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 been studied by photoluminescence spectroscopy. Several intense excitonic 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 has been analysed with two different fits, the gap shift proportional to T-2/(T+theta(D)) and 1/[exp(theta(E)/T)-1] respectively. Information on the scattering processes is obtained from the peak broadening, which is due to exciton-phonon interactions. Both the free exciton energies and their temperature behaviour agree with those observed in bulk and homoepitaxial GaN, and therefore the studied GaN/Si layers are strain-free. Up to four extrinsic transitions at 3.4755 eV, 3.4714 eV, 3.456 eV and 3.450 eV have also been observed, and their assignment to bound excitons and donor to band transitions is discussed. Finally, a band at 3.41-3.42 eV is attributed to a donor-to-acceptor transition. This interpretation implies the presence of an acceptor lying at 70 meV above the valence band, shallower than those usually employed for p-type doping.