Epitaxial strengthening of crystals

#The invention relates to the properties of crystals, more particularly to the strengthening of crystals, especially for use in lasers.

#Single crystals are used in a variety of applications such as in the electronics industry and in the optics industry. In the optical arts, single crystals are grown for laser media, laser amplifiers, harmonic conversion and other uses. Well known examples of laser media single crystals are yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), and gadolinium scandium gallium garnet (GSGG).

#Much of the work in lasers today is in further development of high average power lasers. Such lasers must not only have high energy in a given laser pulse, but must also have a high repetition rate of those pulses. In the past, laser media were predominately in two geometries: the rod for smaller sizes and the disk for larger sizes. Rods could be crystalline or glass, and larger sizes were made of glass. Due to its low thermal conductivity, glass cannot rapidly remove the heat generated during laser operation. Also, most crystals are inherently stronger than glass and have other desirable spectroscopic properties. Thus, crystals are the favored material for the new laser media.

#FIG. 1 is a "zig-zag" slab amplifier geometry in which the slab is pumped through the large faces and the extraction beam zig-zag through the slab by total internal reflection; and

#FIG. 2 is a graph strength of GGG substrates with, and without, repressive epitaxial layers as a function of surface abrasion treatment.

#In choosing a crystalline material to be grown on the surface of another crystal in order to place that crystal in compression according to the present invention, a mismatch in the relative size of the lattice spacings of the crystals are used. The basic requirement is that the lattice spacing of the epitaxial crystal material be slightly larger than the lattice spacing of the substrate crystal to be strengthened. A preferred method for achieving this is to dope the epitaxial material with a dopant having a larger lattice spacing than in the unsubstituted crystal. Otherwise the epitaxial material can be the same or similar material as the substrate. The strain on the substrate surface has been found to be best in the 0.01% to 0.3% range with about 0.1% as the optimum. Strain is defined as substrate characteristic length subtracted from the corresponding epitaxial layer characteristic length divided by the substrate characteristic length. The characteristic length may be an average crystal lattice spacing.

#The following is a description of epitaxial layers grown according to the present invention which were used to demonstrate the concept. The substrates were 111.vertline.-oriented GGG prepared as one inch diameter, 0.020 inch thickness, wafers with low subsurface damage polish and a final colloidal silica polish on both sides. These were the same type of substrates as are used for the epitaxial growtn of iron garnets for bubble memory devices in the electronics industry. These wafers had small orientation flats in the 112.vertline. and 110.vertline. directions and were specified to have less than five dislocations over the central 80% of area, less than two defects per square centimeter over the central 80% of area, flatness better than six fringes using green (546 nm) light over the central 80% of area, and a taper of less than 0.0015 inch across the diameter. While the tests have been thus far on these substrates, the application could have been to the slab surfaces of a laser medium as well.

#Epitaxial growth was performed in a class 100 clean hood. The substrates were cleaned thoroughly with a mildly caustic cleaning solution and rinsed in deionized water before growth. A supercooled lead oxide flux (see Table 1) was used for the liquid phase epitaxial growth using the isothermal dipping techniques with rotation (200 revolutions/minute) of the horizontally held substrates. For more on this well known technique see H. J. Levinstein, et al., Appl. Phys. Lett. 19, 486 (1971).