EXCITON SPECTRA AND SPIN-ORBIT-SPLITTING IN GAN EPITAXIAL-FILMS

This paper is concerned with the interpretation of experimental data o n free exciton energies in GaN epitaxial films grown on various substr ates (sapphire, SiC and GaN). In hexagonal (wurtzite) films the degene racy in the valence band is completely lifted and three excitons can b e observed by reflectivity and photoluminescence excitation spectrosco py. Their energies can be interpreted in terms of the quasi-cubic mode l of Hopfield which involves two parameters, Delta(so) the spin-orbit splitting and Delta(CR) the axial crystal field energy, which may, in principle, be derived from measurement of the energy differences E(C) - E(A) and E(B) - E(A). In practice, experimental data show considerab le variation; this variation is thought to be due to strain in the fil ms introduced as a result of differential thermal expansion during coo l-down from the growth temperature to room temperature, and direct cal culation of the spin-orbit and crystal field parameters for each sampl e results in apparently random values. In particular, the values of De lta(so) obtained differ considerably from that measured (unambiguously ) on a cubic (zinc blende) film, Delta(so) = 17 meV. We show that it i s possible to resolve these dificulties, following an idea introduced by Gil at al (1995), by plotting E(C) and E(B) aginst E(A) for the var ious samples and obtaining a fit to the data thus displayed. We then f ind that an excellent fit can be obtained using the quasi-cubic model with Delta(so) = 17 meV and Delta(CR) ranging from 13 meV to 37 meV ov er the range of samples for which appropriate data have been published .