Nanomechanics of Wide-Bandgap Semiconductors (GaN and 4H-SiC)


Student thesis: Doctoral Thesis

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Award date19 Aug 2020


Despite that the size effect of metallic materials has been well studied, mechanical properties of semiconductor materials have been much less studied. At bulk scales, they are usually brittle, resulting in that major focus was in its fracture mechanics. Our group recent years showed that silicon and diamond could have considerably enhanced elasticity and potential plasticity, suggesting the necessity to study elastic and plastic deformation of semiconductors at small scales due to their micro/nanomechanical applications such as MEMS, NEMS, and flexible electronics. In this thesis work, we focus on the size-dependent mechanical properties of two representative wide-bandgap semiconductor materials, silicon carbide (SiC) and gallium nitride (GaN), based on our developed multi-scale in situ quantitative micro-/nanomechanical characterization techniques.

For 4H-SiC, the elastic compression and tensile properties of 4H-SiC were also investigated. single-crystal 4H-SiC nanopillars with diameters ranging from 240 to 720nm were characterized by the combination of in situ SEM compression technique and in situ TEM compression technique. [0001] oriented pillars were fabricated, and loading-unloading tests are done to investigate the elastic compressive deformation of 4H-SiC. The deformations were fully reversible under loading-unloading tests with fully recoverable elastic strain up to ~8.8%, and the failures still occurred in brittle fracture without visible sign of plasticity. Direct tensile properties of 4H-SiC were investigated with top-down FIB fabrication technique, and the result shows that the tensile sample with diameters of ~120nm can be repeatedly stretched ~7.8%. Both the elastic deformation during compression and tensile processes attribute to the robust application of 4H-SiC in MEMS/NEMS and potential ESE by taking advantage of its wide-bandgap feature.

For GaN, we found the brittle-to-ductile transition phenomenon during the compression test of single-crystalline GaN pillars with diameters ranging from 1.5 μm to 400nm. In the uniaxial compression tests, there is a critical diameter value for pillars to develop cracks, while smaller pillars show metal-like plastic flow under room temperature. The brittle-to-ductile transition results from the competition between the generation of cracks and the initiation of dislocations and the TEM analysis indicates the plastic deformation of GaN nanopillars is dominated by dislocations. The tensile property of single-crystalline GaN is also investigated through different techniques. A novel push-to-pull device was used in the in situ mechanical TEM test, and the cold- welding effect of GaN was found during the tensile process. During the cold-welding process, samples were taken out from TEM to vacuum storage place, so that this process is a kind of cold welding without the irradiation of electron beam. Mechanical tests were done to verify the cold-welding process, and the result showed that the elongation and fracture force was higher than before. High-resolution TEM on the fracture surface indicated that recrystallization and surface attraction is responsible for the rebonding of monocrystalline GaN. The direct tensile elastic properties of top-down FIB fabricated [0001]-oriented gallium nitride was also investigated by in situ electron microscopy at room temperature. It shows that the GaN tensile sample with diameters of ~150nm can be repeatedly stretched ~6.6% elastic strain at room temperature, and fracture strain reaches 6.8% with fracture stress up to ~10GPa. This result attributes to the application of GaN in ESE. Dislocation activities in local microstructure were also found in the TEM fracture morphology analysis.

Our research provides a comprehensive and in-depth understanding of their micro/nano-mechanical properties and structure reliability, essential for technological perspectives in their reliable and high-performance devices applications. Moreover, the way that we can apply uniform, continuous, and reversible strains into to nanostructured SiC and GaN could be adopted for the so-called "elastic strain engineering" (ESE), which would play a vital role in the semiconductor industry, and some preliminary simulation studies showed that outstanding mechanical properties of SiC and GaN could lead to unusual physical features and unprecedented functions.