17 research outputs found
Thermal stability of internal gettering of iron in silicon and its impact on optimization of gettering
The redissolution behavior of gettered iron was studied in p-type Czochralski-grown silicon with a doping level of 2.5×10exp14 cm−3 and an oxide precipitate density of 5×10exp9 cm−3. The concentrations of interstitial iron and iron–boron pairs were measured by deep level transient spectroscopy. It was found that the dependence of redissolved iron concentration on annealing time can be fitted by the function C(t)=C_0[1−exp(−t/tau_diss)], and the dissolution rate tau−1diss has an Arrhenius-type temperature dependence of tau−1diss=4.01×10exp4 × exp[−(1.47±0.10) eV/k_BT] s−1. Based on this empirical equation, we predict how stable the gettered iron is during different annealing sequences and discuss implications for optimization of internal gettering.Peer reviewe
Muon Spin Relaxation Study of (La, Ca)MnO3
We report predominantly zero field muon spin relaxation measurements in a
series of Ca-doped LaMnO_3 compounds which includes the colossal
magnetoresistive manganites. Our principal result is a systematic study of the
spin-lattice relaxation rates 1/T_1 and magnetic order parameters in the series
La_{1-x}Ca_xMnO_3, x = 0.0, 0.06, 0.18, 0.33, 0.67 and 1.0. In LaMnO_3 and
CaMnO_3 we find very narrow critical regions near the Neel temperatures T_N and
temperature independent 1/T_1 values above T_N. From the 1/T_1 in LaMnO_3 we
derive an exchange integral J = 0.83 meV which is consistent with the mean
field expression for T_N. All of the doped manganites except CaMnO_3 display
anomalously slow, spatially inhomogeneous spin-lattice relaxation below their
ordering temperatures. In the ferromagnetic (FM) insulating
La_{0.82}Ca_{0.18}MnO_3 and ferromagnetic conducting La_{0.67}Ca_{0.33}MnO_3
systems we show that there exists a bi-modal distribution of \muSR rates
\lambda_f and \lambda_s associated with relatively 'fast' and 'slow' Mn
fluctuation rates, respectively. A physical picture is hypothesized for these
FM phases in which the fast Mn rates are due to overdamped spin waves
characteristic of a disordered FM, and the slower Mn relaxation rates derive
from distinct, relatively insulating regions in the sample. Finally, likely
muon sites are identified, and evidence for muon diffusion in these materials
is discussed.Comment: 21 pages, 17 figure
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Analysis of copper-rich precipitates in silicon: chemical state, gettering, and impact on multicrystalline silicon solar cell material
In this study, synchrotron-based x-ray absorption microspectroscopy (mu-XAS) is applied to identifying the chemical states of copper-rich clusters within a variety of silicon materials, including as-grown cast multicrystalline silicon solar cell material with high oxygen concentration and other silicon materials with varying degrees of oxygen concentration and copper contamination pathways. In all samples, copper silicide (Cu3Si) is the only phase of copper identified. It is noted from thermodynamic considerations that unlike certain metal species, copper tends to form a silicide and not an oxidized compound because of the strong silicon-oxygen bonding energy; consequently the likelihood of encountering an oxidized copper particle in silicon is small, in agreement with experimental data. In light of these results, the effectiveness of aluminum gettering for the removal of copper from bulk silicon is quantified via x-ray fluorescence microscopy (mu-XRF), and a segregation coefficient is determined from experimental data to be at least (1-2)'103. Additionally, mu-XAS data directly demonstrates that the segregation mechanism of Cu in Al is the higher solubility of Cu in the liquid phase. In light of these results, possible limitations for the complete removal of Cu from bulk mc-Si are discussed
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Analysis of copper-rich precipitates in silicon: chemical state, gettering, and impact on multicrystalline silicon solar cell material
In this study, synchrotron-based x-ray absorption microspectroscopy (mu-XAS) is applied to identifying the chemical states of copper-rich clusters within a variety of silicon materials, including as-grown cast multicrystalline silicon solar cell material with high oxygen concentration and other silicon materials with varying degrees of oxygen concentration and copper contamination pathways. In all samples, copper silicide (Cu3Si) is the only phase of copper identified. It is noted from thermodynamic considerations that unlike certain metal species, copper tends to form a silicide and not an oxidized compound because of the strong silicon-oxygen bonding energy; consequently the likelihood of encountering an oxidized copper particle in silicon is small, in agreement with experimental data. In light of these results, the effectiveness of aluminum gettering for the removal of copper from bulk silicon is quantified via x-ray fluorescence microscopy (mu-XRF), and a segregation coefficient is determined from experimental data to be at least (1-2)'103. Additionally, mu-XAS data directly demonstrates that the segregation mechanism of Cu in Al is the higher solubility of Cu in the liquid phase. In light of these results, possible limitations for the complete removal of Cu from bulk mc-Si are discussed
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Impact of iron contamination in multicrystalline silicon solar cells: origins, chemical states, and device impacts
Synchrotron-based microprobe techniques have been applied to study the distribution, size, chemical state, and recombination activity of Fe clusters in two types of mc-Si materials: block cast mc-Si, and AstroPower Silicon Film(TM) sheet material. In sheet material, high concentrations of metals were found at recombination-active, micron-sized intragranular clusters consisting of micron and sub-micron sized particles. In addition, Fe nanoparticles were located in densities of ~;2'107 cm-2 along recombination-active grain boundaries. In cast mc-Si, two types of particles were identified at grain boundaries: (1) micron-sized oxidized Fe particles accompanied by other metals (Cr, Mn, Ca, Ti), and (2) a higher number of sub-micron FeSi2 precipitates that exhibited a preferred orientation along the crystal growth direction. In both materials, it is believed that the larger Fe clusters are inclusions of foreign particles, from which Fe dissolves in the melt to form the smaller FeSi2 nanoprecipitates, which by virtue of their more homogeneous distribution are deemed more dangerous to solar cell device performance. Based on this understanding, strategies proposed to reduce the impact of Fe on mc-Si electrical properties include gettering, passivation, and limiting the dissolution of foreign Fe-rich particles in the melt