We report on direct evidence of ultrafast carrier dynamics displaying features on the picosecond time scale in microcrystalline silicon (c-Si:H). The dynamics of photogenerated carriers is studied by using above-band-gap optical excitation and probing the instantaneous carrier mobility and density with a THz pulse. Within the first picoseconds after excitation, the THz transmission transients show a fast initial decay of the photoinduced absorption followed by a slower decrease due to carrier recombination. We propose that the initial fast decay in the THz transients is due to carrier capture in the trapping states.

Using a mixed type-I/type-II GaAs/AlAs multiple-quantum-well sample, we have demonstrated an optically controllable and tunable terahertz (THz) filter. Long-lived electron–hole pairs in the quantum wells allow for efficient THz attenuation over a large THz spot size (2 mm) for extremely low optical cw power. This sample can also be used as an optically tunable THz phase shifter. The optically induced change of the GaAs quantum wells from a dielectric to a conducting material leads to the observed attenuation and the shifting of the THz wave forms.

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We report the controlled PL peak shifting of porous silicon (PoSi) by the method of atomic layer etching (ALEP). We hereby investigate the dependence of the crystallite size on the PL peak position of this material. By this method of repeated oxidation by and stripping of the oxidized surface layer, we were able to reduce the size of the clusters layer by layer [1]. In all previous reports the PoSi PL displayed a natural lower energy limit of eV. We are the first to report a continuous PoSi PL peak shift between 1.01 and 1.20 eV.From our experiments we draw several conclusions for the luminescence mechanism: we demonstrate the responsibility of geometrical quantum confinement in silicon crystallites for the efficient room-temperature PL in PoSi near the indirect bandgap of c-Si. For the relatively small shifts from the bulk Si bandgap we employ the effective-mass theory to calculate the cluster-diameter dependence of the bandgap. Along with observations of size-independent PL peaks around 1.6 eV in thermally oxidized samples our measurements indicate that the PoSi PL cannot be described by one origin alone. Both the existence of molecular centers and the geometrical quantum confinement are responsible for the PoSi PL in their specific range of etching and post-anodic treatment parameters.This work was done in collaboration with C. Voelkmann, V. Petrova-Koch, and F. Koch.[1]I.H. Libon, C. Voelkmann, V.

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In this work we describe the controlled shifting of the PL peak of p+ (10 mΩcm) porous silicon (PoSi) by means of atomic layer etching (ALEP). We hereby study the cluster-size dependence of the PL of this material. By this technique of repeated oxidation by H2O2 and stripping of the oxidized surface layer, we reduced the size of the crystallites layer by layer. In all previous reports the PoSi PL appeared to have a natural lower energy limit of ≈ 1.4 eV. We report for the first time a continuous shift of the PoSi PL peak between 1.01 and 1.20 eV. This observation allows us to draw conclusions for the luminescence mechanism: it proves that geometrical quantum confinement in Si crystallites is responsible for the efficient room-temperature PL in PoSi near the indirect bandgap of c-Si. Together with observations of size-independent PL peaks around 1.6 eV in thermally oxidized samples this result indicates that the PoSi PL cannot be described by one origin alone. Both the existence of molecular centers and the geometrical quantum confinement are valid in their specific range of etching and post-anodic treatment parameters.

370 / CLE0'99 / THURSDAY MORNING cations of the phenomena; mapping of supercurrent distribution and magnetic-flux trap memory.(a) Two-dimensional mapping of supercurrent distributionSince the amplitude of the THz radiation excited by the fs laser pulses is proportional to the supercurrent density at the laser spot, the two-dimensional supercurrent can be obtained from the THz radiation intensity by scanning the excitation laser beam. Figure 1 shows the distribution of the THz amplitude (root of the intensity) near the bridge of the bow-tie antenna type device under a bias current of 100 mA. It is seen that the supercurrent flows near the edge of the bridge. Until now, the supercurrent distribution is obtained indirectly from the magnetic field measured by a magneto-optical film or Hall sensor. The present method provides a new direct noncontact method for measuring the supercurrent distribution.(b) Magnetic-flux trap memory We found that the THz radiation is emitted into free space from the YBCO films subjected to an external magnetic field and magneticflux trapped state without a bias current. Here, we propose and demonstrate a new type of superconducting optical flux-trap memory using the THz radiation from magnetic-flux trapped states. Figure 2(a) shows the structure of the memory cell and Fig. 2(b) shows the change of the waveform with the bias current and the laser spot position. The cell has a hole in the center of the bridge to trap the magnetic flux. The direction of the magnetic flux trapped in the hole can be changed by the combination of the bias current direction and the laser spot position. The polarity ofthe THz radiation (read-out signal) reflects the direction of the magnetic flux. By integrating this memory cell two-dimensionally, it is possible to make a new type of superconducting optical flux-trap memory.