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In the present study we combined, in the same synchrotron x-ray experiment, reciprocal space mapping, multiwavelength anomalous diffraction and diffraction anomalous fine structure, to determine the strain, crystallographic polarity, alloy composition and ordering at the atomic scale in [0001]-oriented AlGaN nanowires grown by molecular beam epitaxy on GaN nanowire bases. The information that we obtained was averaged over a macroscopic ensemble of NWs. We found from the diffraction anomalous fine structure that there were an isotropic increased number of Ga-Ga pairs in the Ga next nearest coordination shell (cation sublattice) with respect to what is expected for the AlGaN alloy composition determined by anomalous diffraction. This significant deviation from random alloy atomic distribution is present whatever the AlN molar fraction and growth conditions. Our results are consistent with nanoscale composition fluctuations expected from both alloy disorder or kinematically driven spontaneous ordering, both effects being suspected to account for the physical properties of AlGaN ternary alloys.

©2019 American Chemical Society Artículo firmado por más de diez autores. This work has been supported by the bilateral collaboration program Pessoa and the project NASIB funded by FCT and FEDER (PTDC/CTM-CTM/28011/2017, LISBOA-01-0145-FEDER-028011). M.P. acknowledges support by FCT Portugal through grant SFRH/BPD/111285/2015. B.M. and E.N. acknowledge support by MINECO (projects no MAT 2015-65274-R-FEDER, no MAT 2016-81720-REDC). A.R.C. acknowledges the grant by Ramon y Cajal programs (under contract number RYC-2015-18047). The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement 312483-ESTEEM2 (Integrated Infrastructure Initiative-I3), the ITN project "Supporting Postgraduate Research with Internships in industry and Training Excellence (SPRITE)", and the infrastructure project "Research And Development with Ion Beams Advancing Technology in Europe (RADI-ATE)". We thank I. S. Rogan (KAUST, Saudi Arabia) for the MOCVD GaN reference layers, M. Bockowski (Unipress, Poland) for the HTHP annealing of the thin film reference sample, and I. Kieffer (FAME beamline, ESRF, France) and E. Salas-Colera (SPLINE beamline, ESRF, France) for their help in recording good-quality XAFS spectra and ESRF for the allocated beamtime (MA-2151, 30-02-1067, MA-2964)., Rare earth (RE)-doped GaN nanowires (NWs), combining the well-defined and controllable optical emission lines of trivalent RE ions with the high crystalline quality, versatility, and small dimension of the NW host, are promising building blocks for future nanoscale devices in optoelectronics and quantum technologies. Europium doping of GaN NWs was performed by ion implantation, and structural and optical properties were assessed in comparison to thin film reference samples. Despite some surface degradation for high implantation fluences, the NW core remains of high crystalline quality with lower concentrations of extended defects than observed in ion-implanted thin films. Strain introduced by implantation defects is efficiently relaxed in NWs and the measured deformation stays much below that in thin films implanted in the same conditions. Optical activation is achieved for all samples after annealing, and while optical centers are similar in all samples, Eu^3+ emission from NW samples is shown to be less affected by residual implantation damage than for the case of thin films. The incorporation of Eu in GaN NWs was further investigated by nano-cathodoluminescence and X-ray absorption spectroscopy (XAS). Maps of the Eu-emission intensity within a single NW agree well with the Eu-distribution predicted by Monte Carlo simulations, suggesting that no pronounced Eu-diffusion takes place. XAS shows that 70-80% of Eu is found in the 3+ charge state while 20-30% is 2+ attributed to residual implantation defects. A similar local environment was found for Eu in NWs and thin films: for low fluences, Eu is mainly incorporated on substitutional Ga-sites, while for high fluences XAS points at the formation of a local EuN-like next neighbor structure. The results reveal the high potential of ion implantation as a processing tool at the nanoscale., Unión Europea. FP7, Ministerio de Economía y Competitividad (MINECO), FCT/ FEDER, Unión Europea / ITN, FCT Portugal, Programa Ramón y Cajal, bilateral collaboration program Pessoa, Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub

Rare earth (RE)-doped GaN nanowires (NWs), combining the well-defined and controllable optical emission lines of trivalent RE ions with the high crystalline quality, versatility, and small dimension of the NW host, are promising building blocks for future nanoscale devices in optoelectronics and quantum technologies. Europium doping of GaN NWs was performed by ion implantation, and structural and optical properties were assessed in comparison to thin film reference samples. Despite some surface degradation for high implantation fluences, the NW core remains of high crystalline quality with lower concentrations of extended defects than observed in ion-implanted thin films. Strain introduced by implantation defects is efficiently relaxed in NWs and the measured deformation stays much below that in thin films implanted in the same conditions. Optical activation is achieved for all samples after annealing, and while optical centers are similar in all samples, Eu 3+ emission from NW samples is shown to be less affected by residual implantation damage than for the case of thin films. The incorporation of Eu in GaN NWs was further investigated by nano-cathodoluminescence and X-ray absorption spectroscopy (XAS). Maps of the Eu-emission intensity within a single NW agree well with the Eu-distribution predicted by Monte Carlo simulations, suggesting that no pronounced Eu-diffusion takes place. XAS shows that 70-80% of Eu is found in the 3+ charge state while 20-30% is 2+ attributed to residual implantation defects. A similar local environment was found for Eu in NWs and thin films: for low fluences, Eu is mainly incorporated on substitutional Ga-sites, while for high fluences XAS points at the formation of a local EuN-like next neighbor structure. The results reveal the high potential of ion implantation as a processing tool at the nanoscale.