500
Y. Yoshida
measuring the time necessary for one picture becomes drastically shorter than that of
the mapping technique. We can apply these imaging technologies for a wide variety
of Fe containing materials, such as silicon solar cells, steels, and even biomaterials
soon.
9.6 Diffusion and Segregation Studies by Mössbauer
Spectroscopic Camera
Recently, the imaging techniques in Mössbauer spectroscopy are opening a new
possibility to study the diffusion process not only in homogeneous but also inhomogeneous materials for a diffusion-length from μm to mm. The materials provide
different spectral components due to their atomic structure as well as complex
micro-structures. For instance, a Si-solar cell is often contaminated with Fe impurities, leading to severe degradation in the light-energy conversion efficiency [71]. To
clarify the origin of the Fe contaminations, however, one has to investigate the Fe
diffusion and segregation processes in the p–n junction, which contains electrodes,
anti-reflection coating layer, crystal grains, grain boundaries, dislocations, and strain
distributions. In such systems, the atoms are moving and gathering on different lattice
sites with other chemical states under operation.
In the next two paragraphs, we are going to explain the diffusion studies of Fe
in Si wafer [87] and also in a multi-crystalline Si wafer [90]. The former system is
supposed to be homogeneous, and the latter inhomogeneous. The MSM technique
provides unique information on the diffusion process, i.e., on the diffusion profiles
belonging to the spectral components selectively.
9.6.1 Fe Impurity Diffusion in Single-Crystalline Si
Wafer [87]
We deposited a 2 nm-thick
57 Fe layer in an area of 10 mm × 10 mm on a CZSi wafer (10
15 B/cm
3 ) with 30 mm × 30 mm × 500 μm shown in Fig. 9.54(a) and
subsequently annealed at 430 °C for 1 h in a vacuum furnace. A Mössbauer spectrum
at room temperature consists of three components corresponding to substitutional
Fe s
0 , and interstitial Fe i
0 and Fe i
+ , which are shown in Fig. 9.55. Subsequently,
1/3 of the sample was polished with an angle of 6° to the surface, as is seen in
Fig. 9.54(a). Three MSM electron mapping images were obtained corresponding to
the three spectral components. The focal size of the lens is 250 μmφ. We evaluate the
diffusivity of Fe in Si in two mapping areas of (A) and (B) as indicated in Fig. 9.54(b):
(A) along the grinded surface, and (B) along the original wafer surface crossing
through the
57 Fe-deposition boundary. By integrating the mapping intensity, we can
evaluate the diffusivity of each Fe component along with two different directions by
Y. Yoshida
measuring the time necessary for one picture becomes drastically shorter than that of
the mapping technique. We can apply these imaging technologies for a wide variety
of Fe containing materials, such as silicon solar cells, steels, and even biomaterials
soon.
9.6 Diffusion and Segregation Studies by Mössbauer
Spectroscopic Camera
Recently, the imaging techniques in Mössbauer spectroscopy are opening a new
possibility to study the diffusion process not only in homogeneous but also inhomogeneous materials for a diffusion-length from μm to mm. The materials provide
different spectral components due to their atomic structure as well as complex
micro-structures. For instance, a Si-solar cell is often contaminated with Fe impurities, leading to severe degradation in the light-energy conversion efficiency [71]. To
clarify the origin of the Fe contaminations, however, one has to investigate the Fe
diffusion and segregation processes in the p–n junction, which contains electrodes,
anti-reflection coating layer, crystal grains, grain boundaries, dislocations, and strain
distributions. In such systems, the atoms are moving and gathering on different lattice
sites with other chemical states under operation.
In the next two paragraphs, we are going to explain the diffusion studies of Fe
in Si wafer [87] and also in a multi-crystalline Si wafer [90]. The former system is
supposed to be homogeneous, and the latter inhomogeneous. The MSM technique
provides unique information on the diffusion process, i.e., on the diffusion profiles
belonging to the spectral components selectively.
9.6.1 Fe Impurity Diffusion in Single-Crystalline Si
Wafer [87]
We deposited a 2 nm-thick
57 Fe layer in an area of 10 mm × 10 mm on a CZSi wafer (10
15 B/cm
3 ) with 30 mm × 30 mm × 500 μm shown in Fig. 9.54(a) and
subsequently annealed at 430 °C for 1 h in a vacuum furnace. A Mössbauer spectrum
at room temperature consists of three components corresponding to substitutional
Fe s
0 , and interstitial Fe i
0 and Fe i
+ , which are shown in Fig. 9.55. Subsequently,
1/3 of the sample was polished with an angle of 6° to the surface, as is seen in
Fig. 9.54(a). Three MSM electron mapping images were obtained corresponding to
the three spectral components. The focal size of the lens is 250 μmφ. We evaluate the
diffusivity of Fe in Si in two mapping areas of (A) and (B) as indicated in Fig. 9.54(b):
(A) along the grinded surface, and (B) along the original wafer surface crossing
through the
57 Fe-deposition boundary. By integrating the mapping intensity, we can
evaluate the diffusivity of each Fe component along with two different directions by