Thermal modulation spectroscopy for bandgap determination in plasma-assisted molecular beam epitaxy-grown AlN

A thermal modulation reflectance (TMR) technique is developed and implemented as a contactless, nondestructive optical characterization method for wide-bandgap semiconductor thin films. The physical principles underlying TMR are presented and applied to AlN films grown on Si(111) substrates by plasma-assisted molecular beam epitaxy. The influence of the Al/N growth ratio on the structural, optical, and morphological properties of the films is systematically investigated. The films exhibit a predominantly planar surface morphology with some hillocklike features. TMR measurements enable the determination of the optical bandgap of AlN. A correlation is observed between the bandgap and the Al/N growth ratio. As the samples approach stoichiometric conditions (Al/N = 1), the bandgap energy increases, reaching a maximum value of 6.07 eV.