Fermi level pinning in heavily neutron-irradiated GaN

Abstract

Undoped n-GaN grown by two different metallorganic chemical vapor deposition (MOCVD) techniques, standard MOCVD and epitaxial lateral overgrowth, and Mg-doped p-GaN prepared by hydride vapor phase epitaxy and molecular beam epitaxy were irradiated with fast reactor neutrons to the high fluence of 1018 cm−2. In such heavily irradiated samples the Fermi level is shown to be pinned in a narrow interval of Ec−(0.8−0.95) eV, irrespective of the starting sample properties. The Fermi level pinning position correlates with the measured Schottky barrier height in n-type GaN. The results are interpreted from the standpoint of the existence of the charge neutrality level in heavily disordered material. Based on published theoretical calculations and on deep level transient spectroscopy (measurements and lattice parameter measurements in irradiated material), it is proposed that the Fermi level could be pinned between the gallium-interstitial-related deep donors near Ec−0.8 eV and nitrogen-interstitial-related acceptors near Ec−0.9 eV

Neutron Radiation Effects in Epitaxially Laterally Overgrown GaN Films

“…12 For high neutron doses of ELOG samples, the Fermi level is pinned at approximately the same position as in standard samples. 11 Preliminary results suggest that the types of deep centers created by neutrons in ELOG n-GaN are similar to the ones observed for highdislocation-density MOCVD material, but the average carrier removal rate and the average deep defects introduction rates are lower for ELOG. 14 Concerns in the study of radiation effects in ELOG material include the non-uniformity of structuralproperties (the existence of the highdislocation-density and the low-dislocation density regions within one sample) and electrical and recombination properties (the doping level for the material grown in the windows of the SiO 2 mask is several times higher than in the ELOG region, while the diffusion lengths in the former areas are lower).…”

“…), the entire volume of the samples became the highly resistive n-type, with the Fermi level pinned near E c -0.95 eV. 10,11 Deep Electron and Hole Traps from DLTS, ODLTS, PICTS The evolution of DLTS spectra with neutron irradiation of ELOG samples is shown in Fig. 4.…”

“…11 Although the standard DLTS technique was not applicable to the samples heavily compensated with radiation, PICTS measurements 27 could still be performed. Figure 7 shows PICTS spectra taken on the sample irradiated with 2 · 10 17 cm -2 neutrons.…”

“…These effects are similar to the ones observed for neutron irradiation of undoped n-GaN produced by standard MOCVD. 10,11 As in standard MOCVD GaN, the main contribution to the carrier removal rate in ELOG samples comes from disordered regions that are also responsible for the quasi-hole-trap features in ODLTS of irradiated samples. 10,26 The Fermi level pinning position in heavily irradiated ELOG samples comes from the overlapping of the disordered regions and characterizes the position of the Fermi level in the core of these regions.…”

“…Radiation effects for the high-dislocation-density n-GaN films have been reported for electron, proton, neutron, and heavy ions irradiation. [4][5][6][7][8][9][10][11][12][13] For electron irradiation, the dominant radiation defects are shallow electron traps with activation energies 0.13, 0.16, and 0.18 eV that were attributed to nitrogen vacancies or their complexes. [5][6][7] For heavier particles, or for high electron doses, deeper electron traps with levels near E c -0.8 eV and E c -(1-1.2) eV were dominant.…”