Ekyo Kuroda

The effect of temperature oscillations at the growth interface on crystal perfection

Abstract Dislocation-free silicon crystals are grown with the Czochralski technique under various growth conditions, including changed heater positions, pulling rates, and crystal and crucible rotation rates. The correlation between the microdefect density in crystals and temperature oscillations near the growth interface is investigated. It is found that the microdefect density decreases with higher heater position, increased pullong rate and decreased crystal rotation rate. Theseresults are in accord with the behavior of the temperature oscillation amplitude. Thus, the density of microdefects decreases as the oscillation amplitude is reduced. The relation between microdefect density and temperature oscillation amplitude is well explaines by the crystal remelt model. The microdefect density in a crystal varies for annealing at more than 1000δC immediately after growth. Thus, the reduction of microdefect density by annealing depends on the defect distribution in the as-grown crystals.

Influence of growth conditions on melt interface temperature oscillations in silicon czochralski growth

Abstract The gradient and oscillation amplitude of temperature are accurately measured in the melt and solid during silicon crystal growth by the Czochralski technique. Temperature oscillations near the growth interface are closely related to the growth of high quality silicon crystals. The oscillation amplitude decreases as the heater position is raised or the pulling rate is increased. The amplitude also decreases with a reduction of the crystal rotation rate but is independent of the crucible rotation rate. The amplitude depends on the melt temperature gradient near the growth interface and on the shape of the growth interface. These results should contribute to the fabrication technology of semiconductor substrates for very large scale integrated circuits (VLSI).

Temperature oscillation at the growth interface in silicon crystals

Abstract Temperature oscillations in the melt and in the solid were accurately measured with a fine thermocouple having a diameter of 0.1 mm during silicon crystal growth. The temperature oscillation amplitude is large in the bulk melt, small near the growth interface, and nearly zero in the crystal. In particular, the oscillation amplitude becomes maximum in the melt near the growth interface under the growth conditions of a small oscillation amplitude. The amplitude near the growth interface varies largely from 0.8 to 4°C when the heater position is changed.

Growth and characterization of polycrystalline silicon ingots from metallurgical grade source material

Abstract Silicon crystals are grown from 98% pure metallurgical-grade source material by the Czochralski technique in an attempt to apply them to solar cells. Their crystallinity, impurity content and electrical properties are investigated. Inclusions, observed in the crystals grown at higher rates than 1 mm/min, or when the solidified fraction is over 0.4, are identified as Si-A1 alloys by electron probe micro-analysis. The impurity concentration in the crystals depends mainly on the growth rate. The growth rate of 0.5 mm/min is found to be the optimum for preparing relatively pure crystals. The impurity concentration cannot be decreased to the level expected from the segregation coefficient. This reason is considered to be due to the existence of small-sized inclusions and the formation of cellular structure.

Czochralski Growth of Square Silicon Single Crystals

Square single crystal silicon ingots with 3-inch sides are grown by forming an essentially uniform temperature distribution around the growing ingots. The resistivity distribution pattern for wafers from these ingots is generally square. The square single crystals have etch pit densities of 1-2×103/cm2. The growth for square crystals is explained by a model in which supercooling occurs in the radial direction of the ingots.

Problems in Growth of 4-Inch Wide Silicon Ribbon Crystals

Ribbon crystals with 4-inch width are grown using a specially designed rectangular heater. Wide ribbons crack spontaneously during crystal growth or handling. Thermal stress in crystals is calculated using the observed temperature distribution in crystals. It is clarified that larger maximum shear stress is concentrated around the center of the crystal in wider ribbons. Local thermal stress leads to residual stress, which is the main reason for spontaneous cracks in these ribbons.

Wide silicon ribbon crystals

Abstract An improved edge-defined film-fed growth (EFG) apparatus is used for the growth of wide silicon ribbon crystals. This apparatus utilizes a specially designed heater and a thermal radiation modifier, which facilitate an appropriate temperature profile at the die top where crystals grow. A ribbon crystal as wide as 84 mm is obtained.

Efficient Solar Cells from Metallurgical-Grade Silicon

In order to examine the possibility of utilizing metallurgical-grade silicon (MG-Si) for terrestrial photovoltaic use, solar cells are fabricated either by epitaxial growth or by direct diffusion on a substrate which is obtained by Czochralski crystallization conducted once or twice. Even by single crystallization of this material it is possible to obtain 10% efficiency if a 50 µm active layer is grown on the substrate crystal. A 10.5% (11.9% on the active area) cell is realized for the case where this material is subjected to acid-leaching prior to crystallization. A conversion efficiency as high as 8.5% is obtained for a directly diffused cell when a proper gettering process is applied.

Impurity Gettering of Diffused Solar Cells Fabricated from Metallurgical-Grade Silicon

Damage gettering of undesirable impurities is attempted for crystalline wafers grown from commercial and refined metallurgical-grade silicon. The feasibility of impurity gettering is investigated by fabricating diffused solar cells and evaluating the photovoltaic characteristics. In an O2 annealing, the fill factor of solar cells decreases to 0.25 due to junction leakage, while the factor is drastically improved by annealing in N2. A maximum conversion efficiency is attained to be 9.1% for single crystals grown from refined metallurgical-grade source. Dark current-voltage characteristics are investigated by preparing small-sized diodes into gettered wafers. It is found that damage gettering affects the remarkable reduction of junction leakage and also enhances minority carrier lifetimes in the bulk crystals.

Fabrication and Properties of Silicon Solar Cells Using Squarely Shaped Crystals

Square single crystal silicon ingots with 3-inch sides are grown by producing uniform melt temperature distribution in the crucible. The growth of square ingots is described in a model where supercooling is present at the growth interface, and where growth rates vary for different faces. A relatively high conversion efficiency of 12–13% is obtained from square solar cells. Inhomogeneities in the solar cells are revealed by studying laser-beam induced current images and crystal defect density distribution. The packing density in modules increases up to the 0.93–0.95 level. As a result, modular efficiency also increases by 20%, compared with when using circular wafers.