Origin of a nonradiative center in AlGaAs grown by molecular-beam epitaxy was investigated by secondary ion mass spectroscopy and deep-level transient spectroscopy, from the change of photoluminescence intensity with anneal treatment, and from its comparison with GaAs. Aluminum-oxygen complex is a most probable defect which acts as a nonradiative center in AlGaAs, and the oxygen would evaporate as Al2O from the surface, which would be a reason why the photoluminescence intensity increases under higher substrate temperature and lower V/III flux ratio. The electron-trap level at 0.76 eV from the conduction band would be the recombination center, and the photoluminescence intensity is inversely proportional to the trap concentration.

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We measure, in real time (t), the fluctuating component of the flux-flow voltage V(t), deltaV(t) identical withV(t)-V0, about the average V0 in the vortex-liquid phase of amorphous MoxSi1-x films. For the thick film, deltaV(t) originating from the vortex motion is clearly visible in the quantum-liquid phase, where the distribution of deltaV(t) is asymmetric, indicative of large velocity and/or number fluctuations of driven vortices. For the thin film the similar anomalous vortex motion is observed in nearly the same (reduced-)temperature regime. These results suggest that vortex dynamics in the low-temperature liquid phase of thick and thin films is dominated by common physical mechanisms, presumably related to quantum effects.

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We have measured current-induced voltage noise S V of thick amorphous Mo x Si 1−x films in various fields H from zero to high fields near the vortex-glass (VG) transition H g . Irrespective of temperature T, S V is highest in the Meissner phase ͑H Ͻ H c1 ͒. With applying H that exceeds H c1 , S V decreases abruptly and falls to the background level. With further increasing H, S V then starts to rise at characteristic field H Lg 0 ͑ӶH g ͒ and the broad peak resulting from a plastic-flow motion of VG occurs before H g is approached. Combined with the S V data for the Corbino disk, we suggest that the edge-contamination effects reported in the clean NbSe 2 single crystals are not important in our disordered films. Within the T range studied, H Lg 0 ͑T͒ shows an increase on cooling, which is different from the T-independent disorder-driven transition. We interpret the field regime H c1 ͑T͒ Ͻ H͑T͒ Ͻ H Lg 0 ͑T͒ as the vortex state different from the VG phase.