Relaxation Of Excited Carriers Research Articles

Time resolved study of carrier relaxation dynamics in α-Al2O3

The relaxation of excited carriers in α–Al2O3 is complex, depending for instance on the type of ionizing radiation. Using femtosecond time-resolved absorption spectroscopy, we can induce a controllable excitation density on a wide range, and follow the relaxation dynamics from 30 fs to 7 ns. We show that the excited carrier decay is non-exponential: it is dependent on the pump intensity, i.e. on the initial carrier concentration. We describe the relaxation as a two-steps process, involving the trapping of initially free electron–hole pairs, followed by recombination. A numerical model taking into account the initial electronic excitation by multiphoton absorption and the subsequent relaxation allows to quantitatively reproduce the amplitude of the measured absorption and its temporal evolution.

Perturbo: A software package for ab initio electron–phonon interactions, charge transport and ultrafast dynamics

Perturbo is a software package for first-principles calculations of charge transport and ultrafast carrier dynamics in materials. The current version focuses on electron–phonon interactions and can compute phonon-limited transport properties such as the conductivity, carrier mobility and Seebeck coefficient. It can also simulate the ultrafast nonequilibrium electron dynamics in the presence of electron–phonon scattering. Perturbo uses results from density functional theory and density functional perturbation theory calculations as input, and employs Wannier interpolation to reduce the computational cost. It supports norm-conserving and ultrasoft pseudopotentials, spin–orbit coupling, and polar electron–phonon interactions for bulk and 2D materials. Hybrid MPI plus OpenMP parallelization is implemented to enable efficient calculations on large systems (up to at least 50 atoms) using high-performance computing. Taken together, Perturbo provides efficient and broadly applicable ab initio tools to investigate electron–phonon interactions and carrier dynamics quantitatively in metals, semiconductors, insulators, and 2D materials. Program summaryProgram Title:PerturboCPC Library link to program files:https://doi.org/10.17632/34m2p6v79t.1Developer’s repository link:https://perturbo-code.github.ioLicensing provisions: GNU General Public Licence 3.0Programming language:Fortran, PythonExternal routines/libraries:LAPACK, HDF5, MPI, OpenMP, FFTW, Quantum-ESPRESSO, Wannier90Nature of problem: Computing transport properties from first-principles in materials, including the electrical conductivity, carrier mobility and Seebeck coefficient; Simulating ultrafast nonequilibrium electron dynamics, such as the relaxation of excited carriers via interactions with phonons.Solution method: We implement the first-principles Boltzmann transport equation, which employs materials properties such as the electronic structure, lattice dynamics, and electron–phonon collision terms computed with density functional theory and density functional perturbation theory. The Boltzmann transport equation is solved numerically to compute charge transport and simulate ultrafast carrier dynamics. Wannier interpolation is employed to reduce the computational cost.Additional comments: Hybrid MPI plus OpenMP parallelization is implemented to run large calculations and take advantage of high-performance computing. Most results are output to HDF5 file format, which is portable and convenient for post-processing using high-level languages such as Python and Julia.

Interaction of intense femtosecond laser pulses with KDP and DKDP crystals in the short wavelength regime

We investigate the electronic photo-excitation and relaxation mechanisms involved in the optical breakdown of potassium dihydrogen phosphate crystal (KH2PO4) and its deuterated form. The dynamics and spectroscopic properties of electron–hole pair formation are investigated using time-resolved measurement of the dielectric function, and luminescence spectroscopy. The non-common mechanical and electronic characteristics of these dielectric materials are revealed by the particular structure of ablation craters and also by the complex dynamics observed in the relaxation of excited carriers. This relaxation occurs in two steps, and varies with the initial carrier density and thus with the laser intensity. We show that the defect states play a key role in the excitation pathways, and also determine the relaxation stage. The latter also depends upon the initial amount of energy of the electron–hole pair after photo-excitation. A model based on kinetic equations describing the evolution of the different level populations allows us to successfully interpret and reproduce the experimental data.

Fast relaxation of hot carriers by impact ionization in semiconductor nanocrystals: Role of defects

We present calculations predicting that defects at the surface of semiconductor nanocrystals have a strong influence on the dynamics of hot carriers after photoexcitation. The relaxation of excited carriers by impact ionization of carriers trapped on deep levels is efficient, in particular in the presence of a band of defect states in the gap. Impact ionization of defects can also induce single-carrier multiplication but carriers generated in this way have a 1--100 ps lifetime due to multiphonon capture by the defects. These results are used to discuss recent experimental studies on carrier relaxation and multiplication in nanocrystals.

Spatial evolution of the generation and relaxation of excited carriers near the breakdown of the quantum Hall effect

We have measured the generation and relaxation of excited carriers along their drift direction near the breakdown of the quantum Hall effect (QHE). The dissipative resistivity ρ xx ( x) at current densities close to the critical value for the QHE breakdown was measured as a function of the distance x from the electron injection at x=0. By injecting “cold” electrons into constrictions at supercritical current levels, the evolution of the breakdown along the drift direction was monitored. After a smooth increase of the resistivity with the drifting distance, an avalanche-like rise towards a saturation value occurs. Drastic changes of the resistivity profiles with the applied current were found in a narrow range around the critical current. The observed behavior is attributed to impurity-assisted tunneling between Landau levels. By injecting hot electrons (excited in a periodic set of constrictions) into a region with subcritical current density, the relaxation process was analyzed. Inelastic relaxation lengths with typical values in the range from 0.3 to 4 μm were found, which agree within 10% with the elastic mean free path determined from the Hall mobility at zero magnetic field. We conclude that the energy relaxation process is triggered by scattering at impurity potentials.

Radiative recombination in short-period a-Si/SiO 2 superlattices

Photoluminescence and photoluminescence excitation spectra of short-period (⩽20 Å) a-Si/SiO 2 superlattices have been investigated under continous and picosecond laser excitation. Three types of radiative transitions (intersubband luminescence in a-Si layers, cross-luminescence between states of well and barrier layers and impurity luminescence in a-SiO 2 layers) were observed in ps excited SL, as contrasted to continous excitation when only impurity recombination in a-SiO 2 layers took place. A significant increase (by about 10 4 times) in the impurity luminescence intensity in a-SiO 2 was observed in superlattices compared to that in isolated a-SiO 2 layers. It is caused by a transfer of excited carriers between SL layers. Two types of hot luminescence were observed at transitions between the higher subbands of conduction and valence bands of a-Si layers and between subbands of a-Si nad a-SiO 2 impurity states in SLs. Relaxation of excited carriers between the subbands within the same band is low. No evidence was found of the influence of quantum size effects on deep impurity states in a-SiO 2. Fast nonradiative recombination in a-Si layers of SLs results in the absence of luminescence in these layers under continuous excitation.

Two types of hot luminescence in Si/SiO 2 superlattices

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of short-period (⩽ 20 A ̊ ) a-Si/SiO 2 superlattices (SLs) have been investigated under picosecond excitation. Two types of hot luminescence were observed at transitions between the higher subbands of conduction and valence bands of a-Si layers and between the subbands of a-Si and impurity states of a-SiO 2 in SLs. Relaxation of excited carriers between subbands within the same band was not observed.