Electronic excitations in magnesium epitaxy: Experiment and theory
The nonadiabatic response of the electronic system during growth of Mg films is investigated both experimentally by measuring chemicurrents in Mg/p-Si(001) Schottky diodes, and theoretically by time-dependent perturbation theory applied to first-principles electronic-structure calculations. Reverse currents are detected in the diodes when they are exposed to thermally evaporated Mg atoms. Dissipation of condensation energy to the electronic system as well as absorption of infrared photons due to heat radiation are the current-generating mechanisms. They can be distinguished by studying the dependence of the currents on the evaporator temperature and on the Mg film thickness. In contrast to the photocurrents, the chemicurrent is proportional to the Mg atom flux as it reproduces the enthalpy of Mg sublimation in an Arrhenius diagram. Independent measurements of photocurrents by use of an empty evaporator as a source of heat radiation provide further evidence for a chemicurrent contribution to the overall signal. The presence of chemicurrents in Mg epitaxy is further supported by simulations of monolayer growth and calculations of the pertinent rates for nonadiabatic electronic transitions in Mg adsorption. The simulations show that the grown surface is atomically rough with many step and kink sites. Adsorption at these sites is sufficiently exothermic to induce energetic electron-hole pairs that give rise to a detectable current across the Schottky barrier of the diode. The calculated spectra of the excited electrons and holes are found to display high-energy tails above 0.4 eV. While the contribution of the electronic channel to the dissipation of condensation energy is very small (less than 1%), the calculated probability for high-energy electronic excitations in
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