Temperature-dependent Carrier Dynamics Research Articles

A realistic model of temperature dependent carrier diffusion constant in MAPbI3 films

The charge carrier mobility in perovskites is known to be affected by temperature. However, at low temperatures the perovskite film bears interfacial strains by deformation between the film and its substrate with difference coefficients of thermal expansion. Herein we provide a more realistic temperature-dependent carrier dynamics model involving both interfacial strain effect and intrinsic optoelectronic properties. The interfacial strain of a MAPbI3 film is confirmed through in-situ temperature-dependent X-ray diffractions. The carrier dynamics at different temperatures are unraveled using carrier dynamics fittings to time-resolved photoluminescence decays. An abnormal drop in carrier mobility is revealed with continuously decreasing temperature. The strain affects the trap density in films thus declines the carrier mobility at low temperatures. The carrier losses at the buried surface become significant at low temperatures due to the interfacial strain. This work provides a reminder that substrates should be carefully considered in temperature-related applications of perovskites.

Temperature-Dependent Ultrafast Spectral Response of FAPb(Br0.4I0.6)3 Nanocrystals

We employed transient absorption spectroscopy to investigate the temperature-dependent carrier dynamics of FAPb(Br0.4I0.6)3 nanocrystals (NCs). The carrier dynamics involved the hot-carrier (HC) cooling process, carrier–carrier interactions, and carrier recombination. With an increase in the absorbed photon flux or a decrease in the temperature, the HC cooling process slowed down and other carrier dynamics processes accelerated simultaneously. Moreover, the decrease in temperature caused the FAPb(Br0.4I0.6)3 NCs to pass through a cubic α-phase (Pm3̅m, #221) to a tetragonal β-phase (P4/mbm, #127) at around 175 K, and then retain the tetragonal β-phase (P4/mbm, #127) until 77 K. The β-phase showed a narrowing band gap, a small longitudinal-optical (LO) phonon energy, and a reduction in shallow defects compared with the α-phase. These factors together influence the carrier dynamics and the carrier temperature (TC) cooling process in different crystal structures. These results demonstrate that temperature-dependent spectral analysis is essential for unraveling carrier dynamics in organic–inorganic halide perovskites.

Carrier Transport Limited by Trap State in Cs2AgBiBr6 Double Perovskites.

Understanding the photoinduced carrier dynamics in Cs2AgBiBr6 double perovskites is essential for their application in optoelectronic devices. Herein, we report an investigation on the temperature-dependent carrier dynamics in a Cs2AgBiBr6 single crystal (SC). The time-resolved photoluminescence (TRPL) measurement indicates that the majority of carriers (>99%) decay through a fast trapping process at room temperature, and as the temperature decreases to 123 K, the population of carriers with a slow fundamental decay kinetics rises to ∼50%. We show that the carrier diffusion coefficient (theoretical diffusion length) varies from 0.020 ± 0.003 cm2 s-1 (0.70 μm) at 298 K to 0.11 ± 0.010 cm2 s-1 (2.44 μm) at 123 K. However, in spite of the long diffusion length, the population of carriers that can perform long-distance transport is restricted by the trap state, which is likely a key reason limiting the performance of Cs2AgBiBr6 optoelectronic devices.

Temperature-dependent carrier dynamics of InP/InGaP quantum structures grown at various growth temperatures using migration-enhanced epitaxy

The optical properties of InP/InGaP quantum structures (QSs) grown using migration-enhanced molecular beam epitaxy at growth temperatures ranging from 440 to 520 °C were investigated using temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopies. As the growth temperature increased from 440 to 520 °C, the PL peak energies originating from quantum dots and quantum dashes were blue-shifted, which is attributed to the intermixing of InP and Ga in the InGaP wetting layer. The structural and luminescence properties of InP/InGaP QSs strongly depend on the growth temperatures. The sample grown at 460 °C exhibited the strongest PL intensity at room temperature. In addition, as the temperature increased from 10 to 220 K, the PL decay time of the sample grown at 460 °C increased up to 100 K and then, gradually decreased up to 220 K. The PL decay times of the sample grown at 520 °C increased up to 60 K and then, rapidly decreased up to 120 K. The strongest PL intensity at room temperature and the longest decay time at 100 K for the sample grown at 460 °C indicate that the temperature of 460 °C is optimal for the formation of the InP/InGaP QSs.

Application of emitter-sample hybrid terahertz time-domain spectroscopy to investigate temperature-dependent optical constants of doped InAs.

We investigate temperature-dependent carrier dynamics of InAs crystal by using reflection-type terahertz time-domain spectroscopy, particularly with a recently developed emitter-sample hybrid structure. We successfully obtain the optical conductivity in a terahertz frequency of bulk InAs whose dc conductivity is in the range of 100-150 Ω-1 cm-1. We find that both real and imaginary parts of the optical conductivity can be fit well with the simple Drude model, and the free-carrier density and the scattering rate obtained from the fit are in good agreement with corresponding values obtained by using other techniques, such as the Hall measurement and the dc-resistivity measurement. These results clearly demonstrate that the proposed technique of adopting the emitter-sample hybrid structure can be exploited to determine temperature-dependent optical constants in a reflection geometry and hence to investigate electrodynamics of bulk metallic systems.

Annealing time on carrier dynamics of ZnTe nanoparticles embedded in a near ultraviolet-transparent glass

ZnTe nanocrystals were successfully grown by heat-treating a host glass matrix that had been synthesized by fusion. Room temperature optical absorption and photoluminescence excitation spectra and atomic force microscopy images showed the formation of three well-defined, different sized groups of nanocrystals: quantum dots and bulk-like nanocrystals. Photoluminescence spectra showed emissions belonging to both of these groups as well as deep defects in the electronic structure of the ZnTe nanocrystals. Strong agreement between the fit of the model based in rate equation and the experimental integrated photoluminescence intensity data showed that this described the temperature dependent carrier dynamics of ZnTe nanocrystals.

Temperature dependent carrier dynamics in telecommunication band InAs quantum dots and dashes grown on InP substrates

InAs quantum dots (QDs) grown on InP substrates can be used as light emitters in the telecommunication bands. In this paper, we present optical characterization of high-density circular quantum dots (QDots) grown on InP(311)B substrates and elongated dots (QDashes) grown on InP(001) substrates. We study the charge carrier transfer and luminescence thermal quenching mechanisms of the QDots and QDashes by investigating the temperature dependence of their time-integrated and time-resolved photoluminescence properties. This results in two different contributions of the thermal activation energies. The larger activation energies are attributed to the carrier escape to the barrier layer and the wetting layer (WL) from QDots and QDashes, respectively. The smaller activation energies are found to be originated from inter-dot/dash carrier transfer via coupled excited states. The variation of the average oscillator strength associated with the carrier re-distribution is discussed. The relation of the two activation energies is also quantitatively studied with the measurements of excited-state and ground-state energy separations. Finally, we show an approach to isolate individual quantum dots or dashes in a suitable nanostructure.

Characteristics of THz carrier dynamics in GaN thin film and ZnO nanowires by temperature dependent terahertz time domain spectroscopy measurement

We present a comprehensive study of the characteristics of carrier dynamics using temperature dependent terahertz time domain spectroscopy. By utilizing this technique in combination with numerical calculations, the complex refractive index, dielectric function, and conductivity of n-GaN, undoped ZnO NWs, and Al-doped ZnO NWs were obtained. The unique temperature dependent behaviors of major material parameters were studied at THz frequencies, including plasma frequency, relaxation time, carrier concentration and mobility. Frequency and temperature dependent carrier dynamics were subsequently analyzed in these materials through the use of the Drude and the Drude–Smith models.

Carrier dynamics in the luminescent states of Cd1−xMnxS nanoparticles: effects of temperature and x-concentration

Optical absorption (OA), magnetic force microscopy (MFM), and photoluminescence (PL) measurements were employed to study Cd(1-x)Mn(x)S nanoparticles (NPs), grown in a glass matrix, at different x-concentrations. The formation of two well defined groups of NPs with different sizes was detected by OA spectra at room temperature and confirmed by MFM images, from which they were identified as quantum dots (QDs) and bulk-like nanocrystals (NCs). Emissions from luminescent states were observed in the temperature dependent PL spectra of both groups of NPs, including those from deep defects which were attributed to the presence of divacancies (V(Cd)-V(S)) in the hexagonal wurtzite structure. Furthermore, we have come up with a model based on rate equations that describes energy transfers involving the excitonic states of QDs, the conduction band of bulk-like NCs, and the shallow virtual levels of NPs. This model was used to fit the integrated PL intensity of the corresponding NP groups, and a good agreement between them confirms that the model suitably describes the temperature dependent carrier dynamics of Cd(1-x)Mn(x)S NPs.

Ultrafast carrier dynamics in an InAs/InGaAs quantum dots-in-a-well heterostructure

Ultrafast differential transmission spectroscopy is used to explore temperature-dependent carrier dynamics in an InAs/InGaAs quantum dots-in-a-well heterostructure. Electron-hole pairs are optically injected into the three dimensional GaAs barriers, after which we monitor carrier relaxation into the two dimensional InGaAs quantum wells and the zero dimensional InAs quantum dots by tuning the probe photon energy. We find that carrier capture and relaxation are dominated by Auger carrier-carrier scattering at low temperatures, with thermal emission playing an increasing role with temperature. Our experiments provide essential insight into carrier relaxation across multiple spatial dimensions.

Temperature-dependent carrier dynamics in metallic carbon nanotubes

The low energy states in metallic armchair carbon nanotubes can decay by the electron–electron Coulomb interactions with the intraband and the interband excitations. The inelastic scattering rate is very sensitive to the changes in the electronic distribution and the state energy (or the wavevector). The temperature dependence is linear for the Fermi-momentum state. Such dependence is hardly affected by the nanotube radius and the Fermi energy. The electron–electron interactions are more efficient in electronic deexcitations compared with the electron–phonon interactions. The calculated results are roughly consistent with the experimental measurements from the time-resolved photoemission spectroscopy.

Temperature-dependent carrier dynamics in self-assembled InGaAs quantum dots

We measured the transient temperature-dependent carrier population in the confined states of self-assembled In0.4Ga0.6As quantum dots as well as those of the surrounding wetting layer and barrier region using differential transmission spectroscopy. Results show directly that thermal reemission and nonradiative recombination contribute significantly to the dynamics above 100 K. We offer results of an ensemble Monte Carlo simulation to explain the contribution of these thermally activated processes.