(Invited) Unified Understanding of the Shockley Stacking Fault Formation in 4H-SiC Crystals under Thermal Equilibrium and Non-Thermal Equilibrium Conditions Based on the Quantum Well Action Concept
Authors: Ohtani Noboru 
Published: February 2020

It is well-known that SiC crystal deficiencies are delaying the market penetration of outstandingly superior SiC power devices. Efforts to date have centred on eradicating extended defects from 4H-SiC bulk crystals and epitaxial thin films. Among the extended defects existing in these materials, basal plane stacking faults (SFs) are one of the most serious concerns in SiC power devices because they are formed during the forward-bias operation of 4H-SiC PiN diodes and detrimental to SiC power devices, such as SiC MOSFETs as well as SiC BJTs and PiN diodes. In this phenomenon, i.e., the so-called “bipolar degradation”, single Shockley-type SFs (SSSFs) expand via the recombination of minority carriers (holes) at the partial dislocations in the Si core, which encompass the SSSFs. Recombination-enhanced dislocation glide (REDG) is widely accepted as the most probable kinematic mechanism of this phenomenon, and several other kinematic and energetic effects have been discussed in conjunction with the REDG mechanism.

Another negative effect of SF formation in SiC crystals is the spontaneous formation of double Shockley SFs (DSSFs) in heavily nitrogen-doped 4H-SiC crystals, which largely increases the resistivity of the crystals. When the nitrogen concentration exceeds the critical value (2–3 × 1019 cm−3), DSSFs are spontaneously formed in the crystals during high-temperature annealing (> 1100°C). DSSFs have been shown to heal (shrink) upon annealing at even higher temperatures (1800°C), and the co-doping of aluminum acceptors tends to reduce the density of DSSFs in heavily nitrogen-doped 4H-SiC crystals.

The above-mentioned two phenomena can be explained based on the concept of quantum well action (QWA). In the QWA, electrons in the conduction band of 4H-SiC enter the SF-induced quantum well, consequently decreasing the electronic energy of the system. However, from the viewpoint of thermodynamics, the two phenomena differ from each other. Whereas DSSF formation is a thermodynamically equilibrium process, SSSF expansion in SiC PiN diodes occurs under non-equilibrium conditions, such as the forward biasing of the PiN diode or ultraviolet light illumination of the materials. In this respect, it is worthy to note that SSSF expansion only occurs under non-equilibrium conditions and has never been observed in thermal equilibrium even when the nitrogen concentration in 4H-SiC crystals exceeds 1 × 1020 cm−3.

In this paper, I will try to clarify the physical origin of the driving force of the SSSF expansion during the forward-biasing of 4H-SiC PiN diodes via theoretical investigation of the energetics of the SF formation in 4H-SiC crystals under non-equilibrium conditions. Unified understanding of the SF expansion under thermal equilibrium and non-thermal equilibrium conditions will also be attempted based on the QWA concept. The similarity and dissimilarity in underlying physics between the SF formation in forward-biased 4H-SiC PiN diodes and that in heavily nitrogen-doped 4H-SiC crystals during high temperature annealing are discussed, taking into account the electron trapping into the SF and the resulting positive charge formation in the materials. The difference in the dimensionality of the electron systems in SFs (two-dimensions) and bulk 4H-SiC (three-dimensions) provides different energy dependencies of the density of states for electrons in the two systems and plays an important role in the temperature dependence of the SF expansion in 4H-SiC crystals.