Illuminating the second conduction band and spin-orbit energy in single wurtzite InP nanowires.

We use polarized photoluminescence excitation spectroscopy to observe the energy and symmetry of the predicted second conduction band in 130 nm diameter wurtzite InP nanowires. We find direct spectroscopic signatures for optical transitions among the A, B, and C hole bands and both the first and the second conduction bands. We determine that the splitting between the first and second conduction bands is 228 ± 7 meV in excellent agreement with theory. From these energies we show that the spin-orbit energy changes substantially between zinc blende and wurtzite InP. We discuss the two quite different solutions within the quasi-cubic approximation and the implications for these measurements. Finally, the observation of well-defined optical transitions between the B- and C-hole bands and the second conduction band suggests that either the theoretical description of the second conduction band as possessing Γ8 symmetry is incomplete, or other interactions are enabling these forbidden transitions.

Unexpected benefits of rapid growth rate for III-V nanowires.

In conventional planar growth of bulk III-V materials, a slow growth rate favors high crystallographic quality, optical quality, and purity of the resulting material. Surprisingly, we observe exactly the opposite effect for Au-assisted GaAs nanowire growth. By employing a rapid growth rate, the resulting nanowires are markedly less tapered, are free of planar crystallographic defects, and have very high purity with minimal intrinsic dopant incorporation. Importantly, carrier lifetimes are not adversely affected. These results reveal intriguing behavior in the growth of nanoscale materials, and represent a significant advance toward the rational growth of nanowires for device applications.