Bulk Minority Carrier Lifetime Research Articles

Determination of recombinative lifetime and surface recombination velocities from wavelength dependence of photocurrent in insulator-semiconductor structures

A method is described for extracting bulk minority carrier lifetimes and surface recombination velocities from insulator-semiconductor structures. Recombination in the bulk and at the two surfaces of the wafer are distinguished by using the wavelength dependence of the penetration depth of the incident light flux. Pulse modulated light from various wavelength light-emitting diodes is used to create excess minority carrier holes which are collected in the depletion region of an inverted surface. The spectral dependence of the resulting photocurrents can then be used to determine the surface recombination velocities and the bulk lifetime. A model is described which provides relationships between the bulk minority carrier lifetime, the surface recombination velocities, the absorption coefficient of the incident light, and the resulting capacitive current. The experimental values of lifetime and surface recombination velocity are combined to obtain an effective lifetime which is in good agreement with a combined lifetime obtained from the photoconductive decay technique.

Sensitivity analysis for the determination of recombination parameters in Si wafers using harmonic carrier generation

The sensitivity of a contactless measurement procedure to determine the minority-carrier recombination parameters in Si wafers is analyzed. The measurement is based on the harmonic generation of excess carriers, whose time dependence is measured either by μ-wave reflection or free-carrier absorption. Both the measurement equipment and the model used to extract the surface recombination velocity and the bulk carrier lifetime are introduced and the sensitivity of the parameter fit procedure is examined. The results of this uncertainty analysis are applied to typical experimental situations, e.g., bulk minority carrier lifetime measurement of a monocrystalline wafer with equal surfaces or a bulk minority carrier lifetime mapping done on an as-grown multicrystalline Si wafer before it obtained saw damage removal. The method proves to be very useful, if the interpretation of the results is carefully done and an appropriate experiment is chosen to produce the best possible results.

Analysis of the effect of surface passivant charges on HgCdTe photoconductive detectors

The effect of fixed surface charge density on the performance of HgCdTe (MCT) photoconductive detectors is presented in this study. The figure of merit such as the specific detectivity of photoconductive detectors operating at 77 K is calculated with 300 K background and field of view (FOV). The equations leading to the above calculations are developed for the case of a one-dimensional model. Our calculations show that: (i) shows maxima with varying ; (ii) improvement in due to accumulation is observed for values of up to for materials with low as well as high bulk minority carrier lifetimes ; and (iii) is limited by shunt resistance due to passivants having for low- materials. Furthermore, is found to degrade by an order of magnitude from its peak value with increase in from to in a material having ns, whereas no such degradation in is seen for the same range of in a material having . In addition, our calculations predict that the accumulation layer improves by a factor of 1.5 for surfaces having an initial surface recombination velocity as high as .

Improved device technology for epitaxial Hg1-xCdxTe infrared photoconductor arrays

The performance of Hg1-xCdxTe infrared photoconductors is strongly dependent on the semiconductor surface conditions and, in particular, the degree to which the surface contributes to recombination of photogenerated excess carriers. Although published photoconductor fabrication processes based on bulk Hg1-xCdxTe address this issue by fully passivating both major surfaces (i.e. front and back) with anodically grown native oxide, passivation of the sidewalls is neglected. In this paper it is shown both theoretically and experimentally that leaving the sidewalls unpassivated can result in approximately a factor of two reduction in responsivity for long-wavelength infrared (LWIR) detectors used in high-resolution thermal imaging systems. Detector arrays are typically fabricated on x=0.23 Hg1-xCdxTe representing a cut-off wavelength of 9.4 mu m and use individual element sizes of approximately 50*50 mu m2. We describe in detail for the first time a device technology which enables the fabrication of Hg1-xCdxTe photoconductor arrays such that the entire surface of the semiconductor is effectively passivated, including the sidewalls. Of particular interest is the fact that this improved device technology is compatible with present-day Hg1-xCdxTe epitaxial growth processes. This is in contrast to current photoconductor technology which is primarily based on bulk Hg1-xCdxTe. Experimental results are presented which compare device performance of LWIR detectors fabricated using the improved photoconductor technology with current published photoconductor technology. These results clearly indicate that detectors fabricated on liquid phase epitaxially (LPE) grown x=0.23 Hg1-xCdxTe material using the improved photoconductor device technology achieve much higher responsivities and detectivities. Furthermore, it is shown that only a fully passivated device structure is capable of exploiting any future improvements in bulk minority carrier lifetime as it approaches the Auger recombination limit.

Minority carrier diffusion length effects on light-addressable potentiometric sensor (LAPS) devices

Alternating photocurrent measurements with light-addressable potentiometric sensors (LAPSs) have been used to monitor pH, redox potential, and ionic concentrations at discrete locations in an electrolyte in contact with LAPS devices. We report here the results of AC photocurrent measurements with insulated semiconductor LAPS devices where the semiconductor either is illuminated through the insulator (frontside) or alternatively from the opposite side (backside). Such comparative AC photocurrent measurements were made with semiconductors of varied thickness, at varied frequency of light intensity modulation, and at several different photoexcitation wavelengths. The results are fit to a theoretical expression which predicts the dependence of photocurrent on modulation frequency, wafer thickness, bulk minority carrier lifetime, and surface recombination velocity. The results are useful to optimize the design of LAPS devices with regard to these parameters. The results also predict optimal conditions for minimal lateral spacing of adjacent sensing areas in LAPS devices.

A Chemical/Microwave Technique for the Measurement of Bulk Minority Carrier Lifetime in Silicon Wafers

A chemical/microwave technique for the measurement of bulk minority carrier lifetime in silicon wafers is described. This method consists of a wet chemical treatment (surface cleaning, oxidation in solution, and measurement in solution) to passivate the silicon surfaces, a laser diode array for carrier excitation, and a microwave bridge measuring system which is more sensitive than the microwave systems used previously for lifetime measurement. Representative experimental data are presented to demonstrate this technique. The result reveals that this method is useful for the determination of bulk lifetime of commercial silicon wafers.

Influence of junctions on photoluminescence decay in thin-film devices

The photoluminescence lifetime of III-V thin films is a major tool for the determination of minority-carrier lifetime. Prior work has dealt with the effects of surface recombination for various device geometries. Other work has shown the effect of self-absorption and photon recycling on the effective lifetime. We describe deep homojunctions thin-film devices. Model calculations show that the effective lifetime, as measured in the photoluminescence experiment, is always less than the bulk minority-carrier lifetime. In high-mobility materials, the effective photoluminescence lifetime may be dominated by diffusion and unrelated to recombination. An analysis of the data produces the minority-carrier mobility.

Effects of Pre-Gate Oxidation Intrinsic Gettering Upon Thin Gate Oxide Integrity In High Carbon Content CZ Si

AbstractEffects of post-well drive intrinsic gettering (PWIG) upon the integrity of thin gate oxide in Cz Si wafers with carbon levels, Cs, ranged from 0.2 - ∼ 4 ppma were investigated. A 10 nm thick gate oxide capacitor was used to study its time-dependent breakdown characteristics and minority carrier lifetime. Our data have shown that PWIG cycles and/or carbon impurity affect both bulk oxygen precipitation and minority carrier lifetime, but not the oxide integrity.

Determination of bulk minority-carrier lifetime and surface/interface recombination velocity from photoluminescence decay of a semi-infinite semiconductor slab

The analytical expression is derived for the time dependence of the total minority-carrier content in a semi-infinite semiconductor slab, after excitation with a delta function light pulse. It is shown that to this end it is sufficient to solve the continuity equation for the minority carriers in one dimension only. This implies that the excitation may have an arbitrary lateral spatial distribution. The photoluminescence, probing the total minority-carrier content, turns out to be a nonexponential function of time when the surface/interface recombination velocity differs from zero. A practical method is developed to deduce accurate values of the bulk lifetime and the interface recombination velocity from such a nonexponential decay curve, based on a special way of plotting. Moreover, the analytical expressions are derived for the photoluminescence decay in the case of finite slab thicknesses and finite interface recombination velocities. From these results it is evident that only for large ratios (>10) of slab thickness and minority-carrier diffusion length can the semiconductor be considered as being semi-infinite.

Determination of surface- and bulk-generation parameters from dark-current measurements in surface-channel CCD's

A technique for determining surface-generation velocity and bulk minority-carrier generation lifetime from measurements in surface-channel charge-coupled devices (SCCD) is described. Depleted surface-generation velocity of 1.1 cm/s and bulk minority-carrier generation lifetime of 130 µs have been determined in the channel region for the devices used, and are in good agreement with data obtained by other techniques.

A comparison of Nd:YAG fundamental and second-harmonic Q-switched laser-beam lifetime doping in single-crystal silicon

Silicon devices including bipolar transistors, junction diodes, and MOS capacitors were scanned by a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -switched Nd:YAG (1.06 µm) and frequency-doubled Nd:YAG (0.53 µm) radiations under various conditions. The electrical characteristics of these devices were measured before and after scanning and again after thermal annealing. The data includes transistor gain versus laser power; junction diode leakage current versus junction depth; MOS <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C-V</tex> lifetime versus laser power and the effects of subsequent thermal anneals on all of these. The results are that bulk minority-carrier lifetime decreases of several orders of magnitude will be produced by either of these radiations at peak power levels below those which will produce any visible surface damage. The changes in minority-carrier lifetime are stable for post scanning thermal anneals up to 400°C and are almost completely removed from an 800°C anneal. The depths within which minority-carrier lifetime changes significantly are 0.7 and 1.8 µm for 0.53- and 1.06-µm wavelength laser radiations, respectively. The results indicate that the recombination centers produced by the scanning are point defects and their density decreases exponentially with the distance into the silicon. The average power thresholds for point defect production (for both 0.53- and 1.06-µm wavelengths) were determined and are observed to increase with increased laser wavelength and pulse width. Potential applications in silicon devices and integrated circuits such as selective lifetime doping, β trimming, and selective-link making without passivation damage are possible.

ChemInform Abstract: PROCESS FOR HIGH PHOTOCURRENT IN IBC SOLAR CELLS

AbstractDie IBC‐Sonnenzellen (interdigitated back contact solar cells) eignen sich besonders für hohe Intensitäten, haben keine Abschattung durch das Kollektionsgitter und niedrigere Kontakt‐ und Elektrodenwiderstände.

Incoherent-light-flash annealing of phosphorus-implanted silicon

Incoherent light pulses emitted from a xenon flash lamp were used to anneal radiation damage in (100) silicon implanted with 2×1015 31P+/cm2 at 100 keV. Electrical carrier concentration has been determined by means of differential sheet resistivity and Hall effect together with the anodic oxidation stripping technique; the surface photovoltage technique has been used to evaluate bulk lifetime and Rutherford backscattering and transmission electron microscopy for analysis of radiation damage. Damage recovery appears to take place via a solid phase epitaxial process. Electrical activity and carrier mobility values of samples annealed by incoherent light are similar to those obtained by laser, electron beam, and furnace annealing. The bulk lifetime of minority carriers is not degraded.

Minority-carrier lifetimes and internal quantum efficiency of surface-free GaAs

Minority-carrier lifetimes, internal quantum efficiencies, and values of the radiative recombination coefficient B are determined from photoluminescence time-decay and external quantum efficiency data taken for LPE GaAs samples (germanium doped or undoped) in the doping range 1.9×1015?p0 ?1×1019 cm−3 at 300 °K. Measurements are made on isotype double heterostructure samples where the optically exicted GaAs layer is bounded by wider-band-gap Ga0.5Al0.5As layers which provide for confinement of minority carriers and also minimize the importance of surface or interfacial recombination on the measured lifetimes. Comparison with time-decay data for samples without the ternary cladding layers shows the dominant effect of surface and substrate recombination on the decay time if confinement layers are not provided. Luminescence decay times are observed from 1.2 nsec for heavily doped samples up to 1.3 μsec for lightly doped samples. Values of the bulk minority-carrier lifetimes, radiative lifetimes, and internal quantum efficiencies are determined. The experimental values for B agree well with existing theory for heavily doped material. For lightly doped material, the experimental B coefficient is larger than expected from the single-electron model.

Application of scanning electron microscopy to determination of surface recombination velocity: GaAs

A method is reported for the determination of the surface recombination velocity by scanning electron microscopy; this method is based on an established relationship between the effective diffusion length of the minority carriers, the penetration depth of the electron beam, and the surface recombination velocity. Values of surface recombination velocity, up to about 2.5×106 cm/sec were determined in n-type GaAs with a bulk minority-carrier lifetime of the order of 10−8–10−10 sec; in GaAs, with carrier concentrations exceeding 1018 cm−3, recombination velocity of about 3×106 cm/sec represents a saturation value.

Carrier generation at the Si-SiO2 interface under pulsed conditions

A simple analysis is given of the displacement current transient following an application of a depleting pulse to the gate of a metal-insulator-semiconductor capacitor. It is shown that a plot of the current vs reciprocal time may be used to obtain the surface state density near midgap, the majority-carrier capture cross section, and the bulk minority-carrier lifetime. Experimental results for a typical MOS capacitor are presented.

Determination of the minority carrier bulk lifetime in rod-like germanium samples at temperatures between 5 and 77 K

The dependence of the effective minority carrier lifetime τeff on a longitudinal magnetic field is investigated up to 140 kG in rod-like germanium samples at temperatures between 5 and 20 K and at 77 K. The effective lifetime increases with increasing magnetic field and reaches a saturation value in the high field region. In accordance with the theoretical model of Lautz and Schulz this value is taken as the average bulk lifetime τv of electrons and holes in the samples. Theory and experimental results are in a good agreement at 77 K; from the comparison an estimation for the surface-recombination velocity s can be obtained. Im Temperaturbereich zwischen 5 und 20 K und bei 77 K wird die Abhängigkeit der effektiven Minoritätsträgerlebensdauer τeff von einem longitudinalen Magnetfeld bis zu 140 kG in stäbchenförmigen Germaniumproben untersucht. Die effektive Minoritäts-trägerlebensdauer wächst mit steigendem Magnetfeld an und erreicht einen Sättigungswert. Dieser Wert entspricht, in Übereinstimmung mit dem theoretischen Modell von Lautz und Schulz, einer mittleren Volumenlebensdauer τv der Elektronen und Löcher. Bei 77 K stimmen Theorie und experimentelle Ergebnisse gut überein aus dem Vergleich kann eine Abschätzung für die Oberflächenrekombinationsgeschwindigkeit s gefunden werden.

Theory, design, and performance of low-blooming silicon diode array imaging targets

The increase in size of an image produced by a high intensity source is particularly troublesome in silicon diode array imaging devices. This increase is caused by inversion of Si under the SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> between the diodes or by the lateral diffusion of the excess minority carriers in the field-free region of the sensor array. The formation of inversion layers can be eliminated by proper choice of the sheet resistance of the resistive sea when the target is operated above flat-band voltage. However, when the diodes in the image are fully discharged in a time interval shorter than the frametime, the excess carriers diffuse to the adjacent diodes and cause an increase in the size of the image. Such "blooming" is related to the effective minority carrier lifetime that is a function of surface recombination velocity at the imaging side of the target, surface recombination velocity at the diode side of the target, and bulk minority carrier lifetime. Two methods for controlling blooming will be discussed. In one case, the effective minority carrier lifetime is reduced by adjusting the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> doping profile at the imaging side of the array. The second technique requires diffusion of a separate p-type region, placed between the sensing diodes elements, which provides a high recombination region when the sensing diodes are discharged due to the image overload. Devices have been constructed with good imaging quality, where a 1 percent scan diagonal bright spot bloomed to 4 percent in the first technique, and to 2.5 percent in the second technique with a 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> overload above saturation. The data are in good agreement with the theory.

A novel method for the direct determination of the small signal bulk minority carrier lifetime in the middle region of a semiconductor sandwich†

A modified method involving the observation of the decay of photoconductivity in silicon is described. This variation enables the hole lifetime in thin n-type regions forming the centre region of a p-n-p sandwich to be determined. The p-n junctions are suitably biased so that injected carriers are kept away from the junction region and are thus less likely to be.controlled by ‘ surface ’ recombination. The technique may be useful in evaluating the parameters of thin slices of silicon after an impurity diffusion treatment.

Influence of CO60 Gamma Irradiation on the Surface and Bulk Recombination Rates in Silicon

An experimental investigation has been conducted to determine the influence of Co60 gamma irradiation on the surface recombination velocity in 100 ohm-cm n- and p-type silicon. Utilization of the technique of photoconductive decay to measure the surface recombination velocity requires the knowledge of the bulk minority carrier lifetime in the material. For completeness, the effects of the irradiation on this material parameter are also presented. When etched samples of n-type float zone refined phosphorous doped silicon were irradiated with Co60 gamma rays, the surface recombination velocity exhibited a relative minimum as a function of the total gamma ray exposure dosage. In general, the surface recombination velocity was strongly influenced by the gamma irradiation for n-type material. The surface recombination velocity of irradiated samples of p-type float zone refined boron doped silicon seemed to be only slightly sensitive to gamma radiation. The velocity was a very slowly monotonically increasing function of total gamma ray flux. In order to explain these observed variations of surface recombination velocity, a more complete mathematical model was formulated. The model includes the contribution due to recombination in the space charge layer near the surface to the effective surface recombination velocity at the beginning of this space charge region. Use of this model allowed the correlation of experimental and theoretical values of surface recombination velocity. When bulk minority carrier lifetime versus reciprocal temperature data was curve-fitted to the Shockley-Read lifetime, the position of the radiation-induced bulk recombination center in the forbidden region could be determined.