The effects of fast neutron (up to 1.75 x 10(20) f.n. cm(-2) ) and fast plus thermal neutron (up to 3.5 x 10(20) f.t.n. cm(-2) ) irradiation on the electrical properties and crystal lattice of GaN films have been studied. It is shown that high-dose neutron irradiation induces a transition of n-type samples into a semi-insulating state with resistivity of similar to 10(10)Omega cm at room temperature, while p-type films undergo transition to a high-resistance state. The subsequent irradiation of the samples leads to a decrease of resistivity to -10(5)Omega cm at the final neutron fluence. Raman spectra show appearance of the disorder-activated Raman scattering modes at 210, 300 and 670 cm(-1) linked to defects in Ga and N sublattices. X-ray diffraction measurements reveal an expansion of the GaN c-lattice parameter up to a saturation level of 0.42% at the final fluence, while the a-lattice parameter remains nearly unchanged. The initial value of c restores at 1000 degrees C annealing temperature, with the main annealing stage at 150 degrees C-400 degrees C. The significant role of thermal neutrons (E <= 0.1 MeV) in the damage build-up in GaN is shown.

Neutron irradiation is a unique tool for forming new structural states of ferroelectrics, which cannot be obtained by conventional methods. The inf luence of the irradiation by two doses of fast neutrons ( F = 1 × 10 17 and 3 × 10 17 cm –2 ) on the structure and properties of KNbO 3 single crystals has been considered for the first time. The developed method for taking into account the experimental correction to the diffuse scattering has been used to analyze the structural changes occurring in KNbO 3 samples at T = 295 K and their correlations with the behavior of dielectric and nonlinear optical characteristics. The irradiation to the aforementioned doses retains the KNbO 3 polar structure, shifting Т С to lower temperatures and significantly affecting only the thermal parameters and microstructure of single crystals. Neutron irradiation with small atomic displacements provides a structure similar to the high-temperature modification of an unirradiated KNbO 3 crystal.

The electronic properties and the limiting position of the Fermi level in p -GaSb crystals irradiated with full-spectrum reactor neutrons at up to a fluence of 8.6 × 10 18 cm −2 are studied. It is shown that the irradiation of GaSb with reactor neutrons results in an increase in the concentration of free holes to p lim = (5−6) × 10 18 cm −3 and in pinning of the Fermi level at the limiting position F lim close to E V + 0.02 eV at 300 K. The effect of the annealing of radiation defects in the temperature range 100–550°C is explored.

The neutron irradiation of ferroelectrics is efficiently used to form structural states that cannot be obtained by conventional technologies. To date, the effect of neutron irradiation on the structure and properties of BaTiO 3 has been studied for only ceramic materials. We have considered the influence of fast-neutron irradiation ( F = 1 × 10 17 cm −2 ) on the structure and properties of BaTiO 3 single crystals for the first time. The structural changes occurring in irradiated BaTiO 3 and their correlation with the behavior of dielectric and nonlinear optical characteristics are analyzed with the aid of a specially developed method for taking into account the experimental correction to diffuse scattering. Neutron irradiation to the aforementioned dose retains the polar structure of the material and only slightly changes atomic displacements. The radiationinduced structural changes occur according to the high-temperature type to form a structure similar to the cubic modification of unirradiated BaTiO 3 crystal.

The effect of irradiation with full-spectrum reactor neutrons and predominantly fast reactor neutrons (up to a fluence of 8 × 10 18 cm −2 ) on the electrical properties of epitaxial p -GaN(Mg) films at different initial doping levels (in the range of hole concentrations p = 10 17 –10 19 cm −3 ) is analyzed. It is found that neutron irradiation induces an increase in the resistivity of the initial material to 10 10 Ω cm at 300 K. It is shown that, at high neutron fluences, the resistivity of the material decreases because of the hopping conduction of charge carriers over radiation defect states. The study of isochronous annealing at 100–1000°C reveals stages of donor-defect (100–300°C, 500–700°C, 750–850°C) and acceptor-defect (300–500°C, 650–800°C) annealing in the neutron-irradiated p -GaN(Mg) samples.

The effect of electron irradiation (E = 7, D = 1016–1018 cm–2) followed by heat treatment in the temperature range 100–1000°С on the electrical properties of undoped (n = 1⋅1014–1⋅1016 сm–3), medium- ((n = 1.2–2)⋅1017 сm–3), and heavily silicon-doped (n = (2–3.5)⋅1018 сm–3) epitaxial n-GaN films grown on the Al2O3(0001) substrates using MOCVD technology is discussed. Under electron irradiation, an increase in the specific resistivity of n-GaN and the pinning of the Fermi level in the limit position near E c – 0.9 eV is observed. Restoration of the initial properties of irradiated material is investigated in the temperature range 100–1000°С. A stage of a “reverse” annealing is revealed in the temperature range 300–400°C.

The study is concerned with the effect of electron irradiation (with the energies E = 7 and 10 MeV and doses D = 1016−1018 cm−2) and subsequent heat treatments in the temperature range 100–1000°C on the electrical properties and the spectrum of deep traps of undoped (concentration of electrons n = 1 × 1014−1 × 1016 cm−3), moderately Si-doped (n = (1.2−2) × 1017 cm−3), and heavily Si-doped (n = (2−3.5) × 1018 cm−3) epitaxial n-GaN layers grown on Al2O3 substrates by metal-organic chemical vapor deposition. It is found that, on electron irradiation, the resistivity of n-GaN increases, this is due to a shift of the Fermi level to the limiting position close to E c −0.91 eV. The spectrum of deep traps is studied for the initial and electron-irradiated n-GaN. It is shown that the initial properties of the irradiated material are restored in the temperature range 100–1000°C, with the main stage of the annealing of radiation defects at about 400°C.

We studied 10 MeV electron irradiation effects in a group of n-GaN films grown by standard metalorganic chemical vapor deposition (MOCVD) and by epitaxial lateral overgrowth (ELOG) techniques. The samples were either undoped or Si-doped, so that the shallow donor concentrations ranged from 1014 cm−3 to 3 × 1018 cm−3. It was found that electron irradiation led to the compensation of n-type conductivity and that the carrier removal rate substantially increased with an increase in the starting donor concentration. For the MOCVD samples, it was observed that the main compensating defect introduced by electrons was a 0.15 eV electron trap detected by admittance spectroscopy. Once the Fermi level crossed the level of these traps two other centers with activation energies of 0.2 and 1 eV were found to contribute to the compensation, so that after high doses, the Fermi level in moderately doped samples was pinned near Ec −1 eV. In ELOG samples the 0.15 eV electron traps were not detected. Instead only the 0.2 and 1 eV traps were introduced by irradiation. The carrier removal rate in the ELOG n-GaN was found to be measurably lower than for MOCVD samples with a similar doping level. The results are compared to previously published data and possible models of compensation are discussed.

Carrier removal rates and deep trap spectra were measured in neutron irradiated n-GaN samples grown by metalorganic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE, and epitaxial lateral overgrowth (ELOG). The carrier removal rates were found to significantly increase with donor doping and to decrease in the sequence MOCVD/ELOG/HVPE. The most prominent traps created by irradiation were quasi-hole traps with energy 0.6-0.7 eV and electron traps with energy 0.45 eV. The former were associated with disordered regions in GaN and determine the carrier removal rate in undoped films. The latter were attributed to radiation defect complexes with shallow donors. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3607986] All rights reserved.

Effect of irradiation with high reactor-neutron fluences (Φ = 1.5 × 10 17 -8 × 10 19 cm −2 ) and subsequent heat treatments in the temperature range 100–1000°C on the electrical properties and lattice constant of epitaxial GaN layers grown on an Al 2 O 3 substrate is considered. It is shown that, with the neutron fluence increasing to (1–2) × 10 18 cm −2 , the resistivity of the material grows to values of about 10 10 Ω cm because of the formation of radiation defects, and, with the fluence raised further, the resistivity passes through a maximum and then decreases to 2 × 10 6 Ω cm at 300 K, which is accounted for by the appearance of a hopping conductivity via deep defects in the overlapping outer parts of disordered regions. With the neutron fluence raised to 8 × 10 19 cm −2 , the lattice constant c increases by 0.38% at a nearly unchanged parameter a . Heat treatment of irradiated samples at temperatures as high as 1000°C does not fully restore the lattice constant and the electrical parameters of the material.

Changes in the structural parameters of epitaxial GaN films on sapphire ( n -GaN/Al 2 O 3 (0001)) induced by irradiation with reactor neutrons with integrated fluences up to 7.25 × 10 19 fn cm −2 (φ fn /φ tn ≈ 1) and subsequent isochronal annealing at temperatures up to 1000°C are studied. Measurements of the lattice parameters a and c of the irradiated n -GaN films show that the parameter c increases by 0.38% and the parameter a remains almost unchanged. From theoretical estimations, it follows that, in the irradiated n -GaN film, the elastic tensile stress along the c axis is as high as ∼1.5 GPa, whereas the compression stress in the basal plane of the unit cell is about −0.5 GPa. The tension of the irradiated GaN film along the hexagonal axis induces a decrease in the band gap E g by 37 meV and a lowering of the charge neutrality level by 22 meV with respect to the corresponding parameters in the initial GaN film on sapphire. The parameter c changed by irradiation with reactor neutrons by Δ c can be recovered by annealing in the temperature range 100–1000°C, with the basic stage of annealing at about 400°C.

Thermal neutron irradiation and annealing effects were studied for undoped n-GaN prepared by epitaxial lateral overgrowth (ELOG). Electron beam induced current (EBIC) imaging and profiling prior to irradiation showed that the residual donor doping in our ELOG samples was about three times higher in the high-dislocation-density ELOG wing than in the low-dislocation-density ELOG window regions. Irradiation with thermal neutrons and subsequent annealing led to greatly improved doping uniformity, as evidenced by EBIC imaging. The neutron transmutation doping avoids the anisotropy of donor incorporation efficiency for different planes during ELOG and provides a uniform doping environment. Capacitance-voltage profiling on such samples showed the presence of electrically active centers with concentration close to the concentration of Ge donors produced by Ga interaction with thermal neutrons.

In neutron transmutation doped n-GaN, the electrical properties are found to be dominated not by shallow Ge donors produced by interaction of thermal neutrons with Ga, but by electron traps at 0.45 or 0.2 eV. The traps switch from the former to the latter when the anneal temperature increased from 800 to 1000 °C. The concentrations of both traps rose linearly with neutron fluence and were close to the concentration of Ge donors, suggesting they are Ge complexed with different radiation defects. The authors note the similarity of the properties of these traps to the properties of the dominant electron traps in as-irradiated n-GaN. They also observed prominent hole traps with a level near Ev+1.2 eV. These traps were not detected in virgin or as-irradiated samples. The concentration of the 1.2 eV hole traps increased linearly with neutron fluence, and these traps were assigned to Ga vacancy complexes with oxygen.

The effects of neutron transmutation doping were studied for undoped (residual donor concentrations <1015 cm−3) GaN films grown by metalorganic chemical vapor deposition. After irradiation with reactor neutrons (equal fluences of 1.5×1017 n/cm2 of thermal and fast neutrons) the sample became semi-insulating, with the Fermi level pinned near Ec−0.8 eV. Isochronal annealing from 100 to 1000 °C showed three stages—slight recovery of conductivity at 200–300 °C, reverse annealing at 300–500 °C, and a broad recovery stage from 600 to 1000 °C. After annealing at 1000 °C, the donor concentration in the sample was close to the expected concentration of Ge donors transformed from Ga atoms upon interaction with thermal neutrons (2×1016 cm−3). Admittance spectroscopy showed that the donors had ionization energies ∼Ea=0.2 eV, much deeper than substitutional Ge donors. For intermediate annealing temperatures of 800 °C the donors were deeper (Ea=0.47 eV), but the proximity of concentrations of all these different centers suggests that they are due to transformation of complexes of Ge donors with radiation defects.

Ni/GaN Schottky diode radiation detectors were fabricated on 3-μm-thick unintentionally doped n-GaN films grown by molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) and on 12-μm-thick undoped n-GaN layers prepared by epitaxial lateral overgrowth (ELOG). The reverse current of all detector structures was <10−9 A for bias voltages necessary for detector operation, with the level of background donor doping of <1015 cm−3. With this doping level the space charge region of the Schottky diode could be extended to the entire thickness of the films. The charge collection efficiency of the detectors was close to 100% for MOCVD and ELOG detectors for α-particles with range comparable to the thickness of the layer. Electrical properties and deep trap spectra were also studied. The collection efficiency decreased when the concentra-tion of deep electron traps, particularly Ec-0.6 eV traps, increased in MBE grown films.