One-dimensional diffusion of excitons in GaAs quantum wires was observed by using microphotoluminescence measurements at low temperature. The observed diffusion length increased with decreasing wire width from 30 to 15 nm, and decreased from 15 to 7 nm, where maximum diffusion length was about 4 μm for the 15 nm quantum wire, which is the largest value so far reported. It is considered that the change of diffusion length versus wire width is caused by the competition between one-dimensional character and the interface fluctuation.
We investigated the quantum confined Stark effect in GaAs quantum wires formed in a V-groove structure, demonstrating observation of a blueshift of the photoluminescence peak with the increase of electric fields at 50 K. This blueshift is attributed to the fact that the change in enhanced binding energy of excitons due to the electric field is larger than that in quantized energy levels of electrons and holes. Time-resolved photoluminescence was also measured. The photoluminescence decay time is decreased in small quantum wires of 8 nm width with the increase of electric fields, while the decay time is increased in the quantum wires with a size of 35 nm. These results indicate that the escaping of carriers is more dominant in smaller structures than reduction of the oscillator strength due to the electric fields.
An ultrashort lifetime of photocarriers as short as 600 fs has been obtained in ion-implanted Ge thin films grown on sapphire substrates. The photocarrier mobility determined by photoconductivity measurements is found to be reasonably high (∼100 cm2/V s). We have observed terahertz (THz) radiation from a photoconductive dipole antenna geometry.
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