Toshinori Taishi

Optical properties of dislocations in wurtzite ZnO single crystals introduced at elevated temperatures

Optical properties of wurtzite ZnO bulk single crystals in which an arbitrary number (typically 109–1010u2002cm−2) of fresh dislocations were introduced intentionally by the plastic deformation at elevated temperatures (923–1073 K) were examined. Deformed specimens showed excitonic light emission with photon energies of 3.100 and 3.345 eV, as well as their LO phonon replicas at 11 K. The light intensities increased with increasing dislocation density. The activation energy for a thermal quenching of the 3.100 or 3.345 eV emission band, which corresponds to the depth of the localized energy level associated with the emission band, was estimated to be 0.3±0.1 or 0.05±0.01u2002eV, respectively. The origin of the energy levels was proposed as point defect complexes involving dislocations. The introduction of the dislocations at the elevated temperatures above 923 K did not influence the intensities of the emission bands except the dislocation-related emission bands.

High-Quality p-Type ZnO Films Grown by Co-Doping of N and Te on Zn-Face ZnO Substrates

This article will report the epitaxial growth of high-quality p-type ZnO layers on Zn-face ZnO substrates by nitrogen and tellurium (N+Te) co-doping. ZnO:[N+Te] films show p-type conductivity with a hole concentration of 4×1016 cm-3, while ZnO:N shows n-type conduction. The photoluminescence of ZnO:N shows broad bound exciton emission lines. Meanwhile, ZnO:[N+Te] layers show dominant A0X emission line at 3.359 eV, with a linewidth as narrow as 1.2 meV. Its X-ray linewidth shows narrower line width of 30 arcsec. Detailed investigation of photoluminescence properties of (N+Te) codoped ZnO layers suggest that the binding energy of N acceptors lies in a range of 121–157 meV.

High-temperature strength and dislocation mobility in the wide band-gap ZnO: Comparison with various semiconductors

The mechanical strength of bulk single crystal wurtzite ZnO was investigated at elevated temperatures by means of compressive deformation. The yield stress of ZnO in the temperature range of 650–850°C was found to be around 10–20MPa, i.e., extremely lower than that of GaN, a typical wide band-gap semiconductor. On the basis of the observed temperature dependence of yield stress, the activation energy for dislocation motion at elevated temperatures in ZnO is deduced to be 0.7–1.2eV, which follows the relation of activation energy for dislocation motion versus band-gap energy known in a variety of semiconductors.

Investigation of the crystallinity of N and Te codoped Zn-polar ZnO films grown by plasma-assisted molecular-beam epitaxy

We report on the crystallinity, N incorporation efficiency, optical properties, and electrical properties of N and Te codoped ZnO films grown by plasma-assisted molecular-beam epitaxy. Te improves the surface morphology and roughness of ZnO films in terms of both streak reflection high energy electron diffraction pattern and atomic force microscopy observations. Also, N and Te codoping is helpful to improve the crystallinity and N incorporation efficiency simultaneously. We found that; (a) narrower x-ray linewidth and higher N concentration were obtained by codoping. (b) Nitrogen related emission lines including donor-acceptor pair and acceptor-bound exciton dominantly emerged in photoluminescence spectra. (c) Codoping enhanced the carrier compensation of native donors in ZnO films and suppressed the dislocation scattering. As a consequence, we concluded that N and Te codoping is very effective for the growth of reliable p-type ZnO films which fulfill the controversial requirements; high N concentration a...

Interaction of dopant atoms with stacking faults in silicon crystals

Variation in stacking fault energy with annealing at 1173 K were identified in Czochralski-grown silicon crystals heavily doped with n- or p-type dopant atoms. In n-type crystals, the energy decreased with increasing annealing time. The higher the concentration of dopant atoms, the larger the degree of the decrease. On the other hand, the energy was unchanged during annealing in p-type and nondoped crystals. These results imply that n-type dopant atoms segregate nearby a stacking fault, via their thermal migration, under an electronic interaction leading to a reduction in the stacking fault energy.

Interaction of phosphorus with dislocations in heavily phosphorus doped silicon

Effects of annealing at 1173 K, that is comparable to the typical temperatures for the fabrication of Si-based devices, on the dissociated dislocations in Czochralski-grown silicon crystals heavily doped with phosphorus atoms were determined. Dislocation segments with edge component are constricted. They climbed out of the slip plane toward the compression side, forming complete dislocation segments. The dissociation width of the rest segments is increased. These results suggest that phosphorus atoms segregate nearby dislocations and the high doping level at the dislocations lowers the formation energy of negatively charged vacancies.

Influence of Isoelectronic Te Doping on the Physical Properties of ZnO Films Grown by Molecular-Beam Epitaxy

In molecular-beam epitaxy, isoelectronic Te doping induces the serious change of the physical properties of ZnO films: i) Incorporated Te concentration is proportional to the 2.2th power of injected Te flux in the logarithmic scale; ii) Te-doped ZnO lattices are dominated by the relaxation mechanism of compressive strain; iii) incorporated Te atoms substitute to the O sites of ZnO lattices; iv) the low-level injection of Te atoms below ~1019 cm-3 improves the crystalline quality in the ZnO films; and v) isoelectronic Te centers act as donor impurities, resulting in the increase of electrons in the ZnO films.

Interstitial oxygen behavior for thermal double donor formation in germanium: Infrared absorption studies

The kinetics of the reduction of interstitial oxygen (Oi) due to the formation of thermal double donors (TDDs) upon heat treatment in an oxygen-rich Ge crystal were investigated at various temperatures. Specimens were prepared from a Ge crystal with oxygen at a concentration of 4–5u2009×u20091017u2009cm−3 grown by a new Czochralski method and were heat-treated in the temperature range 300–500u2009°C. Shrinkage of a dissolved oxygen absorption peak at 855u2009cm−1 and simultaneous development of a thermal double donor peak at 780u2009cm−1 were observed by infrared absorption spectroscopy at room temperature. The formation of TDDs was also detected electrically. Reduction of dissolved oxygen concentration upon the heat treatments was kinetically analyzed. The activation energy of the reduction of Oi concentration was evaluated to be 1.7 and 2.0u2009eV in the early and prolonged stages of the heat treatment, respectively, close to that of Oi diffusion. From the results, TDD development process was indicated to be the merge reaction of ...

Solution Growth of SiC from the Crucible Bottom with Dipping under Unsaturation State of Carbon in Solvent

SiC crystals are grown using a Si-Cr-based solvent by a top-seeded solution growth (TSSG) method by changing the dipping time after when the growth temperature is reached. Step-flow-like curve morphologies were observed for a dipping time after 15 min, while polycrystallization occurred at the periphery for that after 120 min, which corresponded to the dipping under unsaturated and supersaturated carbon in the solvent, respectively. Furthermore, the solution growth of SiC with dipping under unsaturated carbon was easily realized by the growth from the crucible bottom, step-flow-like growth was achieved. Using this technique, dominant polytypes of SiC in various growth conditions after stable seed dipping under the unsaturation in the solvent can be demonstrated.