Spectroscopic Characterization of Ion-Implanted GaN
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Spectroscopic Characterization of Ion-Implanted GaN L. Chen and B.J. Skromme Department of Electrical Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, AZ, 85287-5706, U.S.A. ABSTRACT We investigate implantation of high purity HVPE GaN with Mg, Be, C, Zn, Cd, Ca, N, O, P, As, Ne, and Ar. After annealing at 1300 oC, the material is characterized using low temperature photoluminescence (PL). The Mg acceptors exhibit much better optical activation than Be, C, Zn, Cd, or Ca acceptors implanted and annealed under the same conditions. Acceptor-bound exciton peaks and well-resolved donor-acceptor pair bands are observed for both Mg and Zn. A broad peak centered near 2.78 eV is obtained for Cd, confirming that it is deeper than Zn. Isoelectronic As or P exhibit sharp no-phonon bound exciton lines at 2.952 and 3.200 eV, respectively. Defect-related bands centered at 2.2 and 2.35 eV are studied. Both Be and C strongly enhance the yellow (2.2 eV) PL band, but no other impurities do so, including O. INTRODUCTION GaN is a developing wide gap semiconductor material with important applications to both optical and electronic devices. Diffusion of most impurities in GaN is impracticably slow, so that ion implantation is the preferred method for selective doping of GaN. Ion implantation also permits the easy, controlled introduction of dopant species without complications due to unintentional passivating agents such as H. The behavior of implanted impurities and defects in GaN is not yet well understood. Therefore, it is useful to survey the properties of various dopant species and defects. In previous work, we studied Mg, C, and Be implants, sometimes together with Ne, Al, P, or Ar co-implantations, using low temperature photoluminescence (PL) [1, 2]. Good optical activation of Mg acceptors was obtained, yielding a spectroscopic binding energy of 224 meV from the conduction band-to-acceptor (e-Ao) PL peak position. A green band at 2.35 eV and a weaker red band at 1.73 eV were also observed. Co-implantation of Mg, Be, and C acceptors with various other species showed that Mg acceptors incorporate well, whereas Be and C only enhance the yellow PL band. We found that P forms an isoelectronic hole trap on the N site in n-type GaN, but a helium-like double donor on the Ga site in p-type (Mg-doped) GaN. Here, we extend this work to additional impurities and defect states. Low dose implantations into high purity starting material are combined with high temperature annealing to obtain highly resolved optical spectra. We study Zn, Cd, and Ca as p-type dopants at various doses and compare the optical activation of Mg, Be, C, Zn, Cd, and Ca acceptors. More detailed studies of the yellow PL band at 2.2 eV and the green band at 2.35 eV are performed. Implantation of As and P into n-type GaN is studied to compare the behavior of these isoelectronic hole traps. Control samples implanted with inert species and annealed or just annealed are used to confirm the association of PL peaks with specifi
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