No abstract

A comparison of the photoluminescence of Er-doped hydrogenated amorphous silicon and crystalline silicon a-Si:H(Er) and c-Si(Er), is presented. It is shown that a-Si:H(Er) exhibits efficient room-temperature photoluminescence at 1.537 μm which is as strong as the emission from optimized c-Si(Er) at 2 K. Most remarkably, there is practically no temperature quenching of the emission intensity in the range 2–300 K. The experiments suggest that the lifetime connected with the Er-induced emission is considerably shorter in a-Si:H(Er) than in c-Si(Er) which may be responsible for the different dependences of the photoluminescence intensity on the temperature, chopping frequency, and excitation power.

Based on a detailed consideration of excitation mechanisms of erbium in crystalline and amorphous matrix we present an analysis of the physical meaning of the Auger excitation cross section of Er 3ϩ ions in semiconductor. It is demonstrated that large values of Auger excitation cross sections under optical pumping in semiconductor matrices are due to large values of band-to-band absorption coefficient exceeding by several orders of magnitude the absorption coefficient of erbium in dielectric SiO 2 and Al 2 O 3 matrix. The Auger excitation cross section of Er 3ϩ in semiconductor matrices is roughly given by the ratio of the matrix absorption coefficient to concentration of Er 3ϩ ions. While the analysis of the excitation cross section is carried out for Er-doped crystalline and amorphous silicon, the results are expected to be applicable to the other rare-earth doped semiconductors. Based on low-temperature experimental results for crystalline silicon we get the Auger excitation coefficient of c A cryst Ϸ7ϫ10 Ϫ10 cm 3 s Ϫ1 and the effective excitation cross section eff cryst ϭ(2Ϫ8) ϫ10 Ϫ12 cm 2. For amorphous silicon at 100 K we obtain eff amorph Ϸ1.4ϫ10 Ϫ14 cm 2 .

We conclusively establish a direct link between formation of an Er-related donor gap state and the 1.5 microm emission of Er in Si. The experiment is performed on Si/Si:Er nanolayers where a single type of Er optical center dominates. We show that the Er emission can be resonantly induced by direct pumping into the bound exciton state of the identified donor. Using two-color spectroscopy with a free-electron laser we determine the ionization energy of the donor-state-enabling Er excitation as E(D) approximately 218 meV. We demonstrate quenching of the Er-related emission upon ionization of the donor.