Method and apparatus for producing a single crystal

1. A method for producing a single crystal having a diameter D by means of a floating zone method, wherein a precursor feed material is melted by an induction heating coil to form a melt zone, the single crystal crystallizes below a melt zone at a crystallization boundary, and the emission of crystallization heat is impeded by a reflector surrounding the single crystal, comprising heating the single crystal in a first zone in a region of an outer edge of the crystallization boundary by means of a heating device, wherein that first zone of the single crystal which is heated by means of the heating device has, in the longitudinal direction of the single crystal from an outer triple point T a at the outer edge of the crystallization boundary, a length L that is not longer than a distance Δ, the distance Δ being the axial distance between the outer triple point T a , and a center Z of the crystallization boundary.

2. The method of claim 1 , wherein the single crystal is heated with the aid of the heating device such that the distance Δ between the outer triple point T a at the outer edge of the crystallization boundary and the center Z of the crystallization boundary is not more than 90% of a distance Δ′ between the outer triple point T a and the center Z when the heating device is not used.

3. The method of claim 1 , wherein the electrical power of the induction heating coil is regulated in such a way that a chosen absolute value of a distance H in the melt zone between an inner triple point T i at an upper end of the melt zone and the outer triple point T a at the outer edge of the crystallization boundary does not change.

4. The method of claim 2 , wherein the electrical power of the induction heating coil is regulated in such a way that a chosen absolute value of a distance H in the melt zone between an inner triple point T i at an upper end of the melt zone and the outer triple point T a at the outer edge of the crystallization boundary does not change.

5. The method of claim 1 , wherein the single crystal is heated by means of a radiant heating system as a heating device.

6. The method of claim 1 , wherein the single crystal is heated by means of an induction heating system as a heating device.

7. The method of claim 1 , wherein the electrical power of the heating device is varied according to the length of the single crystal.

8. The method of claim 1 , wherein energy having an electrical power of not less than 2 kW is fed to the single crystal by the heating device.

9. The method of claim 1 , wherein the reflector impedes the emission of crystallization heat in a second zone, which is adjacent to and below the first zone and which has a length S in the longitudinal direction of the single crystal, wherein the sum of the lengths S and L corresponds to 0.5 to 1.5 times the diameter D of the single crystal.

10. The method of claim 9 , wherein a thermal radiation absorbing body surrounds the single crystal and absorbs thermal radiation facilitating emission of crystallization heat in a third zone which follows the second zone and which begins at a distance from the outer triple point T a which has at least the length of the diameter D of the single crystal.

11. The method of claim 1 , wherein the melt zone is formed by melting of a polycrystalline feed rod composed of silicon.

12. The method of claim 1 , wherein the melt zone is formed by melting of polycrystalline granular silicon.