Raman scattering study of the long-wavelength longitudinal-optical-phonon–plasmon coupled modes in high-mobility InN layers

Abstract

We have studied the longitudinal-optical (LO)-phonon--plasmon coupled modes in high-mobility InN layers with free-electron densities ranging from $2.3\ifmmode\times\else\texttimes\fi{}{10}^{18}$ to $1.6\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3}$ by means of Raman scattering. The observed ${L}^{\ensuremath{-}}$ coupled-mode peak displays the usual behavior for the low energy branch of the long-wavelength coupled modes, increasing in frequency and phononlike character as the electron density increases. The ${L}^{\ensuremath{-}}$ mode behavior can be satisfactorily explained by the standard dielectric model developed by Hon and Faust which takes into account wave-vector conserving scattering processes governed by the dipole-allowed deformation potential and electro-optic mechanisms. The free-electron density obtained from line-shape fits to the ${L}^{\ensuremath{-}}$ peak agrees well with Hall-effect measurements. The ${E}_{2}^{\text{high}}$ mode shifts to lower frequencies as the electron density increases, suggesting that strain relaxation has a bearing on the residual electron density in the InN layers. The ${L}^{\ensuremath{-}}$ frequency exhibits also a dependence on the excitation wave vector, which further indicates that wave-vector conserving scattering by LO-phonon--plasmon coupled modes takes place in these high-mobility samples. The presence of a relatively strong LO signal is attributed to surface-field-induced scattering in the accumulation region.