Identification of As, Ge and Se Photoluminescence in GaN Using Radioactive Isotopes
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to these results, Boguslawski et al. [15] calculate the GaN antisite level at around EV +1.4 eV, and Neugebauer et al. state that antisites in GaN are energetically less favorable [16] than in other III-V semiconductors. As shown by Magerle et al. [17], another advantage of radioactive isotopes in PL experiments is the chance to create antisites intentionally with suitable isotopes. We report on experiments with the isotope 71As, which decays via 71 Ge into stable 71Ga. Provided that after implantation and annealing all 71As occupies a N site and no site changes are taking place during the decay, then all 71Ga atoms form GaN antisite defects. The aim of this report is the unequivocal chemical assignment of optical transitions found in As, Ge, and Se doped GaN, and to investigate the existence of optically active GaN levels. EXPERIMENTAL Nominally undoped GaN layers (n ≈ 5 × 1016cm-3) grown by metal organic vapor phase epitaxy (MOVPE) on AlN/c-sapphire substrate by Cree Research were implanted either with radioactive 72Se or 71As. The implantations were carried out at the on-line mass separator ISOLDE at CERN with an energy of 60 keV and a dose of 3 × 1012 ions/cm2. The ions end up within a Gaussian shaped profile centered at 21 nm depth with a width of 10 nm and a peak concentration of about 2 × 1018 cm-3. To serve as reference, a small part of each sample was not implanted. The implantation induced damage was reduced by annealing the samples at 1270 K for 10 min in sealed quartz ampoules filled with nitrogen gas at a pressure of 1 bar at room temperature. The isotope 72Se transmutes via the decay chain 72Se (8.4 d) → 72As → (26 h) into stable 72Ge while the isotope 71As (64.28 h) first decays into 71Ge (11.43 d), which finally transmutes into stable 71Ga. The half-life of each decay is given in parentheses. These chemical transmutations were monitored by PL spectroscopy at 4 K using a He flow cryostat. The 325 nm line of a HeCd-laser with an excitation density of 160 Wcm-2 was used to excite the samples. The luminescence was dispersed with a 0.75 m monochromator and detected with a cooled GaAs-photomultiplier. RESULTS AND DISCUSSION Figure 1a shows a selection of the 22 recorded PL spectra of 71As-doped GaN successively taken within 56 days after ion implantation and annealing. The spectra are not corrected for the spectral response of the measurement system. The common features of all spectra are the transition DX at 3.471 eV resulting from a donor-bound exciton [18] and the broad band centered at 2.2 eV labeled with YL, known as the “yellow luminescence” in GaN [9]. The spectrum recorded 12 hours after implantation shows clearly a new broad and intense PL band centered at 2.58 eV, in contrast to the unimplanted part of the sample (not shown). In addition, a new transition labeled with Ge at 3.398 eV and its LO phonon replica Ge-LO at 3.306 eV can be seen. The intensity of the PL band at 2.58 eV decreases continuously during the whole measuring period, in contrast to the intensities of the PL transitions at
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