Silicon carbide (SiC) is a semiconductor material that outperforms pure silicon-based semiconductors in a variety of applications. SiC devices, which are mostly used in power inverters, motor drivers and battery chargers, offer advantages such as high power density and less power loss at high frequencies and even high voltages. While these features and relatively low cost make SiC a promising competitor in various 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 errors in 4H-SiC crystals. Simply put, small dislocations in the crystalline structure over time develop into large defects called “single Shockley stacking faults” that gradually degrade performance and cause device failure. While some methods exist to alleviate this problem, they make the device fabrication process more expensive.
Fortunately, a research team from Japan led by Associate Professor Masashi Kato of the Nagoya Institute of Technology has found a workable solution for this problem. In their study, available online on November 5, 2022 and published in Scientific Reports on November 5, 2022, they present a fault suppression technique called “proton implantation” that can prevent bipolar degradation in 4H-SiC semiconductor wafers when applied before the device. production process. Explaining the motivation for this study, Dr. “Even with the recently developed SiC epitaxial wafers, bipolar distortion persists in the substrate layers. We wanted to help the industry overcome this challenge and find a way to develop reliable SiC devices and, therefore, decided to explore this method to eliminate bipolar distortion,” Kato said. ” Associate Professor Shunta Harada of Nagoya University and academic researcher Hitoshi Sakane from SHI-ATEX, both based in Japan, were also part of the study.
Proton implantation involves “injecting” hydrogen ions into the substrate using a particle accelerator. The idea is to detect partial dislocations in the crystal, preventing the formation of single Shockley stacking errors, which is one of the effects of introducing proton impurities. However, proton implantation itself can damage the 4H-SiC substrate due to the use of high temperature annealing as an additional processing step to repair this damage.
The research team wanted to confirm whether proton implantation would be effective when applied before the device fabrication process, which typically includes a high temperature annealing step. Accordingly, they implanted different doses of protons on 4H-SiC wafers and used them to fabricate PiN diodes. They then analyzed the current-voltage properties of these diodes and compared them with those of a normal diode without proton implantation. Finally, they captured electroluminescent images of the diodes to check for stacking errors.
Overall, the results were very promising, as diodes that had undergone proton implantation performed as well as normal ones, but did not show signs of bipolar decay. The deterioration in the current-voltage characteristics of the diodes caused by proton implantation at lower doses was not significant. However, it was important to suppress the expansion of single Shockley stacking faults.
The researchers hope these findings will help realize more reliable and cost-effective SiC devices that can reduce power consumption in trains and vehicles. “Although the additional manufacturing costs of proton implantation will need to be considered, they will be similar to those occurring with aluminum-ion implantation, which is currently a key step in the fabrication of 4H-SiC power devices.” Kato speculates. “Furthermore, by further optimizing the implantation conditions, there is the possibility that this method could be applied 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.
materials provided by Nagoya Institute of Technology. Note: Content can be edited for style and length.