Optical absorption and photoluminescence studies of β-FeSi₂ prepared by heavy implantation of Fe⁺ ions into Si

Mass-separated ⁵⁶Fe⁺ ions were implanted into Si(100) at 350°C using three different energies and doses of 140 keV (1.32×10¹⁷ cm^-2), 80 keV (6.20×10¹⁶ cm^-2), and 50 keV (3.56×10¹⁶ cm^-2). Their optical properties were investigated as a function of subsequent annealing temperature and its duration time. X-ray diffraction analysis revealed that polycrystalline semiconducting β-FeSi₂ layers are formed in the as-implanted and annealed samples. From Rutherford backscattering spectrometry analysis, the formation of β-FeSi₂ up to the surface was confirmed, and the average thickness and composition of the layers at peak concentration were estimated to be 70-75 nm and Fe:Si = 1:2.0-2.2, respectively. The types of optical transition and the inverse logarithmic slope (E₀) of the Urbach tail were investigated using room temperature optical absorption measurements. All the synthesized β-FeSi₂ layers exhibited a direct transition with direct band-gap energies (E^dir _g) of 0.802-0.869 eV and with high optical absorption coefficients extending to 10⁵ cm^-1 at photon energy above 1.0 eV. The E₀ value characteristic of the Urbach tail was observed to decrease from 260 to 100 meV with elevating annealing temperature. Some of the materials having E₀<160 meV showed two strong photoluminescence (PL) emissions peaked at 0.805-0.807 eV (No. 1) and 0.840-0.843 eV (No. 2) at 2 K, whereas those with E₀>250 meV exhibited only weak emissions. From the results of the temperature- and excitation power-dependent PL measurements, emissions Nos. 1 and 2 were attributed to the trap-related recombinations related to β-FeSi₂, with thermal activation (quenching) energies of 54.7 and 46.7 meV, respectively.