Transport and optics in quantum wires fabricated by MBE overgrowth on the (110) cleaved edge

Abstract

The MBE double-growth technique that we call cleaved-edge overgrowth has, over the past several years, proved itself to be especially suitable for making quantum wires of the very highest quality. We will review our recent progress in measuring the transport and quantum optics characteristics of these wires, and the MBE growth issues that arise with cleaved-edge overgrowth fabrication. Our transport experiments have resulted in 250 Å wide quantum wires with ballistic mean free paths exceeding 10 μm. We verify the prediction that in the ballistic regime the electron conductivity in a quantum wire is independent of the wire length and shows quantized steps proportional to e2/h. The deviation of our observed step heights from exactly e2/h is taken as evidence for correlated electron behaviour. The electrons are tightly confined on three sides by atomically smooth GaAs/AlGaAs heterojunctions and in the fourth direction by an electric field. This results in a quantum wire of nominal square cross-section 250 × 250 Å. Magneto-transport measurements reveal quantum wire sub-band separations in excess of 20 meV as well as the symmetries of the wave functions of the one-dimensional modes. For optics studies our quantum wires are made using cleaved-edge overgrowth to form a line junction as two quantum wells are made to intersect with the cross-section forming a letter ‘T’. This line intersection separately forms a quantum wire bound-state for holes, for electrons, and even for excitons. We have characterized our optical wires by PL, by PLE, and by scanning near-field optics. An important application of this work is our demonstration of the first quantum laser using this T-geometry.