EPR study of conduction electrons in heavily doped n-type 4H SiC

Abstract

The magnetic properties of conduction electrons (CE) in heavily doped n-type 4H SiC have been studied with electron paramagnetic resonance (EPR) in the temperature interval from 7 to 140 K. An EPR line with “Dyson lineshape” was observed in 4H SiC wafers with (ND–NA) ≈ 1019 cm−3. The EPR line is composed of two overlapping lines with Lorentzian and Dysonian lineshape, which are responsible for the localized and delocalized spin systems attributed to electrons localized on the nitrogen donors (LE) and CE, respectively. The Dysonian line is characterized by an asymmetry parameter R. The coupling between the two spin systems, which are characterized by S = 1/2 is realized via an exchange interaction of CE with the LE. As a result the g-factor of the Dysonian spectral line is governed by an exchange interaction between LE and CE and was evaluated as g|| = 2.0045, g⟂ = 2.0003. The temperature dependence of the integral intensity of the CE EPR signal is described by the sum of Curie–Weiss and Pauli paramagnetism. The Curie-like contribution arises from unpaired spins in the impurity band of heavily doped 4H SiC, while a Pauli-like contribution that becomes noticeable at T > 80 K indicates the presence of the CE in the conduction band. From the analysis of the temperature dependence of R(T) and the linewidth ΔH(T) of the CE EPR signal the activation energy of the CE into the conduction band was estimated to be 39.4 meV. The obtained value of the activation energy coincides with that for nitrogen donors forming an impurity band in heavily doped n-type 4H SiC. Thus, the temperature behavior of the CE spin system in highly doped 4H SiC can be interpreted in terms of electrons that are freely moving over the impurity and conduction band. The CE present in the impurity band are strongly coupled with LE by cross-relaxation processes.