertainty of 4.07%. The constructed PCB EC was deemed suitable as an absorbed dose standard. A repeatable rotational alignment was achieved using the differential capacitance technique. The metal electrodes on the PCB made a difference of<1.00% on the backscatter correction when compared to the EC comprised of polystyrene-equivalent collector. A 20% difference in the surface absorbed dose rate to water was found between the two ECs, which is attributed to the cavity diameter differences leading to different magnitudes of dose fall-off along the lateral direction.

Development of pseudocapacitive materials based on cobalt and iron oxide compounds for an asymmetric energy storage device

Composite electrodes based on cobalt hydroxide and iron oxide, respectively, with silica as an under-layer were developed with the use of sol-gel and electrochemical deposition techniques. The electrodes demonstrate similar surface morphologies of the upper pseudocapacitive layer formed by lamellas. The composite Co-based electrodes as well as Fe-based electrodes with silica reveal higher values of capacitance retention after multiple charge-discharge process than the ones without it. The influence of OH−concentration upon pseudocapacitive behavior was estimated for Co–based electrodes. The mixed electrolyte with an extremely low alkali concentration proved to increase capacitance retention of the electrodes. A model of an asymmetric pseudocapacitor was fabricated with the use of the Co and Fe–based electrodes, pre-coated with silica. The use of differential capacitance technique in a full electrochemical cell allowed balancing charges of positive and negative electrodes within their potential windows. The maximum voltage range of 1.4 V was achieved for the cell which exceeds the window of electrochemical stability of water.

Compensation effect and differential capacitance analysis of electronic energy band structure in relaxed InAs quantum dots

The use of a differential capacitance technique for analyzing the effect of strain relaxation on the electronic energy band structure in relaxed InAs self-assembled quantum dots (QDs) is presented. Strain relaxation is shown to induce a deep defect state and compensate the ionized impurity in the bottom GaAs layer, leading to a double depletion width and a long emission time. An expression of capacitance at different frequency and voltage is derived for analyzing the experimental data. It has been shown that the relationship between the low-frequency and high-frequency capacitances can be well explained by a Schottky depletion model with a compensated concentration in the bottom GaAs layer. A simple expression is presented to account for the modulation of the free electrons in the top GaAs layer. This capacitance analysis shows a long low-energy tail for the electron ground state, suggesting not very uniform strain relaxation. The results of this study illustrate a carrier compensation effect of the defect state on the electronic energy band structure near the QDs.

Estimation of parameters characterizing the adsorption of sodium, potassium, and rubidium cryptates at the mercury electrode/solution interface

The adsorption of complexes formed by sodium, potassium, and rubidium cations with macrobicyclic ligand (kryptofix 222 with C18H36N2O6 composition) is studied as a function of the ligand concentration on a stationary mercury drop in 0.1 M solutions of corresponding sulfates and chlorides by using the differential capacitance technique. Based on the model of two parallel capacitors supplemented by the Frumkin isotherm, the adsorption parameters of studied cryptates are estimated by using the regression analysis technique. Differential capacitance curves calculated with the parameters found are compared with experimental data. The comparison of the found adsorption parameters makes it possible to reveal the effects of the nature of included cations and specifically adsorbed supporting-electrolyte anions on the adsorption behavior of cryptates under study.

A contribution to the theory of the C–V technique for the evaluation of carrier-concentration profiles in semiconductors

An extension of the previously developed theory (the integral capacitance technique (ICT)) for the evaluation of the carrier-concentration profile in semiconductors is presented. It is shown that there is a possibility to determine the space-charge density at the edge of a depletion layer straightforwardly from experiment, without additional calculation procedures. This gives a sufficient advantage of the method proposed against those previously developed owing to excluding from the consideration the whole range of the C–V data, which are necessary for the ICT or DCT (differential capacitance technique) implementation. The method is based on the combination of a traditional C–V differential capacitance measurement and the measurement of the proper capacitance (integral capacitance) performed separately and at the same value of the applied voltage.

New interpretation of C–V measurements for determining the concentration profile in a semiconductor

The well-known C–V technique for determining the doping profile in a semiconductor is re-examined. Based on an analysis of the Poisson equation, a modification of the conventional procedure for evaluating the space-charge density distribution within the depletion layer of a semiconductor is presented. This procedure involves a developed integral-capacitance technique, which proves to be generally valid and gives the correct basis for determining the space-charge density near the edge of the depletion layer rather than the real doping profile. The relationship between the proposed method and the conventional differential-capacitance technique is revealed and a comparison of the effectiveness of both of them is also made. The method proves to be useful if shallow diffusion profiles within low-doped substrates are analyzed, when the conventional C–V profiling technique is not applicable. Experimental results obtained with an n+/n epitaxial layer are given and discussed as an illustration of the represented study.

Differential capacitance measurements of relaxation-induced defects in InGaAs/GaAs Schottky diodes

The use of a differential capacitance technique for characterizing the relaxation-induced defect states in Schottky diodes has been studied. Based on a proposed equivalent circuit including the effect of potential drop across the carrier-depletion layer, a simple equation of capacitance at different voltages and frequencies is derived and compared with experimental data obtained from relaxed In/sub 0.2/Ga/sub 0.8/As/GaAs samples. It is shown that the carrier-depletion layer will introduce capacitance dispersion over frequency like traps; from it the device's geometric parameters, the resistance of the carrier-depletion layer and the ionization energy of the deep level that gives rise to this resistance can be obtained. The relation between the low-frequency capacitance and reverse voltage can be explained well by the depletion of the free carriers between the Schottky depletion and the carrier-depletion layer. The relaxation-induced traps are believed to be at 0.535 and 0.36 eV, respectively, in the GaAs and In/sub 0.2/Ga/sub 0.8/As regions.

Stark shift of an interband transition in Cu determined by surface charge measurements

An external (d.c.) electric field, applied across a metal/liquid interface, can shift the electronic levels of the metal surface. If not resolved properly, this effect can be problematic in voltage controlled surface studies, particularly in spectroscopy where photon induced interband transitions are active. A simple method, based on surface charge measurements by differential capacitance technique, is presented to account for such energy shifts. The technique is applied to estimate voltage induced shifts in the d-Fermi level transition threshold in the surface layer of a polycrystalline Cu electrode. The results are in striking agreement with those indicated by previously reported optical experiments on Cu.

Effect of hydrogen plasma treatment on doping profiles in crystalline silicon

Abstract Changes in dopant concentration profiles in crystalline silicon induced by rf hydrogen plasma treatment of Si-SiO2 structures were studied by using differential capacitance technique. It was found that such a treatment influences p-type and n-type silicon controversially. In p-type Si compensation of boron acceptors by neutral H-B complex formation was observed. It was suggested that the increase of doping concentration in n-type Si is due to the generation of donor-like defects in the bulk Si by plasma radiation.

Electrical determination for frost depth in soil

An instrument is described for measuring the state of water in a soil profile (at depths up to 100 cm) as temperatures approach the freezing point. Measurements are made using an apparent differential capacitance technique. Circular stainless steel electrodes are placed on a hollow polyvinyl chloride tube at predetermined distances and the tube is placed into the soil. Each of the electrodes pairs contains a thermal sensor to measure the temperature of the soil at the corresponding electrodes and soil depths. In a laboratory study, under a constant sub-zero temperature environment (− 27°C), the temperature of a moistured soil sample was slowly reduced from + 2 to −3°C at a rate of 1°C per 80 minutes. During the loss of the latent heat (freezing) at the freezing threshold (⩽ 0°C), the soil temperature stabilized temporarily. Following the loss of the latent heat the temperature of the soil began falling again and the apparent capacitance dropped sharply to zero. At the point when the apparent capacitance approaches zero for each electrode pair the “pore” water was considered frozen.

Silicon doping and impurity profiles in Ga0.47In0.53As and Al0.48In0.52As grown by molecular beam epitaxy

Silicon-doped n-type Ga0.47In0.53As and Al0.48In0.52As epitaxial layers lattice matched to InP substrates have been grown by molecular beam epitaxy. Doping levels up to 7×1018 cm−3 vary proportionally with the arrival rate of Si. For the same Si arrival rate, the carrier concentrations in both ternary epitaxial layers are identical. Mobility studies showed that the variations of electron mobility as a function of carrier concentration in Si-doped Ga0.47In0.53As are in good agreement with the theoretically calculated results involving the alloy scattering mechanism at both 77 and 300 K. This alloy scattering mechanism is attributed to the defects induced at lower growth temperature. Doping profile measurements by the differential capacitance technique show that very abrupt changes in carrier concentration can be realized in Si-doped Ga0.47In0.53As and Al0.48In0.52As and Sn-doped Al0.48In0.52As. In the case of Sn-doped Ga0.47In0.53As, the sharpness of the doping profile is limited by the surface segregation of Sn. Schottky barrier height on Ga0.47In0.53As was enhanced with the aid of a thin n-type Al0.48In0.52As surface layer.

Channeling of boron ions into silicon

Channeled and random distributions of boron ions implanted over the energy range 50 keV–1.8 MeV into silicon have been measured using the differential capacitance technique. When implantations are performed along the 〈110〉 or 〈111〉 axis, profiles exhibit a strong orientation dependance. The best channeled profiles shows that more than 70% of the implanted dose is in the channeled peak.

Beryllium doping and diffusion in molecular-beam epitaxy of GaAs and AlxGa1−xAs

p-type doping levels of up to 5×1019 cm−3 in GaAs and 3×1019 cm−3 in Al0.3Ga0.7As have been obtained in molecular-beam epitaxy (MBE) with Be doping. These doping levels are more than one order of magnitude higher than can be achieved with other acceptors excepting ionized Zn, and are obtained while maintaining surface morphologies comparable to those of undoped layers. Mobilities of the Be-doped GaAs MBE layers are comparable to those obtained in liquid-phase epitaxy (LPE) material. Mobilities of Be-doped Al0.3Ga0.7As MBE layers studied in this experiment are lower than those measured in LPE crystals and do not increase with decreasing doping level which indicates a high degree of donor compensation in these samples. Room-temperature photoluminescent efficiencies obtained from the p-MBE GaAs and AlxGa1-xAs layers grown under As-stabilized surface conditions also are significantly lower than those of comparably doped LPE material. This lower efficiency may be related to impurity contamination specific to the dopant source and is not observed during n-type (Sn-doped) GaAs layer growth under otherwise identical conditions. p (Be) -n (Ge) junctions have been evaluated and the results indicate that Be should be a suitable dopant for device fabrication. Doping profile measurements by the differential capacitance technique show that very abrupt (half-width ?350 Å) Be doping pulses can be realized. By observing the out diffusion of Be from these doping pulses during annealing of the diodes a value of D?1×10−15 cm2/sec for the diffusion coefficient of Be at 800° was established. It is concluded that, with the possible exception of ionized Zn, Be should emerge as the preferred acceptor in MBE growth of III–V compounds.

Correlation of photocurrent-voltage curves with flat-band potential for stable photoelectrodes for the photoelectrolysis of water

Using the differential capacitance technique, the flat-band potential of n-type TiO2, SnO2, SrTiO3, KTaO3, and KTa(0.77)Nb(0.23)O3 electrodes has been determined as a function of pH in aqueous electrolytes. Plots of flat-band potential vs. pH are linear in all cases and have a slope of approximately 0.059 V/pH unit. The flat-band potential correlates nicely with the onset for photoanodic currents corresponding to O2 evolution at the n-type semiconductor and H2 at the dark Pt cathode. The ordering of flat-band potentials at a given pH is SrTiO3 of the order of KTaO3 of the order of KTa(0.77)Nb(0.23)O3 greater than TiO2 greater than SnO2 (SnO2 most positive vs a saturated calomel electrode).

Measurement of the electrical impurity profile of implanted ions, using the pulsed MOS C-V technique

The differential capacitance technique used on a MOS structure with a pulsed bias applied, is well suited for the determination of low dose impurity implantations. It is a nondestructive method and requires no extra processing steps.The limitations restricting the use of this technique are discussed. It is shown that for steep profiles a correction due to the existence of a diffusion potential has to be taken into account. The range in which this diffusion voltage is important is the same as for the correction proposed by Kennedy and O'Brien which arises from the difference between the impurity profile and the majority carrier profile in the quasi-neutral condition. The method of calculation, and the experimental verification are discussed.

Ion-Implanted Phosphorous in Silicon: Profiles Using C-V Analysis

Depth distributions are presented for 200-, 300-, and 600-keV phosphorus ions either channeled along the 〈111〉 axis or implanted away from any major axis or plane in silicon. The profiles were obtained using the differential capacitance technique, and the technique is discussed along with a description of an automatic data-acquisition system which allows precise C-V data to be accumulated. The ``random''-direction implant profiles exhibit a definite exponential tail on the deep side of the implant. The characteristics of this tail appear to be material dependent but independent of the implantation energy. Thus, the tail probably results from an anomalous diffusion mechanism. The observed projected range for the random implants is 1.1 μm/MeV, with a range straggle nearly equal to that predicted by the range theory of Lindhard, Scharff, and Schiott (LSS). The channeled profiles exhibit a near flat characteristic to a depth of 2.2 μm/(MeV)1/2.

A technique for directly plotting the doping profile of semiconductor wafers

Abstract : The note describes a simple method of automating the differential capacitance technique using an analog system which provides a direct plot of the doping profile. The technique described can be used in conjunction with a Schottky diode, an abrupt p+n junction diode or an MIS capacitor. (Author)

Evaluation of an approximated doping profile for diffused junctions from capacitance measurements

The differential-capacitance technique for the evaluation of doping profiles of the low-doped side of strongly asymmetrical p-n junctions may be extended to diffused junctions, provided that the actual profile is approximated by an exponential function. An easy fit between experimental and theoretical curves allows evaluation of the two parameters of the exponential approximation.

On the measurement of impurity atom distributions in silicon by the differential capacitance technique : D. P. Kennedy, P. C. Murley and W. Kleinfelder, IBM J. Res. Dev., September (1968), p. 399

On the measurement of impurity atom distributions in silicon by the differential capacitance technique : D. P. Kennedy, P. C. Murley and W. Kleinfelder, IBM J. Res. Dev., September (1968), p. 399

On the Measurement of Impurity Atom Distributions by the Differential Capacitance Technique [Letter to the Editor

In a recent paper, 1 it was shown that the profile inferred from differential capacitance measurement 2–5 of semiconductor junctions is not that of the impurity atom distribution but, instead, that of the majority carrier distribution. For this reason, conventional differential capacitance measurements can be used to evaluate the impurity atom distribution only in charge neutral semiconductor material (where the majority carrier density equals the density of ionized impurity atoms). This requirement of charge neutrality limits the applicability of this measurement technique to semiconductor material containing a minimum impurity atom density of about 10 16 atoms/cm 3 .