First-Principles Study of p-n-Doped Silicon Quantum Dots: Charge Transfer, Energy Dissipation, and Time-Resolved Emission

Electron-phonon coupling controls nonradiative relaxation dynamics of the photoexcited electron-hole pair in semiconductor nanostructures. Here the optoelectronic properties for Al- and P-codoped silicon quantum dots (QDs) are calculated by combining time-dependent density matrix methodology and ab initio electronic structure methods. The energy-band landscape of the codoped Si QD is elucidated via time evolution of population density distributions in energy and in coordinate space. Multiple nonradiative relaxation pathways result in a specific charge-separated state, where a hole and an electron are localized on Al and P dopants, respectively. Analysis of the simulated nonradiative decay shows that high-energy photoexcitation relaxes to the band gap edge within 10 ps, forming the final charge-transfer state. We also simulate time-resolved emission spectra of the codoped Si QD that reveals optical and IR emissions below the optical band gap. These emission features are attributed to the intraband transitions introduced by doping.