  Fully-processed SiC power metal-oxide-semiconductor field-effect transistors
(MOSFETs) emit light during switching of the gate terminal, while both drain
and source terminals are grounded. The emitted photons are caused by
defect-assisted recombination of electrons and holes at the 4H-SiC/SiO$_2$
interface and can be detected through the SiC substrate. Here, we present
time-gated spectroscopic characterization of these interfacial point defects.
Unlike in previous studies, the devices were opened in such a way that the
drain-contact remained electrically active. A separate examination of the
photons emitted at the rising and falling transitions of the gate-source
voltage enabled the extraction of two different spectral components. One of
these components consists of a single transition with phonon replicas of a
local vibrational mode (LVM) with an astonishingly high energy of 220 meV
$\unicode{x2013}$ well above the highest phonon modes in 4H-SiC and SiO$_2$ of
120 meV and 137 meV, respectively. Based on a quantum mechanical model, we
successfully fitted its emission spectrum and assigned it to donor-acceptor
pair recombination involving a carbon cluster-like defect. Other transitions
were assigned to EH$_{6/7}$-assisted, EK$_2$-D, and nitrogen-aluminum
donor-acceptor pair recombination. Due to the relevance of these defects in the
operation of SiC MOSFETs, these novel insights will contribute to improved
reliability and performance of these devices.
