Ordinary optical dielectric functions of anisotropic hexagonal GaN film determined by variable angle spectroscopic ellipsometry

Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nit ride (GaN) thin films-an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam ep itaxy on c-plane sapphire substrates (alpha-Al2O3). Room temperature isotro pic and anisotropic mode VASE measurements were made at angles of incidence between of 20 degrees and 80 degrees. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite c rystal structure of GaN. The sizable off-diagonal elements (A(ps) and A(sp) ) of the Jones matrix indicate that the optical axis < c > of the c-plane s ample are slightly off from the surface normal due to a small miscut of sub strates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angle of i ncidence. Thus the ordinary optical dielectric functions (E perpendicular t o < c >) are precisely determined by the isotropic mode VASE measurements a t angles of incidence between 20 degrees and 40 degrees in the range of 0.7 5-6.5 eV. The VASE data were analyzed by a model dielectric function based on the GaN critical point structure, which allows for a nonzero extinction coefficient k below the band gap. The thicknesses of these GaN films are ac curately determined via the analysis as well. (C) 2000 American Institute o f Physics. [S0021- 8979(00)03319-3].