9 Mössbauer Spectroscopic Studies on Atomic Diffusion in Materials
493
the spectrum (e) under 5 keV electron irradiations. Based on our assignment model
[91], the spectrum consists of substitutional Fe s
0 (green), interstitial Fe i
0 with neutral
charge state (yellow), and interstitial Fe i
2+ state adjacent to the defects (brown) such
as dislocations and grain boundaries. During 5 keV-electron irradiation, however,
Fe i
2+ state changes to Fe i
+ (red) by a reaction between the injected electrons and
Fe i
2+ , forming Fe i
+ state. After irradiation, the spectrum in the dark becomes again
more or less the same as before irradiation. The linewidth of Fe i
2+ seems broader than
the others, which could arise from a dynamical fluctuation due to electron trapping
and de-trapping at Fe i
2+ in the dark condition. Accordingly, it becomes clear that we
should measure four Fe components of Fe s
0 , Fe i
0 , Fe i
+ and Fe i
2+ for the mapping
experiment.
Based on the above-mentioned spectral information, we obtained the MSM
mapping images of a mc-Si solar cell without any intentional
57 Fe deposition.
Figures 9.48 corresponds to the square area of 4 × 4 mm, comparing with the observations in Fig. 9.49 obtained by the complemental evaluation techniques installed in
our MSM set-up in Fig. 9.39. Figure 9.48(a) is the photograph corresponding to the
mapping area of the MSM images of the mc-Si solar cell, which were measured by
counting (b) the transmitted 14.4-keV-γ-rays and (c) the conversion and Auger electrons, respectively. The MSM images measure with a constant velocity mode which
generates reciprocal oscillations in two different speeds at +0.30 (ON-resonance) and
−10 mms
-1 (OFF-resonance) corresponding to the spectrum component of a neutral
interstitial Fe int
0 in the n-region of the solar cell (pn-junction), and to the background,
respectively. We perform the mapping at 80 × 80 points by using an MCX lens with
Fig. 9.47 (a) A schematic band structure of the pn-junction with the Fermi level (blue) comparing
with the energy level of Fe interstitial impurity (brown), (b) an expected charge states of Fe interstitial
based on this model, (c) photos of the mc-Si solar cell, (d) the spectrum in dark, (e) the spectrum
under electron irradiation, and (f) the spectrum in dark after the electron irradiation
493
the spectrum (e) under 5 keV electron irradiations. Based on our assignment model
[91], the spectrum consists of substitutional Fe s
0 (green), interstitial Fe i
0 with neutral
charge state (yellow), and interstitial Fe i
2+ state adjacent to the defects (brown) such
as dislocations and grain boundaries. During 5 keV-electron irradiation, however,
Fe i
2+ state changes to Fe i
+ (red) by a reaction between the injected electrons and
Fe i
2+ , forming Fe i
+ state. After irradiation, the spectrum in the dark becomes again
more or less the same as before irradiation. The linewidth of Fe i
2+ seems broader than
the others, which could arise from a dynamical fluctuation due to electron trapping
and de-trapping at Fe i
2+ in the dark condition. Accordingly, it becomes clear that we
should measure four Fe components of Fe s
0 , Fe i
0 , Fe i
+ and Fe i
2+ for the mapping
experiment.
Based on the above-mentioned spectral information, we obtained the MSM
mapping images of a mc-Si solar cell without any intentional
57 Fe deposition.
Figures 9.48 corresponds to the square area of 4 × 4 mm, comparing with the observations in Fig. 9.49 obtained by the complemental evaluation techniques installed in
our MSM set-up in Fig. 9.39. Figure 9.48(a) is the photograph corresponding to the
mapping area of the MSM images of the mc-Si solar cell, which were measured by
counting (b) the transmitted 14.4-keV-γ-rays and (c) the conversion and Auger electrons, respectively. The MSM images measure with a constant velocity mode which
generates reciprocal oscillations in two different speeds at +0.30 (ON-resonance) and
−10 mms
-1 (OFF-resonance) corresponding to the spectrum component of a neutral
interstitial Fe int
0 in the n-region of the solar cell (pn-junction), and to the background,
respectively. We perform the mapping at 80 × 80 points by using an MCX lens with
Fig. 9.47 (a) A schematic band structure of the pn-junction with the Fermi level (blue) comparing
with the energy level of Fe interstitial impurity (brown), (b) an expected charge states of Fe interstitial
based on this model, (c) photos of the mc-Si solar cell, (d) the spectrum in dark, (e) the spectrum
under electron irradiation, and (f) the spectrum in dark after the electron irradiation