Atsushi Ikari

Temperature Dependence of the Electrical Resistivity of Molten Silicon

The temperature dependence of the electrical resistivity of molten silicon was measured based on the direct-current four-probe method in the temperature range from the melting point (1,415° C) to 1,630° C. The variation of the resistivity in this temperature region was less than 0.7%, which is much smaller than previously reported values. The measured resistivity near the solodification point was about 72×10-6 Ω cm, which is about 8% smaller than previously reported values. The resistivity of molten silicon showed a local minimum in the range from 1,450° C to 1,500° C. The resistivity of molten silicon was calculated based on Zimans formula. The temperature dependence of the measured resistivity was not reproduced when the structure factor S(Q) calculated by a simple hard-sphere model was substituted into Zimans formula, but was reproduced by using the experimental data of S(Q) measured by Waseda which shows the first peak of asymmetric shape. This result suggests that the specific melt structure of molten silicon has a significant effect on the resistivity.

Microvoid defects in nitrogen- and/or carbon-doped Czochralski-grown silicon crystals

Microvoid defects in nitrogen- and/or carbon-doped Czochralski-grown silicon crystals grown under various growth conditions were investigated by transmission electron microscopy. The morphology and volume of the void defects depended strongly on the nitrogen concentration and the cooling rate. With increasing nitrogen concentration, the shape of the voids changed from {111} octahedron to {111} parallelepiped-plate or -rod via an unfaceted structure. The average volume of the voids decreased exponentially with the increase in nitrogen concentration. On the other hand, multiple voids consisting of octahedral segments, whose average volume is small, were observed in carbon-doped crystals. In nitrogen- and carbon-codoped crystals, multiple voids consisting of plate and rod segments were observed, whose average volume was very small. The thickness of the inner oxide layers of voids was influenced by the cooling rate, and not by nitrogen and carbon. From these results, it was assumed that nitrogen and carbon play different roles in the void formation. The change of void morphology by nitrogen-doping is discussed in terms of anisotropic void growth.

Crystal Defects in Epitaxial Layer on Nitrogen-doped Czochralski-grown Silicon Substrate (II) –Suppression of the Crystal Defects in Epitaxial Layer by the Control of Crystal Growth Condition and Carbon Co-doping–

Two types of crystal defects, stacking fault induced by a nitrogen-doped substrate (N-SF) and elliptical pit (E-pit), are generated in the epitaxial layer due to grown-in defects in a nitrogen-doped Czochralski-grown silicon (CZ-Si) substrate. We investigate the dependence of N-SF and E-pit formation on the quality of nitrogen-doped substrates. It was revealed that the control of both nitrogen concentration and crystal growth parameter of the CZ-Si ingot is effective for suppressing N-SF and E-pit formation. We also examined crystal defect in the epitaxial layer on a nitrogen and carbon co-doped substrate, which is the other candidate for realizing the epitaxial wafer having a high intrinsic gettering ability. It was clarified that carbon co-doping in addition to nitrogen doping suppresses N-SF and E-pit generation.

Positron Annihilation in Germanium in Thermal Equilibrium at High Temperature

Annihilation characteristics of positrons in Ge in thermal equilibrium at high temperature were studied using a monoenergetic positron beam. Precise measurements of Doppler broadening profiles of annihilation radiation were performed in the temperature range between 300 K and 1211 K. The line shape parameters of Doppler broadening profiles were found to be almost constant at 300–600 K. The changes in these parameters were observed to start above 600 K. This was attributed to both the decrease in the fraction of positrons annihilating with core electrons and the lowering of the crystal symmetry around the region detected by positron-electron pairs. This suggests that behaviors of positrons are dominated by some form of positron-lattice coupling in Ge at high temperatures. The temperature dependence of the diffusion length of positrons was also discussed.

Influence of Atmosphere on Molten Silicon Density.

The influence of atmosphere on the density of molten silicon has been studied. The density was measured by an improved Archimedean method with the accuracy of 1.1%. The density under argon gas mixed with hydrogen 7.15% is found to be equivalent to that under pure argon gas at the melting point, but showed higher values at higher temperatures (1430-1500° C) with the thermal volume expansion coefficient of about 8.8×10-5 K-1. The density under the hydrogen-containing argon also shows a time-dependent variation over 5 h after melting. The solubility of hydrogen in molten silicon is estimated to be below 3×1018 atoms cm-3. This result suggests that the increase of the density in the presence of hydrogen is caused by the change of the arrangement of the silicon atoms.

Crystal defects in epitaxial layer on nitrogen-doped Czochralski-grown silicon substrate (I): Investigation of the crystallographic structure

We have investigated crystal defects in the epitaxial layer on nitrogen-doped Czochralski-grown silicon (CZ-Si) substrate. It was found that two types of crystal defects are generated in the epitaxial layer on the nitrogen-doped CZ-Si substrate. One is revealed to be a stacking fault and the other is a perfect dislocation pair. The origin of these defects is a rod like void and a dislocation loop in the nitrogen-doped CZ-Si substrate. We discuss the formation mechanism of the crystal defect in the epitaxial layer caused by the grown-in defect.

Influence of the thermal history of melts on the formation of grown-in defects in silicon

We have investigated the influence of time-dependent changes of melt properties on the formation of grown-in defects in silicon single crystals grown by the Czochralski method. It is found that the density of grown-in defects increases if the crystal is pulled immediately after melting. The results indicate that the change in the state of melts can influence the formation of grown-in defects in silicon single crystals.

Electron Spin Resonance Studies on Spots with Brown Rims on a Vitreous Silica Surface formed by a Silicon Melt

Vitreous silica plates were immersed separately in a silicon melt at 1460°C for 4 h and 25 h, respectively in argon atmosphere. Electron spin resonance (ESR) and optical microscopy observations showed that the paramagnetic species (g=2.0049, peak-to-peak line width (ΔHpp) =7.0 G) existed at the center and the periphery of the spots with brown rims in the silica plate subjected to 4 h immersion. It was revealed that both silicon oxide (SiO1.49) and silicon monoxide (SiO1.04) have silicon dangling bonds (e.g., •Si≡Si3, •Si≡Si2O, and •Si≡SiO2) which have ESR parameters (g=2.0048, ΔHpp=7.5 G). The paramagnetic species in the spots with brown rims were estimated to be the silicon dangling bonds (e.g., •Si≡Si3, •Si≡Si2O, and •Si≡SiO2) in the silicon monoxides.

Influence of hydrogen and oxygen on the melt properties and the crystal growth in silicon

Abstract The influence of hydrogen and oxygen on the density of the silicon melt has been studied. The density of the oxygen-containing melt is identical with that without oxygen and shows an anomalous increase of the density near the melting point. The melt density in an argon gas ambient mixed with 7.15% hydrogen is equivalent to that in a pure argon gas at the melting temperature, but showed greater values at higher temperatures (1430–1500 °C). The density of the silicon melt at 1430 °C increases over 5 h after completion of melting. We found that such time-dependent change of the melt properties influences the formation of grown-in defects during crystal growth.