Temperature Dependence Of Carrier Dynamics Research Articles

Auger-type process in ultrathin ReS2

The dramatic enhancement of charge carrier interaction makes many-body effects of great prominence in two-dimensional materials. Here we report the defect-assisted Auger scattering combined with band-to-band Auger recombination as playing the dominant recovery mechanism in the charge carriers of atomically thin-layered ReS2. Time resolved transient absorption spectra investigation reveals two different decay processes over the visible and near- infrared range, which is attributed to the shallow and deep defects introduced by the existence of sulfur (S) vacancy. A rate equation system is invoked to rationalize our peculiar pump and temperature dependence of carrier dynamics quantitatively. These findings provide theoretical insights into the significant role played by nonradiative Auger processes and may pave the way for the development of diverse ReS2-based high performance photonic and optoelectronic devices.

Temperature evolution of carrier dynamics in GaNxPyAs1−y−xalloys

The temperature dependence of carrier dynamics in GaNxAs1−yPy alloys has been investigated by time resolved photoluminescence. This investigation has shown that the decay time constant does not change significantly up to 100 K, and then starts to decrease rapidly above this temperature. Additionally, the decay times at the high-energy side of the spectrum decrease faster than those at the low-energy side. The effects have been explained by the interplay between carrier capture by radiative and nonradiative recombination centers. Detailed simulations show that the effect of carrier localization in the investigated materials is better described by double-scale potential fluctuations that are related to (i) distribution of localized states energy and (ii) bandgap fluctuations. In addition, it was observed that the increase in nitrogen concentration leads to a shorter decay time at room temperature, which is attributed to a larger concentration of non-radiative recombination centers. Furthermore, a post-growth annealing step leads to a longer decay time at room temperature, which is attributed to a reduction in non-radiative recombination centers. At low temperatures, the role of non-radiative centers is suppressed, and therefore the decay time does not differ significantly for samples with either different nitrogen concentrations or in both the as-grown and annealed samples.

Carrier Dynamics in a Series of Organic Magnetic Superconductors

Understanding how electrons behave, i.e., carrier dynamics, in superconductors is indispensable for understanding the mechanism of superconductivity. Recently we have reported the carrier dynamics of κ- and λ-(BETS)2MCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene, M = Ga, Fe) based on ultrafast spectroscopy, but the interpretation of the results remains an open question. In this paper we interpreted the results with the aid of newly measured magnetic susceptibility, X-ray single crystal structural analysis, and band calculation. Observation of coherent phonons only in the λ-type salts indicated that e-ph interaction should be characteristically strong in the λ-type salts. By comparison of the observed carrier dynamics with a two-temperature model, the temperature-dependence of carrier dynamics is consistently explained by the different strengths of the e-ph interaction between the λ- and the κ-type salts. The difference in strength of the e-ph interactions is related to their crystal structures. In conclusion, their carrier dynamics is consistently interpreted and classified by their crystal structures.

Temperature dependence of carrier dynamics for InAs∕GaAs quantum dot infrared photodetectors

Temperature-dependent micro-photoluminescence (μ-PL) spectra and the spectral response for the 30-period undoped InAs∕GaAs quantum-dot infrared photodetectors (QDIPs) are investigated. The competition of different transition levels within the spectra is observed. Also observed is the influence of dot size and density on the μ-PL characteristics. Consistently, the mid-infrared spectral response for the fabricated QDIPs exhibit the same energy position as the shifted PL spectra relative to the energy of wetting layer, which indicates the multi-transition mechanisms responsible for the QDIP spectral response.

Density and temperature dependence of carrier dynamics in self-organized InGaAs quantum dots

We have used two- and three-pulse femtosecond differential transmission spectroscopy to study the dependence of quantum dot carrier dynamics on temperature. At low temperatures and densities, the rates for relaxation between the quantum dot confined states and for capture from the barrier region into the various dot levels could be directly determined. For electron–hole pairs generated directly in the quantum dot excited state, relaxation is dominated by electron–hole scattering, and occurs on a 5 ps time scale. Capture times from the barrier into the quantum dot are of the order of 2 ps (into the excited state) and 10 ps (into the ground state). The phonon bottleneck was clearly observed in low-density capture experiments, and the conditions for its observation (namely, the suppression of electron–hole scattering for nongeminately captured electrons) were determined. As temperature increases beyond about 100 K, the dynamics become dominated by the re-emission of carriers from the lower dot levels, due to the large density of states in the wetting layer and barrier region. Measurements of the gain dynamics show fast (130 fs) gain recovery due to intradot carrier–carrier scattering, and picosecond-scale capture. Direct measurement of the transparency density versus temperature shows the dramatic effect of carrier re-emission for the quantum dots on thermally activated scattering. The carrier dynamics at elevated temperature are thus strongly dominated by the high density of the high energy continuum states relative to the dot confined levels. Deleterious hot carrier effects can be suppressed in quantum dot lasers by resonant tunnelling injection.

Analysis of traps in high quality CVD diamond films through the temperature dependence of carrier dynamics

The analysis of the time behavior of pulses generated by CVD diamond films irradiated with α-particles has been recently shown to be a general and powerful tool to investigate trap properties in diamond films (M. Marinelli et al., Phys. Rev. B 64 (2001) 195205). We use here this technique to measure the activation energy of traps in CVD diamond films, through the analysis of the temperature dependence of the dynamics of carriers generated by α-particle irradiation. The samples used in this study are high quality films deposited in a microwave tubular reactor and show very narrow diamond Raman peaks (FWHM approximately 2.4 cm −1) and extremely low photoluminescence background. The time evolution of the response of α-particle detectors built from these films exhibit both a fast and a slow component. Quantitative analysis leads to the conclusion that only deep traps limit the electron mean free path before trapping, while for holes both deep and shallow centers must be taken into account, the latter becoming the limiting factor in the pumped state. The changes in the pulse shapes are analyzed when the film temperature T is varied from −40 to 20 °C. A systematic speed-up of the response is found with increasing temperature, confirming that the slow component is due to thermally activated detrapping from the relative shallow defects. The detrapping time constant τ D is connected to the activation energy E D of the defects through the formula 1 /τ D = s exp(− E D / kT), where s is the attempt frequency. Plotting ln( τ D ) vs. 1/ T allows to determine the activation energy of the shallow defects, which is found to be 0.35 eV.

Temperature dependence of carrier dynamics in CdSexS1 −x doped glasses studied by two-color picosecond spectroscopy

We have examined the high temperature (25–360 °C) dependence of carrier dynamics in commercially available CdSexS1 −x nanocrystallite doped glasses using picosecond pump-probe measurements. Our results show that two samples of glass with similar absorption edges and size distributions exhibit different behavior in relaxation characteristics as a function of temperature.