Crystalline and optical properties of AlN films with varying thicknesses (0.4-10 µm) grown on sapphire by metalorganic chemical vapor deposition

Abstract

Structural and optical properties of AlN films grown on c-sapphire by metalorganic chemical vapor deposition (MOCVD) with varied thicknesses (0.4-10 µm) are investigated. AlN with good crystal quality, large grain size, low dislocation density and micro-strain were grown. The band gap Eg values obtained from Optical transmission (OT) measurement (5.9-6.0 eV) are lower than from spectroscopic ellipsometry (SE) experiments (6.1-6.2 eV). This is because SE measures bandgap primarily due to the lattice structure while OT also includes effects of impurities- and defects-related transitions in the bandgap measurement. The absorption tails below the band gap, i.e., the Urbach band tails, from OT are relatively wider than from SE. OT measurements showed the Urbach band tails from thick AlN films larger than from thinner films, indicating that more unintended doped impurities and defects were incorporated in thicker samples. As per multi-wavelengths (532-325-266nm) excitation Raman spectroscopy, AlN films have compressive stress as whole and tensile stress towards the surface. E2(high) phonon lifetime reduces with an increase of temperature, due to lattice thermal expansion and increased phonon-phonon interaction. This work provides insight into the structural and optical properties of AlN and could help in understanding AlN for optoelectronic applications.