Patent Number: 042773075
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention provides a method of restoring Si crystal lattice order after such crystal has been subjected to neutron irradiation whereby stable and reproducible electrical properties are attained in the so-treated crystals. In accordance with the principles of the invention, a neutron-irradiated carbon containing Si crystal is annealed over a time period of at least 30 minutes at a minimum temperature adjusted in accordance with the make-up of the irradiation flux (i.e., in accordance with the ratio of thermal to fast neutrons within such flux) and in accordance with the carbon concentration within the irradiated Si crystal. In an exemplary embodiment of the invention, a carbon containing Si crystal is irradiated with a neutron flux comprised of at least 99% thermal neutrons (i.e., the ratio of thermal to fast neutrons within such a flux is 100:1) and the so-irradiated crystal is then annealed for at least 30 minutes at a temperature at least equal to 700.degree. C., independently of the carbon concentration within the irradiated crystal. In another embodiment of the invention, a carbon containing Si crystal is irradiated with a neutron flux comprised of less than 99% thermal neutrons (i.e., the ratio of thermal to fast neutrons within such a flux is smaller than 100:1) and the so-irradiated crystal is then annealed for at least 30 minutes at a temperature adjusted in accordance with the actual make-up of the neutron flux and in accordance with the carbon concentration within the irradiated crystal. In instances in the foregoing embodiment wherein the proportion of thermal to fast neutrons within the irradiation flux is in the range of 1:1 to less than 10:1 and the carbon concentration within the irradiated crystal is greater than 3.multidot.10.sup.16 atoms/cm.sup.3, the annealing temperature is adjusted to be greater than 1100.degree. C. However, with the foregoing neutron flux, when the carbon concentration within an irradiated crystal is less than 3.multidot.10.sup.16 atoms/cm.sup.3, the annealing temperature is adjusted to be in the range of 750.degree. to 1000.degree. C. In instances in the foregoing embodiment wherein the proportion of thermal to fast neutrons within the irradiation flux is in the range of 10:1 to less than 100:1 and the carbon concentration within the irradiated crystal is greater than 3.multidot.10.sup.16 atoms/cm.sup.3, the annealing temperature is adjusted to be greater than 1000.degree. C. However, with the foregoing neutron flux, when the carbon concentration within an irradiated crystal is less than 3.multidot.10.sup.16 atoms/cm.sup.3, the annealing temperature is adjusted to be at least 750.degree. C. In other words, the foregoing embodiments of the invention comprise annealing neutron-irradiated carbon containing Si crystals at temperature conditions which are determined by the ratio of thermal to fast neutrons within the neutron flux produced by a nuclear reactor utilized to irradiate the crystals and by the carbon concentration within the so-irradiated crystals so that the annealing temperature is greater than 1000.degree. C. when the carbon concentration within the irradiated crystal is greater than 3.multidot.10.sup.16 atoms/cm.sup.3 and the annealing temperature is at least equal to 750.degree. when the carbon concentration within the irradiated crystal is less than 3.multidot.10.sup.16 atoms/cm.sup.3. In this manner, the annealing effect is independent from the furnace atmosphere existing during an annealing process. In the development of the invention, it was also determined that the minimum temperature for annealing Si crystal discs having a thickness of less than 2 mm and with the same carbon concentration as in relatively large diameter silicon rods, may be even further lowered relative to the annealing temperature required for such rods. As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention, excepting as it is set forth and defined in the hereto-appended claims.