Silicon carbide (SiC) is a very promising material for high temperature, high frequency and high power applications in electronic devices. However, the commercialization of many SiC-based electronic devices has been very challenging due to the presence of a wide variety of extended defects. In order to improve the performance of SiC, numerous studies about the formation and the propagation of defects during crystal growth have been carried out. Although the results contributed to major advancements in the technology, facilitating the commercialization of these materials, the formation and proliferation of extended defects has not been fully understood yet.
While there is a wide array of different extended defects within SiC, three of the most detrimental have been threading dislocations, in-grown stacking faults and recombination-induced stacking faults (RISFs). In particular, RISFs have been difficult to manage, because they expand during device operation and lead to continuous increases in the turn-on voltage of bipolar devices, such as pin diodes. The expansion is induced through the recombination of free carriers near the RISFs. Understanding the mechanism for their motion is tantamount to their mitigation.
Electroluminescence is commonly used to identify the extended defects: RISFs emit in the violet at 2.89 eV (430 nm) and the partial dislocations that bound the faulted regions are known to emit in the red at 1.8 eV (690 nm). In 4H-SiC, partial dislocations were also recently observed to develop a green luminescence along the carbon-core partial dislocations during device operation. This emission is retained, even if the RISFs are contracted via annealing. Video 1 shows how the RISFs expand along various current injection times, and how the green luminescent centers move along the partial dislocations. This implies that not only RISFs do move under carrier injection within SiC, but that point defects such as boron impurities, can also be induced to move under such conditions.
IMA-EL™, Photon etc. hyperspectral imager, which is specifically designed to study electroluminescent materials, was used to acquire spectral and spatial information of the defects simultaneously. This exceptional technique allowed a rapid and accurate identification of the class of defects that contributes to the green emission in 4H-SiC.
The electroluminescence imaging of the SiC pin diode wasperformed after successive periods of device operation and subsequent annealing in nitrogen atmosphere at 700ºC to contract the RISFs (Figure 1a). Following the expansion of the RISFs again, the electroluminescence from the device was collected in the 400-780 nm spectral range, with a step of 2 nm and an exposition time of 30 s. The single monochromatic images collected with IMA™ allowed the separation of the various classes of defects. In particular, Figure 1b shows the peak emission of RISFs, centered at 424 nm, and Figure 1c-d the partial dislocations at 534 nm and 720 nm. The spectral response of the two regions labelled with “1” and “2” (Figure 2) confirmed that the PDs show a similar sharp emission at 424 nm due to the RISFs, and a broader emission at 530-540 nm. Combining spectral and spatial information, it was possible to attribute the latter emission to mobile boron impurities.
Photon etc's hyperspectral imager was paramount to identify the luminescence band of the various classes of faults, and will empower a better understanding about the formation of defects and their propagation in SiC materials.
Video and images used with permission from J.D. Caldwell, A. Giles, D. Lepage, D. Carrier, K. Moumanis, B.A. Hull, R.E. Stahlbush, R.L. Myers-Ward, J.J. Dubowski, and M. Verhaegen Appl. Phys. Lett. 102, 242109 (2013). Copyright 2013, AIP Publishing LLC. This material may be downloaded for personal use only. Any other use requires prior permission of the author and the AIP Publishing LLC.
Further reading on RISFs and their expansion and contraction:
J.D. Caldwell, R.E Stahlbush, K.D. Hobart, O.J. Glembocki, and K.X. Liu Appl. Phys. Lett. 90, 143519 (2007).
J.D. Caldwell, O.J. Glembocki, R.E. Stahlbush, and K.D. HobartAppl. Phys. Lett. 91, 243509 (2007).
J.D. Caldwell, R.E. Stahlbush, M.G. Ancona, O.J. Glembocki, and K.D. Hobart J. Appl. Phys. 108, 044503 (2010).