Naoki Ebisawa

Yellowish-white luminescence in codoped zinc oxide

The broad-band visible emission of codoped ZnO was studied. The codoped ZnO specimens were intentionally and simultaneously doped with IIIa elements (donors) and Li (acceptor). Broad-band emission covering nearly the whole visible range was achieved. The emission was found to be yellowish white to the naked eye. The visible band was composed of two components, i.e., a green emission having a peak at 2.2 eV and a yellow emission having a peak at 2.0 eV. The peak at 2.2 eV was distinct from the nonstructured green emission at 2.45 eV. The 2.2 and 2.0 eV peaks were attributed to donor-acceptor pair transitions involving the zinc vacancy and lithium, respectively.

Three-dimensional evaluation of gettering ability of Σ3{111} grain boundaries in silicon by atom probe tomography combined with transmission electron microscopy

Three-dimensional distribution of impurities (boron, phosphorus, oxygen, and copper) at Σ3{111} grain boundaries was determined in a Czochralski-grown silicon single crystal by laser-assisted atom probe tomography (APT) combined with transmission electron microscopy, with a detection limit as low as the order of 0.001 at. %. The location of a boundary was determined by APT even when the boundary was not contaminated. Unlike the boundaries in multicrystalline silicon grown by the casting method, the impurities did not segregate at the boundaries even when the impurity concentrations were high. The gettering ability of the boundaries was discussed.

Recombination activity of nickel, copper, and oxygen atoms segregating at grain boundaries in mono-like silicon crystals

Three-dimensional distribution of impurity atoms was determined at functional Σ5{013} and small-angle grain boundaries (GBs) in as-grown mono-like silicon crystals by atom probe tomography combined with transmission electron microscopy, and it was correlated with the recombination activity of those GBs, CGB, revealed by photoluminescence imaging. Nickel (Ni), copper (Cu), and oxygen atoms preferentially segregated at the GBs on which arrays of dislocations existed, while those atoms scarcely segregated at Σ5{013} GBs free from dislocations. Silicides containing Ni and Cu about 5 nm in size and oxides about 1 nm in size were formed along the dislocation arrays on those GBs. The number of segregating impurity atoms per unit GB area for Ni and that for Cu, NNi and NCu, were in a trade-off correlation with that for oxygen, NO, as a function of CGB, while the sum of those numbers was almost constant irrespective of the GB character, CGB, and the dislocation density on GBs. CGB would be explained as a linear co...

Three-dimensional evaluation of gettering ability for oxygen atoms at small-angle tilt boundaries in Czochralski-grown silicon crystals

Three-dimensional distribution of oxygen atoms at small-angle tilt boundaries (SATBs) in Czochralski-grown p-type silicon ingots was investigated by atom probe tomography combined with transmission electron microscopy. Oxygen gettering along edge dislocations composing SATBs, post crystal growth, was observed. The gettering ability of SATBs would depend both on the dislocation strain and on the dislocation density. Oxygen atoms would agglomerate in the atomic sites under the tensile hydrostatic stress larger than about 2.0 GPa induced by the dislocations. It was suggested that the density of the atomic sites, depending on the tilt angle of SATBs, determined the gettering ability of SATBs.

Nanoscopic analysis of oxygen segregation at tilt boundaries in silicon ingots using atom probe tomography combined with TEM and ab initio calculations

We have developed an analytical method to determine the segregation levels on the same tilt boundaries (TBs) at the same nanoscopic location by a joint use of atom probe tomography and scanning transmission electron microscopy, and discussed the mechanism of oxygen segregation at TBs in silicon ingots in terms of bond distortions around the TBs. The three‐dimensional distribution of oxygen atoms was determined at the typical small‐ and large‐angle TBs by atom probe tomography with a low impurity detection limit (0.01 at.% on a TB plane) simultaneously with high spatial resolution (about 0.4 nm). The three‐dimensional distribution was correlated with the atomic stress around the TBs; the stress at large‐angle TBs was estimated by ab initio calculations based on atomic resolution scanning transmission electron microscopy data and that at small‐angle TBs were calculated with the elastic theory based on dark‐field transmission electron microscopy data. Oxygen atoms would segregate at bond‐centred sites under tensile stress above about 2 GPa, so as to attain a more stable bonding network by reducing the local stress. The number of oxygen atoms segregating in a unit TB area NGB (in atoms nm−2) was determined to be proportional to both the number of the atomic sites under tensile stress in a unit TB area nbc and the average concentration of oxygen atoms around the TB [Oi] (in at.%) with NGB ∼ 50 nbc[Oi].

Elemental Distribution in Multilayer Systems by Laser-Assisted Atom Probe Tomography with Various Analysis Directions.

Elemental distributions in a magnetic multilayer system with the structure Si substrate/Ta/NiFe/Ru/CoFeB/Ru/NiFe were studied using atom probe tomography (APT) along different analysis directions. The distributions of Ru and B atoms, which require a high evaporation field, were strongly influenced by the APT analysis direction. In particular, B in the CoFeB layer appeared near the interface with the lower Ru layer when the analysis was anti-parallel to the film growth direction, while B atoms were observed at the other side of the CoFeB layer when the analysis was parallel to the film growth direction. Moreover, when the analysis was perpendicular to the film growth direction, a homogenous distribution of B atoms was found within the CoFeB layer. Owing to this B behavior, the underlying Ru layer was affected in both of these analysis directions. In APT measurements of such a multilayer system composed of a stack of different evaporation field materials, evaluation of the elemental distribution around interfaces should be performed from more than one analysis direction.

Impact of local atomic stress on oxygen segregation at tilt boundaries in silicon

Using the atom probe tomography, transmission electron microscopy, and ab initio calculations, we investigate the three-dimensional distributions of oxygen atoms segregating at the typical large-angle grain boundaries (GBs) ( Σ 3 { 111 } , Σ 9 { 221 } , Σ 9 { 114 } , Σ 9 { 111 } / { 115 }, and Σ 27 { 552 }) in Czochralski-grown silicon ingots. Oxygen atoms with a covalent radius that is larger than half of the silicons radius would segregate at bond-centered positions under tensile stresses above about 2 GPa, so as to attain a more stable bonding network by reducing the local stresses. The number of oxygen atoms segregating in a unit GB area N GB (in atoms/nm2) is hypothesized to be proportional to both the number of the tensilely-stressed positions in a unit boundary area n bc and the average concentration of oxygen atoms around the boundary [ O i] (in at. %) with N GB ∼ 50 n bc [ O i ]. This indicates that the probability of oxygen atoms at the segregation positions would be, on average, fifty ti...