Abstract
Using out-of-plane and in-plane X-ray diffraction techniques, we have investigated the structure at the interface between GaAs nanowires [NWs] grown by Au-assisted molecular beam epitaxy and the underlying Si(111) substrate. Comparing the diffraction pattern measured at samples grown for 5, 60, and 1,800 s, we find a plastic strain release of about 75% close to the NW-to-substrate interface even at the initial state of growth, probably caused by the formation of a dislocation network at the Si-to-GaAs interface. In detail, we deduce that during the initial stage, zinc-blende structure GaAs islands grow with a gradually increasing lattice parameter over a transition region of several 10 nm in the growth direction. In contrast, accommodation of the in-plane lattice parameter takes place within a thickness of about 10 nm. As a consequence, the ratio between out-of-plane and in-plane lattice parameters is smaller than the unity in the initial state of growth. Finally the wurtzite-type NWs grow on top of the islands and are free of strain.
Highlights
Semiconductor nanowires [NWs] are of particular interest due to the ability to synthesize single-crystalline onedimensional [1-D] epitaxial structures and heterostructures in the nanometer range
One route of NW growth is the vaporliquid-solid [VLS] mode realized by metal-organic vapor phase epitaxy [6] or molecular beam epitaxy [MBE] by a solution from a molten eutectic alloy formed by a metallic seed
It was found that nearly any AIIIBV semiconductor material can be grown as NWs onto another AIIIBV or group IV (111) substrate independent from the lattice mismatch [7]
Summary
Semiconductor nanowires [NWs] are of particular interest due to the ability to synthesize single-crystalline onedimensional [1-D] epitaxial structures and heterostructures in the nanometer range. For gold-assisted NW growth, several authors reported on alloy formation between the seed and the precursor material at the substrate/NW interface [8,9]. Reflection high-energy electron diffraction and high-resolution transmission electron microscopy [HRTEM] could identify the evolution of the crystal structure to be zinc blende [ZB] in traces and islands but wurtzite [WZ] in NWs, the strain release between the substrate and NWs could not be quantified. This missing information motivated additional high-resolution X-ray diffraction [HRXRD] measurements at similar
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