Koichi Kakimoto

Direct observation by X-ray radiography of convection of molten silicon in the Czochralski growth method

Abstract Convection of molten silicon during Czochralski single crystal growth was directly observed using X-ray radiography. The melt flow pattern was monitored using a tracer method. The tracer, whose density and wettability were adjusted to that of the molten silicon, was developed. The observed convection of the molten silicon in the crucible was not only steady but also transient, and not axisymmetric but asymmetric. This asymmetry is attributed to the asymmetric temperature distribution within the crucible. The flow velocity of the molten silicon in the 75 mm diameter crucible was 10 to 20 mm/s.

Effects of sodium on electrical properties in Cu2ZnSnS4 single crystal

We have studied the effect of sodium on the electrical properties of Cu2ZnSnS4 (CZTS) single crystal by using temperature dependence of Hall effect measurement. The sodium substitution on the cation site in CZTS is observed from the increasing of unit-cell size by powder X-ray diffraction. Sodium increases the effective hole concentration and makes the thermal activation energy smaller. The degree of compensation decreases with sodium incorporation, thus the hole mobility is enhanced. We revealed that sodium is important dopant in CZTS to control the electrical properties.

Temperature and energy dependences of capture cross sections at surface states in Si metal‐oxide‐semiconductor diodes measured by deep level transient spectroscopy

A new deep level transient spectroscopy technique is presented to determine capture cross sections at metal‐oxide semiconductor (MOS) surface states. The technique enables us to determine energy and temperature dependences of capture cross sections separately. Applying this method, electron capture cross sections at surface states in Si MOS diodes were measured and found to have strong energy dependence and rather weak temperature dependence. It was also found that there was an effect to increase the emission rate, which may be attributed to barrier lowering at the Si‐SiO2 interface.

Spoke patterns on molten silicon in Czochralski system

Abstract Asymmetric temperature profiles similar to the spoke patterns and related asymmetric flow in Si melts of the Czochralski (CZ) system are verified for the first time by three-dimensional (3D) numerical simulation of heat and momentum transfer and by X-ray radiography technique. These profiles appear as the temperature difference between the wall and the crystal becomes large with symmetric boundary conditions. The 3D simulation leads to the conclusion that the vertical temperature gradient in the unstable layer near the free surface is an important cause of making the asymmetric profile. The profile is estimated to be caused by both Rayleigh-Benard and Marangoni-Benard instabilities in the Si melt. It is shown that the relative strength of these two instabilities depends on the coefficients of temperature dependence of the density and surface tension. If the temperature coefficient of surface tension (∂γ/∂T) is greater than 1 x 10-4 N/m ⋅, the Marangoni-Benard instability mainly causes asymmetry, while if ∂γ/∂T is less than this value, the Rayleigh-Benard instability mainly causes asymmetry.

Raman spectra from Ga1−xInxAs epitaxial layers grown on GaAs and InP substrates

Raman spectra from Ga1−xInxAs epitaxial layers of various compositions were studied. Both disorder‐activated acoustic and optical phonons appeared in the midrange of composition independent of substrate materials. Broadening in the LO phonon due to stress was also observed near the interface region between the epitaxial layer and the substrate when there was lattice mismatch between them even if the amount was as small as 0.7%.

Correlation between intrinsic defects and electrical properties in the high-quality Cu2ZnSnS4 single crystal

Temperature dependent Hall effect measurements from 20 to 300 K have been performed on the quaternary compounds Cu2ZnSnS4 (CZTS) single crystals. The conductivity mechanisms can be described by a two-path system using Mott variable range hopping and typical thermal activation conduction. The center level of the acceptor band is 132 meV above the valence band maximum and is of width 40 meV. A correlation between the activation energy and acceptor concentration in CZTS is observed.

Natural and forced convection of molten silicon during Czochralski single crystal growth

Abstract Natural and/or forced convection of molten silicon during Czochralski single crystal growth was directly observed using X-ray radiography with solid tracers for various crystal and crucible rotation speeds, and temperature distribution in a crucible holder. Downflow attributed to natural convection in the center of a crucible which had been simulated by numerical calculation was scarcely observed with and without crucible rotation. Numerical simulation of the molten silicon was carried out by a packaged code of “FLUENT”; in the calculation, measured non-axisymmetric temperature distribution in a crucible holder was adopted. Unidirectional flow with and without crucible rotations can be qualitatively explained by the numerical simulation with non-axisymmetric temperature distribution in the crucible holder. The particle path attributed to natural convection near the solid-liquid interface was suppressed downward with increase in crystal rotation speed. The phenomena can be explained by a generation of forced convection beneath the rotating crystal.

Oxygen transfer during single silicon crystal growth in Czochralski system with vertical magnetic fields

Oxygen transfer in silicon melts during crystal growth under vertical magnetic fields is investigated numerically and experimentally. A three-dimensional numerical simulation, including melt convection and oxygen transport, is carried out to understand how oxygen transfers in the melt under magnetic fields. Oxygen concentrations in single silicon crystals grown from the melt under these magnetic fields are experimentally measured by using an infrared absorption technique. The results obtained are compared to results from a numerical simulation. An anomalous increase is observed in the oxygen concentration of the grown crystals under a magnetic field of about 0.03 T. The cause of this anomaly is identified as Benard instability, since the temperature at the bottom of the crucible is higher than that at interface. When the temperature at the bottom is decreased, the Benard cell can be removed, and a monotonic decrease in the oxygen concentration in the single silicon crystals can be observed.

Global Simulation of Coupled Carbon and Oxygen Transport in a Unidirectional Solidification Furnace for Solar Cells

For an accurate prediction of carbon and oxygen impurities in multicrystalline silicon material for solar cells, global simulation of coupled oxygen and carbon transport in a unidirectional solidification furnace was implemented. Both the gas flow and silicon melt flow were considered. Five chemical reactions were included during the transportation of impurities. The simulation results agreed well with experimental data. The effects of flow rate and pressure on the impurities were examined. An increase in the flow rate can reduce both carbon and oxygen impurities in the crystal, though the reduction of carbon is more obvious. An increase in gas pressure can also obviously reduce the oxygen impurity but has only a small effect on the carbon impurity.

Flow instability of molten silicon in the Czochralski configuration

Abstract The flow instability of molten silicon in the Czochralski configuration has been studied by in-situ observation of melt convection using X-ray radiography and by temperature fluctuation measurement during crystal growth. Flow mode was dependent on an aspect ratio of the melt. For a deep, low aspect ratio melt, with growing crystal which is identical to shouldering process of the growth, the flow was unsteady and non-axisymmetric. For a shallow melt without crystal and crucible rotations, the flow was relatively steady and axisymmetric. However, flow became unsteady and non-axisymmetric for a shallow melt with crystal rotation. Amplitude of directly measured temperature fluctuation in the molten silicon for the case of unsteady and non-axisymmetric flow was larger than that for the relatively steady and axisymmetric flow. The flow instability area, which was also thermally unstable, was found to be larger in the crystal/crucible iso-rotation condition. In contrast Munakata and Tanasawa reported at the International Symposium on Supercomputers for Mechanical Engineering, September 1988, Tokyo that flow instability area was small for the silicone oil with larger Prandtl number.