Modulated free carrier absorption characterization of semiconductor wafers by frequency scans at different pump-to-probe separations
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Abstract:
Based on a three-dimensional modulated free carrier absorption （MFCA） model，simulation was performed to investigate the dependences of MFCA amplitude and phase on the electronic transport properties （the carrier lifetime，the carrier diffusivity，and the front surface recombination velocity） of semiconductor wafers at different pump-to-probe separations and different modulation frequencies. Experiments were performed with a silicon wafer，in which amplitude and phase were recorded as functions of modulation frequencies at several different two-beam separations. The electronic transport properties of semiconductor wafers were determined simultaneously via multi-parameter fitting procedure. These results showed that the method is capable of improving the measurement precision of the simultaneous multi-parameter determination of transport properties.Keywords:
Free carrier absorption
Carrier lifetime
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Results of carrier lifetime studies in low-doped epitaxial 4H SiC layers are reported. The free carrier absorption (FCA) technique was applied to extract carrier lifetime parameters and their spatial distribution in a wide photoexcitation range. The FCA magnitude is shown to scale linearly with the photoinjected carrier concentration, while the absorption cross section increases according to a λ4.4 law for near infrared wavelengths. High spatial resolution carrier lifetime mapping of large 4H SiC areas revealed features related to structural imperfections of epilayers. Finally, a density dependent fast lifetime component was observed at high injection levels and attributed to band-to-band Auger recombination.
Photoexcitation
Carrier lifetime
Free carrier absorption
Auger effect
Wide-bandgap semiconductor
Free carrier
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A three-dimensional model for the modulated free carrier absorption (MFCA) is developed to measure the electronic transport properties (the carrier lifetime, the carrier diffusivity, and the front surface recombination velocity) of semiconductor wafers. The dependence of MFCA amplitude and phase on the electronic transport properties at different pump-probe-beam separation and different modulation frequencies is investigated. It is found that the sensitivities of MFCA signal to individual transport parameters increase with increasing two-beam separation. An experiment with a silicon wafer is performed and the carrier lifetime, carrier diffusivity, and front surface recombination velocity are determined simultaneously and unambiguously by fitting the observed values of the MFCA amplitude and phase as functions of the separation between the pump and probe laser spots, measured at several modulation frequencies covering an appropriate range.
Free carrier absorption
Carrier lifetime
Modulation (music)
SIGNAL (programming language)
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Free carrier
Free carrier absorption
Carrier lifetime
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White light was temporally modulated and subjects gave magnitude estimations of the apparent depth of modulation. Three stimulus parameters were varied: level of time-average luminance, amplitude of modulation, and frequency of modulation. At the lowest luminances, apparent depth of modulation decreased monotonically as modulation frequency increased toward fusion. At the higher luminances, only large amplitudes of modulation produced monotonic functions relating apparent depth of modulation to frequency; smaller amplitudes of modulation produced nonmonotonic functions with maxima in the region of 5–10 Hz. Derived contours relating modulation amplitude to frequency for constant apparent depth of modulation generally resemble functions relating modulation amplitude to frequency for threshold.
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Microwave absorption caused by free carriers was investigated. A 9.35 GHz microwave interferometer was constructed. The transmissivity of 525-µm-thick silicon substrates decreased from 60.4 to 3.8% as the resistivity decreased from 1000 to 4 Ω cm. This characteristic was explained well by a numerical analysis using the free carriers absorption theory. Microwave free carrier photo absorption caused by light-induced carriers was also investigated for p-type silicon samples coated with 100 nm thermally grown SiO2 layers as well as SiOx layers deposited by the vacuum evaporation method. The effective minority carrier lifetime and recombination velocity were analyzed in the case of the photo induced carrier generation with 532 nm light illumination. The effective minority carrier lifetime was increased from 360 to 540 µs and the recombination velocity was decreased from 78 to 30 cm/s by 1.3 ×106 Pa H2O vapor heat treatment at 260 °C for 3 h for light illumination at 0.315 mW/cm2 in the case of the thermally grown SiO2/Si because of the passivation of SiO2/Si interfaces. They were markedly increased from 30 to 380 µs and from 1300 to 60 cm/s, respectively, by the H2O vapor heat treatment in the case of the vacuum-evaporated SiOx/Si.
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Passivation
Charge carrier
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Vacuum evaporation
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A novel noncontact technique for semiconductor wafer mapping of the charge carrier lifetime is reviewed. The principle is based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. The technique is demonstrated here for Si wafer lifetime mapping, both for homogenous wafers as well as for wafers having pn junctions, and the performance and limitations are addressed. In a companion article J. Appl. Phys. 84, 275 (1998), (part I), the injection dependence is treated in more detail. We demonstrate the broad range of accessible injections, allowing on-wafer lifetime mapping both of the minority carrier lifetime and of the high injection lifetime as well as of Auger recombination effects. Furthermore, problems of surface recombination for bare Si surfaces are elucidated while pn junctions or other barriers are shown to suppress diffusion of carriers to surfaces. Also, a scheme for low temperature surface passivation is demonstrated and the resulting surface recombination velocity is derived. Finally, a few examples of characteristic lifetime maps from processed samples are discussed. We conclude that the technique is one of the few available for monitoring of the carrier lifetime during a full device process sequence.
Carrier lifetime
Free carrier absorption
Passivation
Auger effect
Charge carrier
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Laterally resolved modulated free-carrier absorption (MFCA) is applied to the simultaneous determination of the electronic transport properties of semiconductor wafers. A rigorous three-dimensional carrier diffusion model is used to fit the observed dependences of the MFCA signal amplitude and phase on the separation between the pump and probe laser spots, measured at several modulation frequencies covering an appropriate range. This leads to a simultaneous and unambiguous determination of the values of three transport parameters, namely, the minority-carrier lifetime τ, the carrier diffusivity D, and the front surface recombination velocity s1. The extracted values for a n-type Si wafer with a resistivity of 7–10Ωcm are 53μs (τ), 16.6cm2∕s (D), and <200cm∕s (s1), respectively.
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The main objective of this study was to determine the detection thresholds of random amplitude modulation (RAM) as a function of modulation and carrier frequency. Two experiments were performed. Experiment 1 concerned the detection thresholds of AM stimuli for only random amplitude changes at constant modulation frequency. Experiment 2 dealt with detection thresholds for simultaneous, random changes in amplitude and modulation frequency. The data obtained showed that for low modulation frequency, the detection thresholds for sinusoidal amplitude modulation (SAM), random amplitude modulation at constant modulation frequency [RAM(fm const)], and random amplitude and modulation frequency [RAM(fs random)] overlapped one another in the limit of SD. However, for higher carriers and modulation frequencies the RAM(fm const) thresholds were, in a limited range offm , much lower than the SAM ones. When random changes in modulation frequency were combined with random changes in amplitude, the RAM(fm random) thresholds decreased relative to the RAM(fm const) thresholds. a)Permanent address: Institute of Acoustics, Adam Mickiewicz University, Poznan ul. Matejki 48/49, Poland.
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Modulated free-carrier absorption (MFCA) is introduced as a novel contactless lifetime mapping technique with high spatial resolution. The advantages of MFCA mapping as compared to other methods are discussed. Examples of lifetime maps on multicrystalline silicon are presented. Solar cells fabricated from identical wafers have been investigated using light beam induced current mapping. Excellent agreement between structures obtained in the measured lifetime and in the current maps was observed. In addition, a quantitative comparison with diffusion lengths determined by local internal quantum efficiency evaluations is presented.
Carrier lifetime
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Quantum Efficiency
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A contactless, all-optical technique for semiconductor charge carrier lifetime characterization is reviewed. The technique is based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. Main features are a direct probing of the excess carrier density coupled with a homogeneous carrier distribution within the sample, enabling precision studies of different recombination mechanisms. We show that the method is capable of measuring the lifetime over a broad range of injections (1013–1018 cm−3) probing both the minority carrier lifetime, the high injection lifetime and Auger recombination, as well as the transition between these ranges. Performance and limitations of the technique, such as lateral resolution, are addressed while application of the technique for lifetime mapping and effects of surface recombination is outlined in a companion article [J. Appl. Phys. 84, 284 (1998), part II]. Results from detailed studies of the injection dependence yield good agreement with the Shockley–Read–Hall theory, whereas the coefficient for Auger recombination shows an apparent shift to a higher value, with respect to the traditionally accepted value, at carrier densities below ∼2–5×1017 cm−3. Data also indicate an increased value of the coefficient for bimolecular recombination (radiative or trap-assisted Auger) from the generally accepted value. Measurements on an electron irradiated wafer and wafers of exceptionally high carrier lifetimes are also discussed within the framework of different recombination mechanisms.
Carrier lifetime
Auger effect
Free carrier absorption
Charge carrier
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