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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 cm 2 /Vs, polarization  E ⊥ c -axis) exceeds the vertical mobility (µ ||  ∼ 24 cm 2 /Vs,  E || c ) by one order of magnitude.

Phonons and Free Carriers in a Strained Hexagonal GaN-AlN Superlattice Measured by Infrared Ellipsometry and Raman Spectroscopy