Low-energy cathodoluminescence spectroscopy of erbium-doped gallium nitride surfaces

We have used cathodoluminescence spectroscopy with variable incident beam e nergies to study the energy levels and activation of Er impurities in GaN a s a function of depth below the free surface. The GaN films were doped in s itu during either metalorganic molecular-beam epitaxy (MOMBE) or molecular- beam epitaxy (MBE). Besides the well-known Er3+ luminescence at 0.80 eV, we observe emissions at 1.2, 1.8, 2.2, and 2.3 eV, corresponding to higher en ergy Er 4f shell transitions. For unannealed MOMBE-grown GaN:Er, these high er energy emissions appear only for excitation depths of hundreds of nanome ters. The MOMBE-grown GaN;Er annealed to 500 degrees C shows a dramatic inc rease in the 1.8, 2.2, and 2.3 eV peak intensities at shallow probe depths, with its yield increasing with increasing depth. These three features beco me pronounced at all depths after a 700 degrees C anneal, MBE-grown GaN:Er grown with lower C and O impurity levels than the MOMBE-grown sample exhibi ts strong emission at all these energies without annealing. The decreased e mission at shallow (tens of nanometer) probe depths suggests a depletion of activation Er in the near-surface region. Enhancement of near-surface Er3 luminescence with annealing may be due to lattice reordering as well as im purity redistribution. (C) 1999 American Vacuum Society. [S0734-2101(99)001 06-2].