LO phonon-plasmon coupled mode in hexagonal InGaN alloy

Raman scattering spectra of hexagonal InGaN/GaN film, excited with 532 and 488 nm visible laser lines and 325 nm UV laser line, were investigated at room temperature and 78 K. The sample was grown by metalorganic chemical vapor deposition on a sapphire substrate. Excited with 532 and 488 nm visible laser lines, the E2 and A1(LO) modes were observed at about 571.3 and 736.4 cm-1, respectively. These scattering signals mainly originate from GaN layer. Excited with 325 nm UV laser line, the E2 mode shifts to 569.7 cm-1, while A1(LO) mode shifts to 730.3 cm-1 and resonance is enhanced. These scattering signals originate from InGaN layer. A broad feature at high frequency of A1(LO) mode was observed in spectroscopy excited with visible laser line, and was attributed to the LO phonon-plasmon coupled mode of InGaN layer. The electron concentration of InGaN film determined from the frequency of the coupled mode is n(e) = 1.61 × 1018 cm-3. Excited with 325 nm UV laser light, the LO phonon-plasmon coupled mode is absent, and the scattering signal of A1(LO) mode mainly originates from the surface depletion layer of the sample. The thickness of the surface depletion layer is about 40 nm. Furthermore, the scattering intensities of LO phonon-plasmon coupled mode at room temperature and 78 K were compared and analyzed. The screening wave vectors of plasmon at different temperatures were calculated. At low temperature, the screening wave vector increases, the damping of the plasmon decreases, so the LO phonon-plasmon coupled mode becomes stronger. This work is helpful in understanding the property of InGaN and in developing the optoelectronic devices of nitride.