Changing the properties of GaAs via strain engineering in core/shell nanowires

III-V compound semiconductors have fueled many breakthroughs in photonics owing to their direct optical band gap and the possibility to tailor it in ternary or quaternary alloys by selecting the chemical composition appropriately. More recently, III-V semiconductors in the form of free-standing nanowires have found new strengths for a wide range of future applications in nanotechnology, including nano-photonics. Here we explore the great possibilities for strain engineering in core/shell nanowires as an alternative route to tailor the optical band gap of III-V semiconductors without changing their chemical composition. In particular, we demonstrate that the GaAs core in GaAs/InxGa1-xAs or GaAs/InxAl1-xAs core/shell nanowires can sustain unusually large misfit strains that would have been impossible in equivalent thin-film heterostructures, and undergoes a significant modification of its electronic proper-ties.

Core/shell nanowires were grown in the self-catalyzed mode on SiOx/Si(111) substrates by molecular beam epitaxy [1, 2]. Strain analysis was performed using synchrotron X-ray diffraction and Raman scat-tering spectroscopy, and showed that for a thin enough core, the magnitude and the spatial distribution of the built-in misfit strain can be regulated via the composition and the thickness of the shell. Beyond a critical shell thickness, we obtain a heavily tensile-strained core and an almost strain-free shell. The tensile strain of the core exhibits a predominantly-hydrostatic character and causes the reduction of the GaAs band gap energy (Figure 1) in accordance with our theoretical predictions using deformation-potential theory and first-principle calculations. For 7 % of strain (x = 0.54), the band gap energy was reduced to 0.87 eV at 300 K, i.e. a remarkable reduction of 40 %. This is particularly important for ap-plications in optical fiber telecommunications because the emission from strained GaAs nanowires can now cover the O-band and potentially the S-band of telecommunication wavelengths.

Besides the optical band gap, a similar reduction is expected for the effective mass of free electrons in tensile-strained GaAs. The corresponding electron mobility was estimated by time-domain terahertz spectroscopy to be in the range of 4000 – 5000 cm2/V·s at 300 K (core diameter = 22 nm, x = 0.39–0.49). These values are the highest reported, even in comparison to GaAs/AlxGa1-xAs nanowires with double the core thickness. This means that high-mobility transistors could now be possible with strained GaAs nanowires.

All in all, our results demonstrate that strained GaAs in core/shell nanowires can resemble the electronic properties of InxGa1-xAs, which makes it suitable for near-infrared nano-photonics. The use of a binary alloy instead of a ternary one would be advantageous because phenomena like phase separation, surface segregation or alloy disorder that typically exist in ternary alloys and limit the performance of photonic or electronic devices, become now irrelevant.