Colorless silicon carbide gemstones

Details Colorless silicon carbide gemstones Colorless silicon carbide gemstones

2000-02-14

2001-03-13

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Synthetic precious stones

C501S088000, C501S089000, C063S032000

Reexamination Certificate

active

06200917

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the growth of silicon carbide crystals. More particularly, the invention relates to the growth of transparent, monocrystalline, colorless silicon carbide crystals. Because of their durability and other advantageous physical and crystal properties, these crystals may be cut and fashioned into finished gemstones having the fire and brilliance of diamond.
BACKGROUND OF THE INVENTION
Silicon carbide (SiC) is rarely found in nature. It has, however, been manufactured for more than eighty years, in crystalline form, for abrasive products. Silicon carbide crystals found in nature and in abrasive products are generally black and not translucent because they contain substantial levels of impurity atoms.
Because of the theoretically quite favorable electronic properties of silicon carbide, significant development activities were initiated during the 1960s and 1970s with the objective of growing large (bulk) crystals of low impurity silicon carbide for use in the production of semiconductor devices. These efforts finally resulted in the commercial availability of relatively low impurity, translucent silicon carbide crystals. These silicon carbide crystals are fabricated and marketed as very thin, green, amber or blue (175 &mgr;m-400 &mgr;m) slices useful for semiconductor devices.
Recently, it has been discovered that relatively low impurity, translucent, single crystal silicon carbide may be grown with a desired color and thereafter cut and fashioned into synthetic gemstones. These gemstones have extraordinary hardness, toughness, chemical and thermal stability, and a high refractive index that produces unparalleled brilliance.
The single crystals from which the gemstones are produced have been grown by sublimation according to techniques of the type described in U.S. Pat. No. Re. 34,061.
Silicon carbide crystals can be grown in a wide range of colors (including green, blue, red, purple. yellow, amber and black) and shades within each color by the appropriate selection of dopants (e.g., nitrogen and aluminum) and by varying the net doping densities (concentrations). Because of its wide bandgap, undoped (“intrinsic”) silicon carbide crystals in the hexagonal or rhombohedral forms are inherently colorless. Thus, silicon carbide crystals offer the potential to be cut and fashioned into gemstones of many various appearances, including that of diamond.
Although the colored crystals have proven relatively easy to produce, problems have been encountered in creating the exceedingly impurity-free sublimation system environment necessary for growing undoped, colorless silicon carbide. Because colorless silicon carbide gemstones have an extraordinary appeal, there is a need for a more cost effective and reliable process for growing large single crystals of silicon carbide in colorless form.
SUMMARY OF THE INVENTION
The present invention, in one broad aspect, is the discovery that large, transparent, colorless silicon carbide crystals may be grown in a sublimation system where compensating levels of n-type and p-type dopants are introduced into the crystals's lattice structure. The compensated levels of p-type and n-type dopants (i.e., roughly equal levels of the two dopants) serve to produce a colorless crystal by canceling each other in terms of the production of color centers in the crystal. The compensation is best carried out at low carrier levels. For example, the preferred n-type dopant, nitrogen, may be introduced into the lattice only at those low levels dictated by “background” atmospheric nitrogen present in the sublimation system. A like amount of p-type dopant, e.g., aluminum, may be introduced via the sublimation powder or gas at a level sufficient to compensate for the background level of nitrogen. Thus, in one aspect, the present invention may be described as a colorless single crystal of silicon carbide grown with compensated levels of ntype and p-type dopants. These crystals may be cut and fashioned into brilliant colorless synthetic gemstones.
In another aspect, the invention may be defined as a method of producing a colorless single crystal of silicon carbide comprising the step of growing the single crystal of silicon carbide by a sublimation technique wherein compensated levels of p-type and n-type dopants are introduced into the crystal lattice structure.


REFERENCES:
patent: Re. 34861 (1995-02-01), Davis et al.
patent: 3956032 (1976-05-01), Powell et al.
patent: 4866005 (1989-09-01), Davis et al.
patent: 4966860 (1990-10-01), Suzuki et al.
patent: 5030580 (1991-07-01), Furukawa et al.
patent: 5363798 (1994-11-01), Yoder
patent: 5433167 (1995-07-01), Furukawa et al.
patent: 5723391 (1998-03-01), Hunter et al.
Introduction to Ceramics, W.D. Kingery et al., Second edition, John Wiley & Sons, pp. 676-679, No Date.
Optical and Electronic Properties of SiC, W. H. Choyke, The Physics and Chemistry of Carbide, Nitries and Borides, Manchester, England, Sep. 1989, pp. 1-25.
Woo Sik Yee, “Bulk Crystal Growth of 6-H-Sic on Polytype Controlled Substrates through Vapor Phase and Characterization,” Journal of Crystal Growth; Dec. 2, 1991; vol. 15, No. 1/04 Amsterdam.

Affiliated with

Also associated with

Rating

Colorless silicon carbide gemstones does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Colorless silicon carbide gemstones, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Colorless silicon carbide gemstones will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2548827