Phonons and free carriers in strained hexagonal GaN/AlGaN superlattices measured by infrared ellipsometry and Raman spectroscopy

Phonon and free-carrier effects in strained hexagonal (α) {GaN}l−{AlxGa1−xN}m superlattice (SL) heterostructures are studied by infrared spectroscopic ellipsometry (IRSE) and micro (μ)-Raman scattering. Growth of the heterostructures was performed by metal–organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy on (0 0 0 1) sapphire. Unstrained 0.5–1 μm-thick α-GaN layers were deposited prior to the SLs. SL phonon modes are identified combining results from both IRSE and μ-Raman techniques. The average compressive SL stress can be estimated from the shift of the GaN-sublayer phonon modes. The IRSE data are sensitive to free carriers within the GaN sublayers. For the MOVPE grown SL structures, the free-carrier mobility is anisotropic which indicates vertical carrier confinement.