OPTICAL-PROPERTIES OF QUANTUM-WIRE ARRAYS IN (AL,GA)AS SERPENTINE-SUPERLATTICE STRUCTURES

Serpentine superlattices with a built-in quantum-wire array have been grown on vicinal (100) GaAs substrates by molecular-beam epitaxy. The quantum wires have parabolic cross sections with confinement dimensions on the order of 100 angstrom. The goal of this paper is to make a comprehensive optical study of two different serpentine-superlattice samples. The serpentine structures have been characterized by low-temperature cathodoluminescence, photoluminescence, and photoluminescence-excitation measurements. In photoluminescence, a rather sharp peak (typically 7 meV full width at half maximum) is attributed to excitonic recombination in the built-in quantum-wire array. The linear polarization dependence of the serpentine-superlattice emission has been measured with a photoelastic modulation technique, showing a pronounced polarization anisotropy in both photoluminescence and photoluminescence excitation. The serpentine-superlattice photoluminescence emission normal to the vicinal surface shows a linear polarization along the wires of up to about 30% due to the lateral confinement. The carrier confinement has been further characterized by measuring the linear polarization dependence of the photoluminescence normal to the cleaved edges. The measured polarization anisotropy has been compared with the calculated polarization dependence as a function of intermixing between the lateral barriers and wells. It is found that there is a substantial intermixing between the barriers and wells, with at least 30% of the Al intended for the barrier ending up in the well. This results in hole states confined to one dimension, while the electron states are two dimensional due to coupling through the lateral barriers. Linearly polarized photoluminescence excitation has been used to reveal the laterally induced heavy-hole-light-hole splitting. Photoluminescence decay-time measurements of the serpentine-superlattice emission (390 ps) show a longer radiative decay time at low temperature than for a reference alloy-well structure (260 ps), indicating a reduced recombination rate in the serpentine structure. The linear polarization of the photoluminescence is found to be rather constant over large areas of the wafer even though the growth rate varies by a few percent. This indicates that uniform quantum-wire-like states are formed and demonstrates the intended advantage of the serpentine superlattice, which avoids the tilt sensitivity of the tilted superlattice.