Magnesium concentration of the investigated GaN NWs as a function of Mg BEP measured by secondary ion mass spectroscopy.
Raman line for undoped, Si-doped, and Mg-doped GaN NWs as well as for two undoped GaN NWs with different degrees of coalescence. A strain-induced shift is only observed for coalescing samples. A scanning electron microscopy (SEM) image of the fully coalesced sample (solid line) is shown in Fig. 6(d).
PL spectra over a wide energy range indicating the three energy regimes discussed in separate sections (sample a: undoped GaN NW; b: ; c: ).
Temperature dependent PL spectrum of the near band-edge regime of an undoped GaN NW sample, containing three bands at 3.41–3.42, 3.45, and 3.47 eV, respectively. Curves are vertically shifted for clarity.
Low temperature PL spectrum of undoped (sample a) and Si-doped GaN NW samples (sample b: ; c: ; d: ). For the three-band structure is no longer resolved.
PL spectra of four undoped GaN NW samples with increasing degree of coalescence from a to d measured at and their and corresponding top view SEM micrographs. Sample a is the undoped reference from Fig. 4.
PL spectra of the undoped reference sample (NW length 500 nm), one sample with long NWs (2400 nm) grown with , one Si-doped sample, and one Mg-doped sample. Curves are vertically shifted for clarity. Arrows indicate (A) the transition at 3.47 eV, (B) the 3.45 eV band, (C) the 3.41–3.42 eV band which originates at the substrate interface as can be seen by the suppression for the long NWs, (D) the DAP band at 3.27 eV which is strongly enhanced for Mg-doping, and (E) the 3.21 eV emission that is attributed to defects which occur due to coalescence for long NWs. The LO phonon energy (92 meV) is also given to mark replica of peaks (B) and (C).
Low temperature PL spectrum of several Mg-doped GaN NWs compared to an undoped reference. The is increased from a to c (sample a: ; b: ; c: ). For weak Mg-doping (samples a and b) the emission from acceptor-bound exciton recombination can be seen at 3.4655 eV.
PL in the DAP region for one undoped and four Mg-doped samples. The is increased from sample a to c (same samples as shown in Fig. 8).
Temperature dependent PL spectra of Mg-doped NWs grown with (same sample as curve a in Figs. 8 and 9).
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