EXPERIMENTAL-DETERMINATION OF THE N-P-PARTIAL DENSITY-OF-STATES IN THE CONDUCTION-BAND OF GAN - DETERMINATION OF THE POLYTYPE FRACTIONS IN MIXED-PHASE SAMPLES

Hexagonal, cubic, and mixed phase GaN samples grown by molecular beam epitaxy are studied using near-edge x-ray absorption fine structure (N EXAFS) spectroscopy. The spectra were recorded at the N-K-edge, at sev eral angles of incidence. It is shown that the N-K-edge NEXAFS spectra , which are proportional to the p-partial density of states in the con duction band, are characteristic of the cubic or hexagonal structure o f the examined crystal. The spectra of the cubic sample do not depend on the angle of incidence (theta), contrary to the spectra of the hexa gonal sample in which the areas under the NEXAFS resonances depend lin early on cos(2) theta. From the fitting of the lines A(i) = A + B cos( 2) theta, where A(i) are the areas under the resonances and A and B ar e constants, the directions of maximum electron charge density with re spect to the normal to the surface are determined for the hexagonal sa mple. The energy positions of the absorption edge and the NEXAFS reson ances in the spectra from the cubic sample are different from those of the hexagonal sample and in either case are independent of theta. Con trary to that, the energy positions of the NEXAFS resonances in a mixe d phase sample have a characteristic angular dependence and shift betw een the energies corresponding to the cubic and hexagonal polytypes. B ased on this observation, we propose that the spectrum of the mixed-ph ase sample can be approximated as the weighted average of the spectra from the pure cubic and hexagonal samples. From this approximation the coexisting fractions of alpha- and beta-GaN in a mixed-phase sample a re determined and they are found in good agreement with results from h igh resolution transmission electron microscopy and x-ray diffraction. (C) 1998 American Institute of Physics. [S0021-8979(98)02603-6].