Neutron Irradiated GaN Research Articles

Temperature-dependent electrical characteristics of neutron-irradiated GaN Schottky barrier diodes

Temperature-dependent electrical characteristics were explicitly investigated for a 400-μm diameter neutron-irradiated (NI) GaN Schottky barrier diode (SBD). Based on C-V measurements, a marked decrease in electron concentration has been revealed for the NI diode compared with the pristine sample, suggesting a thermal-enhanced carrier removal effect. Neutron irradiation causes noticeable Schottky barrier height inhomogeneity, which was studied by a two-barrier model. Data indicates that neutron irradiation affects a small but measurable suppression of leakage current as well as low frequency noise level. Despite a new deep-level trap was identified, the temperature-dependent electrical results specified GaN SBD's outstanding resistance to neutron irradiations and robustness in extreme operation temperatures.

Lattice displacement and electrical property of Li-ion implanted GaN single crystal

Ga and N lattice displacements in Li-ion implanted GaN are evaluated by Rutherford backscattering using proton beam. Li-ion implantation into GaN single crystal wafer with 450 μm in thickness is performed with energies of 50 keV (ion fluence; 2 x 1014 cm-2), 80 keV (3 x 1014 cm-2) and 120 keV (5 x 1014 cm-2) at room temperature. The aligned spectra are obtained from scattering along the <0001> channeling direction. The number of displaced Ga and N atoms estimated from χmin (the ratio between aligned and random) were 3.5 × 1020 cm-3 and 8.7 × 1020 cm-3, respectively. The displacement concentration of N atoms is larger than that of Ga atoms. This suggests that N interstitial (Ni) relating to the displacement of N atoms is introduced in Li-ion implanted GaN. This result is similar to that of the neutron irradiated GaN. The sheet resistance is 2.6 x 102 Ω/sq. for un-implanted samples and 1.3 x 103 Ω/sq. for Li-ion implanted ones. The origin of the high resistivity after the Li-ion implantation is mainly attributed to the carrier compensation effect due to the deep level of Ni. The resistance decreases to 1.1 x 102 Ω/sq. by annealing at 973 K for 20 min in N2 flow, suggesting the recovery of the implantation induced defects.

(Invited) Neutron Irradiation Effect on GaN-Based Materials

Effects of neutron irradiation on GaN-based materials were investigated. Temperature dependent Hall measurements were used to study the electrical property of two dimensional electron gases (2DEG). The mobility of 2DEG was sensitive to neutron, while sheet carrier density (ns) showed nearly no decrease until a fluence of 3.66×1015cm-2. Raman measurement was used to examine the strain in AlGaN/GaN heterostructure, and no peak shift was observed in the spectrum. In order to investigate the influence of neutron irradiation on the optical properties of GaN, persistent photo conductivity (PPC) and low temperature photoluminescence (PL) were measured. Pronounced PPC was observed, but no yellow luminescence (YL) band appeared in PL spectrum measured at 5K, suggesting that PPC and YL were not related. Thermal stimulated current (TSC) and low temperature photoluminescence (PL) techniques were used to determine the position of deep levels in GaN with high resistance. Four traps are identified in neutron irradiated GaN.

Persistent photoconductivity in neutron irradiated GaN

Unintentionally doped GaN films grown by MOCVD were irradiated with neutrons at room temperature. In order to investigate the influence of neutron irradiation on the optical properties of GaN films, persistent photoconductivity (PPC) and low temperature photoluminescence (PL) measurements were carried out. Pronounced PPC was observed in the samples before and after neutron irradiation without the appearance of a yellow luminescence (YL) band in the PL spectrum, suggesting that the origin of PPC and YL are not related. Moreover, PPC phenomenon was enhanced by neutron irradiation and quenched by the followed annealing process at 900 °C. The possible origin of PPC is discussed.

Photoluminescence of Neutron-irradiated GaN Films and Nanowires

Photoluminescence (PL) of neutron-irradiated GaN films and nanowires is investigated in this study. The GaN films and nanowires were irradiated by neutron beams in air at room temperature, and the neutron-irradiated films and nanowires were annealed in an atmosphere of at temperatures ranging from 500 to . The line-shapes of the PL spectra taken from the neutron-irradiated GaN films and nanowires were changed differently with increasing annealing temperature. In this study, light-emitting centers created in the neutron-irradiated GaN films and nanowires are examined and their origins are discussed. In addition, it is suggested here that the neutron-transmutation-doping is a simple and useful means of homogeneous impurity doping into nanowires with control of the doping concentration.

Defect studies on fast and thermal neutron irradiated GaN

Single crystalline epitaxial GaN films were irradiated with fast (E>1MeV) or with fast and thermal neutrons. These irradiation conditions allow the separation of the effect of transmutational doping with Ge due to nuclear reactions with thermal neutrons on the damage production. High resolution X-ray diffraction showed an expansion of the c-lattice parameter after irradiation which is reversed after annealing at 1000°C. The effect of neutron irradiation on the optical properties of GaN samples was investigated using photoluminescence and Raman spectroscopies. With above band gap excitation the PL spectra of the as-irradiated sample with fast and thermal neutrons is dominated by broad emission bands in the UV and yellow spectral range whereas no PL is observed for the fast neutron as-irradiated sample. Annealing the as-irradiated samples promotes the damage recovery and noticed changes are observed in the PL spectra. Raman scattering spectra indicate an increase of the intensity of the disorder activated phonons revealing higher lattice damage for the irradiation with fast and thermal neutrons.

Fast neutron irradiation effects in n-GaN

The electrical properties and deep level spectra in undoped n-GaN films irradiated by fast neutrons are reported. The electron removal rate was ∼5cm−1, and the dominant deep states introduced by neutron damage were electron traps with activation energy of 0.75eV. For high doses of 1.7×1017–1018cm−2 the material becomes semi-insulating n-type with the Fermi level pinned near Ec−0.85eV. Deep level spectra are dominated by electron traps with activation energy of 0.75eV, close to the energy of the Fermi level pinning in heavily irradiated material. Neutron irradiation also introduces a high density of centers giving rise to strong persistent photocapacitance. The observed phenomena are explained under the assumption that the dominant defects in neutron irradiated GaN are disordered regions produced by high-energy recoil atoms.

Fermi level pinning in heavily neutron-irradiated GaN

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

Lattice disorder in neutron irradiated GaN: Nuclear reaction analysis and Rutherford backscattering studies

Disorders of N and Ga lattice in GaN, introduced by fast neutron irradiation with a fluence of 6.7×1018cm−2, are investigated by nuclear reaction analysis (NRA) using 14N(d,p)15N reaction with 2.6MeV D2+ ions and by Rutherford backscattering (RBS) with 1.5MeV 4He+ ions. The 〈0001〉 aligned NRA yield measured in as-irradiated GaN slightly increases compared with that of un-irradiated crystals, indicating that primary knock-on (PKO) produced by the neutrons results in ∼7.2×102 displaced N atoms. The slight increase in the aligned RBS yield for as-irradiated samples relative to that of un-irradiated ones indicates that the ∼1.8×102 displaced Ga atoms are produced by PKO. The displacement of N atoms is four times larger than that of Ga atoms, reflecting the lighter weight of N than Ga, although both the displacements are recovered by annealing over 1000°C. The displacement of Ga atoms in GaN is two times smaller than that in GaP (∼3.0×102 atoms/PKO), showing the stronger bonds in GaN than GaP.

Defect production in neutron irradiated GaN

Rutherford backscattering in the channelling mode (RBS/C) and high resolution X-ray diffraction (XRD) were used to characterize GaN films after irradiation with fast and thermal neutrons to fluences of 1.0×1019n/cm2 and 4.8×1019n/cm2, respectively. RBS/C experiments indicate that Ga atoms are preferentially displaced along the c-axis. In good agreement, XRD shows an expansion of the lattice along the c-axis after irradiation, while no change of the in-plane lattice parameter was observed. Both fast and thermal neutrons contribute to the lattice damage and to the introduction of perpendicular elastic strain; the former due to direct knock-on atoms, the latter due to recoil processes during the neutron activation of Ga and N atoms.

Large lattice relaxation deep levels in neutron-irradiated GaN

Deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) measurements have been carried out in neutron-irradiated n-type hydride-vapor-phase-epitaxy-grown GaN. A defect center characterized by a DLTS line, labeled as N1, is observed at EC−ET=0.17eV. Another line, labeled as N2, at EC−ET=0.23eV, seems to be induced at the same rate as N1 under irradiation and may be identified with E1. Other defects native to wurtzite GaN such as the C and E2 lines appear to enhance under neutron irradiation. The DLOS results show that the defects N1 and N2 have large Frank-Condon shifts of 0.64 and 0.67eV, respectively, and hence large lattice relaxations. The as-grown and neutron-irradiated samples all exhibit the persistent photoconductivity effect commonly seen in GaN that may be attributed to DX centers. The concentration of the DX centers increases significantly with neutron dosage and is helpful in sustaining sample conductivity at low temperatures, thus making possible DLTS measurements on N1 an N2 in the radiation-induced deep-donor defect compensated material which otherwise are prevented by carrier freeze-out.

Ion channeling and Raman scattering studies of the lattice disorder and residual strain in neutron irradiated GaN

Ion channeling and Raman scattering studies of the lattice disorder and residual strain in neutron irradiated GaN

Ion channeling and Raman scattering studies of the lattice disorder and residual strain in neutron irradiated GaN

The lattice disorder in GaN produced by fast neutrons with a fluence of 6.7×10 18 cm −2 has been investigated with 1.5 MeV 4He + channeling. The slight increase in the 〈0001〉 aligned yield for as-irradiated crystals indicates that each primary knock-on produces approximately 1.7×10 3 displaced atoms. The displaced atoms recover by annealing at 700 °C but the biaxial compressive stress σ xx of ∼0.19 GPa remains in the 1000 °C annealed GaN. There is no change in the lattice spacing of the (0002) plane for both irradiated and annealed samples, suggesting the existence of the strain perpendicular to the [0001] axis. The residual strain correlates with the neutron irradiation-induced complex defect consisting of the transmuted Ge Ga donor and Ga vacancy, acting as an attractive force in Ga planes in 〈0001〉 oriented GaN.

Paramagnetic defects in neutron irradiated bulk GaN crystals

The local structure of defects in undoped, Si-doped, and neutron irradiated free standing GaN bulk single crystals, grown by the hydride vapor phase epitaxy (HVPE), has been investigated by employing an X-band electron spin resonance (ESR) spectrometer. The undoped sample was irradiated to a dose of 2 × 1017 neutrons in an atomic reactor at Korea Atomic Energy Research Institute. The ESR spectrum measured at 8 K of the Si-doped sample shows a broader line width than the undoped one. It reveals that both have only one ESR center with the g-value of gII = 1.9503 and g⟂ = 1.9476. The ESR spectrum of neutron irradiated GaN sample shows also the same center but broader line width than that before irradiation. Although no further paramagnetic centers have been detected, the line broadening in both ESR and Raman spectra reveals that neutron irradiation induced some sort of structural disorder in the crystal.