Silicon carbide (SiC) is a semiconductor material that outperforms silicon-based semiconductors in several applications. Commonly used in power converters, motor drives and battery chargers, SiC devices offer advantages such as high power density and reduced power losses at high frequencies, even at high voltages. While these properties and relatively low cost make SiC a promising competitor in several sectors of the semiconductor market, its poor long-term reliability has been an insurmountable barrier for the past two decades.
One of the most pressing problems with 4H-SiC – a type of SiC with superior physical properties – is bipolar degradation. This phenomenon is caused by the expansion of stacking faults in 4H-SiC crystals. Simply put, small dislocations in the crystal structure grow over time into large defects called “single Shockley stacking errors” that gradually degrade performance and cause the device to fail. While some methods exist to fix this problem, they make the device manufacturing process more expensive.
Fortunately, a team of researchers from Japan led by Nagoya Institute of Technology Associate Professor Masashi Kato has now found a viable solution to this problem. In their study made available online November 5, 2022 and published November 5, 2022 in the journal Scientific Reports, they present an error suppression technique called “proton implantation” that can prevent bipolar degradation in 4H-SiC semiconductor wafers when placed ahead of the device. applied manufacturing process. Dr. Explaining the motivation for this study, Kato says, “Even in the recently developed SiC epitaxial wafers, bipolar degradation persists in the substrate layers. We wanted to help the industry navigate this challenge and find a way to deliver reliable SiC devices, and , therefore decided to investigate this method for eliminating bipolar degradation.” Associate Professor Shunta Harada of Nagoya University and Hitoshi Sakane, an academic researcher from SHI-ATEX, both in Japan, were also part of this study.
Proton implantation involves “injecting” hydrogen ions into the substrate using a particle accelerator. The idea is to prevent the formation of some Shockley stacking faults by pinning down partial dislocations in the crystal, one of the effects of introducing proton impurities. However, proton implantation itself can damage the 4H-SiC substrate, so high-temperature annealing is used as an additional processing step to repair this damage.
The research team wanted to verify whether proton implantation would be effective when applied before the device fabrication process, which typically includes a high-temperature annealing step. Accordingly, they applied proton implantation at different doses to 4H-SiC wafers and used them to fabricate PiN diodes. They then analyzed the current-voltage characteristics of these diodes and compared them to those of an ordinary diode without proton implantation. Finally, they took electroluminescence images of the diodes to check whether stacking faults had developed or not.
Overall, the results were promising, as diodes that had undergone proton implantation performed as well as regular diodes, but with no signs of bipolar degradation. The degradation of the current-voltage characteristics of the diodes caused by proton implantation at lower doses was not significant. However, the expansion suppression of some Shockley stacking errors was significant.
The researchers hope that these findings will help realize more reliable and cost-effective SiC devices that can reduce power consumption in trains and vehicles. “Although the additional fabrication costs of proton implantation should be considered, they would be comparable to those of aluminum ion implantation, currently an essential step in the fabrication of 4H-SiC power devices.” speculates Dr. Kato. “In addition, with further optimization of implantation conditions, there is an opportunity to apply this method for the fabrication of other types of devices based on 4H-SiC.”
Hopefully, these findings will help unlock the full potential of SiC as a semiconductor material to power next-generation electronics.
Material supplied by Nagoya Institute of Technology. Note: Content is editable for style and length.