Displacement damage effects on thermalmechanical properties of 4H-SiC
Date
2023
Authors
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Publisher
University of Delaware
Abstract
Silicon Carbide (SiC) is a wide bandgap semiconductor material recently being used in replacement of traditional semiconductors for high-voltage power device applications. Radiation environments that exist terrestrially and in outer space pose a noticeable risk to electronics which can lead to cumulative degradation effects within the material known as displacement damage (DD). DD characterizes the defects in the lattice that can accumulate over periods of high energy particle exposure, which include the formation of vacancies, interstitials and Frenkel pairs. This thesis explores the mechanics and physics of thermomechanical property changes in single crystal 4H-SiC at various defect concentrations. The goal and motivation behind this research is to provide insight to the degradation of the material in extreme environments to ultimately assist the development of radiation-hardened electronics with applications in nuclear reactor monitoring, aerospace, and deep space exploration. ☐ Displacement damage effects are inherently atomistic spanning nano to micro level length scales through long range elastic fields and their spatiotemporal interactions. To map out the physical mechanisms that govern damage evolution and degradation of material properties, an atomistic simulation approach is applied to model vacancies, interstitials, and Frenkel pairs at various defect concentrations. Uniaxial loading is applied along the [100] direction to investigate the effect of softening from damage evolution in the lattice. The results on mechanical effects show that: (1) the defect concentration affects elastic modulus linearly; (2) elastic softening is insensitive to the defect type; (3) interstitials make the lattice behave as a ductile-type material; and (4) increases in defect density decrease the yield strength at a decreasing rate. Regarding thermal effects, linear thermal expansion coefficient (TEC) and the specific heat capacity at constant volume (cv) are explored. Results show that (1) Frenkel pair defects have the most significant effect, (2) vacancies have the second greatest impact on the TEC, while the interstitials do not show defect density dependency until a concentration of 10%, (3) defect concentrations beyond a critical value induce negative thermal expansion (NTE) in the lattice, (4) interstitials have a much larger impact on the internal energy of the system than it has on thermal expansion, (5) at room temperature, interstitials drop the energy needed to raise the temperature of the system over twice as much as that by vacancies, and (6) Frenkel pairs as a function of concentration are the dominating defect for cv, dropping the energy needed to increase the temperature system at an exponential rate. ☐ Overall, the effects from displacement damage show significant alterations to the thermomechanical properties of 4H-SiC as defect concentration increases. The findings and insights reported in this thesis are expected to offer solutions to the challenge in determining the root cause of macroscale thermomechanical degradation or premature burnouts of electronic devices under extreme conditions.
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Keywords
Displacement damage, Radiation, Silicon Carbide, Specific heat capacity, Thermal expansion, Young's modulus