Oxygen in the Czochralski (CZ) silicon crystal is incorporated during the crystal growth in which a silica crucible is used for containing the silicon melt and some silica dissolves into the silicon melt. As the first process of the oxygen transportation in CZ-Si crystal growth, the silica dissolution process, including the dissolution rate and formation of the interfacial phase (brownish ring) have been investigated. It is found that the dissolution rate can be con-trolled by changing the sample assembly. The dissolution rate is much faster in the region near triple junction of Si melt, silica glass and Ar atmosphere, which is the fundamental concept for the proposal of the sessile drop method. An approximate intrinsic silica dissolution rate has been obtained with the sessile drop method. The inter-facial phase of so-called "brownish rings" has been investigated using an in situ observation method. A formation mechanism of the brownish rings has been proposed based on the observation results. As another oxygen transportation process, oxygen equilibrium segregation coefficient has been investigated as follows. Single silicon crystals have been grown successfully under the equilibrium condition of a SiO_2-Si-SiO system in a closed silica ampoule with a vertical directional freezing method. The oxygen concentration in the grown single crystals was obtained using Fourier transform infrared spectroscopy (FT-IR) and secondary ion mass spectroscopy (SIMS) techniques as 1.7±0.1 (×10^<18> atoms/cm^3). The equilibrium segregation coefficient of oxygen was obtained by comparing the oxygen concentration in the silicon crystal to the oxygen solubility in the silicon melt. Then, the equilibrium oxygen segregation coefficient was determined to be 0.8±0.1.
When using the Czochralski method for growing large-diameter (>300 mm) silicon crystals, the control of oxygen concentration and its distribution in the crystals by crucible rotation is a serious problem. We have developed a new method to control oxygen concentration and its distribution by the electromagnetic force without crucible rotation. The electromagnetic force in the azimuthal direction in the silicon melt is generated by the interaction between the electric current (/) through the melt (in the radial direction) using an electrode and the vertical magnetic field (5). Using this method, the oxygen concentration in a small crystal (the diameter of 40 mm and the length of 200mm) was continuously changed from 10^<17> to 10^<18> atoms/cm^3. The homogeneous oxygen distributions in both the radial and the pulling directions were also achieved by gradually increasing melt rotation rate. Numerical simulation results of melt flow during EMCZ (Electromagnetic Czochralski) crystal growth showed that natural convection is almost suppressed and forced flow generated by electromagnetic force is dominant in the melt. From the results of experiment and numerical simulation, the continues change of oxygen concentration and the homogenization of oxygen distribution along the radial direction are attributed to the control of the diflusion-boundary-layer at both the melt/crucible and crystal/melt by forced flow due to the electromagnetic force. These things suggest that EMCZ method would be effective for the control of oxygen concentration and its distribution even in the large diameter silicon crystal.
Behavior of point defects in growing silicon crystal is discussed. Local equilibrium concentration is derived and compared to reported conventional ones. Stress effects on point defects are described such as concentration change around dislocation loops, effects of thermal stress and impurity doping effects. Temperature gradient and heat balance at the solid/liquid interface is discussed in detail because of their important role in determining point defect behavior.
We present the effect of nitrogen doping on the formation of grown-in defects and on the behavior of oxygen precipitate. By nitrogen doping, the morphology of voids becomes plate shape triclinic and the size of voids decreases (nitrogen concentration 〜10^<14> atoms/cm^3), while the oxygen precipitates are already existent at grown-in state with high volume density (nitrogen concentration >2×10^<15> atoms/cm^3) . The oxygen precipitate density after heat treatment shows constant value and is in-dependent of heat treatment temperature in nitrogen doped crystals. Implications for the mechanism of nitrogen doping effects are discussed.
This report involves the point defects behavior in heavily boron-doped Si crystals investigated by the observation of AOP (Anomalous oxygen precipitation) and dislocation clusters. With increasing boron concentration, it becomes difficult to observe the AOP and dislocation clusters. The reason is thought to be that both excess vacancy and interstitial silicon atoms decrease with increasing boron concentration. Various models for point defects behavior in heavily boron-doped Si crystals are reviewed and the shortcomings of each models are discussed.
The formation process of a dislocation cluster outside the oxidation induced stacking fault (OSF)-ring region during Czochralski (CZ) silicon crystal growth was investigated using a quenching and in situ annealing technique. Dislocation clusters were determined to be interstitial type from inside-outside contrast analysis of the transmission electron microscope (TEM) image. From the quenching experiment, the clustering temperature of dislocations which were formed by supersaturated self-interstitials and/or small dislocation loops was found to be about 1000℃ during growth. Furthermore, the characteristic axitial distribution of etching pit size was observed in the halted crystal. This could be explained by the change in concentration of supersaturated self-interstitials and the formation of a dislocation cluster due to the enhancement of diffusion of point defects.
"Sakurai-program" to draw crystal form as a function of time was capable to study the growth process of crystals such as quartz, pyrite and others. However, the program is restricted to be operated on specific type of computers be-cause the program was written by "N-88 BASIC". Therefore, the Sakurai-program was improved to operate on WINDOWS-95/98 by applying of "Visual Basic". The improved Sakurai-program provides several unique functions to draw crystal forms with faces≦100 and to demonstrate the changes of crystal forms as a function of growth rates ratios of the specific faces on crystals such as quartz and pyrite, as examples. The improved Sakurai-program is expected to be applied for educations in mineralogy and crystallography from the veiw point of crystal growth, in addition to the study tool of crystal growth process.