EXCITON AND DONOR-ACCEPTOR RECOMBINATION IN UNDOPED GAN ON SI(111)

The optical transitions in undoped, hexagonal GaN layers, grown on Si( 111) by molecular beam epitaxy under nitrogen-rich conditions, have be en studied by photoluminescence spectroscopy. Several intense excitoni c emissions, of free and bound character, are detected as narrow as 1. 7 meV at low temperature. The free A, B acid C excitons, observed at 3 .4786 eV, 3.484 eV and 3.503 eV, respectively, allow the determination of the crystal-field (Delta(cr)=9.9 meV) and spin-orbit (Delta(so)=19 .9 meV) splittings. The evolution of their energies with temperature h as been analysed with two different fits, the gap shift proportional t o T-2/(T+theta(D)) and 1/[exp(theta(E)/T)-1] respectively. Information on the scattering processes is obtained from the peak broadening, whi ch is due to exciton-phonon interactions. Both the free exciton energi es and their temperature behaviour agree with those observed in bulk a nd homoepitaxial GaN, and therefore the studied GaN/Si layers are stra in-free. Up to four extrinsic transitions at 3.4755 eV, 3.4714 eV, 3.4 56 eV and 3.450 eV have also been observed, and their assignment to bo und excitons and donor to band transitions is discussed. Finally, a ba nd at 3.41-3.42 eV is attributed to a donor-to-acceptor transition. Th is interpretation implies the presence of an acceptor lying at 70 meV above the valence band, shallower than those usually employed for p-ty pe doping.