Enhanced Optical Absorption of GaAs Near-Band-Edge Transitions in GaAs/AlGaAs Core-Shell Nanowires: Implications for Nanowire Solar Cells

Dense arrays of core-shell nanowires possess great potential as superabsorptive media for the fabrication of efficient solar cells. We report on GaAs near-band-edge absorption properties of free-standing GaAs- AlGaAs core-shell nanowires having different shell thicknesses, by detailed line-shape analyses of room-temperature photoreflectance (PR) spectra, employing first-derivative Gaussian and Lorentzian models of the GaAs complex dielectric function. Line-shape analyses of the nanowire PR spectra returned a doublet of resonance lines at energies between 1.410 and 1.422 eV, ascribed to strain-split heavy-and light-hole exciton absorption transitions in the GaAs nanowire cores. The optical oscillator strengths of exciton resonances evaluated by Lorentzian analyses of PR features showed a significant enhancement (up to 30x) of GaAs band-edge optical absorption in nanowires with respect to the reference planar structure. Additionally, values of integrated Lorentzian moduli were normalized to the total GaAs core volume fill fraction (estimated in the range 0.5-7.0% with respect to a planar layer of the same height) within each nanowire ensemble, achieving a first ever experimental estimate of the GaAs near band-edge absorption enhancement factor for GaAs-AlGaAs core-shell nanowires in the range 22-190, depending on the nanowire inner core-shell structure. Such strong absorption enhancement is ascribed to improved wave-guiding of incident light into the GaAs cores by the surrounding AlGaAs shell (its average thickness being estimated between similar to 14 and 100 nm in the present nanostructures).