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A new set-up of “Mossbauer Spectroscopic Microscope (MSM)” is applied to study not only the diffusion of Fe in a single-crystalline Si, but also a correlation between Fe impurities and the lattice defects in a multi-crystalline (mc-) Si. In addition to substitutional Fe s 0 and interstitial Fe i 0 components, the Mossbauer spectrum of mc-Si contains a new component assigned to “Fe i -defect associations”. All three components appear to distribute inhomogeneously, and to correlate with the defect distributions. These results are clearly different from that of the single crystalline Si wafer.

A prototype Mossbauer Spectroscopic Microscope is applied for the feasibility study of 57Fe impurity diffusion in Si wafers. A 2nm- 57Fe deposited Si wafer is annealed at 430 °C for 1 hour, and subsequently the 1/3 area of the wafer is further grinded with an angle of 6 degrees to the original surface to get a higher depth resolution for the mapping. Subsequently, the mapping images for Fe $_{\text {sub}}^{\mathrm {0}}$ , Fe $_{\text {int}}^{\mathrm {0}}$ , and Fe $_{\text {int}}^{\mathrm {+}}$ states are measured separately for the two different areas: one area (A) along the grinded surface, and the other (B) along the original wafer surface crossing through the 57Fe-deposition boundary. By integrating the mapping intensity the diffusivity of each Fe component along the two different directions can be evaluated and compared with an average Fe diffusivity in Si, which is measured by EDS mapping data obtained for the same area.

57Fe-diffused multi-crystalline silicon (mc-Si) wafer was studied by both Mossbauer and photoluminescence (PL) microscopes. By observing the 57Fe doped and non-doped areas separated with grain boundaries, substitutional and interstitial Fe impurities appear to influence differently on the PL intensities, which are closely related to the carrier trappings at the Fe.

We have been developing a “Mossbauer Spectroscopic Microscope (MSM)” which consists of a focusing lens for 14.4 keV γ-rays and a high precision X–Y stage. The measuring system both for electrons and γ-rays combined with a new Mossbauer driver, i.e., “a moving coil actuator with a liner encoder” enables us to measure the mapping images simultaneously corresponding to different spectral components. The system has a controlling system based on a LabVIEW program and a LIST mode data acquisition system (NIKI-GLASS/A3100). To investigate a correlation between the microstructure of a sample and 57Fe atoms, a scanning electron microscope (APCO/Mini-EOC) is also installed to this system.

Immediately after the GeV-implantation of 57Mn nuclei produced by the RIKEN-RIBF facility, 57Mn/57Fe Mossbauer spectra in Si-solar cells are measured under light illumination. Comparing with the spectrum of p-type multi-crystalline-Si, the broad spectra of the solar cell under operation can be analyzed as a superposition of interstitial and substitutional Fe components with different charge states. The charge states of Fe impurities are created by the excess carrier injection followed by a directional carrier flow in the p-n junction. The present results provide us a possibility to clarify the carrier trapping process at the Fe impurities in Si-solar cells.

Two different types of “Mossbauer camera” are presented in this review article. The first one uses an imaging technique for γ-rays which are converted to visual light by a Photon-counting Image Acquisition System (PIAS) combined with a Fiber Optic plate with CsI (Tl) Scintillator (FOS), while the second one is a mapping technique for γ-rays which are first focused by a Multi Capillary X-ray lens (MCX), and subsequently electrons and γ-rays are detected as function of the sample position by a micro-channel plate (MCP) and a Si-PIN detector, respectively. A mapping image of a silicon solar cell is shown as an example obtained by the latter system. We also report on the present status in developing a new combined lens for the mapping using a MCX and a Fresnel Zone Plate (FZP) in order to realize a submicron-meter spatial resolution in a laboratory.

External bending stress was applied to a 57Fe-doped n-type silicon wafer, and transmission Mossbauer spectra were measured at room temperature in order to observe directly stress induced diffusion of 57Fe atoms in the silicon matrix. As the stress is increased up to 23 MPa, the resonance area of the substitutional Fes component is found to decrease by more than 30%. In addition, a line broadening can be clearly seen in the interstitial Fe $_{\rm i}^{0}$ component, corresponding to a jump frequency of 106 s − 1. This diffusion process is interpreted to be due to the fast jumps between interstitial sites, following a formation jump of interstitial Fe $_{\rm i}^{0}$ from substitutional Fes.

A newly developed mapping-technique, “Mossbauer microscopy”, for 57Fe probes using a multi capillary X-ray lens (MCX) is tested by observing the two dimensional images of 57Fe deposited Si crystals and a stainless steel. The space resolution is reached down to about 20 μm. A possibility of three dimensional microscope is also mentioned.

A 3D-Mossbauer Spectroscopic Microscope is developed to evaluate Fe impurities in multi-crystalline Si solar cells, which combines the Mossbauer spectroscopic microscope with a scanning electron microscope (SEM), an electron beam induced current (EBIC), an electron backscatter diffraction (EBSD), and an electron energy analyzer (HV-CSA). In addition, a new moving-coil-actuator with a liner encoder of 100 nm-resolution is incorporated for the operations with both a constant velocity and a constant acceleration mode successfully with the same precision as that obtained by the conventional transducers. Furthermore, a new multi-capillary X-ray lens is designed to achieve a γ-ray spot size less than 100 μm in diameter. The new microscope provides us to investigate the space correlation between Fe impurities and the lattice defects such as grain boundaries in multi-crystalline Si solar cells.

At the RI-beam facility of the RIKEN, 57 Mn/ 57 Fe isotopes are separated and implanted into n-type FZ-Si (6 × 10 16 As/cm 3 ), and subsequently the atomic jump processes of 57 Fe in the Si matrix are observed by Mossbauer spectroscopy on a time scale of 100 ns between 330 and 1200 K. The spectra consist of two components due to "interstitial Fe" and "substitutional Fe", both of which yield the same isomer shifts as those reported in p-type Si, showing long-range diffusion with increasing temperature. The spectra are compared with those simulated from a theoretical model.

Unstable isotopes of 57 Mn (τ 1/2 = 1.45m) are separated and implanted into FZ- and CZ-Si wafers using a RIKEN projectile nuclear fragment separator (RIPS). Subsequently, Mossbauer spectra of 57 Fe in Si are measured at high temperatures up to 800 K, in order to study the charge states and the diffusion processes of interstitial and substitutional Fe atoms in Si matrix. The spectra consist of two singlets between 300 and 650K, and only one singlet above 700K. The isomer shifts of the singlets at 300K are 0.809(13) and -0.042(38) mm/s, referring to the value of α-Fe. The values correspond to interstitial and substitutional Fe, respectively. Above 600 K, both the disappearance of the interstitial component and the simultaneous relaxation effects on the centre shifts are clearly observed. These dynamical behaviours can be interpreted as a reaction process of interstitial Fe atoms jumping into vacancies, which are accompanied by changes in the charge states. Above 650K a line broadening of the singlet can be also seen, indicating an enhanced diffusion of the Fe atoms due to the excess vacancies.

Mossbauer spectra of 57Mn/57Fe in FZ-Si are measured between 30 and 800 K during implantation of radioactive 57Mn (a half-life of 1.45 min), which are produced via a projectile nuclear fragmentation. The primary implantation energy and the total fluence of 57Mn are 2 GeV and 101057Mn/cm2 typically for one spectrum, respectively. The spectra up to 600 K can be fitted by two components corresponding to Fe atoms on interstitial and substitutional sites. As the temperature is elevated, the area of the interstitial line decreases above 500 K, while the substitutional line increases. At 700 K, both lines merge into a broad single line, which can be interpreted as a motional averaging due to interstitial Fe jumps into vacancies.

A new implantation technique is developed for the in-beam Mossbauer spectroscopy with implantation energy of several GeV using the RIken Projectile Separator (RIPS). 57Mn (T 1/2 = 1.45 min) ions can be produced and subsequently implanted into any material as a secondary radioactive beam by the projectile-fragmentation reaction of 59 Co beam with a Be target. The Mossbauer spectra of 57Fe in a p-type (100) FZ-Si following β-decay from 57Mn were measured at different temperatures between 30 and 296 K. The spectra can be fitted with two single lines, which are assigned to interstitial and substitutional 57Fe atoms in Si. This technique can be easily applied to other nuclear probes, and therefore, opens a new possibility for Mossbauer spectroscopy complementary to synchrotron techniques.

A "Mossbauer spectroscopic microscope" has been developed using a Multi-Capillary-X-ray (MCX) lens, operating in a laboratory combined with a scanning electron microscope (SEM). This microscope yields a two-dimensional mapping image of 57Fe probes with a space resolution of about 50μm in comparison with the microstructure observed by SEM.