Interplay between GaN and AlN sublattices in wurtzite AlxGa1−xN alloys revealed by Raman spectroscopy

Abstract

The large frequency shift displayed by the longitudinal optical ͑LO͒ phonons A1(LO) and E1(LO) when going from GaN to AlN promises an accurate determination of the composition in Al x Ga 1Ϫx N bulk layers by Raman spectroscopy. However, this determination is affected by a large uncertainty for low Al mole fractions (xϽ0.20), due to the broadened spectral line shape exhibited by these modes. A detailed study of Raman spectra recorded on layers with xϽ0.27, grown either on sapphire or on silicon substrates, has been performed in order to elucidate the origin of that broadening. The influence on the A1(LO) line shape of the sapphire substrate modes, compositional inhomogeneities, residual strain, and those effects inherent to the lattice dynamics of ternary alloys, is analyzed. We conclude that the broadening is caused by intrinsic inhomogeneities of the microscopic polarization fields resulting from alloying. This effect is usually obscured in other III-V compounds, such as arsenides or phosphides, due to their considerably lower LO-transverse optical splitting.

Resonant Raman characterization of InAlGaN/GaN heterostructures

“…The evolution of the full width at half maximum (FWHM) of the phonon mode at Γ with alloy composition is shown in the inset of the figure. The width of the phonon mode increases rapidly with In content in a similar way as in In x Ga 1-x N and Al y Ga 1-y N samples [6,7], while the expected reduction of FWHM due to the lower Al content is not noticeable. Second, we observe a shift to lower energies of the A 1 (LO) mode with increasing In content.…”

AlGaN/GaN HEMT-based ultraviolet photodetectors with enhanced device efficiency

“…The slight variation in the AlxGaN 𝐴 ! O\ position is due to the different buffer geometries and Al mole fraction in the three samples [136]. In addition to these phonon peaks, the spectra show two interesting peaks named IF1 and IF2, referring to interface phonons 1 and 2.…”