Keigo Hoshikawa

Czochralski Silicon Crystal Growth in the Vertical Magnetic Field

Czochralski (CZ) silicon crystals with striation free and microscopic homogeneous dopant concentrations are grown under the presence of an axially symmetric vertical magnetic field. The thermal convections of 3.5 kg of molten silicon in a crucible are successfully suppressed by a magnetic field of more than 1000 Oe in strength. Many advantages are found for the method developed in comparison with the conventional transverse magnetic field method in use.

Crystal growth of completely dislocation-free and striation-free GaAs

Abstract Completely dislocation-free and striation-free, semi-insulating GaAs crystals with 50 mm diameter are grown for the first time by means of the newly developed Czochralski process. These crystals are obtained by combining the following techniques; (i) a dislocation-free seed crystal is used to eliminate grown-in dislocations, (ii) the fully encapsulated Czochralski (FEC) method is applied in combination with indium doping to suppress stress-induced dislocations, and (iii) a vertical magnetic-field is applied to homogenize the distribution of doped indium.

Silicon crystal growth in a cusp magnetic field

Abstract Czochralski silicon crystal growth in the presence of an axially symmetric cusp magnetic field is reported for the first time. The free surface of the melt is centered between two superconducting coils. Oxygen concentration in the crystal is shown to be successfully controlled from 1X10 18 to 2X10 17 atoms/cm 3 by increasing the cusp magnetic field strength up to 3500 Oe at the center of the bottom melt-silica crucible interface, while keeping the crystal rotation constant at 30 rpm and keeping the crucible rotation constant at −10 rpm. Both oxygen and dopant distributions are homogenized by the magnetic field. The controllability of oxygen concentration is due to the advantageous characteristics of the cusp magnetic field to realize localized control of thermal convection at the melt-crucible interface, independent of that at the melt free surface. The good crystal homogeneity results because there is no need to change the crystal and crucible rotations adequately to control the oxygen concentration, in contrast to the previous use of a transverse or vertical magnetic field in which extreme changes in the crucible or crystal rotation rate are required.

Three-dimensional numerical analyses of the effects of a cusp magnetic field on the flows, oxygen transport and heat transfer in a Czochralski silicon melt

Abstract The effects of a cusp magnetic field on the flows, oxygen transport and heat transfer in an actual Czochralski silicon melt are quantitatively analyzed using boundary conditions based on experimental measurements and a numerical method involving the direct solution of the Navier-Stokes and Maxwell equations for three-dimensional unsteady flows. The unstable and instantaneously asymmetric flow components and forced comvections emanating from the rotating crucible bottom in the absence of a magnetic field are effectively suppressed when a cusp magnetic field is applied. On the other hand, the rotational flows caused by crucible rotation become uniform and the circumferential velocity increases upon application of a cusp magnetic field. As a result of these rotating melt flows together with crystal rotation in the opposite direction, strong stable forced convections are produced that move towards the centre directly below the growth interface. Due to these changes in the melt flows, the concentration of oxygen at the growth interface is decreased and the radial uniformity is improved; temperature fluctuations within the melt are significantly reduced; and the radial temperature gradient is increased, whereas the temperature gradient decreases with respect to depth. These results regarding temperature and oxygen concentration are all in good agreement with experimental observations.

Liquid encapsulated, vertical bridgman growth of large diameter, low dislocation density, semi-insulating GaAs

The liquid encapsulated, vertical Bridgman growth process has been carried out for the growth of 3 inch diameter GaAs single crystals. Molten GaAs in a pyrolytic boron nitride curcible is gradually crystallized from seeded bottom to top of the melt by means of pulling down the crucible. In this method, B2O3 is used as an encapsulant to suppress decomposition and evaporation of the arsenic from the molten and crystalline GaAs. It was found that B2O3 prevents GaAs from directly contracting the crucible wall, thus suppressing grain boundary nucleations and enabling reproducible growth of the single crystals. The 3 inch diameter crystals grown by this method have dislocation densities of the 15 to 12 level of those grown by the standard LEC method. They are of high purity, undoped semi-insulating, and are suitable for integrated circuit substrates.

Growth and characterization of sapphire ribbon crystals

Abstract The potential for improving the crystal quality of sapphire prepared by the edge-defined, film-fed growth technique was investigate. Crystal imperfections were observed by optical microscopy and X-ray topography. Saphire ribbons were grown in single or multiple forms in an induction furnace with graphite susceptors and molybdenum reflectors. Voids, grain boundaries and dislocations appeared in the sapphire ribons in relation to the growth conditions. Voids were formed by increasing the growth rate above about 0.7 mm/min. Grain boundaries were formed by imperfect epitaxy between the seed and the melt, or by polygonization of grown-in dislocations. Stress-induced and grown-in dislocations were separately observed in the ribbons free from voids and grain boundaries. The stress-induced dislocations were mostly suppressed by cooling the ribbons at rates below 150°C/h. Grown-in dislocations were completely eliminated by realizing a convex interface by necking down or by using an inclined top die. Dislocation free sapphire ribbons were realized

Oxygen solubility and its temperature dependence in a silicon melt in equilibrium with solid silica

Abstract Oxygen solubility in a silicon melt is measured for the first time in an equilibrium with solid silica, the situation that occurs at the melt-crucible interface in a Czochralski silicon crystal growth. The solubility is well expressed as a function of temperature by C s = 4.0 × 10 23 exp(-2.0 × 10 4 / T) atoms/cm 3 in a range from 1698 to 1820 K (13–135 K above the melting point of silicon). This temperature dependence is considerably smaller than has been thermochemically estimated so far. Standard enthalpy change is obtained from the temperature dependence to be δH ° = 79 kcal / mol for the reaction of SiO 2 (solid) = Si (melt) + 2O (in melt), which is consistent with the known values of other oxygen related reactions. The solubility extrapolated to the melting point is 2.8 × 10 18 atoms/cm 3 , which is a little larger than those measured so far of 1.8 × 10 18 and 2.2 × 10 18 atoms/cm 3 for a floating melt in an oxygen atomosphere. Equilibrium with gaseous silicon monoxide or with gaseous oxygen may be a factor contributing to the small solubilities in the previous experiments.

Low Oxygen Content Czochralski Silicon Crystal Growth

A low oxygen content silicon crystal ingot with 2.5×1017 to 3.5×1017 atomscm-3 was obtained. It was grown by using the Czochralski crystal growth furnace heated with a graphite heater connected to a three phase electrical source, and by controlling the seed and the crucible rotation in conjunction with the fluid rotation caused by the rotating magnetic field of the system. Neither bulk stacking faults after oxidation in wet oxygen at 1100°C for 2 hours, nor oxide precipitates after being annealed in an argon atmosphere at 1000°C for 12 hours, were observed in the crystal.

Homogeneous Dopant Distribution of Silicon Crystal Grown by Vertical Magnetic Field-Applied Czochralski Method

Phosphorus-doped Czochralski (CZ) silicon crystals with less than 5% macroscopic radial and microscopic axial resistivity variations are grown in the presence of 1000 Oe vertical magnetic field strength. Crystals with extremely improved dopant homogeneity are facilitated by determining the appropriate crystal and crucible rotation rates corresponding to the residual thermal convection in the molten silicon. Experimental results can be consistently explained by flow model of forced and thermal convections which are stabilized by a magnetic field.

Growth and characterization of calcium niobium gallium garnet (CNGG) single crystals for laser applications

Abstract We have successfully grown new laser crystals with a disordered type structure, calcium niobium gallium garnet (CNGG), by the Czochralski technique. Undoped, Nd and, Tm and Ho doped CNGG single crystals about 25 mm in diameter and 70 mm in length were grown using Pt and Ir crucibles. The doped CNGG single crystals were found to have broad absorption spectra which are attractive for laser diode pumped lasers. The growth conditions, crystal quality and spectroscopic properties of the CNGG for laser applications are reported.