The present invention relates to the growth of silicon carbide for semiconductor purposes, and to the seeded sublimation growth of large, high quality silicon carbide single crystals. The invention particularly relates to improvements that reduce the defect density in large single crystals grown using seeded sublimation techniques.
Silicon carbide has found use as a semiconductor material for various electronic devices and purposes in recent years. Silicon carbide is especially useful due to its physical strength and high resistance to chemical attack. Silicon carbide also has excellent electronic properties, including radiation hardness, high breakdown filed, a relatively wide band gap, high saturated electron drift velocity, high temperature operation, and absorption and emission of high energy photons in the blue, violet, and ultraviolet regions of the spectrum.
Single crystal SiC is often produced by a seeded sublimation growth process. In a typical silicon carbide growth technique, a seed crystal and a source powder are both placed in a reaction crucible which is heated to the sublimation temperature of the source and in a manner that produces a thermal gradient between the source and the marginally cooler seed crystal. The thermal gradient encourages vapor phase movement of the materials from the source to the seed followed by condensation upon the seed and the resulting bulk crystal growth. The method is also referred to as physical vapor transport (PVT).
In a typical silicon carbide growth technique, the crucible is made of graphite and is heated by induction or resistance, with the relevant coils and insulation being placed to establish and control the desired thermal gradients. The source powder is silicon carbide, as is the seed. The crucible is oriented vertically, with the source powder in the lower portions and the seed positioned at the top, typically on a seed holder; see U.S. Pat. No. 4,866,005 (reissued as No. RE34,861). These sources are exemplary, rather than limiting, descriptions of modern seeded sublimation growth techniques.
Current seeded sublimation techniques for the production of large bulk single crystals of SiC typically result in a high concentration of defects on the growing surface of the SiC crystal. High concentrations of defects cause significant problems in limiting the performance characteristics of devices made on the crystals, or substrates resulting from the crystals. For example, a typical micropipe defect density in some commercially available silicon carbide wafers can be on the order of 100 per square centimeter (cm−2). A megawatt device formed in silicon carbide, however, requires a micropipe defect free area on the order of 0.4 cm−2. Thus, obtaining large single crystals that can be used to fabricate large surface area devices for high-voltage, high current applications remains difficult.
Common defects found in crystals produced in the seeded sublimation production of SiC crystals include screw dislocations, particularly 1c screw dislocations. The nature and description of specific defects is generally well understood in the crystal growth art. In particular, a screw dislocation is defined as one in which the Burgers Vector is parallel to the direction vector. On an atomic scale, the resulting dislocation gives the general appearance of a spiral staircase. Other defects include threading dislocations, basal plane dislocations and micropipes. Clusters of Ic screw dislocations result in micropipes. These defects are present in crystal seeds as background defects, originating at the bottom of the seed and migrating to the surface.
More defects are introduced as a result of mechanical polishing of the surface of the crystal seed. These newly introduced defects typically reach 5-10 microns below the polished surface and are sometimes referred to as “subsurface defects.” They have characteristics of 1c or threading edge or basal plane defects, but tend to loop back to the crystal surface. If these defects remain in the seed crystal, they will tend to propagate into the growing crystal under growth conditions.
The presence of subsurface defects in bulk single crystals of SiC may also interfere with single-polytype crystal growth. The 150 available polytypes of SiC raise a particular difficulty. Many of these polytypes are very similar, often separated only by small thermodynamic differences. Maintaining the desired polytype identity throughout the crystal is only one difficulty in growing SiC crystals of large sizes in a seeded sublimation system. When surface defects are present, there is not enough polytype information on the crystal surface for depositing layers to maintain the desired polytype. Polytype changes on the surface of the growing crystal result in the formation of even more surface defects.
One technique used to remove such defects is hydrogen etching of the seed wafer at temperatures of 1600° C. or greater. Hydrogen etching, however, is a difficult and expensive process, and often results in etching of the silicon face of the seed as well as the growing surface of the seed. An etched Si face is undesirable because the etching process may enlarge pits and micropipes on the Si face, or create new ones, or both. Under growth conditions, these defects may then transmit as open void spaces through the seed into the growing crystal.
Another problem with current etching technology is that the etching is only efficient to depths of about 1 μm. It is estimated that subsurface damage resulting from the crystal growth process reaches depths of at least about 5 μm, and possibly deeper than about 10 μm. If these defects are not removed, the resultant devices grown on the SiC seed will have an unacceptable defect level.
Accordingly, it would be desirable to develop a method for efficiently removing subsurface damage on the growing surface of bulk single crystals of SiC, while protecting the opposing crystal face in order to produce large, high quality bulk single crystals of SiC.