Minority Carrier Lifetime Values Research Articles

Characterization of GaInP/Ge heterostructure solar cells by capacitance measurements at forward bias under illumination

The n-GaInP/n-p Ge heterostructure sloar cells grown by OMVPE were studied by new technique based on the diffusion capacitance measurements under illumination at open circuit voltage conditions. The value of the effective minority carrier lifetime in the p-Ge base τeff= 1.5×10 -6 s was determined from the frequency dependence of the diffusion capacitance, C(f). The numerical simulation model was developed, which is in a good agreement with the experimental C(f), capacitance-voltage and dark current-voltage measurements. The value of the bulk minority lifetime equal to τ b= 5×10 -6 s was obtained by the simulation fit of the experimental data. The simulations have also demonstrated that the diffusion capacitance is very sensitive to the recombination velocity at the back contact, Sbc, for Sbc<10 5

Dry texturing of mc‐Si wafers

AbstractTexturing of mc‐Si is a prevailing research topic to improve solar cell efficiency in production. Surface texturing for enhanced absorption in Si has been historically obtained by creating randomly distributed pyramids using anisotropic etchants; but this preferential etching works only on single crystalline silicon because of its crystallographic orientations. A low‐cost, large area, random, mask‐less texturing scheme is expected to significantly impact terrestrial PV technology and reduce the amount of wet‐chemical waste. We propose an approach based on randomly etched mc‐Si by RIE system using NF3 instead of SF6 or CF4 to reduce the detrimental formation of carbonaceous or sulfurous contamination at the silicon surface, which results in a surface recombination. To obtain a fast process we have investigated the effect of the chemical etching due to the NF3 radicals and the ion bombardment induced by Ar. We have found that Arions promote a helpful surface pre‐conditioning, while fluorine radicals, produced by NF3 dissociation, are needed to increase the Si etching rate. Different combinations of flux ratios, gas pressures and RF power have been explored. Efforts have been devoted in obtaining a homogeneous texture on large area wafers, which is inescapable for industrialization. After 10 minutes process effective reflectance values have been measured within the range of 12‐14%, and with a‐Si/SiNx the value reduced to 7%. Post‐processing minority carrier lifetime values in the range of 10 microseconds have been measured without applying any further chemical cleaning. Additionally, microscopic analysis has been performed to evaluate the surface microstructure morphology (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A new approach to determine accurately minority-carrier lifetime

Abstract Electron or proton irradiations introduce recombination centers, which tend to affect solar cell parameters by reducing the minority-carrier lifetime (MCLT). Because this MCLT plays a fundamental role in the performance degradation of solar cells, in this work we present a new approach that allows us to get accurate values of MCLT. The relationship between MCLT in p -region and n -region both before and after irradiation has been determined by the new method. The validity and accuracy of this approach are justified by the fact that the degradation parameters that fit the experimental data are the same for both short-circuit current and the open-circuit voltages. This method is applied to the p + / n -InGaP solar cell under 1 MeV electron irradiation.

Evaluation of Laser Doping of Si from MCLT Measurement

The measurement of minority carrier lifetime (MCLT) ofp-n Si fabricated with aid of laser doping technique was reported. The measurement is achieved by using open circuit voltage decay (OCVD) technique. The experiment data confirms that the value of MCLT and proftle of Voc decay were very sensitive to the doping laser energy.

Investigation of Czochralski Silicon Grown with Different Interstitial Oxygen Concentrations and Point Defect Populations

Czochralski silicon crystals, grown with different interstitial oxygen concentrations (group I) or different types of intrinsic defects (group II), were analyzed by X-ray section topography, diffuse X-ray scattering, surface photovoltage, microwave photoconductive decay, and Fourier infrared spectroscopy. The samples of group I showed that the main factor influencing the X-ray section topography images is the amount of precipitated oxygen. The increase in this amount involves a decrease in size and an increase in density of the oxygen precipitates. A strong reduction in the minority carrier diffusion length is observed after precipitation. These effects seem to be dependent on the positions along the ingot axis at which the samples were taken, which would indicate the influence of the sample thermal history on the detectability of oxygen precipitates by means of these techniques. The samples of group II are characterized by vacancy-rich, interstitial-rich, and defect-free regions. After copper decoration, the different defects were observed to have different sizes, densities, shapes, and depth distributions. This feature allows one to clearly distinguish between crystal zones where vacancy or interstitial clusters prevail and where defects are absent. Minority carrier lifetime values are directly related to the defect volume densities. The combined use of different characterization techniques proved itself a powerful tool to study different defect types in Czochralski silicon. © 2001 The Electrochemical Society. All rights reserved.

Crystal Surface Defects and Oxygen Gettering in Thermally Oxidized Bonded SOI Wafers

This work investigates the role of interstitial oxygen content in the starting Czochralski bonding wafers, and the effects of buried layered implants and thermal processing on the formation of crystalline defects in the surface of bonded silicon on insulator (SOI) material. We found that unimplanted SOI material originating from Si with high oxygen levels were populated with heavy oxygen precipitation and related crystal defects, and were also characterized by low minority carrier lifetime values. However, a dramatic improvement was observed in the behavior of SOI with low oxygen levels where the development of oxygen precipitation in the SOI layer was inhibited. Incorporation of a buried As ion implanted layer in high oxygen level SOI increased the lifetime of the material and suppressed the formation of oxygen precipitates through an oxygen gettering effect. The gettering efficiency of the buried defect layer was found to be related to the dose of the As ion implantation, the annealing temperature, and type of starting silicon material. A dose of was sufficient for effective gettering. © 2001 The Electrochemical Society. All rights reserved.

Electron transport in AlGaInP quantum well lasers

By placing a direct-gap monitor layer (or collector) in the p-cladding layer of red-emitting AlGaInP laser diode structures, we have studied the transport of electrons through this layer by observation of spontaneous emission. Pulsed optical excitation superimposed on cw electrical injection has been used to determine the delay time between optical injection of carriers into the well and radiative recombination from the monitor pit. Computer simulations using measured values of minority carrier lifetime for the well and monitor layer show that the transit time for electrons through the p-cladding layer correspond to an electron mobility of 160 cm2/V s.

Modeling the [dV/dt] of the IGBT during inductive turn off

Insulated gate bipolar transistor (IGBT)-based pulsewidth modulation (PWM) inverters are commonly used in inductive load circuits such as motor control. During clamped inductive load turn off of the IGBT, high-power losses occur during two phases. Due to the large inductive motor load, the voltage across the IGBT rises to the bus voltage while carrying the full-rated current. In the second phase, the current decreases as the IGBT goes into its forward blocking mode. In this paper, the turn-off process during the first phase is analyzed in detail for the first time. A simple analytical model has been derived, based upon the initial steady-state minority carrier distribution, which allows predicting the rate of rise of the voltage during this time period where the collector current remains constant. The predictions of the analytical model are in excellent agreement with results obtained from two-dimensional (2-D) numerical simulations over a broad range of minority carrier lifetime values. This analytical model provides a good estimate (within 10%) of the power losses incurred during the first phase of turn off.

The Hydrophilization of Process Wafers in Dilute Hydrogen Peroxide Solutions and Ozonated Deionized Water and Its Effects on Defects and Gate Oxide Integrity

The purpose of this study is to explore the effect of hydrophilization of wafer surfaces on defects and gate oxide integrity in metal-oxide semiconductor (MOS) devices. After HF treatment, dilute H2O2 solutions and ozonated deionized (DI) water were used as the final cleaning chemical to hydrophilize wafer surfaces. The defects created on patterned hydrophilic wafers were the least compared with patterned hydrophobic wafers or wafers with a mixture of both surfaces. The defects created on spin dried patterned hydrophobic wafers were water marks. The concentrations of Al, Ca and K observed in dilute H2O2 solutions and ozonated DI water were slightly higher than in DI water. Minority carrier lifetime values were the highest in HF last treated wafers and the lowest in those treated in dilute H2O2 solutions. MOS capacitors were fabricated to evaluate the gate oxide integrity when different cleaning procedures were applied. The distribution of breakdown range of devices shifted to a higher region on the application of HF and ozonated DI water. Dilute H2O2 solutions resulted in a poor breakdown field distribution. Ozonated DI water treated wafers showed higher breakdown field distributions than HF-last.

Electrical Properties of Silver Impurities and their Annealing Behaviour in p-Type Fz Silicon

The electrical activity of silver as well as its annealing properties in 10 Ω cm p-type Fz silicon substrate are studied by means of the four-point probe and minority carrier lifetime measurements. Silver atom concentration in the range 10 14 to 10 15 cm -3 consistently showed a donor type behaviour in the material and its presence led to a reduction of up to two orders of magnitude in the lifetime of minority carriers by the formation of deep-level traps. Isochronal annealing of silver contaminated specimens showed some gettering of the Ag impurities with resulting temperature dependent changes in the resistivity as well as the minority carrier lifetime values. Analysis of our results shows that a large fraction of the silver impurity atoms present forms the deep level defects and both the deep- and donor-levels appear to originate from the same source.

(001)-oriented lead selenide films grown on silicon substrates

Lead selenide epitaxial films were grown on silicon substrates using a thin (⩽100 nm) YbS buffer layer. The films were (001)-oriented and had structural, electrical and photoelectrical properties comparable with those of PbSe bulk crystals. The X-ray rocking curve halfwidths were of 100–200 arcsec which are within similar values for PbSe/BaF 2(CaF 2)/Si films, thoroughly investigated for infrared detector array applications. The films investigated exhibit relatively large photosensitivity in the temperature range T = 80–300 K. Minority carrier lifetime values τ ≈ 10 −6s at 80K and their temperature dependencies are preferably determined by the Schockley-Read-Hall recombination mechanism through the deep levels in the band gap.

I-V characteristics and performance of silicon solar cells between low- and high-level injection Part I: The modeling method

A method for the analysis of solar cells incorporating high-low junctions, and operating between the well-studied cases of low-and high-level injection, has been developed. It is a numerical extension of the transport velocity transformation method, oriented to the determination of I-V characteristics. The model is derived from a one-dimensional analytical treatment of minority-and majority-carrier transport, approximating the electrostatic field and spatially varying material parameters by piecewise constant functions. The analytical method is extended beyond the charge neutrality case by use of a numerical iteration process to achieve consistency of the fields with the charge imbalances between the minority- and majority-carrier distributions. Between iteration steps, the minority-carrier lifetime values are readjusted to fit the changing carrier concentrations, applying commonly used models. The method has the advantage of providing final I-V data by subdivision of the device regions into a rather small number of layers. Results obtained by applying this model to several structures are presented in Part II of this paper.

Pulsed laser modification of SiO2/Si interface properties and minority-carrier lifetime

Effects of pulsed laser irradiation on the SiO2/Si interface are investigated by means of capacitance-voltage and capacitance-time experiments yielding values of surface state density (Nss), oxide charge (QFC), and minority carrier lifetime (MCL) which prove to be a sensitive probe of the laser damage thresholds. Increases of Nss and QFC and decreases of MCL of two to three orders of magnitude from controlled starting values are observed at incident energy densities from 0.7 to 1.1 J cm−2. Subsequent thermal treatments restore either QFC or both QFC and Nss to control values. The laser method gives a technique for localized lifetime reduction on silicon wafers while maintaining long lifetimes elsewhere on the wafers.

Minority carrier traps in epitaxial gallium arsenide phosphide

The optical injection of minority carriers in the depletion region of a reverse biased Schottky barrier has permitted the study of deep levels in vapour phase epitaxial n-type GaAsP (Te) structures. The deep level transient spectroscopy (DLTS) experiments have yielded two hole traps at (Ev + 0.60)eV and (Ev + O.36)eV. Additional subsidiary transient capacitance, photocurrent and capture experiments are analysed, the details of which give electron and hole capture cross-sections, trap densities and activation energies. Derived values of minority carrier lifetime (~1 ns) agree with experimental values and with injection-level dependent LED efficiency.

Thermal balancing via distributed inert-gas streams for high-meniscus ribbon crystal growth

Superior melt growth of ribbon crystals by thermal profile control that is continuously variable, inert, and capable of heat extraction has been achieved. In the device developed, multiple inert-gas jets are directed at various parts of the hot zone, each with individually controlled flow adjustment. In a furnace at 1200°C, the temperature at one edge of a 38-mm-wide ribbon growth zone can be varied at a rate of 6.8°C per liter per minute of argon flow. Asymmetrical temperature profiles in the zone can be flattened to within ±1°C or symmetrically tailored. Silicon ribbons having uniform widths of 38 and 50 mm and grown with a high melt meniscus have been produced by the capillary action shaping technique with inert-gas thermal balancing. Such crystals show minority carrier lifetime values of up to about 300 μs.

Epitaxial silicon p-n junctions on polycrystalline ``ribbon'' substrates

Silicon epitaxial layers have been deposited on polycrystalline silicon ``ribbon'' substrates. It is shown that the epitaxial layer is structurally much superior to the substrate material. Good quality p-n junctions have been epitaxially grown on these ribbons with reasonable values of minority carrier lifetime and saturation current density, but variations are observed related to structural imperfections.

Effect of Electrostatic Field Gradient in Semiconductors with Diffused Impurities

Previous works on the evaluation of the collection efficiency of diffused radiant energy converters have assumed a constant electrostatic field in the diffused region. In order to match the theoretical spectral response based on this assumption with the experimental results, extremely low values of minority carrier lifetime were further assumed. No physical explanation is available for these assumptions. Using the basic diffusion theory, a theoretical investigation of the effects of impurity distribution in diffused semiconductors has been carried out for diffused impurity profiles produced by (1) diffusion from an infinitesimally thin layer of dopant and (2) diffusion from a constant podant source. It has been established that the electrostatic field gradient in the diffused semiconductor acts as an equivalent recombination effect with an equivalent minority carrier lifetime of the order of 10−10 sec. It has been shown that the effect of an electrostatic field gradient is much more predominant than the radiative and nonradiative recombinations.

Spectrum of fluctuations and lifepath in silicon

The so-called shot noise is classified into two parts. One of them is a shot noise in the narrow sense. The other is a generation-recombination noise. The differences between them are discussed theoretically as well as experimentally. The shot noise in the narrow sense is analysed according to the theory of Hill and Van Vliet. The lifepath in silicon has been measured by the method of noise. The results obtained are compared with the one measured by other methods. Values of minority carrier lifetime by our method agree well with those obtained from other methods.

Confirmation of Lifetimes by Noise and by Haynes-Shockley Method

Values of minority carrier lifetime in germanium filaments deduced from noise produced by photogeneration of carriers agree well with the values obtained by the Haynes-Shockley method.