Temperature dependent excitonic relaxation in CdSe/ZnSe quantum islands: experiment and computer simulation

Abstract

In this contribution, a Monte-Carlo computer simulation of phonon-assisted relaxation processes of excitons in CdSe/ZnSe quantum islands is presented. With the same set of parameters, it was possible to reproduce both the temperature dependence of the energy position and of the full width at half maximum (FWHM) of the luminescence emitted from a ZnCdSe quantum film containing 5 to 10 nm wide Cd-rich islands. With help of the model, the energy dependence of the distribution of the localized states is shown to be weaker than Gaussian. Furthermore, the obtained fitting parameters give evidence for an intra-island excitonic relaxation. Finally, our results, based on a realistic description of the exciton kinetics, are compared with those of a rate equation model. 1 Introduction In localized systems, and among them in quantum-dot systems, thermally activated excitons can transfer to adjacent states with the help of phonon-assisted processes. As a result, the energy position of the photoluminescence (PL) maximum exhibits a "red-blue-red"-shift with increasing temperature [1]. This anormalous shift is well known for various quantum-well systems, where it was observed experimentally [2-4] and described theoretically [5]. For quantum dot systems a rate equation approach has been used [6]. In these systems, the shift has been interpreted in terms of an inter-island relaxation [7]. Interpretation of optical spectra has, however, indicated the fractal nature of the island shape [8,9]. In the resulting potential, intra-island hopping relaxation of excitons would be possible.In this paper, the scenario of an intra-island spatial relaxation is tested. Experimental results for the anomalous PL shift are compared with a Monte-Carlo simulation of phonon-assisted hopping [10]. After a description of the sample and of the experimental data, the operating principle of the simulation and its application to our experimental data are presented. Then the obtained parameter sets and their meaning for the distribution of localization centers for hopping processes will be discussed. Finally, the model is compared with the results of a calculation based on rate equations.

Band‐Tail Recombination in Hybrid Lead Iodide Perovskite

“…Our overall analysis reveals the band‐tail PL emission from FAPbI 3 to be consistent with typical behavior for an inorganic semiconductor, described by a band‐structure picture with a relatively modest degree of energetic disorder as reflected by spatial variation of the bandgap energy. The maximum value of the anomalous relative Stokes shift we observe for FAPbI 3 at low charge‐carrier densities (≈40 meV) is similar to the values observed in other inorganic semiconductors such as AlInGaN (≈45 meV), GaInNAs (≈55 meV), and CdSe/ZnSe (≈20 meV), and band‐tail emission has also been observed in CIGS and CZTS . Despite often being misidentified until represented on a log‐log plot, power‐law luminescence decays have also been identified in a range of systems, including semiconductor quantum dots, dye‐sensitized nanoparticle films, and semiconductors such as hydrogenated amorphous Si,[54a] p‐type GaAs, and GaP .…”

Comment on “Interplay of Structural and Optoelectronic Properties in Formamidinium Mixed Tin‐Lead Triiodide Perovskites”

“…A fit of Δε( x ) and 〈ε〉 Stokes ( x ), as measured by Parrott et al., requires quantitative relations between ε 0 ( x ) on the one hand and Δε( x ) and 〈ε〉 Stokes ( x ) on the other hand. Computer simulations [ 14,15 ] predict that Δε( x ) ≈ βε 0 ( x ) and 〈ε〉 Stokes ( x ) ≈ γε 0 ( x ) with factors β and γ varying in the range 2 ≲ β, γ ≲ 8, depending on material parameters and on temperature. Without requiring the precise values for β and γ, this comment elucidates the excellent agreement between the values x a , b for the maxima of the disorder energy scale ε 0 ( x ), as predicted by Equations (1), (2), and (3), and the Sn concentrations for the maximal disorder effects in the luminescence linewidth Δε( x ) and in the Stokes shift 〈ε〉 Stokes ( x ), observed by Parrott et al.…”