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 ( alpha) {GaN}(l)-{ AlN}(m) superlattice (SL) heterostructure (l = 8 nm, m = 3 nm) are studied by infrared spectroscopic ellipsometry (IRSE) and micro (mu)-Raman scatteri ng. Growth of the heterostructures was performed by metal-organic vapor pha se epitaxy (MOVPE) on (0001) sapphire. An unstrained 1 mum-thick alpha -GaN layer was deposited prior to the SL. SL phonon modes are identified combin ing results from both IRSE and mu -Raman techniques. The shift of the GaN-s ublayer phonon modes is used to estimate an average compressive SL stress o f sigma (xx) similar to - 4.3 GPa. The IRSE data reveal a free-carrier conc entration of n(e) similar to 5x10(18) cm(-3) within the undoped SL GaN-subl ayers. According to the vertical carrier confinement, the free-carrier mobi lity is anisotropic, and the lateral mobility (mu (perpendicular to) simila r to 400 cm(2)/Vs, polarization E perpendicular toc-axis) exceeds the verti cal mobility (mu similar to 24 cm(2)/Vs, E parallel toc) by one order of ma gnitude.