Carrier-carrier Collisions Research Articles

Dissipation-enabled hydrodynamic conductivity in a tunable bandgap semiconductor.

Electronic transport in the regime where carrier-carrier collisions are the dominant scattering mechanism has taken on new relevance with the advent of ultraclean two-dimensional materials. Here, we present a combined theoretical and experimental study of ambipolar hydrodynamic transport in bilayer graphene demonstrating that the conductivity is given by the sum of two Drude-like terms that describe relative motion between electrons and holes, and the collective motion of the electron-hole plasma. As predicted, the measured conductivity of gapless, charge-neutral bilayer graphene is sample- and temperature-independent over a wide range. Away from neutrality, the electron-hole conductivity collapses to a single curve, and a set of just four fitting parameters provides quantitative agreement between theory and experiment at all densities, temperatures, and gaps measured. This work validates recent theories for dissipation-enabled hydrodynamic conductivity and creates a link between semiconductor physics and the emerging field of viscous electronics.

Terahertz radiation generation from short-pulse laser interaction with electron-hole plasmas

In semiconductors, electrons and their corresponding holes exist as free carriers that are created in pairs in inter-band transitions. At very high carrier densities, the carrier-carrier collisions dominate over carrier-lattice collisions and these carriers begin to behave collectively to form a plasma. In this study, we present phase-matched terahertz (THz) radiation generation from laser interaction with electron-hole plasmas. Two collinear laser pulses of finite spot size propagate in an electron-hole plasma to generate THz radiation. The lasers beat and exert a ponderomotive force on electrons and holes. The carriers oscillate with oscillatory velocity at the laser beat frequency. The laser beat wave interacts with the plasma density perturbation and produces a nonlinear transverse current with nonzero curl. This nonlinear current drives coherent THz radiation. Spacial density modulation of hole density is considered to satisfy the phase-matching condition for resonant interaction. The systematic analysis of this mechanism shows an efficient and tunable source of THz radiation, which varies with various parameters such as the density modulation, laser intensity and other laser parameters.

Large-signal characterization of millimeter-wave IMPATTs: effect of reduced impact ionization rate of charge carriers due to carrier-carrier interactions

In this paper, we study the effect of energy loss of charge carriers due to carrier-carrier interactions prior to impact ionization on the static and large-signal characteristics of double-drift region impact avalanche transit time (IMPATT) diodes based on Si designed to operate at millimeter-wave (mm-wave) atmospheric window frequencies such as 94, 140, and 220 GHz. The above mentioned effect has been incorporated in the simulation by taking into account a recently reported generalized analytical model of impact ionization rate of charge carriers based on multistage scattering phenomena in the base semiconductor. Results are compared with static and large-signal signal simulation results of the same diodes that we have reported earlier by taking into account the empirical relation of ionization rates fitted from the experimental data (experiment was carried out on IMPATT structures suitable for operating near 100 GHz). It is observed that both the large-signal RF power output and DC to RF conversion efficiency of the diodes are deteriorated significantly due to reduced ionization rates as a consequence of carrier-carrier collision events prior to the impact ionization. This effect is found to be more pronounced in 140 and 220 GHz diodes due to the enhanced carrier-carrier collisions within those diodes having greater background doping densities as compared to 94 GHz diode. The simulation results presented in this paper found to be in closer agreement with the experimental results as compared to the results that we have reported earlier.

A generalized analytical model based on multistage scattering phenomena for estimating the impact ionization rate of charge carriers in semiconductors

A generalized analytical model based on multistage scattering phenomena has been developed in this paper for estimating the impact ionization rate of charge carriers in semiconductors. The probabilities of impact ionization initiated by electrons and holes have been calculated separately by taking into account all possible combinations of optical phonon scattering and carrier-carrier collisions prior to the impact ionization. Finally the analytical expressions of impact ionization rate of electrons and holes have been developed by using the aforementioned impact ionization probabilities. The impact ionization rates of electrons and holes in 4H-SiC have been calculated within the field range of $$2.5\times 10^{8}$$2.5×108---$$6.5\times 10^{8}\hbox { V m}^{-1}$$6.5×108Vm-1 by using the analytical expressions of those developed in the present paper. Those are also calculated by using the analytical expressions developed by some other researchers earlier without considering the multistage scattering phenomena. Finally the theoretical results obtained from the analytical model proposed in this paper and the analytical model developed by earlier researchers within the field range under consideration have been compared with the ionization rate values calculated by using the empirical relations fitted from the experimentally measured data. Closer agreement with the experimental data has been achieved when the impact ionization rate of charge carriers in 4H-SiC are calculated from the proposed model as compared to the earlier one.

Surface plasmon polariton model of high-spatial frequency laser-induced periodic surface structure generation in silicon

In recent years, high-spatial frequency laser-induced surfaces structures have been generated in a large variety of dielectrics. In silicon subwavelength ripples, some of which featured periodicities below 100 nm, were formed using ultrafast lasers. We demonstrate for Si(100) surfaces that generation of a dense electron-hole plasma in the focal spot of ultrashort-pulsed laser light followed by massive excitation of plasma waves provides an explanation for the formation of such high-spatial frequency surface structures. The applied Drude-like model includes carrier-carrier collisions and is in excellent agreement with the experimentally observed ripple period.

Studies on the Effect of the Size of Polycaprolactone Microspheres for the Dispersion of Salbutamol Sulfate from Dry Powder Inhaler Formulations

To study the effect of the size of the surface-coated polycaprolactone (PCL) microparticle carriers on the aerosolization and dispersion of Salbutamol Sulfate (SS) from Dry Powder Inhaler (DPI) formulations. The microparticles were fabricated using an emulsion technique in four different sizes (25, 48, 104 and 150μm) and later coated with Magnesium stearate (MgSt) and leucine. They were characterized by laser diffraction and SEM. The Fine Particle Fraction (FPF) of SS from powder mixtures was determined by a Twin Stage Impinger (TSI). As the carrier size increased from 25μm to 150μm, the FPF of the SS delivered by the coated PCL particles increased approximately four fold. A linear relationship was found between the FPF and Volume mean Diameter (VMD) of the particles over this range. The dispersion behaviour of SS from PCL carriers was dependent on the inherent size of the carriers and the increased FPF of SS with increased carrier size probably reflects the higher mechanical forces produced due to the carrier-carrier collisions or collisions between the carrier particles and the internal walls of the inhaler during aerosolization.

Resonant third-harmonic generation of a short-pulse laser from electron-hole plasmas

In semiconductors, free carriers are created in pairs in inter-band transitions and consist of an electron and its corresponding hole. At very high carrier densities, carrier-carrier collisions dominate over carrier-lattice collisions and carriers begin to behave collectively to form plasma. Here, we apply a short-pulse laser to generate third-harmonic radiation from a semiconductor plasma (electron-hole plasma) in the presence of a transverse wiggler magnetic-field. The process of third-harmonic generation of an intense short-pulse laser is resonantly enhanced by the magnetic wiggler, i.e., wiggler magnetic field provides the necessary momentum to third-harmonic photons. In addition, a high-power laser radiation, propagating through a semiconductor imparts an oscillatory velocity to the electrons and exerts a ponderomotive force on electrons at the third-harmonic frequency of the laser. This oscillatory velocity produces a third-harmonic longitudinal current. And due to the beating of the longitudinal electron velocity and the wiggler magnetic field, a transverse third-harmonic current is produced that drives third-harmonic electromagnetic radiation. It is finally observed that for a specific wiggler wave number value, the phase-matching conditions for the process are satisfied, leading to resonant enhancement in the energy conversion efficiency.

Dynamics of Photoexcited High Density Carriers in ZnO Epitaxial Thin Films

Time-resolved luminescence spectra of ZnO epitaxial thin films were measured by the optical Kerr gate method in femtosecond time regime under resonant excitation of the excitonic state. We observed the relaxation of hot carriers until the buildup of luminescence due to an electron-hole plasma (EHP) state and recovery of the renormalized bandgap due to radiative recombination of electrons with holes in the quasi-thermalized EHP state. The relaxation of hot carriers mainly comes from carrier-carrier collisions but might include in part carrier-LO phonon scattering.

Power nonlinearities in the luminescence spectrum of [formula omitted] superlattices

We report on a time-correlated nonlinear photoluminescence study of GaAs AlAs short period superlattices at low to medium excitation densities (10 13 – 10 16 cm −3). The nonlinearity of PL intensity at doubling the excitation density is 1–10% and depends on luminescence wavelength revealing the internal structure of the luminescence line. The observed nonlinearities have been attributed to three main phenomena: repopulation of localized excitons by photogenerated hot excitations, filling of the (localized) exciton states and population of higher energy states by carrier-carrier collisions. Observed time-correlations allow one to differentiate between these contributions and establish the characteristic densities and time constants.

Relaxation processes of hot holes in Ge and GaAs investigated by ultrafast infrared spectroscopy

Relaxation phenomena of holes in p-type Ge and GaAs are studied by ultrafast infrared spectroscopy in the wavelength range from 0.8 to 10 mu m. In germanium, carriers are excited to the split-off band by 250 fs pulses and the subsequent scattering dynamics is measured via the transient intervalence band absorption. The excited carriers undergo intervalence band scattering within 100 fs and thermalize with time constants around 700 fs. In contrast, equilibration among unexcited holes proceeds within 100 fs. Inelastic carrier-carrier collisions via the dynamically screened Coulomb interaction are the main mechanism of thermalization. In GaAs, we monitor the picosecond recombination of free holes with shallow accepters. After infrared photoionization of impurity atoms, hole capture occurs within several 10-11 S-1. Emission of single longitudinal optical phonons represents the predominant process by which the impurity ground state is repopulated.

Femtosecond luminescence spectroscopy of carrier thermalization in GaAs and InP

Thermalization of a non-equilibrium distribution of electrons and holes generated by femtosecond photoexcitation is studied in GaAs and InP via spectrally and temporally resolved luminescence. In both materials, a rapid onset of luminescence is observed over a broad spectral range from the bandgap up to 1.7 eV. The data demonstrate the redistribution of both electrons and holes over a wide energy range within 100 fs, even for excitation densities as low as 1017 cm-3. Equilibration is dominated by carrier-carrier collisions with scattering rates that are considerably higher than predicted by theoretical simulations using static screening of the interaction potential. The authors' Monte Carlo calculations, applying a molecular dynamics scheme, give enhanced scattering among the carriers with respect to a static screening approach, resulting in smoother distribution functions and luminescence spectra which are in good agreement with the experimental results.