The in-house High-Temperature Integrated Electronics and Sensors (HTIES) Program at NASA Lewis is currently developing a family of silicon carbide (SiC) semiconductor devices for use in high-temperature, high-power, and/or high radiation conditions. Conventional semiconductors, such as silicon and gallium arsenide (GaAs), cannot adequately perform under these conditions. Integrated electronics and sensors capable of operating in a hostile environment would have numerous important aerospace-related applications, such as propulsion control, power control, radar and communications, and radiation hardened circuits. They also would find numerous spinoff applications in the Earth-bound commercial power and automobile industries.
Theoretical appraisals have indicated that SiC power devices would operate over higher voltage and temperature ranges, have superior switching characteristics, and yet have die sizes nearly 20 times smaller than correspondingly rated silicon based devices. However, these tremendous theoretical advantages have yet to be realized in experimental SiC devices. Prototype SiC devices constructed to date have failed to operate at currents of more than 2 A. Investigations by the NASA Lewis HTIES research group recently identified the crystal defects restricting SiC power devices to these relatively small electrical current levels.
All useful SiC devices are fabricated starting from commercially available silicon carbide wafers. These wafers contain crystal defects called micropipes, small tubular voids that run through the wafers in a direction normal to the polished wafer surface. The NASA Lewis HTIES team showed that micropipe defects originating in SiC wafers are responsible for premature electrical failures in most SiC devices larger than 1 mm x 1 mm in area. In particular, the micropipes were experimentally linked to localized reverse failures at reverse-bias voltages well below the known breakdown voltage inherent to homogeneous SiC. This was found to be the case even when the device is fabricated entirely within high quality SiC epitaxial layers that have been grown on top of the original SiC wafer, due to the observed fact that the defects propagate from the wafer into the SiC epitaxial layers as they are grown. The accompanying figure shows the visible microplasma that is observed within a micropipe when an SiC diode junction fails prematurely.
The establishment of micropipes in SiC wafers as a defect limiting the performance of high-voltage SiC devices has serious implications for the near term realization of high-current SiC power devices. Until micropipe density is significantly reduced, SiC power device ratings will be restricted because the defects prevent scaleup to the large device areas (>1 mm x 1mm) needed to carry high current (>10 A). Higher than existing voltage and current ratings will not be attained until improvements in SiC crystal growth enable larger defect-free areas.
The announcement of these findings by the NASA Lewis HTIES team has triggered intensified research worldwide toward eradicating micropipe defects from SiC wafers. A twofold reduction in micropipes was recently announced by Cree Research, Inc.
Source: 1994 NASA Lewis Research & Technology Report