Phonons and free carriers in a strained hexagonal GaN-AlN superlattice measured by infrared ellipsometry and Raman spectroscopy

Phonon and free-carrier effects in a strained hexagonal (α) {GaN}_l-{AlN}_m superlattice (SL) heterostructure (l = 8 nm, m = 3 nm) 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) on (0001) sapphire. An unstrained 1 μm-thick α-GaN layer was deposited prior to the SL. SL phonon modes are identified combining results from both IRSE and μ-Raman techniques. The shift of the GaN-sublayer phonon modes is used to estimate an average compressive SL stress of σ_xx ∼ - 4.3 GPa. The IRSE data reveal a free-carrier concentration of n_e ∼ 5×10 ¹⁸ cm^-3 within the undoped SL GaN-sublayers. According to the vertical carrier confinement, the free-carrier mobility is anisotropic, and the lateral mobility (μ_⊥ ∼ 400 cm²/Vs, polarization E⊥c-axis) exceeds the vertical mobility (μ_∥ ∼ 24 cm²/Vs, E∥c) by one order of magnitude.