GaAs/AlGaAs quantum wire lasers and other low-dimensional structures fabricated by cleaved edge overgrowth

Abstract

Cleaved edge overgrowth—a molecular beam epitaxy technique which incorporates two sequential growth steps along orthogonal crystal directions —was employed to fabricate lasers containing an array of 22 quantum wires with cross-sections of about 7 by 7 nm. In addition, we have applied this method to prepare single modulation-doped quantum wires exhibiting electron mobilities in excess of 2×10⁵ cm²/Vs.

The active region of the lasers consists of atomically precise quantum wires that form at the T-shaped intersections of [001] oriented quantum wells with those grown, after an in situ cleave, along the [110] crystal axis. The origin of the quantum mechanical bound state is the relaxation of quantum well confinement at this intersection, which leads to an expansion of the electron and hole wavefunctions into the larger available volume at the T-junction. The quantum wires are, in turn, embedded in a T-shaped dielectric waveguide formed by [001] and [110] oriented cladding layers, which confine the optical mode to the vicinity of the one-dimensional quantum structures. The high degree of structural perfection achievable in this way allows the observation of stimulated optical emission in optically as well as in electrically pumped devices. Efficient current injection into the wires by p and n doping in the two growth directions is demonstrated by the suppression of optical emission from the quantum well states as well as by threshold currents as low as 400 μA for uncoated devices at 1.7 K. From the absence of bandgap renormalization effects manifested in the near constancy of the quantum wire emission wavelength with changes in pump power over almost three orders of magnitude, we conclude that the Mott density for ionization of the one-dimensional excitons is never reached. This suggests that gain in these lasers is due to exciton recombination and indicates interesting new behavior of exciton in low-dimensional systems.

In two-terminal magnetoresistance measurements along the modulation-doped quantum wires we see clear evidence for two-dimensional confinement of the electrons. The observed retarded depopulation of Landau levels at low magnetic fields is in close agreement with a calculation of the magnetic field dependence of the number of occupied subbands in a narrow channel.