Excited Carrier Dynamics Research Articles

Ultrafast Carrier Dynamics in Silicon: A Two-Color Transient Reflection Grating Study on a(111)Surface

The dynamics of excited carriers generated near a silicon surface were characterized on femtosecond time scales using the transient grating technique in the reflection configuration. For electrons in the energy range $1.4\ifmmode\pm\else\textpm\fi{}0.6\mathrm{eV}$ above the conduction band edge and their corresponding holes, the lifetime for relaxation through phonon scattering at carrier densities below ${10}^{20}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ was determined to be 240 fs. This relaxation time increased sharply for carrier densities higher than $5\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, providing the first direct evidence for charge screening of carrier-phonon scattering in Si.

Disorder-induced intervalley scattering rates of AlxGa1-xAs by empirical pseudopotential calculations

The probability rates for transitions between different states of the virtual crystal induced by a random distribution of the cations in the ternary alloy system AlxGa1-xAs are calculated in the Born approximation using local empirical pseudopotentials, which are consistent with newer Eg(x) data and always yield the same arsenic potential. Attention is paid to scattering between the inequivalent conduction band valleys at the high-symmetry points Gamma , X and L for material near the transition from direct to indirect band structure (cross-over), which is of importance for application to the dynamics of excited carriers in optical switching and memory devices. In order to describe the net rate of electron transfer between two valleys 1 and 2 by integral S12g1g2(f1-f2) dE, where g(E) and f(E) are the corresponding densities of states and distribution functions respectively, the scattering factor S12 and its energy dependence is obtained exactly by averaging individual transition matrix elements over all energetic resonant states belonging in each case to 1 or 2. In this connection we give for the first time theoretical values for the strength of disorder-induced X-L coupling and present energy-dependent time constants tau 12=(S12(g1+g2))-1, concerning the occupation-probability relaxation by disorder-induced intervalley scattering.

Femtosecond hot carrier energy redistribution in GaAs and AlGaAs

Excited carrier dynamics in GaAs and Al .2Ga .8As are investigated using femtosecond pump and continuum probe techniques. Absorption saturation measurements provide evidence for transient spectral hole burning due to split-off as well as heavy and light hole valence to conduction band transitions. The initial nonthermal carrier distribution thermalizes and assumes a broad energy distribution on a femtosecond time scale.

Femtosecond hot-carrier energy relaxation in GaAs

Excited carrier dynamics in GaAs and Al0.2Ga0.8As are investigated using femtosecond pump and continuum probe techniques. We observe absorption spectral hole burning arising from excited carriers generated by transitions from the split-off band as well as the heavy- and light-hole bands. Transient absorption saturation measurements indicate that the initial nonthermal carrier distribution thermalizes on a time scale of several tens of femtoseconds.

Gloucestershire Medical Association

Understanding and controlling of excited carrier dynamics is of fundamental and practical importance, particularly in photochemistry and solar energy applications. However, theory of energy relaxation of excited carriers is still in its early stage. Here, using ab initio molecular dynamics (MD) coupled with time-dependent density functional theory, we show a coverage-dependent energy transfer of photoexcited carriers in hydrogenated graphene, giving rise to distinctively different ion dynamics. Graphene with sparsely populated H is difficult to dissociate due to inefficient transfer of the excitation energy into kinetic energy of the H. In contrast, H can easily desorb from fully hydrogenated graphane. The key is to bring down the H antibonding state to the conduction band minimum as the band gap increases. These results can be contrasted to those of standard ground-state MD that predict H in the sparse case should be much less stable than that in fully hydrogenated graphane. Our findings thus signify the importance of carrying out explicit electronic dynamics in excited-state simulations.