Fig 8 - uploaded by Richard Ahrenkiel
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The PL spectra of three samples measured in air and in a passivating iodine-methanol solution. Curves A and B are data from a float-zone-grown sample that is boron-doped to a level of 3.63 · 10 15 cm-3. Curves C and D are data from a float-zone sample that is undoped and with a background doping of 2 · 10 12 cm-3 .
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In recent years, intrinsic luminescence has been used as a method to characterize the recombination lifetime of crystalline
silicon. The assumption is that the steady-state intrinsic photoluminescence at 1.09eV (1.134μm) can be related to the recombination lifetime. In this work, we measured the band-edge photoluminescence (PL) intensities
of a num...
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Context 1
... spectra of three samples are shown in Fig. 8. Sample 36 is a float-zone sample that is boron-dope to a level of 3.63 · 10 15 cm -3 . The passivated life- time, as measured by RCPCD, is 47.9 ls, and the unpassivated lifetime is 8.9 ls. Curve A is the spectra in solution and curve B is the spectra in air. The ratio of the area of spectra A/B is 1.33, whereas the ratio of the ...
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ABSTRACT
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Citations
... The advantages and limitations for both approaches are discussed and compared.7.2 Imaging crystal orientations in mc-Si wafers viaThe effective lifetime of any semiconductor may be written as, Where , and represent the bulk lifetime and surface lifetime respectively. Since the PL intensity is proportional to the effective lifetime, as shown in Section 3.1, the PL Hence, the PL intensity is proportional to the bulk lifetime in sufficiently well passivated wafers[176][177][178]. By contrast, in an unpassivated wafer, provided that the bulk lifetime is much higher than surface lifetime (satisfying equation 7.3), the PL intensity reflects the surface recombination velocity of each grain in a mc-Si wafer, which in turn depends on their crystal orientations. ...
This thesis applies photoluminescence imaging technique
to study various carrier recombination mechanisms in
multicrystalline silicon materials. One emphasis of the work has
been recombination at grain boundaries, which is one of the
limiting factors for the performance of multicrystalline silicon
solar cells. An approach for quantifying the recombination
activities of a grain boundary in terms of its effective surface
recombination velocity, based on the photoluminescence intensity
profile across the grain boundary, is developed. The approach is
applied to compare the recombination properties of a large number
of grain boundaries in wafers from different parts of a p-type
boron doped directionally solidified multicrystalline silicon
ingot. The results show that varying impurity levels along the
ingot significantly impact the electrical properties of as-grown
grain boundaries, and also their response to phosphorus gettering
and hydrogenation. The work is then extended to various types of
multicrystalline silicon materials. The electrical properties of
conventionally solidified p-type, n-type and also recently
developed high performance p-type multicrystalline silicon wafers
were directly compared in terms of their electronic behaviours in
the intra-grain regions, the grain boundaries and the dislocation
networks. All studied samples reveal reasonably high diffusion
lengths among the intra-grain regions after gettering and
hydrogenation, suggesting that the main performance limiting
factors are likely to be recombination at crystal defects.
Overall, grain boundaries in the conventional p-type samples are
found to be more recombination active than those in the high
performance p-type and conventional n-type samples. As-grown
grain boundaries and dislocations in the high performance p-type
samples are not recombination active and only become active after
thermal processes. In contrast, grain boundaries in the n-type
samples are already recombination active in the as-grown state,
but show a dramatic reduction in their recombination strength
after gettering and hydrogenation. Apart from recombination
through crystal defects within the bulk, recombination at
surfaces acts as another significant loss mechanism in solar
cells. This thesis also demonstrates the use of the
photoluminescence imaging technique to study surface
recombination in silicon wafers, and provides some examples of
such applications.
... Hence, the PL intensity is proportional to the bulk lifetime in sufficiently well passivated wafers. 12,13 By contrast, in an unpassivated wafer, provided that the bulk lifetime is much higher than surface lifetime, the PL intensity reflects the SRV of each grain, which in turn depends on their crystal orientation. Different crystal orientations have different surface recombination velocities due to the variations in surface structure, such as the density of dangling bonds, and their interactions with surface passivating films, such as deposited films, or even a native oxide. ...
We present a method for monitoring crystal orientations in chemically polished and unpassivated multicrystalline silicon wafers based on band-to-band photoluminescence imaging. The photoluminescence intensity from such wafers is dominated by surface recombination, which is crystal orientation dependent. We demonstrate that a strong correlation exists between the surface energy of different grain orientations, which are modelled based on first principles, and their corresponding photoluminescence intensity. This method may be useful in monitoring mixes of crystal orientations in multicrystalline or so-called “cast monocrystalline” wafers.
... En effet, les défauts en surface sont un canal de recombinaison non radiatif important qui peut fortement diminuer l'intensité de la luminescence. L'influence de la passivation de la surface sur le rendement quantique de photoluminescence a été observée dans plusieurs études [84,85] . Dans notre cas, nous ne serons pas sensibles à ces effets car l'excitation se faisant proche du gap fondamental, la profondeur d'absorption est très grande. ...
Optical pumping techniques are usually well adapted for the study of spin dynamics in semiconductors. Silicon is a special case, because of an indirect bandgap and a low spin-orbit coupling.
Our approach consists in «reducing the lifetime» of conduction electrons, so that their initial polarization is preserved.
First, we carried out a polarized photoluminescence. Reducing lifetime is achieved mainly by varying the acceptor concentration. A highly polarized hot luminescence is evidenced on a heavyly doped sample. However, the depolarization effect of a transverse field is not observed.
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... The above results confirm QSS-PL as a quantitative tool for minority carrier lifetime measurements on silicon wafers. Ahrenkiel et al. recently investigated the relationship between the luminescence signal and the minority carrier lifetime over a wide range of silicon wafer resistivities 18 . The authors plotted "the luminescence intensity" measured on a number of samples as a function of "the minority carrier lifetime" and concluded from this data that PL is "not a reliable measurement tool for minority carrier lifetime" 18 . ...
... Ahrenkiel et al. recently investigated the relationship between the luminescence signal and the minority carrier lifetime over a wide range of silicon wafer resistivities 18 . The authors plotted "the luminescence intensity" measured on a number of samples as a function of "the minority carrier lifetime" and concluded from this data that PL is "not a reliable measurement tool for minority carrier lifetime" 18 . We believe that the origin of this discrepancy is in the comparison by Ahrenkiel et al of single luminescence intensities measured on a variety of wafers with single lifetime values that are measured with a different experimental technique, which is a too simplistic approach as it ignores some fundamental aspects of minority carrier lifetime and QSS-PL measurements: ...
Direct-detection experiments for light dark matter are making enormous leaps in reaching previously unexplored model space. Several recent proposals rely on collective excitations, where the experimental sensitivity is highly dependent on detailed properties of the target material, well beyond just nucleus mass numbers as in conventional searches. It is thus important to optimize the target choice when considering which experiment to build. We carry out a comparative study of target materials across several detection channels, focusing on electron transitions and single (acoustic or optical) phonon excitations in crystals, as well as the traditional nuclear recoils. We compare materials currently in use in nuclear recoil experiments (Si, Ge, NaI, CsI, CaWO₄), a few of which have been proposed for light dark matter experiments (GaAs, Al₂O₃, diamond), as well as 16 other promising polar crystals across all detection channels. We find that target- and dark-matter-model-dependent reach is largely determined by a small number of material parameters: speed of sound, electronic band gap, mass number, Born effective charge, high-frequency dielectric constant, and optical phonon energies. We showcase, for each of the two benchmark models, an exemplary material that has a better reach than in any currently proposed experiment.
Direct detection experiments for light dark matter are making enormous leaps in reaching previously unexplored model space. Several recent proposals rely on collective excitations, where the experimental sensitivity is highly dependent on detailed properties of the target material, well beyond just nucleus mass numbers as in conventional searches. It is thus important to optimize the target choice when considering which experiment to build. We carry out a comparative study of target materials across several detection channels, focusing on electron transitions and single (acoustic or optical) phonon excitations in crystals, as well as the traditional nuclear recoils. We compare materials currently in use in nuclear recoil experiments (Si, Ge, NaI, CsI, CaWO$_4$), a few which have been proposed for light dark matter experiments (GaAs, Al$_2$O$_3$, diamond), as well as 16 other promising polar crystals across all detection channels. We find that target- and dark matter model-dependent reach is largely determined by a small number of material parameters: speed of sound, electronic band gap, mass number, Born effective charge, high frequency dielectric constant, and optical phonon energies. We showcase, for each of the two benchmark models, an exemplary material which has a better reach than in any currently proposed experiment.
Inactivation of non-radiative defects by hydrogen and their thermal stabilities in a high-quality floating-zone Si wafer depending on annealing conditions have been studied using in-situ photoluminescence (PL) and thermal desorption under an ultra-high vacuum. The PL intensity increased to ∼400% of its initial value after annealing at 450°C and decreased to ∼6% of its initial value after annealing at 600°C due to inactivation and activation of non-radiative defects, respectively. Based on the annealing temperature- and duration-dependence of the PL intensity, we propose two types of hydrogenated defects with different thermal stabilities.
Residual stress and crystalline defects in silicon wafers can affect solar cell reliability and performance. Infrared photoelastic measurements are performed for stress mapping in monocrystalline silicon photovoltaic (PV) wafers and compared to photoluminescence (PL) measurements. The wafer stresses are then quantified using a discrete dislocation-based numerical modeling approach, which leads to simulated photoelastic images. The model accounts for wafer stress relaxation due to dislocation structures. The wafer strain energy is then analyzed with respect to the orientation of the dislocation structures. The simulation shows that particular locations on the wafer have only limited slip systems that reduce the wafer strain energy. Experimentally observed dislocation structures are consistent with these observations from the analysis, forming the basis for a more quantitative infrared photoelasticity-based inspection method.
Contactless measurements of the recombination lifetime have become the standard in photovoltaic and electronic materials research, as well as in the associated industries. Fast evaluation is a critical need as well as the need to keep the material under test from becoming contaminated by the measurement apparatus. A technique was developed for measuring the transport properties and recombination kinetics of semiconductors and photoconductors [R. K. Ahrenkiel and S. W. Johnston, Mater. Sci. Eng., B 102, 161172 (2003)]. The primary application of this technique is the measurement of carrier lifetime but carrier mobility can also be linked to the data. The author has named the technique resonance-coupled photoconductive decay and it was developed [R. Ahrenkiel, U.S. patent 5,929,652 (27 July 1999); R. Ahrenkiel and S. Johnston, U.S. patent 6,275,060 (14 August 2001); S. Johnston and R. Ahrenkiel, U.S. patent 6,369,603 (9 April 2002)] at the National Renewable Energy Laboratory. The technique provides the rapid measurement of recombination lifetimes that are of vital importance to electronic and photovoltaic materials. These measurements are also of value to a wide range of optoelectronic technologies. The operating frequencies of the measurements here are in the range of 420 to 430 MHz. The detection is based on the coupling of a high-Q resonant antenna to the sample. The operating frequency is chosen to be in the range of the resonant frequency so that the real and imaginary parts of the system impedance, Z, are changing rapidly. Here, these changes in Z about the resonance produce high sensitivity to changes in carrier concentration produced by photoexcitation.
Application of various characterization methods for the investigation of photovoltaic materials allows fast progress in perfection of their quality. However, capabilities of the methods should be clearly understood and the methods should be applied in the correct manner to avoid false and/or unreliable interpretation of the results. We applied photoluminescence (PL) for characterization of multi-crystalline silicon (mc-Si) samples and compared the obtained results with carrier lifetime measurement data for the same samples. The analyses revealed strong influence of surface recombination and optical shadowing from grain boundaries on the interpretation of the PL results. Proper surface passivation allows application of defect-related luminescence for the characterization of mc-Si along with traditionally used band-to-band luminescence.
