K. Kushida

Thermally stimulated current studies on neutron irradiation induced defects in GaN

The evaluation of the neutron irradiation induced defects in GaN is studied using a thermally stimulated current (TSC) method with excitation above (below) the energy band gap using ultraviolet (blue, green, red, and infrared) emitting diodes. Annealing at 1000°C, a broad TSC spectrum for excitation by the ultraviolet light is resolved by five traps, P1 (ionization energy is 200meV), P2 (270meV), P3 (380meV), P4 (490meV), and P5 (595meV). Infrared illumination shows a remarkable reduction in TSC for the P2 and P3 traps, indicating the photoquenching behavior. The possible origins of the observed five traps are discussed.

Thermally stimulated current studies on deep levels in hydrothermally grown single crystal ZnO bulk

The evaluation of the deep levels in hydrothermally grown ZnO single crystal bulk is studied using a thermally stimulated current (TSC) method with excitation above (below) the band gap. Two broad TSC spectra are resolved by four traps, P1 (165meV), P2 (255meV), P3 (300meV), and P4 (375meV). P2, P3, and P4 traps are responsible for excitation by the blue and green lights, but P1 trap is weakly responsible. Possible origins of P1 and P2 are attributed to native point defects and Li acceptor, respectively. P3 is correlated to oxygen vacancy as an origin of the green luminescence.

Origins of low resistivity in Al ion-implanted ZnO bulk single crystals

The origins of low resistivity in Al ion-implanted ZnO bulk single crystals are studied by combining Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA), photoluminescence (PL), and Van der Pauw methods. The Al-ion implantation (peak concentration: 2.6 × 1020cm−3) into ZnO is performed using a multiple-step energy. The resistivity decreases from ∼104 Ω cm for un-implanted ZnO to 1.4 × 10−1 Ω cm for as-implanted, and reaches 6.0 × 10−4 Ω cm for samples annealed at 1000 °C. RBS and NRA measurements for as-implanted ZnO suggest the existence of the lattice displacement of Zn (Zni) and O (Oi), respectively. After annealing at 1000 °C, the Zni related defects remain and the Oi related defects disappear. The origin of the low resistivity in the as-implanted sample is attributed to the Zni (∼30 meV [Look et al., Phys. Rev. Lett. 82, 2552 (1999)]). In contrast, the origin of the low resistivity in the sample annealed at 1000 °C is assigned to both of the Zni related defects and the elect...

Raman scattering from the filled tetrahedral semiconductor LiMgAs

Ordered LiMgAs (space group: F‐43m), viewed theoretically as a zinc‐blende‐AlAs‐like (MgAs)− lattice partially filled with He‐like Li+ interstitials, is studied using a Raman‐scattering method. The band gap of as‐grown crystals is found to be 2.309 eV with a direct gap from the photoluminescence measurements at 20 K, in consistency with that estimated by the optical absorption. From Raman scattering, two LO and TO phonons at k∼0 for Li‐As and Mg‐As pair are found to be 656 and 638 cm−1 and 329 and 307 cm−1, respectively, indicating that LiMgAs crystallizes with the ordered Li and Mg atoms at their tetrahedral sites. These results confirm a prediction that the ordered LiMgAs is attributed to the low ionic character of both Li‐As (Pauling’s ionicity fi = 0.22) and Mg‐As (fi = 0.15) bonds.

Persistent photoconductivity and photo‐responsible defect in 30 MeV‐electron irradiated single crystal ZnO

Persistent photoconductivity (PPC) in 30‐MeV electron irradiated ZnO single crystals is studied by excitation using light emitting diodes (LEDs) with various wavelengths. The decay transient of the photoconductivity shows relaxation times in the range of a few ten days for the illumination at 90 K and a few hours at room temperature. An electron paramagnetic resonance (EPR) signal with g‐value = 2.005 appears after illumination of blue LED, suggesting the transfer from the artificially introduced oxygen vacancy of 2+ charge state to the metastable + charge state. Once generated, the metastable state does not immediately decay into the 2+ charge state because of energetic barriers of ∼190 meV, supporting the mechanism of PPC proposed by Van de Walle.

Atomic force microscopy observation of the Jahn‐Teller instability in spinel LiMn2O4 embedded in silicon substrates

Surface morphology of 3.5×3.5 μm2 area of spinel LiMn2O4 embedded in Si substrates, a typical cathode material for Li ion secondary batteries, is studied using an atomic force microscopy (AFM) with a conductive probe. Under the negative bias voltage to attract Li+ ions, electric current abruptly increases at 5.5 V, indicating Li+ ion movement toward the surface. Increase in current accordingly, part of scale‐shaped grains covering the whole surface expand and flatten, suggesting that the Jahn‐Teller phase transition was induced by the repulsive interaction between the Mn‐eg and O‐2p electrons in Li accumulated layer.

Neutron-transmuted carbon-14 in neutron-irradiated GaN: Compensation of DX-like center

The transmuted-C related luminescence and net carrier concentration are studied by combining photoluminescence, liquid scintillation, and Raman scattering. GaN single crystal films grown by metalorganic-vapor-phase epitaxy are irradiated with fast and thermal neutrons at fluxes of 3.9 × 1013 cm−2s−1 and 8.15 × 1013 cm−2s−1, respectively. Irradiation time is 48 hours. The calculated 72Ge and 14C concentrations are 1.24 × 1018 cm−3 and 1.13 × 1018 cm−3, respectively. The transmuted 14C is detected by the liquid scintillation method to survey β-rays emitted in the process of 14C decays from 14N. Tritium (3H) is also emitted by a (n,t) reaction of 14N due to the neutron irradiation above 4.5 MeV. Photoluminescence relating to C, DX-like center of Ge and yellow luminescence band are observed in 1000 °C annealed NTD-GaN. The free electron concentration estimated from Raman scattering is 4.97 × 1017 cm−3. This value is lower than that from the transmuted Ge concentration, suggesting the compensation due to the t...

Thermally Stimulated Current Studies On Electron‐irradiated Single Crystal ZnO Bulk: Dual Light Illumination Effect

Conventional Thermally stimulated current (TSC) studies using white light or monochromatic light LEDs cannot discriminate the electron trap from the hole trap. In order to determine the donor like or acceptor like defects, the TSC studies are carried out using the dual light illumination, namely the illumination with wavelength above or near the band gap and subsequent the illumination with wavelength below the band gap. Three broad peaks are observed using ultraviolet or blue LEDs in 30 MeV electron‐irradiated ZnO single crystals. These peaks are resolved by six traps, P1 (149 meV), P2 (256 meV), P3 (317 meV), P4 (381 meV), P5 (525 meV), and P6 (650 meV). The origin of the P2 and P3 traps has been tentatively attributed to Li acceptor and oxygen vacancy, respectively. P1, P2, and P3 traps disappear by the dual light illumination. It is suggested that P1, P2, and P3 traps are the donor like defects located near the conduction band, while P4, P5, and P6 traps are the acceptor like defect related to energy ...

Origins of low resistivity and Ge donor level in Ge ion-implanted ZnO bulk single crystals

The energy level of Ge in Ge-ion implanted ZnO single crystals is studied by Hall-effect and photoluminescence (PL) methods. The variations in resistivity from ∼103 Ωcm for un-implanted samples to ∼10−2 Ωcm for as-implanted ones are observed. The resistivity is further decreased to ∼10−3 Ωcm by annealing. The origins of the low resistivity are attributed to both the zinc interstitial (Zni) related defects and the electrical activated Ge donor. An activation energy of Ge donors estimated from the temperature dependence of carrier concentration is 102 meV. In PL studies, the new peak at 372 nm (3.33 eV) related to the Ge donor is observed in 1000 °C annealed samples.

Optical Band Gap of a New Filled Tetrahedral Semiconductor Li3AlN2

Li3AlN2 can be viewed as the assemblage of eight hypothetical zincblende AlN lattices (Li0.5Al0.5N)− partially filled with He‐like Li+ interstitials, leading to a new type of “filled tetrahedral semiconductor”. Optical absorption studies show that Li3AlN2 is likely to be a direct band gap material (Eg ∼ 4.45 eV), whereas zincblende AlN is indirect. According to “interstitial insertion rule” by Wood et al., these results suggest that the insertion of Li+ ions into the interstitial sites in zincblende‐like (Li0.5Al0.5N)− causes an upward shift of the X conduction band due to a Pauli repulsion of conduction electrons, exposing the Γ point as the conduction band minimum. The band gap is ∼14 % smaller than zincblende AlN because of the relatively high covalency of Al‐N bond. This is also supported by the fact that the Madelung constant (αM=2.3864) of Li3AlN2 with Al‐N bond shrinkage is ∼ 22 % smaller than that without the shrinkage.