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

Mass-separated Fe-56(+) ions were implanted into Si(100) at 350 degrees C using three different energies and doses of 140 keV (1.32X10(17) cm(-2)), 80 keV (6.20X10(16) cm(-2)), and 50 keV (3.56X10(16) 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 beta-FeSi2 layers are formed in the as-implanted and annealed samples. From Rutherford backscattering spectrometry analysis, the formation of beta-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 (E(0)) of the Urbach tail were investigated using room temperature optical absorption measurements. All the synthesized beta-FeSi2 layers exhibited a direct transition with direct band-gap energies (E(g)(dir)) of 0.802-0.869 eV and with high optical absorption coefficients extending to 10(5) cm(-1) at photon energy above 1.0 eV. The E(0) value characteristic of the Urbach tail was observed to decrease from 260 to 100 meV with elevating annealing temperature. Some of the materials having E(0)<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 beta-FeSi2, with thermal activation (quenching) energies of 54.7 and 46.7 meV, respectively. (C) 1996 American Institute of Physics.