Optical properties of hexagonal GaN

Single-crystalline hexagonal GaN (alpha-GaN) films have been grown on (0001) sapphire substrates by metalorganic chemical vapor deposition at 1040 degrees C. The complex dielectric functions, epsilon(E) = epsilon(1)(E) + i epsilon(2)(E), of the epitaxial films have been measured by spectroscopic ellipsometry (SE) for E perpendicular to c in the region between 1.5 and 5.0 eV at room temperature. Previously published ultraviolet SE spectra of alpha-GaN are examined by considering the effects of surface roughness using an analysis based on an effective medium model. Ex situ atomic force microscopy is used to assess independently surface flatness. By mathematically removing the effects of surface roughness, the most reliable epsilon(E) values for alpha-GaN are presented in the 1.25-10 eV photon-energy range, Theoretical dispersion analysis suggests that the E-0 structure could be characterized by a three-dimensional M-0 critical point and the E-1 alpha (alpha=A,B,C) structures by two-dimensional M-1 critical points. To facilitate design of various optoelectronic devices, dielectric-function-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity of alpha-GaN are also presented. (C) 1997 American Institute of Physics.