Eiji Tokizaki

Density Variation of Molten Silicon Measured by an Improved Archimedian Method

The temperature dependence of the density of molten silicon was determined in the range from about 1,420° C to 1,650° C using a technique for accurate Archimedian density measurement. The temperature range was found to be divided into three regions in terms of the temperature coefficient of the density. An anomalous value in the thermal volume expansion coefficient of about 7.6×10-4°C-1 was observed just above the melting point. A thermal volume expansion coefficient of about 1.0×10-4°C-1 was obtained for the range from about 1,430° C to 1,540° C. Scattering of the density data was observed for temperatures higher than 1,540° C, for which the thermal volume expansion coefficient was estimated to be about 2.8×10-4°C-1. The time dependence of the density of molten silicon has also been examined. It was found to clearly increase for few hours after completion of melting, reaching a maximum, and thereafter decrease slowly.

Temperature Dependence of the Viscosity of Molten Silicon Measured by the oscillating Cup Method.

The viscosity of molten silicon was measured using a oscillating cup made of SiC. In the temperature range, from the solidification point of about 1,415° C to about 1,600° C, the viscosity ranged from 0.7 to 0.9 mPa s. The activation energy estimated from the data was 0.20 eV. An anomalous increase in the viscosity with decreasing temperature was observed for temperatures lower than about 1,430° C. In this temperature range, an anomalous temperature dependence of the density and surface tension have been observed. Our results thus indicate that the anomalous behavior of different properties occurs simultaneously, reflecting a kind of essential variation in the melt.

Oxygen Solubilities in Si Melt : Influence of Sb Addition

The oxygen solubility in liquid silicon, both undoped and Sb-doped, in equilibrium contact with SiO2 has been analyzed after quenching by the SIMS (secondary ion mass spectroscopy) technique in order to investigate the influence of antimony addition on the oxygen solubility. It has been shown that oxygen solubility in undoped silicon melt increases from 2.08×1018 to 2.29×1018 atoms/cm3 upon temperature increases from 1447 to 1542°C. The influence of antimony addition in the same temperature range was twofold. Addition of less than 1 at.% Sb caused practically no change in oxygen solubility, while addition of more than 1.3 at.% Sb appreciably increased both its level and its temperature coefficient.

Density measurement of molten silicon by an improved Archimedian method

A convenient technique for practicing accurate Archimedian density measurements of molten silicon was developed and the temperature dependence of the density was obtained in the range of about 1440 to 1640°C. The density at 1440°C was about 1% larger than the previously reported value. A change in the dependence slope was observed near 1540°C. Thermal volume expansion coefficients of about 1.0x10-4 and 2.8x10-4°C-1 were obtained for temperatures below and above 1540°C, respectively.

Emissivity of liquid silicon in visible and infrared regions

Normal spectral emissivity of Si melt in visible and infrared regions was determined by the direct measurement of thermal radiations from the melt and a blackbody cavity which was located close to the melt. The spectral emissivity slightly decreases with wavelength. The emissivity slightly changes with temperature. The spectral emissivity values in visible and in infrared region are 0.27 and 0.21, respectively. The wavelength dependence of the emissivity can be interpreted by a dielectric response of free electrons in the melt.

Evaporation of oxygen-bearing species from Si melt and influence of Sb addition

Evaporation loss from undoped and Sb-doped Si melt in both SiC-coated carbon and a silica crucible was measured in the temperature range of 1440–1560° C. For undoped Si melt in a silica crucible, the evaporation rate changed with temperature in an exponential manner and depended on the area ratio, A c/A s, where A c is the contact area of the Si melt and silica crucible, and A s the free surface area of the Si melt. There were three trends of the dependence, and the evaporation rate tended to reach a certain saturation value when the area ratio approached infinity, which can be regarded as the intrinsic evaporation rate due to oxygen saturation in Si melt. It was concluded that the oxygen concentration in the Si melt was not saturated under the present experimental conditions. Evaporation from Sb-doped Si melt seemed complex. Under the assumption that the evaporating species from an Sb-containing Si melt in a SiC-coated carbon crucible was antimony vapor only and that the Sb evaporation rate did not change in different crucibles, the total evaporation from the silica crucible was analyzed. From the linear time dependence and nonexponential temperature dependence, the evaporation rate excluding Sb evaporation was concluded to be that of some oxide species but not pure SiO. Such evaporation was greater than the evaporation of SiO from undoped Si melt; more than three times as large at the melting point of Si. Therefore oxygen reduction in highly Sb-doped Cz–Si crystals can be attributed mainly to the enhancement of oxygen evaporation.

Density Variations in Molten Silicon Dependent on Its Thermal History

Time-dependent density variations in molten silicon held at 1452° C for about 6 h depended on the thermal process prior to holding. A slow decrease was observed after melting and heating, but a slow increase followed cooling. Melt density reached the same stable value of 2.556 g/cm3 after both variations. Reflecting this variation, the density of the melt measured during heating was slightly higher than that measured during cooling. The effect of the crucible material on melt density was also studied. With an SiO2 crucible, oxygen impurities of about 1018 atoms/cm3 were detected in the resolidified melt. The temperature dependence of the melt density was basically the same as for a SiC-coated graphite crucible, but the anomalous increase in melt density for temperatures near the melting point seemed to be slightly steeper.

Direct measurement of spectral emissivity of liquid Si in the range of visible light

Normal spectral emissivity of liquid Si was determined by direct measurement of thermal radiations from a surface of the Si melt and a blackbody cavity. The spectral emissivity has little dependence on the wavelength. The emissivity is 0.27 for the wavelength from 500 to 800 nm and is about a half of that of solid silicon at the melting point. Temperature dependence of the emissivity is very small in the temperature range from the melting point to 1550 °C. Free‐electron model with a plasma frequency and relaxation time of the order of 1016 Hz gives a good agreement with the experimental result. That indicates the dominant effect of the free electrons on the optical properties of the liquid Si.

Variations in the physical properties of molten lithium niobate caused by doping with magnesium oxide

Abstract The density, surface tension and viscosity of congruently melted LiNbO3 liquids doped with up to 5 mol% of MgO were measured from 1250 to 1450°C. Doping LiNbO3 with 5 mol% MgO caused the density temperature coefficient to increase from 4.45 x 10-4 to 5.72 x 10-4 g/cm3· °C, and the surface tension temperature coefficient to increase from 7.98 x 10-2 to 1.75 x 10-1 dyn/cm·°C as compared with undoped LiNbO3 melt. These variations may explain the melt convection variations.

EMISSIVITY OF LIQUID GERMANIUM IN VISIBLE AND NEAR INFRARED REGION

The normal emissivity of liquid germanium was determined from the direct measurement of the thermal radiation from the liquid surface and a blackbody. The monochromatic emissivity in the visible region and the average emissivity in the near infrared region were measured to investigate the wavelength dependence. The emissivity of the liquid has little dependence on the wavelength and is 0.217±0.002 for the visible region (600–900 nm) and is 0.200±0.005 for the near infrared region at the melting point. The emissivity has a weak dependence on the temperature from 910 to 1160 °C. The weak dependence agrees well with a dielectric response of the free electrons.