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Strong Near-Field Light–Matter Interaction in Plasmon-Resonant Tip-Enhanced Raman Scattering in Indium Nitride
Zitatschlüssel Poliani2020
Autor Poliani, Emanuele and Seidlitz, Daniel and Ries, Maximilian and Choi, Soo J. and Speck, James S. and Hoffmann, Axel and Wagner, Markus R.
Seiten 28178–28185
Jahr 2020
ISSN 1932-7447
DOI 10.1021/acs.jpcc.0c04549
Journal The Journal of Physical Chemistry C
Jahrgang 124
Nummer 51
Monat dec
Verlag American Chemical Society
Zusammenfassung We report a detailed study of the strong near-field Raman scattering enhancement, which takes place in tip-enhanced Raman scattering (TERS) in indium nitride. In addition to the well-known first-order optical phonons of indium nitride, near-field Raman modes, not detectable in the far-field, appear when approaching the plasmonic probe. The frequencies of these modes coincide with calculated energies of second-order combinational modes consisting of optical zone center phonons and acoustic phonons at the edge of the Brillouin zone. The appearance of strong combinational modes suggests that TERS in indium nitride represents a special case of Raman scattering in which a resonance condition on the nanometer scale is achieved between the localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs) of the probe with the surface charge oscillation of the material. We suggest that the surface charge accumulation (SCA) in InN, which can render the surface a degenerate semiconductor, is the dominating reason for the unusually large enhancement of the TERS signal as compared to other inorganic semiconductors. Thus, the plasmon-resonant TERS (PR-TERS) process in InN makes this technique an excellent tool for defect characterization of indium-rich semiconductor heterostructures and nanostructures with high carrier concentrations. ■ INTRODUCTION During the last 2 decades, group-III-nitride semiconductor hetero-and nanostructures have established a dominant role as versatile materials for solid-state light-emitting devices. The large tunability of the direct bandgap in the ternary alloys InGaN and AlGaN combined with the existence of shallow donor and acceptor dopants makes these ternary material systems the best available choices for optoelectronic applications ranging from the ultraviolet to the green spectral region. 1 The bottleneck, which prevents nitride-based devices to be available as longer-wavelength emitters, is the systematic reduction of the internal quantum efficiency (IQE) with increasing emission wavelength (green gap), as well as with increased drive current (droop). 2 Among the reasons for these limitations are hole localization, alloy composition fluctuations, and In clustering, resulting in an increase of nonradiative recombination mechanisms. 2,3 These effects become more pronounced for alloys with high In content and contribute to the limitations of long-wavelength light emitters based on the InGaN material system. Even in the case of pure InN, nanoclusters of metallic In may be present, which modify the electronic and optical properties of the material. 4−6 Raman spectroscopy is recognized as an essential tool for the characterization of strain, compositional fluctuations, and defects in semiconductors; however, it remains a diffraction-limited technique, which cannot access subwavelength information, i.e., it is not suitable for characterization on the nanometer scale. Optical near-field techniques are powerful solutions that circumvent these limitations and enable the characterization of nanostructures with few nanometers of spatial resolution. Tip-enhanced Raman spectroscopy (TERS) exploits the plasmonic and polaritonic properties of a metallic tip to enhance and localize the electromagnetic field, which interacts with the sample. 7 While the majority of TERS studies focus on the vibrational properties of molecules and clusters, 8−10 only few works exist that address near-field Raman scattering in inorganic semiconductors and their nanostructures.
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