Mott transition of excitons in coupled quantum wells

In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.     

Electron transport in modulation-doped GaAs v-groove quantum wires

We report the growth of modulation-doped GaAs/Al_xGa_1−xAs v-groove quantum wires and structural, electrical and optical investigations of their electronic states and transport properties. By using alternative group III precursors on partially SiO₂ masked pre-patterned GaAs substrates, samples have been fabricated which permit electrical measurements of single isolated wire structures without the need for additional electron-beam lithography. Magneto-transport was measured as a function of tilt angle of the incident magnetic field to identify the formation of low-dimensional electron gases in different parts of the structure. Photoluminescence investigations reveal 1D and 2D confined states which show different carrier heating when electric fields are applied along the wire structure.     

Selective growth of GaAs quantum dots and vertical quantum wires in two-dimensional V-grooves

We propose and demonstrate a novel technique for the fabrication of quantum dot (QD) structures using metal organic chemical vapor deposition (MOCVD). The GaAs quantum dots are grown at the bottom of the two-dimensional V-groove (2DVG) structures which are composed of (1 1 1)A and (1 1 1)B-facets on GaAs(1 0 0). The 2DVG is formed by MOCVD selective growth on a SiO₂ patterned substrate. It should be noted that the 2DVGs cannot be formed by a chemical wet etching technique because the facet's anisotropy of etching ratios are different. By changing the growth condition, we can obtain GaAs QD structures which have a size of less than 10 nm, and vertical GaAs quantum wires (V-QWRs) in 2DVGs. We have observed photoluminescence from each structure. We have also demonstrated stacking of GaAs QDs in the 2DVG on GaAs (1 0 0).     

Biexciton quantum coherence in a single quantum dot

Nondegenerate (two-wavelength) two-photon absorption using coherent optical fields is used to show that there are two different quantum mechanical pathways leading to formation of the biexciton in a single quantum dot. Of specific importance to quantum information applications is the resulting coherent dynamics between the ground state and the biexciton from the pathway involving only optically induced exciton/biexciton quantum coherence. The data provide a direct measure of the biexciton decoherence rate which is equivalent to the decoherence of the Bell state in this system, as well as other critical optical parameters.     

Strong Fermi-edge singularity in ultra-high-quality AlGaAs/GaAs quantum wires

We report the observation of a strong Fermi-edge singularity (FES), with the complete suppression of the band-edge peak, in the photoluminescence spectra of ultra-high-quality modulation-doped AlGaAs/GaAs quantum wires (QWRs). We find that the FES effect is very sensitive to the Fermi energy. The strong FES is observed only in QWRs having a Fermi energy of the order of a few meV, and disappears almost completely when the Fermi energy exceeds 10 meV. These results are expected to spark new research activities, both experimentally and theoretically, on many-body effects in one-dimensional electron gas.     

Multilayer self-organization of InGaAs quantum wires on GaAs surfaces

Molecular-Beam Epitaxy growth of multiple In_0.4Ga_0.6As layers on GaAs (311)A and GaAs (331)A has been investigated by Atomic Force Microscopy and Photoluminescence. On GaAs (311)A, uniformly distributed In_0.4Ga_0.6As quantum wires (QWRs) with wider lateral separation were achieved, presenting a significant improvement in comparison with the result on single layer [H. Wen, Z.M. Wang, G.J. Salamo, Appl. Phys. Lett. 84 (2004) 1756]. On GaAs (331)A, In_0.4Ga_0.6As QWRs were revealed to be much straighter than in the previous report on multilayer growth [Z. Gong, Z. Niu, Z. Fang, Nanotechnology 17 (2006) 1140]. These observations are discussed in terms of the strain-field interaction among multilayers, enhancement of surface mobility at high temperature, and surface stability of GaAs (311)A and (331)A surfaces.     

Fermi-edge singularities in the photoluminescence spectrum of modulation-doped GaAs v-groove quantum wires

We report the observation of strong Fermi-edge singularities in the photoluminescence spectrum of strongly-confined, modulation-doped GaAs v-groove quantum wires. The behaviour of the singularity has been investigated at high excitation intensity, and both lattice and electrical heating. The latter produces a strong reduction of the singularity due to Fermi surface smearing, whereas, increased photoexcitation produces complex electron–hole correlation effects.     

Conductivity in intrinsic quantum well wires: Semimetal-semiconductor transition

The effect of size quantization on the conductivity of semimetallic and semiconducting rectangular quantum well wires (QWW) is studied. A semimental-semiconductor transition is predicted to occur at a critical thickness of the wire. For wires of smaller thickness than the critical thickness, semiconducting behavior will occur with the conductivity decreasing exponentially with decreasing transverse dimensions of the wire. That is due to carrier freeze-out because of the increase in effective bandgap resulting from the size quantization. The effect dominates any thickness dependence of the mobility resulting from scattering interactions.     

Raman scattering from confined phonons in GaAs/AlGaAs quantum wires

We report on photoluminescence and Raman scattering performed at low temperature (T =  10 K) on GaAs/Al_0.3Ga_0.7As quantum-well wires with effective wire widths ofL =  100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ο_L10 =  285.6 cm⁻¹forL =  11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderleinas applied to the GaAs/Al_0.3Ga_0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques.