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 (alpha) {GaN}(l) - {A lxGa1-xN}(m) superlattice (SL) heterostructures are studied by infrared spe ctroscopic ellipsometry (IRSE) and micro (mu)-Raman scattering. Growth of t he heterostructures was performed by metal-organic vapor phase epitaxy (MOV PE) and molecular beam epitaxy on (0001) sapphire. Unstrained 0.5-1 mum-thi ck alpha -GaN layers were deposited prior to the SLs. SL phonon modes are i dentified combining results from both IRSE and mu -Raman techniques. The av erage compressive SL stress can be estimated from the shift of the GaN-subl ayer 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. (C) 2001 Else vier Science B.V. All rights reserved.