Semi-insulating monocrystalline silicon carbide substrates are needed in order to provide the high resistivity necessary for RF passive behavior. In addition, resistivities of 5000 ohm-cm or higher are required for device isolation and to minimize back-gating effects; these values are also needed in order to reduce transmission line losses to an acceptable level.
Historically, semiconductor devices using these substrates have been fabricated using techniques that are inherently difficult to implement due to the high costs of equipment, fabrication and testing. These difficulties have been compounded by the fact that atomic-scale defects in crystalline structures deeply modify their basic properties and often limit their applications. In the case of silicon carbide, such imperfections have been shown to affect the ability to achieve superior microwave performance.
As a result, there has been a long and fruitful need for a method of producing semi-insulating silicon carbide single crystal wafers that are free of such undesirable deep level trapping elements. This need has been largely unmet until now.
According to the present invention, a high purity, hydrogen passivated source powder is heated and sublimated at temperatures that induce the formation of an optimal number of point defects for semi-insulating silicon carbide growth upon a seed crystal. The heating, sublimation and condensation steps continue until a desired amount of highly pure, semi-insulating silicon carbide crystal grows upon the seed crystal in an ambient that is not contaminated with deep level trapping elements of an undesirable nature.
The growing crystal preferably has a polytype selected from among the 3C, 4H, 6H and 15R polytypes of silicon carbide. The growth process can be referred to herein as seeded sublimation growth, although in fact the crystal may be grown in a conventional silicon carbide growth crucible and then cut or sliced.
Further, it is desirable to have a method of minimizing the undesirable levels of nitrogen in the silicon carbide source powder. This is particularly important in a seeded sublimation growth system since the silicon carbide source powder is a crucial component of the apparatus that is used for the sublimation growth steps, as described herein.
In a preferred embodiment, the silicon carbide source powder is heated in a hydrogen containing atmosphere to minimize the undesirable nitrogen content in the silicon carbide source powder. The temperature of the silicon carbide source powder is preferably maintained at a preferred upper limit of about 2400deg C., which is a relatively common practical temperature in the field of chemical vapor deposition of silicon carbide from source gases.
After the heating step is complete, the source powder is heated again in a hydrogen containing environment until a desired amount of silicon carbide growth has occurred upon the seed crystal. The temperature of the source powder is preferably maintained at a further preferred upper limit of about 2400deg., which is a relatively common practical thermal limit at atmospheric pressure in the field of silicon carbide growth.
It is an essential step in the sublimation growth process to maintain the source powder and seed crystal at these high temperature limits during the sublimation growth step in order to optimize the number of point defects that are induced to render the crystal semi-insulating. The method of the present invention is a simple, economical and readily available method for achieving these important functional and practical advantages in silicon carbide seeded sublimation growth.
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