X-RAY-ABSORPTION, GLANCING-ANGLE REFLECTIVITY, AND THEORETICAL-STUDY OF THE N K-EDGE AND GA M(2,3)-EDGE SPECTRA IN GAN

A comprehensive study of the nitrogen K edge and gallium M(2,3) edge i n gallium nitride is presented. Results of two different experimental techniques, x-ray absorption by total photocurrent measurements and gl ancing-angle x-ray reflectivity, are presented and compared with each other, First-principles calculations of the (polarization averaged) di electric response epsilon(2)(omega) contributions from the relevant co re-level to conduction-band transitions and derived spectral functions are used to interpret the data. These calculations are based on the l ocal density approximation (LDA) and use a muffin-tin orbital basis fo r the band structure and matrix elements. The angular dependence of th e x-ray reflectivity is studied and shown to be in good agreement with the theoretical predictions based on Fresnel theory and the magnitude of the calculated x-ray optical response functions. The main peaks in the calculated and measured spectra are compared with those in the re levant partial density of conduction-band states. Assignments are made to particular band transitions and corrections to the LDA are discuss ed. From the analysis of the N K and Ga M(2,3) edges the latter are fo und to be essentially constant up to similar to 10 eV above the conduc tion-band minimum. The differences in spectral shape found between the various measurements were shown to be a result of polarization depend ence. Since the c axis in all the measurements was normal to the sampl e surface, p-polarized radiation at glancing angles corresponds to E p arallel to c while s polarization corresponds to E perpendicular to e at all incident angles. Thus, this polarization dependence is a result of the intrinsic anisotropy of the wurtzite structure. Spectra on pow ders which provide an average of both polarizations as well as separat e measurements of reflectivity with s polarization and p polarization were used to arrive at this conclusion.