The possibility to tune the bandgap of (In,Ga)N across the whole visible spectral range makes this alloy attractive for solar harvesting applications. Enhanced light conversion efficiencies are expected when (In,Ga)N is grown in the form of nanowires, since the nanowire geometry allows strain relaxation at the nanowire sidewalls as well as efficient light–matter coupling. However, the dynamics of charge carriers in these structures is complex, due to the compositional fluctuations inherent to (In,Ga)N alloys as well as the strong radial electric fields resulting from the pinning of the Fermi level at the sidewalls. It thus remains unclear if (In,Ga)N nanowires are beneficial for solar energy applications.
Figure: (Left) Schematic representation of the conduction and valence band profiles (CB and VB, respectively) across an (In,Ga)N nanowire. The parabolic potential arising from surface electric fields is superimposed with local potential fluctuations due to alloy disorder. As a result, electrons and holes localize independently, and spatially direct and indirect optical transitions are possible (blue and red arrows, respectively). (Right) Photoluminescence spectrum at 10 K taken on an ensemble of In0.06Ga0.94N nanowires. The bands at 3.2 and 2.4 eV arise from spatially direct and indirect transitions, respectively.
In this work, we demonstrate by photoluminescence and cathodoluminescence experiments that the combination of carrier localization and surface electric fields strongly modifies the optical properties of (In,Ga)N nanowires with a low In content. Compared to the bulk, In0.06Ga0.94N nanowires exhibit a strongly redshifted light emission (800 meV) together with light absorption throughout the visible range. These significant changes arise from the competition between spatially direct and indirect recombination channels. The broadband light absorption in (In,Ga)N nanowires makes them attractive for solar energy applications while limiting the demand for the relatively scarce element In. In the specific case of solar water splitting, the redshift of the bandedge in In0.06Ga0.94N nanowires thanks to the radial Stark effect should lead to an increase in the theoretical solar-to-hydrogen conversion efficiency from 1 to 10%