Ultra-low field frequency-swept electrically detected magnetic resonance

We have developed a new ultra-low field frequency-swept (FS) electrically detected magnetic resonance (EDMR) spectrometer to perform sensitive EDMR measurements of 4H-silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors at sub-millitesla (mT) magnetic fields. The new spectrometer design enables the detection of so-called ultra-strong coupling effects such as multiple-photon transitions and Bloch-Siegert shifts. In this paper, we present a new spectrometer design and discuss ultra-low field FS-EDMR sensitivity to both multiphoton transitions and Bloch-Siegert shifts of the FS-EDMR response. FS-EDMR effectively eliminates the interference of the sub-mT EDMR response from a near-zero field magnetoresistance (NZFMR) phenomenon that pervades the sub-mT regime in a magnetic field-swept EDMR scheme. We discuss an automatic power leveling scheme, which enables frequency sweeping. We also present results illustrating the Bloch-Siegert shift of the FS-EDMR response. Finally, we study the two-photon transition line shape in the 4H-SiC transistor as a function of the static field, in which we observe a collapse of the two-photon linewidth with decreasing static field and compare our results to the theory of two-photon absorption in EDMR.