Direct Observations of Fe Impurities in Si with Different Fermi Levels by Mössbauer Spectroscopy

Abstract

The charge states of Fe interstitial atoms in Si are investigated by Mössbauer spectroscopy. The spectra of 57Fe-diffused Si wafers are measured directly after the deposition at room temperature. The Fermi levels are changed by applying the external voltages to a Schottky junction, and by using different Si wafers with different dopant concentrations, providing different fractions of interstitial Feint0 and Feint+ Mössbauer components which correspond to the isomer shifts of 0.40 and 0.80 mm/s, respectively.

Mössbauer Spectroscopy on Fe Impurities in Si Materials

“…Nowadays, one could use much better theoretical tools [23], but the isomer shifts deduced for the charged interstitial and substitutional Fe atoms do not agree with the experimental values. Our assignments for the charged Fe states are, therefore, proposed by taking into account of the experimental conditions at the 57 Fe nuclear probes in Si materials [17][18][19][20], referring to the theoretical predictions for the Fe deep levels in Si [24]. It should be mentioned that all the spectra did not contain any Fe-silicides and Fe-oxides whose isomer shifts are well known.…”

“…The isomer shift, which is proportional to the electron density at the nucleus, corresponds to the energy shift of the nuclear level for a Mössbauer component on a Doppler velocity scale of mm/s against the spectral centre of α-Fe at room temperature. The data have been obtained in different Mössbauer experiments on 57 Fe impurities, which were differently introduced into Si samples, and subsequently measured at different temperatures: (1) deposition at room temperature, and diffusion and measurements at high temperatures up to 1273K [11,14,15], (2) highly energetic implantation of the mother isotope of 57 Mn/ 57 Fe into Si [12,13,17], and (3) deposition and diffusion, and measurements at room temperature [16,[18][19][20]. All the results can be analysed by the new model based on the interstitial Fe with neutral, +1, and + 2 states (defect associated), in addition to substitutional Fe with neutral, possibly +1 and -1 states.…”

“…Mössbauer spectroscopy provides information corresponding to all dominant Fe components existing in Si matrix, even if they are not electrically active, while all other evaluation techniques can only be accessible to electrically active components. In addition, we have recently found in an absorber experiment on 57 Fe deposited Si wafer using a strong 3.75GBq-57 Co source [20] that the Mössbauer spectrum could change drastically under different experimental conditions such as the γ-ray fluence as well as the electrical contact of the sample surface to the ground. These findings strongly suggest that the charge states of Fe impurities will be changed through the carrier trappings, as one has been expecting.…”

“…Mössbauer spectroscopy appears to be ideal to characterize Fe impurities in Si crystal [6][7][8][9][10]. Recently, we have performed a series of the experimental investigations [10][11][12][13][14][15][16][17][18][19][20] using Mössbauer spectroscopy on Fe impurities in p-type and n-type Si materials such as single crystal Si, multi-crystal line (mc) Si wafers, and even mc-Si solar cells, the results are appealing that we need a new picture for Fe impurities in Si materials: the Fe impurities in the Si matrix exist not only on the interstitial sites with Fe int 0/+ and Fe int + -Bpair, but also on a higher charge state of Fe int 2+ associated with defects as well as on the substitutional sites with Fe sub 0 and Fe sub -. Furthermore, these components are found to transform each other in the Mössbauer spectra by changing experimental conditions such as temperature, external voltage, electron or light irradiation, and external stress as well as by changing the Fermi level, carriers and their concentrations, and the device structures in the vicinity of 57 Fe probes.…”