Two types of excited electron dynamics in zinc oxide

We present first-principle evaluations of the electron-phonon coupling strength parameter and associated characteristics of relaxation for the excited electrons in the conduction band of zinc oxide. The evaluations are based on the pseudopotential plane-wave approach to the electronic band structure, the density-functional perturbation theory for the calculations of phonons and electron-phonon interactions, and on the “Fermi golden rule” for evaluations of the electron relaxation time and the energy-loss time. The calculations demonstrate existence of two types of electron dynamics, the picosecond one for electrons near the bottom of the conduction band and the femtosecond for the higher energies. Sensibly good agreement with experimental data confirms the validity of the calculations.

Figure 1

The DOS calculated for the three grids of phonon wave vectors, and the number of phonon states for the $10\times 10\times 8$ grid normalized at maximum to unity.

Figure 2

The calculated phonon dispersion curves (solid lines) along symmetry directions in the Brillouin zone of the wurtzite-structure ZnO and the corresponding experimental data as well, open and solid circles, and diamonds (Refs. 32, 33, 34).

Figure 3

The dependence of the electron-phonon coupling strength parameter $\lambda$ on the number of phonon $\mathbf{q}$ vectors in the grid and on the broadening width $\eta$ for the three excess energies of the excited electron, 0.03, 1.53, and 3.03 eV. The open diamonds are the data for the $6\times 6\times 4$ grid of $\mathbf{q}$, open squares are for the $8\times 8\times 6$ grid and black diamonds are for the $10\times 10\times 8$ grid.

Figure 4

The dependence of $\lambda$, $\Gamma$, and $\Delta e$ on the excess energy of an excited electron $E-E_{\text{BCB}}$ and on the number of phonon $\mathbf{q}$ vectors in the grid.

Figure 5

The dependence of the relaxation time ${\tau }_{rel}$ and energy-loss time ${\tau }_{en}$ on the excess energy of excited electron in the conduction band of ZnO.