Patent ID: 6969502

Claim:
A large-volume monocrystal having a diameter of at least 300 mm and a height of at least 140 mm, a maximum refractive index variation through a crystal volume less than or equal to 2Ã—10 âˆ’6 and a stress birefringence SBR than 2 nm/cm; wherein said large-volume monocrystal is made by a method comprising the steps of: a) providing a device comprising a housing ( 10 ) with a closable charging and discharging opening, a melting vessel ( 20 ) disposed in said housing ( 10 ) and enclosing a melting space ( 30 ), said melting space ( 30 ) being bounded by a bottom ( 24 ), side walls ( 22 ), an upper opening ( 26 ) opposite the bottom and optionally by a cover ( 28 ) for said upper opening, at least one heating element ( 50 , 50 â€²) for melting a crystal raw material mass placed in the melting space ( 30 ) and/or for maintaining a molten state of an already melted crystal raw material mass, said at least one heating element ( 50 , 50 â€²) being disposed above said melting vessel ( 20 ) and generating an axial heat flow extending axially between the bottom ( 24 ) and the opening ( 26 ) of the melting vessel, and at least one element ( 82 , 54 ) disposed at the side walls ( 22 ) of the melting vessel ( 20 ) so as to prevent a lateral heat flow; b) heating crystal raw material present in said melting vessel ( 20 ) by said at least one heating element ( 50 , 50 â€²) or introducing already melted crystal raw material at a temperature above a melting point of the crystal raw material to form a melt with a melt surface in the melting vessel; c) subsequently lowering the temperature to at least a crystallization point of the crystal raw material so as to form said monocrystal on the bottom ( 24 ) of the melting vessel ( 20 ), said monocrystal forming a solid/liquid phase boundary at which said monocrystal grows toward the melt surface in a direction perpendicular to said phase boundary; and d) maintaining a vertical axial temperature gradient between the bottom ( 24 ) of the melting vessel ( 20 ) and the upper opening ( 26 ) and preventing heat inflow and/or heat outflow through said side walls ( 22 ) by means of the at least one element ( 82 , 54 ), so that said solid/liquid phase boundary has a curvature radius of at least one meter.