Optical absorption of composition-tunable InGaAs nanowire arrays.

InGaAs nanowire (NW) arrays have emerged as important active materials in future photovoltaic and photodetector applications, due to their excellent electronic properties and tunable band gap. Here, we report a systematic investigation of the optical absorption characteristics of composition-tunable vertical InGaAs NW arrays. Using finite-difference time-domain simulations we first study the effect of variable composition (Ga-molar fraction) and NW array geometry (NW diameter, period, fill factor) on the optical generation rate. NWs with typical diameters in the range of ∼100-250 nm lead to generation rates higher than the equivalent bulk case for moderate fill factors (NW period of ∼0.3-0.8 μm), while slightly smaller fill factors and increased diameters are required to maintain high generation rates at increased Ga-molar fraction. The optical absorption was further measured using spectrally resolved ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy on NW arrays transferred to transparent substrates. Interestingly, large variations in Ga-molar fraction (0 < x(Ga) < 0.5) have a negligible influence, while minute changes in NW diameter of less than ±20 nm affect the absorption spectra very strongly, leading to pronounced shifts in the peak absorption energies by more than ∼700 meV. These results clearly highlight the much larger sensitivity of the optical absorption behavior to geometric parameters rather than to variations in the electronic band gap of the underlying NW array.