Strain effects and phonon-plasmon coupled modes in Si-doped AlN

The E _2h and A ₁LO) phonon modes of AlN films grown on sapphire are analyzed by Raman scattering as a function of silicon doping for concentrations covering from 5.5 × 10 ¹⁹ cm ^-3 to 5.2 × 10 ²¹ cm ^-3. For high doping levels the appearance of a mode around 520 cm ^-1 indicates the precipitation of crystalline silicon in the samples and its inhomogeneous incorporation to the AlN layer. The frequency of this mode shifts to lower energies with doping, indicating that the silicon crystals are embedded in the AlN lattice and under tensile strain. On the other hand, the AlN phonon modes are blue-shifted due to the compressive strain as a result of the silicon incorporation. This strain is partially relieved for very high doping levels. The coupling of the polar A ₁LO) mode of the AlN films with the electronic cloud (phonon-plasmon coupling) is analyzed for electron concentrations up to 4 × 10 ¹⁷ cm ^-3. A 2 cm ^-1 blue shift of the phonon frequencies is observed for the highest electron concentrations.