Laser internal modification slicing has emerged as a high-efficiency, low-damage technique for slicing single-crystal semiconductor substrates like 4H-SiC. However, its widespread adoption has been hindered by a limited mechanistic understanding of how laser energy couples with material response to govern phase transitions and crack propagation, especially at the atomic scale. In particular, the roles of nanosecond laser thermal effects in driving controllable phase transformation and the underlying crack dynamics remain unclear, making process optimization largely empirical. Here, by combining energy-controlled experiments with molecular dynamics (MD) simulations, we elucidate an energy-dependent, multistage phase transition pathway in laser-sliced 4H-SiC. This pathway progresses from initial amorphization with Si/C precipitation to thermal-stress-induced plastic slip that generates stacking faults and cubic 3C-SiC at the amorphous–crystalline interface. A critical energy threshold is identified that dictates the transition between distinct modification regimes and governs corresponding crack propagation behavior. Atomistic simulations further reveal the mechanisms of thermal stress-driven crack nucleation and propagation, along with a temperature-dependent fracture strength and shift in crystallographic cracking preference—from low-index planes under high temperature to high-index planes during cooling. The insights presented in this work bridge laser processing parameters with intrinsic material behavior, offering a mechanistic foundation for the rational design of laser-based slicing processes and achieving optimized processing quality. While demonstrated on 4H-SiC, the underlying energy- and stress-governed principles are applicable to a wider class of hard-brittle, anisotropic semiconductors, advancing the transition from empirical tuning to physics-informed manufacturing.
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