Abstract: The cooling speed of the portion near the rear part of a single-crystal body and passing through the defect-forming temperature zone is kept the same as that of the front portion of the single-crystal body. Namely, the heater is kept in operation while pulling the single crystal silicon subsequent to forming the tail of the single crystal silicon and the cooling speed throughout the whole single-crystal body in the defect-forming temperature zone is kept below 15.degree. C./min (levels A and B). Furthermore, the length of the tail is preset in the process of pulling the single crystal silicon so that the single-crystal body cools down slowly while passing through the defect-forming temperature zone (level C).

Abstract: A method for fabricating a single-crystal semiconductor by means of CZ method is disclosed. The method separates the single-crystal semiconductor from the melt by increasing the lift rate when the growth of a crystal body is finished. By controlling the lift rate, the single-crystal semiconductor is then gradually cooled within a range of an arbitrary crystal temperature, thereby forming a concave separated surface. The single-crystal semiconductor is cooled at a rate of lower than 35.degree. C./min when the temperature of the separated surface is within a range between the melting point and 1000.degree. C., or by keeping the temperature of the separated surface within a range between 1250.degree. C. and 1000.degree. C. for more than 30 minutes. Therefore, no dislocation is introduced in the crystal body, and productivity is improved.

Abstract: When the pulling speed is V(mm/min), the temperature gradient along the crystal axis within the temperature range from the melting point of silicon to 1300.degree. C. is G1 (.degree. C./mm), the temperature gradient along the crystal axis within the temperature range from 1150.degree. to 1080.degree. C. is G2 (.degree. C./mm), and the octahedral-shaped void density is d (pieces/cm.sup.3), crystals are grown under a condition satisfying:V/G1>0.581.times.V.times.G2-(d-4.3.times.10.sup.3)/2.65.times.10.sup.6 and V/G1>0.25.In this way, the defect density is reduced to less than 1.times.106 pieces/cm.sup.3 and silicon single crystals having superior gate oxide integrity and semiconductor device yield are obtained.