Nov 12, 2009
Characterization study: wurtzite GaAs/AlGaAs core-shell nanowires
Researchers at Norwegian University of Science and Technology are examining the growth of GaAs/AlGaAs core-shell nanowires on GaAs(111)B substrates by gold-assisted molecular beam epitaxy. Room-temperature and low-temperature (4.4 K) micro-photoluminescence measurements show a much higher radiative efficiency from the GaAs core after the nanowire is overgrown with a radial AlGaAs shell.
Recently, there has been significant progress in developing semiconductor nanowire (NW) heterostructures for future nano-photonic and nano-electronic devices. Such development requires a detailed understanding of the structural and optoelectronic properties of these one-dimensional (1D) heterostructures. Among these NW heterostructures, the radial core-shell and core-multishell structures offer the chance to control both the composition and the size of the core and the radial shell(s) separately. The band offset between the core and the shell(s) provides an effective way of radially confining carriers along the NW axis. Due to the specific structural design of the core-shell NWs, various devices including light-emitting diodes, laser diodes, field-effect transistors, solar cells and sensors can be realized.
Growth and analysis
An epitaxial radial AlGaAs shell is formed on the GaAs core by the vapour-solid growth mechanism where the shell growth rate increases with higher aluminium content, which is attributed to the shorter diffusion length of aluminium adatoms compared to gallium adatoms. In parallel with the radial AlGaAs shell growth there is also some axial AlGaAs growth taking place on top of the GaAs nanowire core forming an AlGaAs layer at the top of the nanowire.
Selected area electron diffraction patterns (see image) reveal a wurtzite crystal structure both in the GaAs core and in the AlGaAs shell. PL spectra of the same single GaAs/AlGaAs core-shell nanowire at 4.4 K and at room temperature are also shown in the figure (top left). We note here that the emission energy (1.489 eV) from the core GaAs is lower than the free exciton energy (1.515 eV) of zinc-blende GaAs. This is probably due to the recombination from the type-II band alignment that can occur due to a stacking fault induced by GaAs zinc-blende segments sandwiched in-between a dominating GaAs wurtzite structure.
The existence of AlGaAs-related PL peaks from the GaAs/AlGaAs core-shell nanowires suggests that there is room for improvement of the growth, especially by reducing the axial growth of AlGaAs.
The researchers presented their work in the journal Nanotechnology.
About the author
Hailong Zhou PhD is now a postdoc in the Robert F Hicks group at the Chemical and Biomolecular Engineering Department, University of California, Los Angeles, US. He is currently exploring self-catalyzed vapour-liquid-solid growth of InP and InAsP nanowires by metalorganic vapour phase deposition, as well as characterizing the structures and progressing their application in electronic and photonic devices.