Semiconductor quantum-wire structures directly grown on high-index surfaces

The direct synthesis of GaAs quantum-wire structures on (311)A oriented substrates by molecular-beam epitaxy has been achieved due to the in situ formation of an array of nanometer-scale macrosteps or facets with a periodicity determined by energy rather than growth-related parameters. These kinds of macrosteps are formed by breaking up a flat surface with high surface energy into facets corresponding to planes with lower surface energy. Reflection high-energy electron diffraction (RHEED) directly reveals the formation of such macrosteps on the GaAs (311)A surface comprised of two sets of {331} facets oriented along the [2¯33] direction. The lateral periodicity of 32 Å is determined from the splitting of the zeroth-order streak observed along [2¯33] into sharp satellites and the height of the steps of 10.2 Å from the splitting along its length. The RHEED intensity dynamics during growth of GaAs/AlAs multilayer structures show a pronounced oscillation at the onset of GaAs and AlAs growth, respectively, due to a phase change of the surface corrugation during the deposition of the first monolayers. The complete structure then contains alternating thicker and thinner channels of GaAs and AlAs forming the quantum wires oriented along [2¯33], which is confirmed by high-resolution transmission-electron microscopy. The GaAs quantum-wire structures grown on (311) substrates exhibit a pronounced anisotropy of the electronic properties. Photoluminescence and photoluminescence-excitation (PLE) measurements reveal distinct energy shifts of the excitonic resonances and a strong polarization anisotropy in agreement with theory. Confinement energies up to 90 meV are determined from the appearance of phonon-related lines in the PLE spectra. A strong anisotropy in conductivity is observed in modulation-doped heterostructures. The integrated luminescence intensity of the GaAs quantum-wire structures does not degrade up to temperatures as high as 400 K. This result is important for applications in light-emitting devices.