Polarized and spatially resolved Raman scattering from composition-graded wurtzite InGaAs nanowires

We report Raman scattering from wurtzite single-crystalline InGaAs nanowires (NWs) to probe optical phonon behaviors associated with spatial grading in alloy composition along the NW length. Polarized Raman spectra revealed several optical phonons and their scattering symmetries: (i) InAs-like A₁(LO) and A₁(TO) phonons and (ii) GaAs-like A₁(LO), A₁(TO), and E₂(high) phonons. In addition, strong anisotropic behavior was observed in the Raman tensor elements of the A₁(TO) phonon mode. Interestingly, a spatial mapping of the GaAs-like A₁(TO) phonon along the NW length direction showed a systematic increase in energy from the NW top (∼255 cm^-1) to the midpoint (∼263 cm^-1), indicating an increase in the Ga mole fraction from about 0.5 to about 0.8. Further toward the NW bottom, the GaAs-like A ₁(TO) phonon energy saturated to the peak value at about 264 cm^-1. In the upper half of the NW, the phonon linewidths broadened significantly due to the spatial grading in In/Ga composition along the NW length. When the composition grading was negligible in the bottom half of the NW, the spectral widths were considerably narrowed. The GaAs-like E₂(high) phonon showed similar variations in both energy and spectral width along the NW length.