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We investigate the 3.45-eV luminescence band of spontaneously formed GaNnanowires on Si(111) by photoluminescence and cathodoluminescence spectroscopy.This band is found to be particularly prominent for samples synthesized atcomparatively low temperatures. At the same time, these samples exhibit apeculiar morphology, namely, isolated long nanowires are interspersed within adense matrix of short ones. Cathodoluminescence intensity maps reveal the3.45-eV band to originate primarily from the long nanowires. Transmissionelectron microscopy shows that these long nanowires are either Ga polar and arejoined by an inversion domain boundary with their short N-polar neighbors, orexhibit a Ga-polar core surrounded by a N-polar shell with a tubular inversiondomain boundary at the core/shell interface. For samples grown at hightemperatures, which exhibit a uniform nanowire morphology, the 3.45-eV band isalso found to originate from particular nanowires in the ensemble and thuspresumably from inversion domain boundaries stemming from the coexistence of N-and Ga-polar nanowires. For several of the investigated samples, the 3.45-eVband splits into a doublet. We demonstrate that the higher-energy component ofthis doublet arises from the recombination of two-dimensional excitons free tomove in the plane of the inversion domain boundary. In contrast, thelower-energy component of the doublet originates from excitons localized in theplane of the inversion domain boundary. We propose that this in-planelocalization is due to shallow donors in the vicinity of the inversion domainboundaries.

Nature of excitons bound to inversion domain boundaries: Origin of the 3.45-eV luminescence lines in spontaneously formed GaN nanowires on Si(111)