Enhanced light scattering of the forbidden longitudinal optical phonon mode studied by micro-Raman spectroscopy on single InN nanowires

Abstract

In the literature, there are controversies on the interpretation of the appearance in InN Raman spectra of a strong scattering peak in the energy region of the unscreened longitudinal optical (LO) phonons, although a shift caused by the phonon-plasmon interaction is expected for the high conductance observed in this material. Most measurements on light scattering are performed on ensembles of InN nanowires (NWs). However, it is important to investigate the behavior of individual nanowires and here we report on micro-Raman measurements on single nanowires. When changing the polarization direction of the incident light from parallel to perpendicular to the wire, the expected reduction of the Raman scattering was observed for transversal optical (TO) and E(2) phonon scattering modes, while a strong symmetry-forbidden LO mode was observed independently on the laser polarization direction. Single Mg- and Si-doped crystalline InN nanowires were also investigated. Magnesium doping results in a sharpening of the Raman peaks, while silicon doping leads to an asymmetric broadening of the LO peak. The results can be explained based on the influence of the high electron concentration with a strong contribution of the surface accumulation layer and the associated internal electric field.

InN Nanowires: Growth and Optoelectronic Properties

“…In contrast, other interpretations attribute this mode to the accumulation layer [45], or to a particular damping of the plasmonic system [45,46]. Non-polar and unscreened modes are observed in Raman spectra on single NWs almost independently of the scattering geometry [47] ruling out the interpretation given in [43,44] and supporting the interpretation of [45,46]. Furthermore, the absence of the peak in the out of resonance UV spectra [48] supports the latest findings.…”

Recent Advances on p-Type III-Nitride Nanowires by Molecular Beam Epitaxy

“…Mg-doped InN nanowires have been investigated previously [68,69]. Nevertheless, these Mg-doped InN nanowires have remained n-type.…”

Photoluminescence properties of Mg-doped InN nanowires

“…Although extensive studies have been performed on Mg-doped InN films, 6-12 the achievement of p-type conduction has remained elusive, due to the presence of high background electron concentration and surface electron accumulation. 13,14 In addition, InN nanowires grown by conventional methods face the same challenges as in InN thin films, [15][16][17][18][19] and there have been very few studies on Mg-doped InN nanowires. 17,18 Recently, with the dramatically improved molecular beam epitaxial (MBE) growth process, 20 intrinsic InN nanowires, with the absence of electron accumulation on the lateral surfaces (m-planes), have been demonstrated.…”

“…13,14 In addition, InN nanowires grown by conventional methods face the same challenges as in InN thin films, [15][16][17][18][19] and there have been very few studies on Mg-doped InN nanowires. 17,18 Recently, with the dramatically improved molecular beam epitaxial (MBE) growth process, 20 intrinsic InN nanowires, with the absence of electron accumulation on the lateral surfaces (m-planes), have been demonstrated. [21][22][23] The removal of surface electron accumulation and the resulting extremely low background electron concentration 4 provides a great promise to the realization of p-type InN nanowires, which was reported recently.…”

“…It can be seen that the nanowires exhibit non-tapered wurtzite structure with welldefined hexagonal morphology, which is in contrast to the commonly reported tapered InN nanowires. [15][16][17][18][19] Figure 1(b) shows a high-resolution transmission electron microscopy (TEM) image of the root region of a typical Mg-doped InN nanowire, which exhibits no stacking faults and misfit dislocations, further indicating high crystalline quality.…”