Optical Transport Experiments in Heterostructures
Various transport processes in GaAs/AlGaAs heterostructures are investigated by optical techniques using ultrashort laser pulses.
The transfer of optically injected electron-hole populations in single heterostructures from AlGaAs to GaAs is observed by studying the luminescence above the AlGaAs bandgap. The technique of “population correlation” allows time-resolved luminescence experiments also at low emission intensities. We find that the “thermal emission” of carriers from AlGaAs to GaAs is limited by ambipolar diffusion, leading to transfer times of 5 to 11 ps depending on the layer thickness.
In double-quantum-well structures consisting of quantum wells with two different well widths the tunneling times from the narrow well (higher ground state) to the wide well are quantitatively measured. The tunneling times are of the order of 200 ps, due to the barrier width and the nonresonant process which requires additional phonon emission or absorption for momentum conservation.
Electron-hole scattering in quantum wells leads to negative mobility of minority carriers in modulation-doped structures due to the “carrier drag” effect. Recently we could observe also the negative photoconductivity associated with the negative mobility. Quantitative evaluation gives more exact values on electron-hole momentum scattering times.
Photoluminescence from materials for subpicosecond photoconductors (radiation-damaged Ga0·47In0·53As) gives direct information on the ultrafast recombination processes. Decay times of 0.9 ps are observed, as well as evidence for extreme nonequilibrium carrier distributions (“inverted” luminescence spectra).
KeywordsLuminescence Spectrum Minority Carrier Ambipolar Diffusion Tunneling Time Intersubband Transition
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