Impact Ionization Of Excitons Research Articles

Transition from Diffusive to Superdiffusive Transport in Carbon Nanotube Networks via Nematic Order Control.

The one-dimensional confinement of quasiparticles in individual carbon nanotubes (CNTs) leads to extremely anisotropic electronic and optical properties. In a macroscopic ensemble of randomly oriented CNTs, this anisotropy disappears together with other properties that make them attractive for certain device applications. The question however remains if not only anisotropy but also other types of behaviors are suppressed by disorder. Here, we compare the dynamics of quasiparticles under strong electric fields in aligned and random CNT networks using a combination of terahertz emission and photocurrent experiments and out-of-equilibrium numerical simulations. We find that the degree of alignment strongly influences the excited quasiparticles' dynamics, rerouting the thermalization pathways. This is, in particular, evidenced in the high-energy, high-momentum electronic population (probed through the formation of low energy excitons via exciton impact ionization) and the transport regime evolving from diffusive to superdiffusive.

Mechanisms of exciton photoluminescence quenching in the electric field of a standing surface acoustic wave

Mechanisms of exciton photoluminescence (PL) quenching in the longitudinal electric field of a standing surface acoustic wave (SAW) have been studied by the example of type II GaAs/AlAs superlattices (SLs). Such SLs with a long lifetime of nonequilibrium carriers have allowed examining the influence of the SAW electric field on the excitonic PL both under the continuous and impulse laser excitations. It has been found that the mechanisms of the interaction of excitons and a SAW electric field depend upon the kinetic energy of excitons and carriers. As for hot excitons and carriers, the standing SAW electric field causes the impact ionization of excitons with a subsequent capture of free carriers at the nonradiative recombination centers, which results in a decrease in the steady-state exciton PL. As for cold excitons and carriers, the impact of excitons with the carriers accelerated by the SAW electric field results mainly in exciton delocalization from the levels of quantum wells formed due to interface roughness with a subsequent capture of excitons at the nonradiative recombination centers, which leads to the acceleration of the PL kinetics.

Impact ionization of excitons in Ge/Si structures with Ge quantum dots grown on the oxidized Si(100) surfaces

Photoconductivity (PC) of Si/Ge structures with Ge quantum dots (QDs) grown on the Si(100) surfaces covered with the ultrathin, about 0.3–0.5 nm thick, SiO2 films is studied as a function of the interband light intensity under various lateral voltages. The structures exhibit PC with steps and a step with a peak at the step edge for low- and high-temperature grown structures, respectively. These PC features are associated with the impact ionization of QD-related excitons. The PC at step edges increases by several orders of magnitude for a certain value which is governed by the balance between rates of photo-generation, recombination, and impact ionization of excitons. The electron localization deeper in Si from the Ge QD layer in conjunction with a narrow binding-energy distribution of excitons is suggested to be the main reason that provides the sharpness of PC steps. The PC appears to be very sensitive to the impact ionization and QD preparation conditions. This allows revealing the specific characteristics of QD structures, related to their electronic and structural properties.

Impact ionization of excitons in single-crystal silicon and its effect on the exciton concentration and luminescence near the fundamental absorption edge

The effect of impact ionization of excitons by free charge carriers on the exciton concentration in single-crystal silicon (c-Si) at room temperature and at a high injection level is investigated. At sufficiently high concentrations of free electrons (n), the impact ionization dominates over thermal ionization. At such n, the effect results in much lower exciton concentrations (nex) compared to those with disregard of it and linear or almost linear portions in the dependences nex(n) and the dependences of the near-band-edge luminescence intensity in c-Si on the intensity of its excitation. The proposed technique for calculating nex can be developed for other semiconductors at other temperatures.

Electric-field-induced impact ionization of excitons in GaN and GaN/AlGaN quantum wells

Impact ionization of exciton states in epitaxial GaN films and GaN/AlGaN quantum-well structures was studied. The study was done using an optical method based on the observation of exciton photoluminescence quenching under application of an electric field. It was established that electron scattering on impurities dominates over that from acoustic phonons in electron relaxation in energy and momentum. The mean free path of the hot electrons was estimated. The hot-electron mean free path in GaN/AlGaN quantum wells was found to be an order of magnitude larger than that in epitaxial GaN films, which is due to the electron scattering probability being lower in the two-dimensional case.

Impact ionization of excitons in an electric field in GaN

The impact ionization of excitonic states is studied in epitaxial GaN filmsand GaN/AlGaN quantum well structures. The investigation is carried out byoptical methods involving the observation of quenching of the excitonphotoluminescence under an applied electric field. It is found that impurityscattering rules the momentum and energy relaxation, rather than the acousticphonon scattering. The effective mean free path of hot electrons is estimated.In GaN/AlGaN quantum wells the mean free paths of hot electrons appear to bean order of magnitude larger than those of GaN films due to the decrease inscattering probability of the electron in the two-dimensional case.

Excitonic photoluminescence quenching by impact ionization of excitons and donors inGaAs/Al0.35Ga0.65Asquantum wells with an in-plane electric field

We present a detailed experimental study on photoluminescence quenching due to exciton and donor impact ionization by accelerated electrons under an in-plane nanosecond duration electric field created in $\mathrm{GaAs}/{\mathrm{Al}}_{0.35}{\mathrm{Ga}}_{0.65}\mathrm{As}$ quantum wells. From the photoluminescence transients measured by the time-correlated single-photon counting technique, we have determined the experimental conditions under which donor impact ionization can have an influence on quenching of the excitonic photoluminescence. The coefficient of two-dimensional exciton impact ionization has been estimated; its dependences on the applied electric field, lattice temperature, and width of the quantum wells are given.

Impact Ionization of Excitons in an Electric Field in GaN

Quenching of the exciton luminescence of GaN epitaxial films and GaN/AlGaN quantum wells in an electric field is observed and attributed to impact ionization of excitonic states by hot electrons. It is found that impurity scattering rules the momentum relaxation of electrons rather than the acoustic phonon scattering. The effective mean free path of hot electrons l(eff) is estimated. The value of l(eff) for quantum wells appears to be one order of magnitude larger than that for epitaxial films.

Impact ionization of excitons by hot carriers in quantum wells

Dissociation of free excitons into electrons and holes, due to collisions between the excitons and charge carriers accelerated by an in-plane electric field in a multiple quantum well structure, is considered. We found that at normally used 2D excitonic concentrations the impact ionization process is rather strong and dominates over the exciton formation process at electric fields of the order of a few hundred V .

Photoluminescence transients due to donor and exciton avalanche breakdown

Time dependence of excitonic, free electron–acceptor and donor–acceptor-pair photoluminescence (PL) was investigated at low temperatures, when impact ionization of shallow donors and excitons predominates. The PL transients were excited by periodically applying short electrical pulses to epitaxial n-GaAs samples illuminated by a continuous laser irradiation. The experimental results are analyzed by using the time-dependent rate equation system, with the donor and exciton impact ionization and thermal recombination processes taken into account. A critical analysis of corresponding rate coefficients and collection of coefficients for simulation of the avalanche breakdown transients are presented.

Optically detected Auger recombinations in erbium- and ytterbium- doped InP

Microwave-induced impact ionization of excitons and shallow donors is studied in Er- and Yb-doped InP. The experimental results indicate a high efficiency of free-electron-related Auger recombination. Yb and Er intrashell emissions are deactivated due to an energy transfer to either free or bound carriers. A new Auger-type nonradiative process is suggested in which the rare-earth-related photoluminescence decays via an energy transfer to a free carrier which has become localized via a short-range potential.

Optically detected microwave-induced impact ionization of ytterbium bound excitons in InP

Optically detected microwave-induced impact ionization of excitons and shallow donors is studied in Yb-doped InP grown by metalorganic chemical vapor deposition. The experimental results directly confirm that Yb3+ intrashell emission is induced by nonradiative recombination of Yb bound excitons due to an impurity Auger effect. Yb3+ ions in InP are found to bind excitons with the electron being localized first, followed by subsequent hole capture.

Impact ionization of free excitons in stressed pure germanium.

The low-temperature photoluminescence of free excitons in pure Ge is studied in a weak electric field under uniaxial stress. The photoluminescence intensity of free excitons suddenly drops due to the impact ionization of excitons with increasing applied electric field. On the other hand, the photocurrent sharply increases for the same reason. The breakdown electric field for the impact ionization of excitons decreases to around 40% at 44 MPa in 〈111〉-stressed Ge from the zero-stress value. We speculate that this decrease is due to the decrease in binding energy of the exciton with stress.

Optically Detected Cyclotron Resonance of Exciton and Electron-Hole Droplet Systems in Pure Germanium

Optically detected cyclotron resonance (ODCR) in pure-Ge has been observed. Experimental results are (1) hole resonance has been observed stronger at ODCR than at usual cyclotron resonance (CR), (2) effective mass dependence of signal intensity of electron resonance at ODCR is different from one at CR, and (3) both free carrier-exciton and free carrier-exciton-electron hole droplet coexisting systems are not expressed by a single carrier temperature when free carriers are rather hot through microwave absorption. By means of kinetic equations it is shown that the origin of ODCR observation is the impact ionization of excitons by free carriers in the free carrier-exciton coexisting system.

Optical and Electrical Bistability Induced by Exciton Ionization Processes

AbstractNonlinear optical effects in the luminescence and the microwave absorption which are induced by heating of electrons in a microwave electric field are revealed in silicon: (1) Hysteresis in the exciton condensation process at T = 2 K which results in the bistability of the recombination radiation, with a hysteresis lifetime of 3 × 10−2s. (2) Self‐oscillations in the electron density during impact ionization of excitons, the frequency of the self‐oscillations varies over the range 0.5 to 5 MHz while changing the light excitation or microwave power. (3) Thermal exciton breakdown which is accompanied by jumps and hysteresis of the sample temperature as the excitation is varied.

Optoelectronic Bistability with Internal Feedback in Cadmium Sulfide

AbstractOptoelectronic bistability with internal feedback in CdS is studied in the presence of an electric field. Feedback results from heating of the crystal by exciton impact ionization current and absorption induced by an electric field.

Impact ionization of excitons and electron-hole droplets in silicon.

The low-temperature photoluminescence of bound excitons and electron-hole droplets in $n$-type Si is studied in a weak electric field for the first time, using simultaneous pulsed excitation for both the laser excitation and the applied electric field. The luminescence is quenched because of impact ionization by field-accelerated electrons and holes. The quenching starts around 50 V/cm, depending on sample purity and excitation intensity.

Exciton Impact Ionization and Electric Conductivity in Germanium at Low Temperature

AbstractThe exciton luminescence intensity and the electric conductivity are measured simultaneously during optical excitation in high‐purity Ge and in p‐doped Ge at 4.2 K as a function of an applied electric field. With increasing field the exciton density decreases due to exciton impact ionization. The carriers which are set free by this process result in a distinct increase in the electric conductivity. For undoped Ge the results agree well with a rate equation model based on impact ionization of the excitons. In the p‐doped Ge impurity impact ionization is found to have a strong influence in addition to impact ionization of the excitons.

Impact ionization of excitons and shallow donors in InP

Quenching of neutral-donor- and exciton-related photoluminescence in high-purity liquid-phase-epitaxial and vapor-phase-epitaxial InP samples is studied as a function of applied weak electric fields at liquid-He temperatures. Field-dependent suppression of neutral-donor-to-acceptor and free- and bound-exciton recombination peaks in the spectrum is attributed to impact ionization of the shallow donors and excitons by hot electrons which were accelerated by the applied field. More rapid quenching of the neutral-donor-bound and neutral-acceptor-bound exciton peaks with increasing field strength as compared to neutral-donor and free-exciton peaks is attributed to the dissociation of free excitons from the neutral centers by impact ionization.

Effect of Electric Fields on Luminescence in GaAs

AbstractPhotoluminescence spectra near the exciton region are investigated under the application of external electric fields parallel and perpendicular to epitaxial GaAs surfaces. Quenching of the luminescence is observed and explained in terms of impact ionization of excitons. The emission peaks of the upper (UPB) and lower polariton branches (LPB) shift to higher and lower energy sides, respectively, with increasing the electric fields or the excitation intensity. The calculated peak of the LPB emission is located around 1.515 eV and shifts to higher energy side and dominates the UPB emission, which is contradictory to the present observation. New emission peak appears at 1.515 eV under an application of an electric field in pure samples with carrier concentrations less than 6×1013 cm−3.