beta -FeSi sub 2 in (111)Si and in (001)Si formed by ion-beam synthesis

Ion-beam synthesis of beta -FeSi sub 2 is demonstrated both in (111)Si and (001)Si substrates by 450 keV Fe ion implantation at elevated temperatures using a dose of 6x10 exp 17 Fe/cm exp 2 and subsequent annealing at 900 deg C. The structure of the buried layers has been analyzed using Rutherford backscattering spectrometry, x-ray diffraction, and (cross-section) transmission electron microscopy. In (111)Si an epitaxial layer is formed consisting of grains with lateral dimensions of approximately 5 mu m. Epitaxy of beta -FeSi sub 2 (110) and/or (101) planes parallel to the (111)Si substrate plane is observed. In (001)Si a layer is formed consisting of grains with lateral dimensions of typically 0.5 mu m. Several grain orientations have been observed in this material, among others beta -FeSi sub 2 {320}, {103}, and {13,7,0} parallel to (001)Si. Selected (111)Si samples were investigated optically using spectroscopic ellipsometry, and near-infrared transmittance and reflectance spectroscopy. The results confirm that the beta -FeSi sub 2 layer has an optical band gap of 0.87 eV. The ellipsometry results indicate that the layers formed by ion-beam synthesis are more dense than those formed by surface growth techniques. Hall measurements show that the beta -FeSi sub 2 layers obtained are p type. Mobilities observed are 1-4 cm exp 2 /V s at room temperature and approx25 cm exp 2 /V s at liquid-nitrogen temperature. These results show that the electrical properties of ion-beam-synthesized beta -FeSi sub 2 is comparable with those of surface-grown material. The results confirm that optoelectronic applications of beta -FeSi sub 2 are limited.