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 (400 µeV) in the photoluminescence spectra of GaN nanowire ensembles grown on Si(111) substrates at very high (>850 ∘C) 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 2×1016cm−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

Importance of the dielectric contrast for the polarization of excitonic transitions in single GaN nanowires

“…This large width partly stems from the surface-induced distortion of the wavefunction of donor-bound excitons [17,18] and results in a spectral overlap of the transitions involved in the radiative decay of excitons, thus complicating the quantitative analysis of their optical polarization. We have recently reported a significant reduction of the contribution of surface effects to the emission broadening for GaN nanowires grown on Si(111) at a temperature larger than 850°C [19]. While the morphology of these samples is similar to those observed for nanowires formed at lower temperatures, the donor-bound exciton line in our GaN nanowire ensembles exhibits a linewidth of about 0.5 meV at 10 K [19].…”

“…We have recently reported a significant reduction of the contribution of surface effects to the emission broadening for GaN nanowires grown on Si(111) at a temperature larger than 850°C [19]. While the morphology of these samples is similar to those observed for nanowires formed at lower temperatures, the donor-bound exciton line in our GaN nanowire ensembles exhibits a linewidth of about 0.5 meV at 10 K [19].…”

“…The ensemble was grown at 865°C using a Ga/N flux ratio higher than one to compensate for the high desorption rate of Ga [23]. The combination of a high substrate temperature and a high Ga flux induces melt-back etching of the Si substrate due to the formation of a Ga-Si eutectic alloy [19]. The melt-back etching of the Si substrate causes an enhanced incorporation of Si in the nanowires as discussed in detail in [19].…”

High-Temperature Growth of GaN Nanowires by Molecular Beam Epitaxy: Toward the Material Quality of Bulk GaN

“…21,25,30−32 For instance, it has been shown previously that the introduction of an AlN buffer layer favors GaN nucleation on Si substrates. 25,30,32 Therefore, for a single-step growth approach and identical impinging fluxes, one would expect to achieve higher growth temperatures on AlN-buffered Si substrates. Figure 2b demonstrates that this is actually the case.…”

“…Afterward, the substrates were exposed to an active N flux of Φ N = (11.0 ± 0.5) nm/min at the growth temperature for 10 min, except for those samples prepared on AlNbuffered Si. Details of the preparation of the AlN buffer layers can be found in ref 25. All samples were grown with Φ N = (11.0 ± 0.5) nm/ min.…”