An Instantaneous Recombination Rate Method for the Analysis of Interband Recombination Processes in ZnO Crystals.

Luigi Santamaria,Pasqualino Maddalena,Stefano Lettieri

doi:10.3390/ma15041515

Abstract

Time-resolved photoluminescence (TRPL) analysis is often performed to assess the qualitative features of semiconductor crystals using predetermined functions (e.g., double- or multi-exponentials) to fit the decays of PL intensity. However, in many cases—including the notable case of interband PL in direct gap semiconductors—this approach just provides phenomenological parameters and not fundamental physical quantities. In the present work, we highlight that within a properly chosen range of laser excitation, the TRPL of zinc oxide (ZnO) bulk crystals can be described with excellent precision with second-order kinetics for the total recombination rate. We show that this allows us to define an original method for data analysis, based on evaluating the “instantaneous” recombination rate that drives the initial slope of the decay curves, acquired as a function of the excitation laser fluence. The method is used to fit experimental data, determining useful information on fundamental quantities that appear in the second-order recombination rate, namely the PL (unimolecular) lifetime, the bimolecular recombination coefficient, the non-radiative lifetime and the equilibrium free-carrier concentration. Results reasonably close to those typically obtained in direct gap semiconductors are extracted. The method may represent a useful tool for gaining insight into the recombination processes of a charge carrier in ZnO, and for obtaining quantitative information on ZnO excitonic dynamics.

Highlights

The phenomenon of band-to-band photoluminescence (PL) in semiconductor materials is caused by the radiative recombination of free charge carriers, generated via the absorption of optical radiation, whose photon energy is larger than the bandgap energy of the material.Surface traps, bulk defects and band offsets at interfaces significantly affect the rates of charge recombination [1,2,3,4] and of PL intensity, by introducing efficient non-radiative recombination pathways and/or by affecting the average spatial separation between electrons and holes
We focused the present work on developing a simple method for the data analysis of Time-resolved photoluminescence (TRPL) in zinc oxide (ZnO), aiming to determine the actual physical quantities that are defined in the second-order rate equation for the electron-hole recombination, namely, the unimolecular lifetime, τ, the bimolecular coefficient, B and the equilibrium density of free carriers, N
The results are presented in two separate subheadings, where the first describes the theoretical model used to set the analytical procedure of data analysis, while the second shows and discusses the experimental results obtained for ZnO bulk crystals

Summary

Introduction

The phenomenon of band-to-band (or “interband”) photoluminescence (PL) in semiconductor materials is caused by the radiative recombination of free charge carriers (i.e., valence holes and conduction electrons), generated via the absorption of optical radiation, whose photon energy is larger than the bandgap energy of the material.Surface traps, bulk defects and band offsets at interfaces significantly affect the rates of charge recombination [1,2,3,4] and of PL intensity, by introducing efficient non-radiative recombination pathways and/or by affecting the average spatial separation between electrons and holes. The optical PL spectroscopy of semiconductors is often employed to assess crystal quality and surface/interface characteristics [5,6,7,8,9,10] Very often, these assessments do not involve quantitative analysis and phenomenological approaches are sufficient for extracting the desired information. A multi-exponential function would be the correct description of a PL decay only if the total recombination was caused by different populations of excited charges all decaying simultaneously via a first-order kinetic process. In reality, this does not occur: it is instead well established in the theory of interband recombination [11,12] that the electron-hole decay does not follow first-order kinetics, except for very low charge densities. There is just one population of excited charges involved in a direct-bandgap semiconductor, namely the free carrier population (electrons energetically close to the minimum of the conduction band and holes close to the maximum of the valence band)

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Conclusion