Well-resolved sharply structured luminescence spectra at 1.54 μm were observed in erbium-implanted GaP, GaAs, InP, and Si. The optical transitions occur between the weakly crystal field split spin-orbit levels, 4I13/2→4I15/2, of Er3+(4f11). Typical spectral linewidths in GaAs are 2 cm−1(0.25 meV) at 6 K and 11 cm−1(1.36 meV) at room temperature.
The feasibility of producing erbium-doped silicon light-emitting diodes by molecular beam epitaxy is demonstrated. The p-n junctions are formed by growing an erbium-doped p-type epitaxial silicon layer on an n-type silicon substrate. When the diodes are biased in the forward direction at 77 K they show an intense sharply structured electroluminescence spectrum at 1.54 μm. This luminescence is assigned to the internal 4f–4f transition 4I13/2→4I15/2 of Er3+ (4f11).
We investigate the dependence of carrier lifetimes in radiation-damaged, GaAs on proton implantation dose by means of time-resolved reflectivity and photoconductivity experiments with subpicosecond resolution. The carrier lifetimes decrease with increasing implantation dose at low implantation levels whereas beyond the ‘‘amorphization dose’’ a saturation at 0.5 ps can be observed due to a saturation of the defect density.
The energy spectra of monochromatic neutrons scattered on selenium and tellurium have been investigated by means of a rotating crystal spectrometer. In polycrystalline selenium the highest spectral line can be identified as an intrachain bond stretching mode. Transforming selenium from its crystalline to its amorphous and liquid state, the shift of the line energy is only a few percent. The other lines in the spectra (from bond shearing and torsional modes of the chains) are influenced more strongly. For tellurium, on the other hand, the spectral lines of the solid disappear completely in the liquid phase. The experimental results are compared with the results of optical spectroscopy. Furthermore, the diffusive motion of the atom has been studied by quasi-elastic neutron scattering for liquid selenium containing various admixtures of iodine (0-6 %), which change the average chain length of the polymeric melt, and for liquid tellurium. For Se, one obtains an effective diffusion constant D which describes the motion of small chain segments over atomistic distances. D is of the order of cm2/s. Its activation energy was about 5 kcallmol.
We present a comprehensive theoretical and experimental analysis of the current response of GaAs metal-semiconductor-metal Schottky photodiodes exposed to 70 fs optical pulses. Theoretical simulations of the carrier transport in these structures by a self-consistent two-dimensional Monte Carlo calculation reveal the strong influence of the distance between the finger electrodes, the external voltage, the GaAs layer thickness and the excitation intensity on the response time and the corresponding frequency bandwidth of these photodetectors. For many experimental conditions, the model demonstrates a clear temporal separation of the electron and hole contributions to the output current due to the different mobilities of the two carrier types. For a diode with an electrode separation of 0.5 μm, an electric-field strength above 10 kV/cm and low intensity of the incident light the theory predicts a pulse rise time below 2 ps, an initial rapid decay as short as 5 ps associated with the electron sweep out and a subsequent slower tail attributed to the hole current. For weaker electric fields and/or higher light intensities a significant slowing down of the detector speed is predicted because of effective screening of the electric field by the photoexcited carriers. Heterostructure layer-based devices are shown to provide superior performance compared to diodes manufactured on bulk substrates. Experimental data obtained by photoconductive or electro-optic sampling on diodes with electrode separation between 0.5 and 1.2 μm agree fairly well with the theoretical predictions.
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