Optical studies of modulation-doped v-groove quantum wires
Experimental studies of optical properties in undoped and modulation-doped v-groove quantum wires (QWR) are presented. The results show good agreement with theoretical predictions. The investigation of undoped samples demonstrates the successful fabrication of high quality samples with small wire dimensions, exhibiting narrow linewidths and large subband spacings. Calculations from the Schrodinger solver show good agreement with the experimental results. Information about the shape of the confining potential is obtained from magneto-optical measurements where anisotropic shrinkage and binding energies of the excitons are measured. In high excitation power experiments the suppression of the excitonic recombination is observed due to screening and phase space filling. Photoluminescence excitation experiments reveal an inefficient carrier intersubband relaxation. Extending the optical investigations to modulation-doped samples, the formation of a one-dimensional electron gas can be observed. As a strong indication for this is the presence of a Fermi edge singularity (FES). Furthermore, Poisson-Schrodinger calculations show that the increased electron density in the conduction band leads to modified confinement energies. This was confirmed in magneto photoluminescence (MPL) experiments, where the diamagnetic shift of the luminescence from the first excited state is stronger than in the undoped case, as the state is squeezed further into the corners of the QWR. Recombination of the ground state electrons with different hole states appears as a fine structure in MPL spectra. Detailed analysis provides clear evidence of the FES. The expected temperature sensitivity of the FES is observed for lattice and electron heating. The FES intensity is also reduced at high excitation powers. The role of hole localisation and subband coupling is discussed. At applied magnetic fields the coupling of bands induces an enhancement of the singularity. Finally, the behaviour of hot carriers is investigated with time-resolved and electro-photoluminescence measurements. Long luminescence lifetimes indicate that electron-hole separation occurs due to the pinch-off between the QWR and the side quantum well. The field dependence of the electron heating shows quite clearly that LO phonon scattering is the dominant relaxation process at electron temperatures above ~40 K.