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Full-Text Articles in Mechanics of Materials
Computational Design Of Two-Dimensional Transition Metal Dichalcogenide Alloys And Their Applications, John Douglas Cavin
Computational Design Of Two-Dimensional Transition Metal Dichalcogenide Alloys And Their Applications, John Douglas Cavin
Arts & Sciences Electronic Theses and Dissertations
The discovery of bronze as an alloy of copper and tin is arguably the earliest form of material design, dating back thousands of years. In contrast, two-dimensional materials are new to the 21st century. The research presented in this dissertation is at the intersection of alloying and two-dimensional materials. I specifically study a class of two-dimensional materials known as transition metal dichalcogenides (TMDCs). Because of the large number of transition metals, there are many combinations of TMDCs that can be alloyed, making experimental exploration of the phase space of possible alloys unwieldly. Instead, I have applied first-principles methods to study …
Mechanical Strength Of Germanium Doped Low Oxygen Concentration Czochralski Silicon And The Effect Of Oxygen On Nitrogen Dissociation In Silicon, Junnan Wu
McKelvey School of Engineering Theses & Dissertations
During the Czochralski growth of silicon, it is inevitable for oxygen to be incorporated into the silicon crystal from the quartz crucible. Interstitial oxygen improves the mechanical strength of silicon by pinning and locking dislocations, but also generates thermal donors during device processes, shifting the electrical resistivity. For silicon wafers used in radio frequency (RF) applications, it is important to ensure the high resistivity of the substrates for good RF characteristics. Therefore, the oxygen level in these high resistivity silicon wafers is kept very low (< 2.5 × 1017 atoms/cm3) by carefully controlling the Czochralski growth conditions, in order to reduce the thermal donor concentration to an acceptable level. Silicon on insulator (SOI) substrates made from high resistivity wafers have been widely used for RF applications. SOI manufacturing includes multiple high temperature thermal cycles (1000 – 1100 °C), during which the high resistivity wafers are prone to slip and warpage. Therefore, it is technologically important to recover some of the lost mechanical strength due to the lack of oxygen by introducing electrically inactive impurities to suppress the dislocation generation and mobility in silicon. Germanium (Ge) as an isovalent impurity is 4% larger in size and forms a solid solution with silicon in the entire concentration range. Previous works have shown Ge doping at high concentrations above 6 × 1019 atoms/cm3 increased mechanical strength of silicon with high oxygen concentration (~ 1 × 1018 atoms/cm3). In this work, we explore the effect of Ge doping (7 - 9 × 1019 atoms/cm3) on the mechanical strength of low oxygen concentration (< 2 × 1017 atoms/cm3) silicon, where the oxygen associated dislocation locking and pinning are very low. A mechanical bending test was used to study the average dislocation migration velocity and the critical shear stress of dislocations motion at 600 – 750 °C for Ge doped, nitrogen doped, and undoped low oxygen samples, as well as nitrogen doped float-zone and un-doped high oxygen concentration samples. Next, we fabricated SOI substrates using these high resistivity wafers and compared their slip generation rates and the slip-free epitaxial grow temperature windows after the high temperature thermal cycles (> 1000 °C). Our results indicate at lower temperature Ge doesn’t affect the dislocation mobility …