Sub-meV linewidth in GaN nanowire ensembles: absence of surface excitons due to the field-ionization of donors

Abstract

We observe unusually narrow donor-bound exciton transitions (0.4 meV) in the photoluminescence spectra of GaN nanowire ensembles grown on Si(111) substrates at very high (> 850 degrees Celsius) temperatures. The spectra of these samples reveal a prominent transition of excitons bound to neutral Si impurities which is not observed for samples grown under standard conditions. Motivated by these experimental results, we investigate theoretically the impact of surface-induced internal electric fields on the binding energy of donors by a combined Monte Carlo and envelope function approach. We obtain the ranges of doping and diameter for which the potential is well described using the Poisson equation, where one assumes a spatially homogeneous distribution of dopants. Our calculations also show that surface donors in nanowires with a diameter smaller than 100 nm are ionized when the surface electric field is larger than about 10 kV/cm, corresponding to a doping level higher than 2 x 10^16 cm^-3. This result explains the experimental observation: since the (D+,X) complex is not stable in GaN, surface-donor-bound excitons do not contribute to the photoluminescence spectra of GaN nanowires above a certain doping level, and the linewidth reflects the actual structural perfection of the nanowire ensemble.

References in corpus (6)

• A growth diagram for plasma-assisted molecular beam epitaxy of GaN nanocolumns on Si(111)
• Spontaneous nucleation and growth of GaN nanowires: Fundamental role of crystal polarity
• Self-regulated radius of spontaneously formed GaN nanowires in molecular beam epitaxy
• Correlation between the structural and optical properties of spontaneously formed GaN nanowires: a quantitative evaluation of the impact of nanowire coalescence
• Statistical analysis of the shape of one-dimensional nanostructures: determining the coalescence degree of spontaneously formed GaN nanowires
• Investigating the origin of the nonradiative decay of bound excitons in GaN nanowires

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• Radial Stark effect in (In,Ga)N nanowires
• Improved control over spontaneously formed GaN nanowires in molecular beam epitaxy using a two-step growth process
• External control of GaN band bending using phosphonate self-assembled monolayers
• Absence of quantum-confined Stark effect in GaN quantum disks embedded in (Al,Ga)N nanowires grown by molecular beam epitaxy
• Enhanced radiative efficiency in GaN nanowires grown on sputtered TiN${}_{\mathbf{x}}$: effects of surface electric fields