Modeling of Defect Related Reliability Phenomena
in SiC Power-MOSFETs
2.4 Magnetic Resonance Methods
Besides electrical device characterization, physical characterization methods allow to gain insight into the nature of the defects at the MOS interfacial and insulator layers. As such the Electron Spin Resonance (ESR) or Electron Paramagnetic Resonance (EPR) spectroscopy based on [164, 165] has served to identify Pb,0 and E′ centers as defects in the Si/SiO2 system. The measurement technique is based on the Zeeman effect that relates the spin energy state change of an electron with an externally applied magnetic field . Two spin states within an orbital exhibit an energy difference in opposing direction. In order to change its spin state, the electron has to absorb a photon of energy . Thus the fundamental EPR resonance equation yields
in which is the Bohr magneton and the g-factor associated with the interaction of both the external field and local magnetic fields due to spin-orbit coupling. In the experimental setup, the sample is placed within a Helmholtz coil to generate a homogeneous magnetic field and a spectrometer employing a microwave radiation source, a resonance cavity and a detector. The absorption spectrum is measured over varying which gives a unique fingerprint of the electron spin state transition, which can be uniquely related to theoretical calculations of atomic configurations employing unpaired electrons. This is typically achieved by extracting the factor at the measured field and frequency of the absorption line according to (2.5). Since the spin-orbit coupling is theoretically well understood, the -factor of the unpaired electron can be associated with a specific orbital.
A variant of EPR spectroscopy is the Electrically Detected Magnetic Resonance (EDMR) characterization method . Thereby the energy levels of electron donors and acceptors are extracted by switching the spin with a microwave pulse (same principle as in EPR). The recombination current that occurs when the electron moves to a lower energy state (which is possible after spin change due to Pauli’s exclusion principle) and recombines with a hole is detected. This technique allows for the detection of only a very small number of defects within a sample.