Bandgap tuning in GaAs_1−xSb_xaxial nanowires grown by Ga-assisted molecular beam epitaxy

Abstract

In this work we present a comprehensive study on the effects of Sb incorporation on the composition modulation, structural and optical properties of self-assisted axial GaAs 1−x Sb x nanowires of 2-6 μm in length grown on (111) Si substrate by molecular beam epitaxy. The Sb composition in the GaAs 1−x Sb x axial nanowire (NW) was varied from 2.8-16 at.%, as determined from energy dispersive x-ray spectroscopy. Lower Sb composition leads to thinner nanowires and inhomogeneous Sb composition distribution radially with a depleted Sb surface region inducing weak type-II optical emission, the presence of an additional peak at higher Bragg angle in the x-ray diffraction spectra and an electric-field-induced strong Raman LO mode. Higher Sb composition of 16 at.% leads to a more uniform Sb compositional distribution radially leading to type-I optical transitions exhibiting the lowest PL peak energy occurring at 1.13 eV. In addition, the high quality of these nanowires exhibiting pure zinc blende crystal structure, largely free of any planar defects, is borne out by high resolution transmission electron microscopy and selected area diffraction patterns. The shift and broadening of the Raman LO and TO modes reveal evidence of increased Sb incorporation in the nanowires. Significant improvement in optical characteristics was achieved by the incorporation of a Al 0.2 Ga 0.8 As passivating shell. The results are very promising and reveal the potential to further red shift the optical emission wavelength by fine tuning of the fluxes during growth.

A Two-Step Growth Pathway for High Sb Incorporation in GaAsSb Nanowires in the Telecommunication Wavelength Range

“…13b in which the relative intensity of the ratio of the LV mode to the normalized TO mode has been plotted as a function of Sb concentration in the NWs. A linear fit resulted in a slope of 2.23, which is comparatively higher than the slope of ~1.2 reported in the literature 20, 22 . The explanation for the observed larger slope in our work is as follows.…”

“…12b,c) accompanied with a larger FWHM (Fig. 12d) of the spectra for 34 at.% Sb, attesting to the increased incorporation of larger atomic size Sb in the lattice that leads to enhanced disorder-induced strain 22, 33 .

Figure 12XRD spectra of GaAs 1−x Sb x NWs for ( a ) different Sb compositions; ( b ) and ( c ) represent the corresponding GaAsSb (111) and GaAsSb (222) Bragg peaks; ( d ) represent the change in FWHM with Sb at.% variation in the NWs.

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“…With increasing Sb content to 34 at.%, both TO and LO mode signals are broadened with a maximum red shifts to 254 cm −1 and 275 cm −1 , respectively. This red shift of the phonon modes is influenced by different material characteristics, namely mass disorder, change in the dielectric medium and strength of the ionic plasma coupling associated with alloying 20, 22 , and both LO and TO modes increase with higher Sb composition in the NWs. The broadening of the modes with Sb composition is indicative of a higher degree of disorder.…”

Recent Progress on the Gold-Free Integration of Ternary III–As Antimonide Nanowires Directly on Silicon

“…Sb-induced morphological transformation is not limited to InAsSb NWs and has also been observed in GaAsSb NWs [ 60 , 61 ]. Ren et al [ 46 ] observed that the morphologies of MBE grown GaAsSb NWs are significantly influenced by the Sb flux, whereas, the diameter of the NW increased, the lengths decreased with increasing Sb flux.…”

“…A comprehensive study of the effect of Sb incorporation on the composition modulation, structural and optical properties of self-assisted GaAsSb NWs on (111) Si substrate was conducted by Ahmad et al [ 61 ] in 2017 using MBE. The NWs exhibited a pure zinc blende crystal structure, largely free of any planar defects with inverse dependence of NWs density as a function of Sb flux, which was associated with the surfactant effect of Sb and droplet size-dependent Gibbs–Thomson effects [ 62 ].…”