This Is AuburnElectronic Theses and Dissertations

Characterization of 4H-SiC/High-κ Dielectric Interfaces via First Principles Calculations




Wang, Lu

Type of Degree

PhD Dissertation




For power electronic devices, wide band gap (WBG) semiconductors are most attractive owing to their excellent physical properties, that they can sustain much higher voltages, frequencies, and temperatures than conventional semiconductors. As the most common power device, the metal–oxide–semiconductor field-effect transistor (MOSFET) has been driving the rapid exponential growth of modern semiconductor technology since the 1960s. With continuous development of device optimization and improvements in the production process, MOSFET fabricated based on the Si is reaching its performance limit. Instead, 4H-SiC has become prevalent for power device applications. However, the greatest challenge in 4H-SiC MOSFET is the low channel mobility that is primarily attributed to the poor SiC/SiO2 interface quality. Various chemical treatments have been adopted to reduce the interface trap densities, the channel mobility is still low. Alternatively, using high-κ dielectrics with suitable band gaps to replace the conventional SiO2 as gate insulators is a promising approach for improving the SiC device performance. In this study, we first produce an array of realistic models of the (0001) Si-face of the 4H-SiC with SiO$_2$ using first principles calculations within the density functional theory (DFT) to understand the role of nitrogen in the improvement of the SiC/SiO$_2$ interface. Electron density profiles of the model interfaces match those obtained by X-ray reflectivity (XRR), suggesting that nitridation of the interfaces from NO annealing yields a higher Si density near the inter-faces. Moreover, we find that the addition of N stabilizes the interface by reducing strain with no introduction of interfacial gap states. Subsequently, we screen high-κ dielectrics for 4H-SiC MOSFETs using first principles calculations within the DFT and identify alkaline earth metal fluorides AF$_2$ (A = Mg and Ca) and LiF as promising high-κ/large band gap dielectrics candidates. The properties of model interfaces formed using similar approaches to those of SiC/SiO2 systems, as well as the 4H-SiC/Al$_2$O$_3$ interface models are analyzed and the band offsets of these 4H-SiC/high-κ dielectrics interfaces are compared against limited experimental data. We find that O terminated 4H-SiC (0001) surface is advantageous to LiF/4H-SiC and AF$_2$/4H-SiC (A = Ca and Mg) interfaces in reducing mid-gap states. We note that both H and N passivated Al$_2$O$_3$/4H-SiC interfaces effectively remove the mid-gap states. Lastly, we investigate other alternative high-κ dielectrics for 4H-SiC, such as MgO and Mg$_x$Ca$_{1-x}$O. The electronic band structures of MgO/4H-SiC and Mg$_x$Ca$_{1-x}$O/4H-SiC heterostructures are analyzed as well as the band offsets. Overall, this work offers insights on the potential use of alternative high-κ dielectric materials in wide band gap electronic applications.