A sampling of high-resolution TEM images at SiC/SiO2 interfaces

Characterization of 4H-SiC MOSFETs

This project was part of my Ph.D. research while at the University of Maryland. It focused on analyzing the impacts of various fabrication processes on 4H-SiC wide bandgap MOSFET devices. Specifically, we were interested in the passivation of electrically active atomic-scale defects at the interface between SiC and SiO$_\sf{2}$. To analyze these defects, we used a combination of high resolution and scanning transmission electron microscopy (HRTEM and STEM), together with analytical TEM using electron energy loss spectroscopy (EELS).

Through the course of this project, we revealed that nitric oxide (NO) device annealing results in a narrowing of the atomic-scale transition layer that exists between SiC and SiO$_\sf{2}$ as the concentration of nitrogen increases. We further discovered using X-ray photoelectron spectroscopy (XPS) that the bonding state for Si atoms within the transition layer changes slightly upon NO annealing. Using advanced multivariate analysis techniques, we were able to probe the nature of these interfacial states using EELS. NO annealing was found to cause changes in the bonding of Si, C, and O at the interface, attributable to the presence of interfacial nitrogen. Finally, we investigated the impacts of boron- and phosphorus-based passivation strategies to see how their effects varied compared to NO. Both of these newer processing approaches had significantly higher influence on the structure of the oxide than NO, explaining the origins of some instabilities observed in their corresponding MOSFET devices.

This project was highly collaborative in nature, involving researchers from the U.S. Army Research Laboratory, Auburn University, and Rutgers University.

Some highlights of the project:

• Used high resolution TEM and electron energy loss spectroscopy to investigate the effects of post-processing on SiC MOSFETs
• Implemented novel EELS methodologies to probe the nature of the interfacial transition layer in SiC MOS devices
• Discovered unique electronic states of silicon in nitric oxide annealed devices using unsupervised machine learning EELS analyses
• Developed oxide spin-etching process with monolayer sensitivity for XPS depth profiling

Publications

Analysis of the electronic and chemical structure in boron and phosphorus passivated 4H-SiC/SiO$_\sf{2}$ interfaces using HRTEM and STEM-EELS

We report a transmission electron microscopy (TEM) study of the impacts of phosphorus and boron passivation processes at 4H-SiC/SiO$_\sf{2}$ interfaces.
Applied Physics Letters 113, 193503, 2018

Advanced Analytical Microscopy at the Nanoscale: Applications in Wide Bandgap and Solid Oxide Fuel Cell Materials

The atomic-level structure and chemistry of materials ultimately dictate their observed macroscopic properties and behavior. As such, an intimate understanding of these characteristics allows for better materials engineering and improvements in the resulting devices. In our work, two material systems were investigated using advanced electron and ion microscopy techniques, relating the measured nanoscale traits to overall device performance. First, transmission electron microscopy and electron energy loss spectroscopy (TEM-EELS) were used to analyze interfacial states at the semiconductor/oxide interface in wide bandgap SiC microelectronics. This interface contains defects that significantly diminish SiC device performance, and their fundamental nature remains generally unresolved. The impacts of various microfabrication techniques were explored, examining both current commercial and next-generation processing strategies. In further investigations, machine learning techniques were applied to the EELS data, revealing previously hidden Si, C, and O bonding states at the interface, which help explain the origins of mobility enhancement in SiC devices. Finally, the impacts of SiC bias temperature stressing on the interfacial region were explored. In the second system, focused ion beam/scanning electron microscopy (FIB/SEM) was used to reconstruct 3D models of solid oxide fuel cell (SOFC) cathodes. Since the specific degradation mechanisms of SOFC cathodes are poorly understood, FIB/SEM and TEM were used to analyze and quantify changes in the microstructure during performance degradation. Novel strategies for microstructure calculation from FIB-nanotomography data were developed and applied to LSM-YSZ and LSCF-GDC composite cathodes, aged with environmental contaminants to promote degradation. In LSM-YSZ, migration of both La and Mn cations to the grain boundaries of YSZ was observed using TEM-EELS. Few substantial changes however, were observed in the overall microstructure of the cells, correlating with a lack of performance degradation induced by the H2O. Using similar strategies, a series of LSCF-GDC cathodes were analyzed, aged in H2O, CO2, and Cr-vapor environments. FIB/SEM observation revealed considerable formation of secondary phases within these cathodes, and quantifiable modifications of the microstructure. In particular, Cr-poisoning was observed to cause substantial byproduct formation, which was correlated with drastic reductions in cell performance.
Ph.D. Thesis, 2016

Characterization of the Oxide-Semiconductor Interface in 4H-SiC/SiO$\sf_2$ Structures using TEM and XPS

Microscopy and Microanalysis, 2015

Systematic structural and chemical characterization of the transition layer at the interface of NO-annealed 4H-SiC/SiO$\sf_2$ metal-oxide-semiconductor field-effect transistors

In this work, we characterize the transition layer at the 4H-SiC/SiO2 interface as a function of nitric oxide (NO) post-annealing time using HRTEM and EELS. We confirm an inverse relationship between NO-anneal time and transition layer width, which correlates with improved channel mobility.
Journal of Applied Physics, 2013

Talks

Analytical Electron Microscopy of Interfacial States in 4H-SiC/SiO$_\sf{2}$ MOS Devices
Nov 30, 2016