Radial Stark effect in (In,Ga)N nanowires

TL;DR

This paper investigates the radial Stark effect in (In,Ga)N nanowires, revealing how built-in electric fields influence luminescence and suggesting potential for broadband solar absorption.

Contribution

It demonstrates the impact of radial electric fields on luminescence in (In,Ga)N nanowires and links the Stark effect to broadband absorption for solar energy applications.

Findings

Radial electric fields cause a Stark shift in nanowire emission.

Luminescence is dominated by surface recombination at elevated temperatures.

Radial Stark effect contributes to broadband visible absorption.

Abstract

We study the luminescence of unintentionally doped and Si-doped In$_x$Ga$_{1-x}$N nanowires with a low In content (x<0.2) grown by molecular beam epitaxy on Si substrates. The emission band observed at 300 K from the unintentionally doped samples is centered at much lower energies (800 meV) than expected from the In content measured by x-ray diffractometry and energy dispersive x-ray spectroscopy. This discrepancy arises from the pinning of the Fermi level at the sidewalls of the nanowires, which gives rise to strong radial built-in electric fields. The combination of the built-in electric fields with the compositional fluctuations inherent to (In,Ga)N alloys induces a competition between spatially direct and indirect recombination channels. At elevated temperatures, electrons at the core of the nanowire recombine with holes close to the surface, and the emission from unintentionally doped nanowires exhibits a Stark shift of several hundreds of meV. The competition between spatially direct and indirect transitions is analyzed as a function of temperature for samples with various Si concentrations. We propose that the radial Stark effect is responsible for the broadband absorption of (In,Ga)N nanowires across the entire visible range, which makes these nanostructures a promising platform for solar energy applications.