Temperature-dependent Recombination Research Articles

Small Polaron Dynamics in Transition Metal Oxide Photoelectrodes

The transition metal oxides are widely used in many optoelectronic applications, such as working as charge-transporting layers in solar cells and photoelectrodes in light-driven water-splitting. Considering the ionic bonding nature, the lattice of these oxides can be polarized by the charge carriers due to the strong electron-phonon coupling, giving rise to lattice distortion that fosters a local potential well. This lattice distortion together with the trapped carrier is regarded as a quasi-particle, i.e., a polaron. Here, the photocarrier dynamics in transition metal oxides were investigated using the transient absorption and time-resolved terahertz spectroscopies. We identified the spectral features of small polarons, self-trapped excitons, and free carriers and extracted their dynamics independently. We proposed a molecular model that could successfully explain the non-barrier self-trapping and quantitatively describe the temperature-dependent recombination of self-trapped excitons and the dopant effect on the photocarrier dynamics. Our model also suggested that the recombination of self-trapped excitons at room temperature was dominated by nuclear quantum tunneling. This model also predicted the micro-heterogeneity of the self-trapped species, and thus be able to quantitatively describe the Auger annihilation dynamics of self-trapped excitons.

Temperature-dependent electroluminescence of red high-In-content MQWs of dual-wavelength micro-LED

Temperature-dependent electroluminescence (TDEL) measurements have been employed to investigate the carrier transport and recombination processes of InGaN red micro-LED based on dual-wavelength InGaN/GaN MQWs structure. EL peak energy and carrier transport of the red micro-LED both show temperature dependence, due to temperature-induced changes in defect activation. In addition, the current density at which the blue peak of the low-In-content appears in the EL spectrum varies with temperature. As the temperature increases, the blue peak of the low In component tends to appear at higher current densities, which may be attributed to the increase in thermally activated defects hindering the injection of holes into the low-In-content MQWs further away from p-GaN. Furthermore, the IQEs of the high-In-content MQWs are estimated from the TDEL method and then reveal the temperature-dependent efficiency droop. The IQE decreases as temperature increases, particularly above 50 K, where it drops sharply due to temperature-dependent nonradiative recombination. And the two different variation trends in IQE of MQWs with high and low In content reveal a competitive mechanism in carrier distribution, implying that more escaping holes from high-In-content MQWs will further reduce red emission efficiency but enhance carrier injection and blue emission in low-In-content MQWs.

Quantifying non-threshold Auger-recombination processes in mid-wavelength infrared range HgCdTe quantum wells

We study photoluminescence temperature quenching in HgTe/CdHgTe quantum wells (QWs) emitting at 3–4 μm wavelengths and recover temperature-dependent interband recombination rates. Recombination coefficients are determined for the process that we identify as a non-threshold ehh Auger process involving valence band continuum states in barriers. With the effective Auger coefficient CA reaching ∼10−13 cm4/s under resonant conditions, such a process is shown to determine stimulated emission thresholds in a wide temperature interval, while the contribution of conventional, activated Auger processes is presumably rather limited. Thus, threshold energy considerations should be used with caution for the optimization of HgCdTe QW lasers operating around 3 μm, and relatively low-barrier QWs may provide better performance than the high-barrier ones despite lower energy thresholds for thermally activated eeh-Auger recombination. It holds as long as the conduction band offset is detuned from the bandgap energy to avoid additional non-threshold eeh-processes and sufficient hole localization at elevated temperatures is maintained.

Manipulation of Temperature-Dependent Luminescence Behaviors of Mn4+-Activated Fluoride Phosphors.

In this paper, the giant tunability of thermal behaviors, i.e., from thermal deterioration to substantial growth, is firmly demonstrated for the vibronic luminescence of Mn4+ ions in fluoride phosphors. Such peculiar behavior is uncovered to be associated with the thermal excitation of a low-frequency phonon bath, and a theoretical model involving the excitation-wavelength-dependent populations of vibronic levels and the temperature-dependent nonradiative recombination processes is successfully constructed. Two main governing parameters, namely, the thermal activation energy Ea and the involved average phonon energy ΔE, are thus determined for the distinct thermal behaviors of Mn4+-ion luminescence. This demonstration may pave the way for manipulating the thermal behaviors of vibronic luminescence in solids to some extent.

Dielectronic recombination rate coefficients of carbon-like Kr30+

Dielectronic recombination (DR) rate coefficients for carbon-like Kr30+ have been measured over the collision energy of 0–60 eV using the heavy-ion storage ring CSRe at the Institute of Modern Physics in Lanzhou, China. The present DR spectrum covers the resonances associated with the , and core excitations. The corresponding DR resonance energies and strengths have been calculated by using the flexible atomic code (FAC) to understand the measured results. An overall agreement has been obtained between the experiment and theory, except for the data at the collision energies below 7 eV and in 35–38 eV, where the electronic correlation effect is strong. In particular, the resonances from the trielectronic recombination due to have been identified with the help of the FAC calculation. Temperature-dependent plasma recombination rate coefficients were derived from the measured DR rate coefficients for the temperature range 103–107 K and compared with our FAC calculations as well as the previous AUTOSTRUCTURE calculations by Zatsarinny et al (2004 Astron. Astrophys. 417 1173–81). The FAC and AUTOSTRUCTURE calculations are in good agreement with the presently derived plasma rate coefficients. The present work provides the benchmark data for astrophysical and laboratory plasma modeling.

Towards understanding photon absorption and emission in MgAl layered double hydroxide

MgAl-LDH, a UV-Vis luminescent material, reveals suitability to study in-depth the charge separation and temperature-dependent recombination pathways in LDHs.

Rate Coefficients for Dielectronic Recombination of Carbon-like 40Ca14+

Abstract Dielectronic recombination (DR) rate coefficients for carbon-like 40Ca14+ forming nitrogen-like 40Ca13+ have been measured using the electron–ion merged-beam technique at the heavy-ion storage ring CSRm at the Institute of Modern Physics in Lanzhou, China. The measured DR rate coefficients in the energy range from 0 to 92 eV cover most of the DR resonances associated with 2s 22p 2 → 2s 22p 2 and 2s 22p 2 → 2s2p 3 core transitions (ΔN = 0). Theoretical calculations of the DR cross sections were carried out by using two different state-of-the-art atomic theoretical techniques, multiconfiguration Breit–Pauli (MCBP) code AUTOSTRUCTURE and relativistic configuration interaction code FAC, to compare with the experimental rate coefficients. The theoretical calculations agree with the experimental results at collision energy higher than 10 eV. However, significant discrepancies of resonance energies and strengths can be found at collision energy below 8 eV. Temperature-dependent plasma recombination rate coefficients were derived from the measured DR rate coefficients in the energy range from 0.1 to 1000 eV and compared with the recommended atomic data from the literature. The theoretical data of Gu et al. and Zatsarinny et al. are 30% lower than the experimental results at the temperatures of photoionized plasmas, but have a very good agreement at the temperatures of collisionally ionized plasmas. Other previously published theoretical data of Jacobs et al. and Mazzotta et al. by using Burgess formula and LS-coupling calculations significantly underestimate the plasma rate coefficients in the low temperature range. The present results comprise a set of benchmark data suitable for astrophysical modeling.

Defect activation and annihilation in CIGS solar cells: an operando x-ray microscopy study

The efficiency of thin-film solar cells with a Cu( Gax)Se2 absorber is limited by nanoscopic inhomogeneities and defects. Traditional characterization methods are challenged by the multi-scale evaluation of the performance at defects that are buried in the device structures. Multi-modal x-ray microscopy offers a unique tool-set to probe the performance in fully assembled solar cells, and to correlate the performance with composition down to the micro- and nanoscale. We applied this approach to the mapping of temperature-dependent recombination for Cu( Gax)Se2 solar cells with different absorber grain sizes, evaluating the same areas from room temperature to . It was found that poor performing areas in the large-grain sample are correlated with a Cu-deficient phase, whereas defects in the small-grain sample are not correlated with the distribution of Cu. In both samples, classes of recombination sites were identified, where defects were activated or annihilated by temperature. More generally, the methodology of combined operando and in situ x-ray microscopy was established at the physical limit of spatial resolution given by the device itself. As proof-of-principle, the measurement of nanoscopic current generation in a solar cell is demonstrated with applied bias voltage and bias light.

Double D-centers related donor-acceptor-pairs emission in fluorescent silicon carbide

A new boron-induced deeper acceptor level (D*-center) different from the D-center in nitrogen-boron co-doped 6H fluorescent silicon carbide (f-SiC) is revealed by measuring the temperature-dependent photoluminescence (PL). The D*-center is correlated to the dominate donor-acceptor-pair (DAP) recombination at low temperature ranges in f-SiC with a PL peak around 1.90 eV. A hole-trap with an energy level that lies between the D*-center and the D-center is predicted to exist in the f-SiC samples. A two-step thermal ionization involving the hole-trap is proposed to explain the evolution of both D*-center and D-center related temperature-dependent DAP recombination.

Strongly temperature-dependent recombination kinetics of a negatively charged exciton in asymmetric quantum dots at 1.55 µm

We report on strongly temperature-dependent kinetics of negatively charged carrier complexes in asymmetric InAs/AlGaInAs/InP quantum dots (dashes) emitting at telecom wavelengths. The structures are highly elongated and of large volume, which results in atypical carrier confinement characteristics with s-p shell energy splittings far below the optical phonon energy, which strongly affects the phonon-assisted relaxation. Probing the emission kinetics with time-resolved microphotoluminescence from a single dot, we observe a strongly non-monotonic temperature dependence of the charged exciton lifetime. Using a kinetic rate-equation model, we find that a relaxation side-path through the excited charged exciton triplet states may lead to such behavior. This, however, involves efficient singlet-triplet relaxation via the electron spin-flip. Thus, we interpret the results as an indirect observation of strongly enhanced electron spin relaxation without a magnetic field, possibly resulting from atypical confinement characteristics.

Thermally Activated Second-Order Recombination Hints toward Indirect Recombination in Fully Inorganic CsPbI3 Perovskites.

The relationship between the dipole moment of the methylammonium cation and the optoelectronic properties of lead halide perovskites remains under debate. We show that both the temperature-dependent charge carrier mobility and recombination kinetics are identical for methylammonium and cesium lead iodide, indicating that the role of the monovalent cation is subordinate to the lead iodide framework. From the observation that for both perovskites the electron–hole recombination is thermally activated, we speculate that the bandgap is slightly indirect.

The relative impact of photoionizing radiation and stellar winds on different environments

Photoionizing radiation and stellar winds from massive stars deposit energy and momentum into the interstellar medium (ISM). They might disperse the local ISM, change its turbulent multi-phase structure, and even regulate star formation. Ionizing radiation dominates the massive stars’ energy output, but the relative effect of winds might change with stellar mass and the properties of the ambient ISM. We present simulations of the interaction of stellar winds and ionizing radiation of 12, 23, and 60 M⊙ stars within a cold neutral (CNM, n0 = 100 cm−3), warm neutral (WNM, n0 = 1, 10 cm−3), or warm ionized (WIM, n0 = 0.1 cm−3) medium. The flash simulations adopt the novel tree-based radiation transfer algorithm TreeRay. With the On-the-Spot approximation and a temperature-dependent recombination coefficient, it is coupled to a chemical network with radiative heating and cooling. In the homogeneous CNM, the total momentum injection ranges from 1.6 × 104 to 4 × 105 M⊙ km s−1 and is always dominated by the expansion of the ionized HII region. In the WIM, stellar winds dominate (2 × 102 to 5 × 103 M⊙ km s−1), while the input from radiation is small (∼ 102 M⊙ km s−1). The WNM (n0 = 1 cm−3) is a transition regime. Energetically, stellar winds couple more efficiently to the ISM (∼ 0.1 percent of wind luminosity) than radiation (< 0.001 percent of ionizing luminosity). For estimating the impact of massive stars, the strongly mass-dependent ratios of wind to ionizing luminosity and the properties of the ambient medium have to be considered.

Nonlinear interaction of an intense radio wave with ionospheric D/E layer plasma

This paper considers the nonlinear interaction of an intense electromagnetic wave with the D/E layer plasma in the ionosphere. A simultaneous solution of the electromagnetic wave equation and the equations describing the kinetics of D/E layer plasma is obtained; the phenomenon of ohmic heating of electrons by the electric field of the wave causes enhanced collision frequency and ionization of neutral species. Electron temperature dependent recombination of electrons with ions, electron attachment to O2 molecules, and detachment of electrons from O2− ions has also been taken into account. The dependence of the plasma parameters on the square of the electric vector of the wave E02 has been evaluated for three ionospheric heights (viz., 90, 100, and 110 km) corresponding to the mid-latitude mid-day ionosphere and discussed; these results are used to investigate the horizontal propagation of an intense radio wave at these heights.

Rethinking the theoretical description of photoluminescence in compound semiconductors

Semiconductor compounds, such as Ga(NAsP)/GaP or GaAsBi/GaAs, are in the focus of intensive research due to their unique features for optoelectronic devices. The optical spectra of compound semiconductors are strongly influenced by the random scattering potentials caused by compositional and structural disorder. The disorder potential is responsible for the red-shift (Stokes shift) of the photoluminescence (PL) peak and for the inhomogeneous broadening of the PL spectra. So far, the anomalous broadening of the PL spectra in Ga(NAsP)/GaP has been explained assuming two coexisting length scales of disorder. However, this interpretation appears in contradiction to the recently observed dependence of the PL linewidth on the excitation intensity. We suggest an alternative approach that describes the PL characteristics in the framework of a model with a single length scale of disorder. The price is the assumption of two types of localized states with different, temperature-dependent non-radiative recombination rates.

Temperature-dependent charge transport in solution-processed perovskite solar cells with tunable trap concentration and charge recombination

Based on control of the perovskite film thickness, we investigate temperature-dependent charge carrier transport, recombination, traps, and solar cell behavior based on methylammonium lead triiodide films.

Microwave extraction method of radiative recombination and photon lifetimes up to 85 °C on 50 Gb/s oxide-vertical cavity surface emitting laser

Optical modulation bandwidth for a semiconductor diode laser is governed by the thermally limited spontaneous radiative recombination lifetime, τrec, photon lifetime, τp, and cavity photon density for stimulated recombination. Thus, temperature dependent recombination lifetime is a critical parameter for the limitation of photonic device operations. Here, we develop a microwave extraction method to accurately determine the radiative recombination and photon lifetimes over a temperature range up to 85 °C through the equivalent circuit modeling based on the measured microwave scattering-parameters. For an 850 nm oxide-vertical cavity surface emitting laser with error free data transmission capability over 50 Gb/s, the extracted lifetimes are τrec = 0.1778 ns and τp = 4.2 ps at 25 °C and τrec = 0.2445 ns and τp = 4.8 ps at 85 °C.

Temperature-dependent recombination coefficients in InGaN light-emitting diodes: Hole localization, Auger processes, and the green gap

We obtain temperature-dependent recombination coefficients by measuring the quantum efficiency and differential carrier lifetimes in the state-of-the-art InGaN light-emitting diodes. This allows us to gain insight into the physical processes limiting the quantum efficiency of such devices. In the green spectral range, the efficiency deteriorates, which we assign to a combination of diminishing electron-hole wave function overlap and enhanced Auger processes, while a significant reduction in material quality with increased In content can be precluded. Here, we analyze and quantify the entire balance of all loss mechanisms and highlight the particular role of hole localization.

Temperature-dependent recombination velocity analysis on artificial small angle grain boundaries using electron beam induced current method

The details of the process of carrier recombination via the Shockley-Read-Hall (SRH) defect level, at the grain boundaries of multicrystalline silicon, were investigated. For this, the temperature-dependent recombination velocities, as determined by experiments, were analyzed by the application of an electron beam induced current method. For the model, the misorientation angles at the grain boundaries were defined using a multi-seed casting-growth method. The results of our experiments indicated different temperature behaviors at low and high temperatures. These can be explained by controlling the process anticipated by the SRH model, that is, the process whereby minority carriers (electrons) are captured at lower temperatures, followed by the reemission of the carriers before recombination with Arrhenius behavior at higher temperatures. The minority capture process appeared to conform to the power law T−α temperature behavior. Thus, there are two candidate electron capture mechanisms, namely, cascade phonon emission capture for shallow centers and excitonic-Auger capture for deep centers. The activation energy for the reemission of carriers was around 0.1 eV. These findings regarding the temperature dependence are essentially independent of the misorientation angles, suggesting a common defect level and recombination mechanism. The difference in the recombination velocities can be regarded as being derived from the difference in the density at the defect level.

A comparison of the optical properties of InGaN/GaN multiple quantum well structures grown with and without Si-doped InGaN prelayers

In this paper, we report on a detailed spectroscopic study of the optical properties of InGaN/GaN multiple quantum well structures, both with and without a Si-doped InGaN prelayer. In photoluminescence and photoluminescence excitation spectroscopy, a 2nd emission band, occurring at a higher energy, was identified in the spectrum of the multiple quantum well structure containing the InGaN prelayer, originating from the first quantum well in the stack. Band structure calculations revealed that a reduction in the resultant electric field occurred in the quantum well immediately adjacent to the InGaN prelayer, therefore leading to a reduction in the strength of the quantum confined Stark effect in this quantum well. The partial suppression of the quantum confined Stark effect in this quantum well led to a modified (higher) emission energy and increased radiative recombination rate. Therefore, we ascribed the origin of the high energy emission band to recombination from the 1st quantum well in the structure. Study of the temperature dependent recombination dynamics of both samples showed that the decay time measured across the spectrum was strongly influenced by the 1st quantum well in the stack (in the sample containing the prelayer) leading to a shorter average room temperature lifetime in this sample. The room temperature internal quantum efficiency of the prelayer containing sample was found to be higher than the reference sample (36% compared to 25%) which was thus attributed to the faster radiative recombination rate of the 1st quantum well providing a recombination pathway that is more competitive with non-radiative recombination processes.

The effects of temperature dependent recombination rates on performance of InGaN/GaN blue superluminescent light emitting diodes

The effects of temperature dependent radiative and nonradiative recombination (Shockley–Read–Hall, spontaneous radiative, and Auger coefficients) on the spectral and power characteristics of a blue multiple quantum well (MQW) superluminescent light emitting diode (SLD or SLED) have been studied. The study is based on the rate equations model, where three rate equations corresponding to MQW active region, separate confinement heterostructure (SCH) layer, and spectral density of optical power are solved self-consistently with no k-selection energy dependent gain and quasi-Fermi level functions at steady state. We have taken into account the temperature effects on Shockley–Read–Hall (SRH), spontaneous radiative, and Auger recombination in the rate equations and have investigated the effects of temperature rising from 300K to 375K at a fixed current density. We examine this procedure for a moderate current density and interpret the spectral radiation power and light output power diagrams. The investigation reveals that the main loss due to temperature is related to Auger coefficient.