Natural Orbital Branching Scheme for Time-Dependent Density Functional Theory Nonadiabatic Simulations

The mechanism of the irradiation synergistic effect of silicon bipolar junction transistors explained by multiscale simulations of Monte Carlo and excited-state first-principle calculations

Neutron and γ-ray irradiation damages to transistors are found to be non-additive, and this is denoted as the irradiation synergistic effect (ISE). Its mechanism is not well-understood. The recent defect-based model [Song and Wei, ACS Appl. Electron. Mater. 2, 3783 (2020)] for silicon bipolar junction transistors (BJTs) achieves quantitative agreement with experiments, but its assumptions on the defect reactions are unverified. Going beyond the model requires directly representing the effect of γ-ray irradiation in first-principles calculations, which was not feasible previously. In this work, we examine the defect-based model of the ISE by developing a multiscale method for the simulation of the γ-ray irradiation, where the γ-ray-induced electronic excitations are treated explicitly in excited-state first-principles calculations. We find the calculations agree with experiments, and the effect of the γ-ray-induced excitation is significantly different from the effects of defect charge state and temperature. We propose a diffusion-based qualitative explanation of the mechanism of positive/negative ISE in NPN/PNP BJTs in the end.

The seeds and homogeneous nucleation of photoinduced nonthermal melting in semiconductors due to self-amplified local dynamic instability

Laser-induced nonthermal melting in semiconductors has been studied over the past four decades, but the underlying mechanism is still under debate. Here, by using an advanced real-time time-dependent density functional theory simulation, we reveal that the photoexcitation-induced ultrafast nonthermal melting in silicon occurs via homogeneous nucleation with random seeds originating from a self-amplified local dynamic instability. Because of this local dynamic instability, any initial small random thermal displacements of atoms can be amplified by a charge transfer of photoexcited carriers, which, in turn, creates a local self-trapping center for the excited carriers and yields the random nucleation seeds. Because a sufficient amount of photoexcited hot carriers must be cooled down to band edges before participating in the self-amplification of local lattice distortions, the time needed for hot carrier cooling is the response for the longer melting time scales at shorter laser wavelengths. This finding provides fresh insights into photoinduced ultrafast nonthermal melting.

Nonadiabatic Dynamics of Photocatalytic Water Splitting on A Polymeric Semiconductor

To elucidate the nature of light-driven photocatalytic water splitting, a polymeric semiconductor-graphitic carbon nitride (g-C₃N₄)-has been chosen as a prototype substrate for studying atomistic water spitting processes in realistic environments. Our nonadiabatic quantum dynamics simulations based on real-time time-dependent density functional theory reveal explicitly the transport channel of photogenerated charge carriers at the g-C₃N₄/water interface, which shows a strong correlation to bond re-forming. A three-step photoreaction mechanism is proposed, whereas the key roles of hole-driven hydrogen transfer and interfacial water configurations were identified. Immediately following photocatalytic water splitting, atomic pathways for the two dissociated hydrogen atoms approaching each other and forming the H₂ gas molecule are demonstrated, while the remanent OH radicals may form intermediate products (e.g., H₂O₂). These results provide critical new insights for the characterization and further development of efficient water-splitting photocatalysts from a dynamic perspective.